August 2012

Introduction to the

Microbiology of

Food Processing

 Small Plant News Guidebook Series

United States 

Department of 

Agriculture

Food Safety and 

Inspection Service
Small Plant News is a four-page, four-color newsletter published by the U.S. 

Department of Agriculture’s (USDA) Food Safety and Inspection Service (FSIS). 

It is targeted to small and very small Federal- and State-inspected establishment 

owners and operators who produce meat, poultry, and processed egg products. 

Small Plant News’s mission is to support the “FSIS’ Strategic Implementation 

Plan for Strengthening Small and Very Small Plant Outreach” by providing 

pertinent information for plant owners and operators so they can produce safe 

food and, ultimately, ensure the success of their livelihoods. 

The newsletter strives to do this through: 

✔	 Informing and educating small and very small plant owners and operators 

on FSIS news with current and meaningful information in an easy-to-read 

format. 

✔	 Assisting plant owners and operators in incorporating FSIS rules and 

regulations into their daily operational practices with “plain language” 

information. 

✔	 Fostering small and very small plants’ ability to stay in business and produce 

the safest food by providing essential tips that will encourage the highest 

sanitation standards, paperwork compliance, and cost-saving measures. 

✔	 Honoring FSIS’ obligations to small and very small plants by providing a 

mechanism that increases two-way dialogue between plants and the Agency. 

Back issues of Small Plant News are available on FSIS’ Web site at 

www.fsis.usda.gov. Or you may call the Small Plant Help Desk at 

(877) 374-7435 to order back copies. 
United States Department of Agriculture

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Table of Contents 

Introduction.................................................................................................... 4
 

Food Intoxication Versus Food Infection ........................................................ 6
 

Significant Microorganisms in Food Production ............................................ 7
 

Foodborne Pathogens...................................................................................... 9
 

Bacteria.................................................................................................... 9
 

General Information on Bacteria .................................................... 9
 

Pathogenic Bacteria of Importance ............................................... 13
 

Molds .................................................................................................... 25
 

Yeasts ..................................................................................................... 26
 

Viruses................................................................................................... 27
 

Parasites................................................................................................. 28
 

Control of Foodborne Pathogens .................................................................. 31
 

Microbiological Sampling Programs............................................................. 52
 

Design of a Sampling Plan .................................................................... 52
 

Essential Components of a Sampling Plan.................................... 53
 

Product Versus Environmental and Food Contact Surface Sampling ... 55
 

Environmental and Food Contact Surface Sampling Programs.... 56
 

Sampling Frequency.............................................................................. 56
 

Analyzing Results .................................................................................. 56
 

Aseptic Sampling Techniques ................................................................ 57
 

Glossary of Useful Terms............................................................................... 58
 

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Introduction to the Microbiology of Food Processing

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Introduction
 

Microbiology refers to the study of microorganisms. As the name suggests, 

microorganisms are organisms that are so small they can only be seen using 

a microscope. Bacteria, fungi (such as yeasts and molds), protozoa, viruses, 

algae, and some parasites are all types of microorganisms. Some people also 

list prions as microorganisms even though they are proteins and not actually 

organisms. (You may remember that prions are the cause of bovine spongiform 

encephalopathy [BSE] and its human variation, Variant Creutzfeldt-Jakob 

Disease [vCJD].) 

Microorganisms are all around us, and they are capable of surviving in a wide 

array of environmental conditions. They are a necessary part of our world and 

perform a variety of useful functions. Here are some examples: 

V Fermentation 

Fermentation, the conversion of carbohydrates into sugar and alcohol, is 

required for the production of beer, wine, many types of cheeses and breads, 

and some sausages. 

V Digestion 

Microorganisms help people digest fruits and vegetables, just as they help 

animals digest plants. 

V Provide vitamins 

Some microorganisms synthesize the vitamins we need to stay healthy. 

V Recycling 

There are other types of microorganisms that have the ability to break down 

organic matter (material containing carbon that comes from what used to 

be a living organism) and return it to the earth in a recycling process to 

form food for plants, which in turn provide food for animals. This process 

of breaking down organic matter is part of what we normally think of as 

spoilage. All raw foods contain microorganisms that will eventually spoil 

and break down the food. Without such microorganisms, the earth would 

accumulate dead animals, plants, and other non-decayed matter. 
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Unfortunately, microorganisms also can be detrimental. They are the 

cause of many diseases in humans, animals, and plants. Disease-causing 

microorganisms are called pathogens. A pathogen, or the substances it 

produces, must invade the human, animal, or plant body to cause illness. 

While many diseases are transmissible from person to person or from animals 

to humans, only a few are transmitted through foods. Diseases that are 

caused by eating food are usually referred to as food poisoning or foodborne 

illnesses. This guidebook serves as a broad and very basic introduction to the 

microorganisms associated with food production, and in particular, bacterial 

pathogens. FSIS hopes it will assist you, the small and very small plant owner 

and operator, with your understanding of the microorganisms responsible 

for foodborne illness and, thus, enhance your ability to use microbial control 

interventions more effectively in your operations. 

C. streptococcus sp 
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Food Intoxication Versus Food Infection
 

Microbial foodborne illness is often divided into two broad categories: food 

intoxication and food infection. 

Food intoxication occurs when the pathogenic microorganism secretes a toxin 

in the food. Eating the food that contains the toxin disrupts a particular 

target, such as the gastrointestinal tract or the nervous system. The symptoms 

of intoxication vary from bouts of vomiting and diarrhea to severely disrupted 

muscle function, as with botulism. 

Food infection occurs when the microorganism multiplies in food until 

it reaches the minimum infective dose (MID), which is the number of 

microorganisms needed to cause illness in humans. When the food is eaten, 

the microorganism acts directly on the intestines. In some cases, the microbes 

infect the surface of the intestine; in others, they invade the intestine and other 

body structures. Most food infections result in some degree of diarrhea and 

abdominal distress. Food infection also can result in toxin production, causing 

the same disease signs. However, in food infection, microbes growing in the 

infected tissue release the toxins, unlike food intoxication, where the toxins are 

already present in the food. 

The MID varies for different pathogens and can range from as few as 10 cells 

for the Shigella species (and possibly Escherichia coli O157:H7) to as many as 

100,000 cells for Staphylococcus aureus. Some of the factors that influence the 

infections include: 

V The immune status of the host—immunodepressed or immunocompetent. 

V How efficient the pathogen is at attaching to, and penetrating, the target
 

tissues.
 

V The number of pathogenic organisms entering the body. 
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Significant Microorganisms in Food Production
	

Microorganisms such as molds, yeasts, and bacteria can grow in food and cause 

spoilage. Bacteria also can cause foodborne illness. Viruses and parasites, such 

as tapeworms, roundworms, and protozoa, can cause foodborne illness, but 

they are not capable of growing in food and do not cause spoilage. 

The following is a list of pathogens and infectious agents of public health 

concern. This list is not exhaustive, but contains most of the foodborne 

pathogens that affect meat, poultry, and processed egg products. 

V Bacteria 

} Bacillus cereus (B. cereus) 

} Brucella species (Brucella spp) 

} Campylobacter spp 

} Clostridium botulinum (C. botulinum) 

} Clostridium perfringens (C. perfringens) 

} Escherichia coli O157:H7 (E. coli O157:H7) and
 

other Shiga-toxin-producing E. coli (STEC)
 

(including O26, O45, O103, O111, O121, and O145)
 

} Listeria monocytogenes (L. monocytogenes) 

} Salmonella spp 

} Shigella spp 

} Staphylococcus aureus (S. aureus) 

} Yersinia enterocolitica (Y. enterocolitica) 
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V Viruses 

} Hepatitis A and D 

} Norovirus 

} Rotaviruses 

V Tapeworms 

} Taenia spp 

V Roundworms 

} Trichinella spp 

V Protozoa 

} Toxoplasma spp 

} Sarcocystis spp 

The U.S. Centers for Disease Control and Prevention (CDC) reports that the 

most common foodborne illnesses for which an organism has been identified 

are those caused by the bacteria Campylobacter, Salmonella, L. monocytogenes, 

and E. coli O157:H7 and the norovirus. 
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Foodborne Pathogens 

Bacteria 

General Information on Bacteria 

Bacteria are the most important and troublesome of all the microorganisms 

for the food processor. Bacteria are single-celled living bodies. Varying in 

length from 1/25,000 to 1/1,000 of an inch, they are among the smallest living 

creatures known. 

There are many different ways to classify and group microorganisms, such as 

microscopic appearance; materials they can use as foods; byproducts resulting 

from the breakdown of these foods; tolerance to oxygen; growth temperatures; 

resistance to destructive agents, such as heat and chemicals; ability to perform 

certain biochemical reactions in the laboratory; and possession of certain 

genetic sequences. 

Viewed with a microscope, bacteria appear in several shapes or forms, but are 

primarily either round in shape (called “cocci”) or rod-shaped (called “rods” or 

“bacilli”). 

Spore Forming and Non-Spore-Forming Bacteria 

Spores are at dormant or resting stage in the normal growth cycle of some 

bacteria. The primary function of most spores is to ensure the survival of the 

organism through periods of environmental stress. Spores have been compared 

to plant seeds because they will “sprout” (germinate) and grow when conditions 

are suitable. When a bacterial spore germinates, it is the same organism 

continuing its growth process. The process of spore formation is called 

sporulation. 
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Bacteria can be placed into two groups based on their ability to form spores: 

spore formers and non-spore formers. As you may have guessed, spore formers 

can form spores, and non-spore formers cannot form spores. The four major 

spore-forming bacteria are C. botulinum, C. perfringens, B. cereus, and Bacillus 

anthracis (B. anthracis). These bacteria are normally present everywhere in the 

environment, which can make them difficult to control in a food-processing 

facility. 

Cells of non-spore formers, cells of spore formers that have not formed spores, 

and cells that have sporulated are referred to as “vegetative cells.” Vegetative 

cells generally have little resistance to heat, drying and other unfavorable 

conditions. However, bacterial spores in general are extremely resistant to heat, 

cold and chemical agents. For example, some bacterial spores can survive in 

boiling water (212 °F or 100 °C) for more than 16 hours, but vegetative cells 

(same organisms in the vegetative state and the non-spore-forming bacteria) 

cannot. Generally, spores that resist heat also are highly resistant to destruction 

by chemicals. There are bacterial spores that can survive more than 3 hours 

in sanitizing solutions normally used in a food-processing plant. These same 

sanitizing agents easily destroy vegetative cells. 

Because bacterial spores can survive in unfavorable conditions, they can be 

present in food before, during, and after lethality treatment is applied. This 

is why the time it takes to heat and/or cool product during food processing 

is so important. If the food product is not heated or cooled appropriately – 

that is, the product stays for too long in a temperature range that is favorable 

to bacterial growth—the spores will change to a vegetative state and start 

reproducing. This is called outgrowth of the spores. The bacterial cells can 

reproduce to reach the MID and cause a food infection, or produce a sufficient 

amount of toxins to create a food intoxication. 

Bacterial Toxins 

Toxins are substances produced by living cells or organisms that are capable 

of causing disease when they come in contact with, or are absorbed by, body 

tissues. They can vary greatly, both in the severity of effect (ranging from 

minor to fatal) and how easily they can be destroyed. One way to categorize 

toxins depends on where they affect the body: 
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V Enterotoxins 

Affect the gastrointestinal tract, causing vomiting, diarrhea, gastrointestinal 

distress and/or pain. 

V Neurotoxins 

Affect the nervous system, causing dizziness, blurred or double vision, the 

inability to swallow, skeletal muscle weakness, and/or flaccid paralysis. 

Depending on the bacteria producing the toxin, it can be a byproduct of the 

sporulation process or a byproduct of normal cell metabolism. Toxins that are 

already present in the food product are often referred to as “preformed toxins.” 

If there is enough of the preformed toxin present, just eating the food will cause 

illness. Some toxins are extremely heat resistant and may pose a hazard even 

if the food is properly cooked. Toxin-producing bacteria include S. aureus, B. 

cereus, C. botulinum, and C. perfringens. 

Bacterial Cell Reproduction 

Bacteria reproduce by division, which is what microbiologists call fission. 

When a bacterial cell is ready to divide, the cell material gradually increases 

until its volume is almost doubled. The cell then constricts in the middle. 

This constriction deepens until the cell contents are held in two distinct 

compartments separated by a wall. These two compartments finally separate to 

form two new cells, which are duplicates of the former cell and each other. 

As the individual cells reproduce on a Petri dish, they form a group known 

as a colony. When we think about measuring bacteria, we usually think 

about colonies rather than individual bacterial cells. Since the reproduction 

of bacteria increases their numbers, the size of the colony grows. This is why 

bacterial reproduction is sometimes referred to as growth. “Bacterial growth” 

refers to an increase in the number of bacteria present, not to an increase in the 

size of the individual bacterial cells. 
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The following graph shows the four phases of bacterial growth (reproduction): 

V Lag phase-Occurs when a bacterial population first enters an environment 

that allows growth. 

} Growth is slow because the bacterial cells are adapting to their new 

environment. 

} Most variable of the phases because the amount of time that is needed 

to adapt depends on factors, such as the characteristics of the bacteria 

and the environment. 

} The length of the phase affects how quickly the MID of pathogenic 

bacteria is reached. 

} The length of the phase affects whether the interventions that your 

plant has in place are sufficient to control the numbers of pathogenic 

bacteria. 

V Log phase or exponential phase-Occurs when the number of cells increases 

exponentially in a favorable environment. 

} Numbers of bacterial cells will double every 20 to 30 minutes. 

•	 The rate at which bacterial cells reproduce is known as growth rate 

(growth rate = k). 

•	 The time it takes for bacterial cell numbers to double is known as 

generation time (generation time = g). 

} Nutrients in the environment are used up quickly until one of the 

nutrients is gone and growth slows down. 
 

 

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V Stationary phase-The phase where the rate of bacterial growth equals the 

rate of bacterial death. 

} The reproduction of cells slows down because nutrients are used up in 

the log phase. 

} The increase of byproducts from bacterial growth can inhibit further 

growth. 

V Death phase-Reproduction rate is lower than the death rate. 

} More bacterial cells are dying than reproducing. 

} There is a net loss of bacterial cells capable of reproduction. 

} Some bacteria can form resistant spores and survive this phase. 

Pathogenic Bacteria of Importance 

Let’s take a closer look at significant bacteria that you should address in your 

operation. 

Salmonella species (Salmonella spp.) 

Transmission fecal-oral ingestion 

Source beef, poultry, milk, or eggs 

Immediate symptoms nausea, vomiting, abdominal cramps, 

diarrhea, fever, and headache 

Long-term 

consequences 

may follow 3-4 weeks after the onset of 

acute symptoms 

Onset time ranges from 6-48 hours 

MID as few as 15-20 cells 

Length of disease averages from 1 to 7 days 

In food processing, Salmonella infection is of concern in raw poultry, swine, 

and ready-to-eat products. There are specific regulatory requirements for 

reduction of Salmonella in ready-to-eat products, raw ground beef, and raw beef 

components that are intended for grinding. 

Salmonella is a rod-shaped, generally motile, non-spore forming, Gramnegative bacterium. It grows at 41-117  ̊F (5-47  ̊C), at a pH as low as 4.2, 
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with or without air, and at a water activity (measure of available water) greater 

than 0.94 (aw >0.94). The optimum growth temperature is at the human body 

temperature (98.6 °F). Salmonella remains viable for a long time in frozen 

foods. All Salmonella serotypes (strains) are considered potential human 

pathogens, but only a few serotypes are associated with foodborne illness 

(salmonellosis): Typhimurium, Enteriditis, Newport, Heidelberg, and Javiana. 

Pathogenic Salmonella possess factors that enhance their ability to invade and 

replicate in the host. 

Salmonella is an enteric microorganism associated with the intestinal tract of 

many animals and, thus, is potentially present in most raw meats. Illness is 

usually caused by the ingestion of a sufficient number microorganisms which 

survive digestion and reproduce in the human intestinal tract. Fortunately, 

Salmonella are heat-sensitive and easily destroyed with the mild heat treatments 

for cooking meat (heating to 145  ̊F [63  ̊C]). Salmonella also are acidsensitive and are not good at competing with other bacteria. However, it 

should be noted that acid resistance can be induced by exposure to mildly 

acidic conditions (pH 5.5-6.0), resulting in tolerance to extremely acidic 

environments (pH 3-4). Salmonella has not been a problem with fermented 

sausages when the product is properly fermented using an appropriate starter 

culture. Salmonella not only survives drying, but also becomes more heatresistant with drying and is more of an issue in non-fermented dried meats, 

such as jerky, and whole meat cuts, such as dried hams. Salmonella remains 

viable for a long time in frozen foods. 

Control mechanisms include temperature control and sanitation prior to 

processing and the use of wet heat early in the heat process to avoid pre-drying. 

In non-heated meats, the proper use of high salt and proper curing techniques 

are most effective. 

The prevalence of Salmonella in beef, lamb, pork, and poultry carcasses varies 

greatly. The overall contamination of meat and poultry carcasses with these 

pathogens depends not only on the numbers of the pathogens on the hair, hide, 

feathers, skin, and in the intestinal tract of the animals, but also on the degree 

of cross-contamination occurring from these sources during slaughter and 

processing. 
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Salmonella has been isolated from a wide variety of foods, such as spices, milk 

and dairy products, fish, shrimp, coconut, salad dressing, cake mixes, creamfilled desserts, dried gelatin, peanut butter, and chocolate. 

Environmental sources of the organism include water, soil, insects, factory 

surfaces, kitchen surfaces, and animal feces, to name a few. 

Escherichia coli (E. coli) 

Transmission fecal-oral ingestion 

Source raw ground and non-intact beef products, raw 

vegetables, raw milk 

Immediate 

symptoms 

severe cramping (abdominal pain) and 

diarrhea, which is initially watery, but 

becomes grossly bloody; occasional vomiting; 

low-grade or no fever 

Long-term 

consequences 

Hemolytic Uremic Syndrome (HUS) – More 

commonly affects children under 10, but may 

also affect adults and the elderly; characterized 

by hemolytic anemia, and thrombocytopenia, 

and kidney failure may follow after a week. 

Onset time approximately 4 days (median) 

MID as few as 10 bacterial cells 

Length of disease average duration of 8 days 

In food processing, pathogenic E. coli infection is of concern in raw ground and 

non-intact beef products and dry-cured salami. There are specific regulatory 

requirements for the reduction of E. coli O157:H7 in raw ground and nonintact beef products. 

E. coli O157:H7 is a rod-shaped, generally motile, non-spore-forming, Gramnegative bacterium. It generally grows at 45-115  ̊F (7-46  ̊C), at a pH range 

between 4.4-9, with or without air, and at an aw >0.95. There are strains of 

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E. coli O157:H7 that can survive in water and possess an unusual tolerance to 

environmental stress, such as temperature, pH, and dryness. 

A minority of E. coli serotypes are capable of causing human illness by different 

mechanisms. The pathogen of primary concern is E. coli O157:H7, which is 

a Shiga-toxin-producing E. coli (STECs). Other types of E. coli, referred to 

as non-O157:H7 STECs, also can cause human illness. The strains of most 

concern are E. coli O26, O103, O111, O121, O45, and O145. 

E. coli O157:H7 causes hemorrhagic colitis (HC) and hemolytic uremic 

syndrome (HUS). All people are believed to be susceptible to HC, but young 

children and the elderly appear to be the most frequently effected by HUS. 

E. coli is a normal inhabitant of the intestines of all animals, including humans; 

and it serves a useful function in the body by suppressing the growth of 

harmful bacterial species and by synthesizing appreciable amounts of vitamins. 

Although always present, E. coli O157:H7 appears to be more prevalent from 

April through October, peaking in the summer months. 

Escherichia coli (E. coli) under a microscope. 
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Listeria monocytogenes (L. monocytogenes) (Lm) 

Transmission oral ingestion 

Source ready-to-eat (RTE) hot dogs, deli 

meats, pâté and meat spreads, 

fermented raw meat sausages, and raw 

poultry and meat (all types) 

Immediate symptoms influenza-like symptoms including 

persistent fever 

non-invasive form – febrile 

gastroenteritis 

invasive form – septicemia, 

meningitis, encephalitis, intrauterine 

or cervical infections in pregnant 

women 

Long-term consequences Not Applicable 

Onset time influenza-like symptoms – may be 

greater than 12 hours 

•	 non-invasive form – probably 

greater than 18-20 hours 

•	 invasive form – may range from a 

few days to 3 weeks 

MID Unknown. May vary with the strain 

and susceptibility of the individual. 

Length of disease days to weeks 

In food processing, L. monocytogenes infection is of concern in ready-to eat 

(RTE) food-products, particularly hot dogs and deli meats. There are specific 

regulatory requirements for control of this pathogen in the product and for 

sanitation in the RTE food-producing facility. 

L. monocytogenes is a rod-shaped, non-spore-forming, Gram-positive bacterium. 

L. monocytogenes is a human pathogen that causes listeriosis. It is motile 

and can grow in cool (temperature range of 32-113  ̊F [0-45  ̊C]) and damp 

environments, at a pH range of 4.4-9.4, and at a aw > 0.92. 

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Some characteristics that make some strains of L. monocytogenes hardy include 

the ability to grow and/or survive in acidic environments (pH 4.39), withstand 

heat treatments, grow at refrigeration temperatures, and survive in a concentrated 

salt solution (as high as 30.5 percent at 39.2 °F). Temperatures higher than 

170 °F will inactivate the Listeria organisms. Food-producing facilities must 

ensure that RTE products are not recontaminated with L. monocytogenes from 

environmental sources after the bacteria have been inactivated by the cooking 

process (“lethality step”). 

L. monocytogenes is found in the environment and can be carried by humans and 

animals. In the plant, it can be found on equipment, utensils, humans, in water, 

air flow, etc., and has been isolated at every level of the meat processing chain in 

slaughter and processing plants. It is of concern in the production of dried meats 

since it can grow aerobically or anaerobically and can survive dry conditions. 

L. monocytogenes can form biofilms, which serve as protective shells. Cells can 

travel to other sites and start the cycle of biofilm formation at a distance from 

the original site. Biofilms can form within a few hours or days depending on 

the number of bacterial cells, available nutrients, surface characteristics, and 

temperature. Once formed, they can persist for a long time (years), be very 

difficult to remove, and provide protection from the chemicals used to clean and 

sanitize surfaces, freezing, drying, high salinity, and heat. 

Its presence in the RTE environment can pose a serious problem, especially in 

the RTE finished product and food contact surfaces. Product flow must be 

designed to segregate finished products from raw products, and personnel who 

handle RTE products should be restricted to specific areas to prevent crosscontamination. 

In addition to good sanitation and avoiding cross-contamination, Listeria can be 

controlled by a combination of lower pH, high-brine concentration, and lactic 

acid starter cultures that provide harmless bacteria for Lm to compete against. 

Generally, listeriosis occurs among the elderly, pregnant women (the outcome 

upon exposure of the fetus to the pathogen results in spontaneous abortion, 

stillbirth, or neonatal sepsis), diabetics, people on kidney dialysis, and the 

immunocompromised (bone marrow transplant patients, cancer patients, etc.). 
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Campylobacter jejuni (C. jejuni) (Campy) 

Transmission fecal-oral ingestion 

Source raw chicken 

Immediate symptoms diarrhea, which may be watery or sticky 

and can contain blood; fever; nausea; 

cramping; abdominal pain; headache; 

and muscle pain 

Long-term consequences Guillain-Barré syndrome, a rare disease 

that affects the nerves of the body; 

begins several weeks after the diarrheal 

illness; 1 in every 1,000 reported cases 

Onset time 2-5 days after exposure 

MID fewer than 500 cells 

Length of disease generally lasts 7-10 days 

In food production, C. jejuni infection is of concern in raw poultry products. 

There are regulatory performance standards for the control of Campylobacter on 

chilled carcasses in young chicken (broiler) and turkey slaughter plants. 

Campylobacter jejuni is a Gram-negative, curved, and motile rod. It is a 

microaerophilic organism, which means it has a requirement for reduced levels 

of oxygen and requires 3-5 percent oxygen and 2-10 percent carbon dioxide for 

optimal growth. C. jejuni is relatively fragile and susceptible to environmental 

stresses, such as 21 percent oxygen, drying, heating, disinfectants, and acidic 

conditions. It can grow at temperatures between 90 and 113  ̊F (32-45  ̊C), at a 

pH range of 4.9-9.0, and at aw > 0.98. Campylobacter dies off rapidly at ambient 

temperatures and atmosphere, and grows poorly in food. 

The illness caused by C. jejuni is called campylobacteriosis, enteritis, or 

gastroenteritis. It is thought that C. jejuni first colonizes the small intestine 

(jejunum and ileum) and then the colon, producing a heat-labile toxin that 

may cause diarrhea. It is one of the most common bacterial causes of diarrheal 

illness in the United States. Campylobacteriosis occurs more frequently in the 

summer months than in the winter. It lives in the intestines of healthy birds and 

is associated mainly with raw poultry and contaminated water sources. It is a 

fragile bacterium and can be killed with oxygen, freezing, or drying. Control 

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measures taken for Salmonella, Listeria, and E. coli O157:H7 will control other 

pathogens such as Campylobacter and Y. enterocolitica, a pathogen associated with 

pork. 

Staphylococcus aureus (S. aureus) (Staph) 

Transmission oral ingestion of vegetative cells 

Source meat and meat products, poultry, and 

egg products 

Immediate symptoms nausea, vomiting, retching, abdominal 

cramping, headaches, muscle cramping, 

and transient changes in blood pressure 

and pulse rate may occur 

Long-term consequences Not Applicable 

Onset time usually rapid (30 min. to 8 hrs.) 

MID greater than 100,000 cells per gram (less 

than 1.0 microgram of enterotoxin A) 

Length of disease generally 2 days (3 days and sometimes 

longer in severe cases) 

In food processing, S. aureus is of concern in ready-to-eat food products. There 

are no specific regulatory requirements for controlling of S. aureus on RTE 

foods. 

Staphylococcus aureus is a Gram-positive bacterium (coccus), which appears in 

clusters resembling grapes under the microscope. It is a non-motile, non-sporeforming facultative anaerobe that can grow with or without oxygen. It grows 

by aerobic respiration or by fermentation yielding lactic acid. It can grow at 

temperatures between 35.9-113  ̊F (7-45  ̊C), with an optimum temperature of 

98.3  ̊F, (37  ̊C), a pH range of 4.2-9.3, and at salt concentrations as high as 25 

percent. It is resistant to drying, and is able to produce enterotoxins in foods with 

aw as low as 0.85. 

S. aureus is associated with mucous membranes (nose and throat) and is 

commonly found on the skin and hair of healthy humans and animals. It also is 

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present in infected wounds, lesions, and boils in both humans and animals. It’s 

easily spread though the air by coughing and sneezing and can contaminate meat 

from the animal skin or tissue during slaughter (especially during hide removal 

and gutting). S. aureus is not a good competitor with other microorganisms 

and, thus, usually is not a problem in raw foods. It becomes a problem when 

competitive microbes are removed by cooking or inhibited by high-salt levels. In 

salted meats, where many microorganisms are inhibited, S. aureus can flourish 

without proper controls. Nitrite does not affect the growth of S. aureus in airdried fresh pork sausage. Even though S. aureus can grow with or without air, it 

grows best aerobically. 

Staphylococcal food poisoning is caused by the consumption of a heat-stable 

enterotoxin produced as a byproduct during the growth of certain strains of S. 

aureus. Toxin production requires considerable growth by the microbe and is 

normally not present until the total cell numbers reach 100,000 per gram of 

meat. Since the microorganism is readily destroyed by heat, but the toxin is heat 

stable, total counts of S. aureus may not indicate if the toxin is present. 

One of the biggest concerns of this pathogen is the increased incidence of 

methicillin-resistant S. aureus (MRSA) and other strains that are resistant to 

a variety of different antibiotics. Furthermore, S. aureus strains can exhibit 

resistance to antiseptics and disinfectants, including quaternary ammonium 

compounds. 

Foods that require considerable handling during preparation and are kept 

at slightly elevated temperatures after preparation are frequently involved in 

staphylococcal food poisoning. Human intoxication is caused by ingesting 

enterotoxins produced in food by some strains of S. aureus, usually because the 

food has not been kept hot enough (140 °F [60 °C] or above) or cold enough (45 

°F [7.2 °C] or below). 

Staphylococci exist in air, dust, sewage, water, milk, and food, or on food 

equipment, environmental surfaces, humans, and animals. Humans and animals 

are the primary reservoirs. Animals and poultry carry S. aureus on parts of their 

body, which can lead to infections. The udders and teats of cows, tonsils and 

skin of pigs, and skin of chickens and turkeys are known sources. Staphylococci 
Introduction to the Microbiology of Food Processing

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are present in the nasal passages and throats and on the hair and skin of 50 

percent or more of healthy people. Although food handlers are usually the main 

source of food contamination in food poisoning outbreaks, equipment and 

environmental surfaces also can be sources of contamination with S. aureus. 

S. aureus is highly vulnerable to destruction by heat treatment and nearly all 

sanitizing agents when correctly applied. While heat processing (pasteurization) 

and normal cooking temperatures are effective in killing the pathogen, food 

establishments have to be alert that the enterotoxins are not inactivated by 

heat. Its heat resistance is increased in dry, high-fat and high-salt foods, and 

the organism survives frozen storage. The presence of this bacterium or its 

enterotoxins in processed foods or on food processing equipment (in areas that 

are difficult to clean) is generally an indication of poor sanitation and/or time and 

temperature abuse. Foods that present the greatest risk are those in which a heat 

treatment (e.g., cooking) or application of an inhibitory agent or treatment (e.g., 

cured, salted meats) has been applied. 

Clostridium botulinum (C. botulinum) (botulism) 

Transmission oral ingestion of neurotoxin 

Source canned or fermented meat, vegetables, 

and fish 

Immediate symptoms dry mouth, dizziness, weakness; 

nausea and vomiting also may occur. 

Neurologic symptoms will follow, 

including blurred or double vision, 

the inability to swallow, difficulty 

speaking, descending weakness of 

skeletal muscles and, ultimately, 

respiratory paralysis. Untreated, this 

can lead to death. 

Long-term consequences Not Applicable 

Onset time 12-48 hours 

MID a few nanograms of toxin 

Length of disease days to months 

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In food processing, C. botulinum is of concern in thermally processed 

(“canned”) and fermented foods. There are specific regulatory requirements for 

controlling of C. botulinum in canned and fermented foods. 

C. botulinum requires anaerobic conditions, so it can only grow in the absence 

of oxygen. Under the right conditions, it creates heat-stable spores that can 

survive in foods that are incorrectly or minimally processed. C. botulinum 

produces a potent neurotoxin, which is formed during vegetative cell growth. 

Illness is caused by the ingestion of foods containing the neurotoxin. The toxin 

is heat labile and can be destroyed if heated to 176 °F (80 °C) or higher for 10 

minutes. Foods with a pH 4.6 or higher (more basic) can support growth and 

toxin production. 

The ability to form spores enables C. botulinum to survive a wide range of 

unfavorable conditions, such as heat and chemicals. The spores survive many 

heat processes that kill other pathogens in meat and poultry. In fact, certain 

types of C. botulinum spores are able to survive 5 to 10 hours in boiling water. 

In canned products, oxygen is excluded from sealed containers, thus providing 

the anaerobic environment for spores to germinate, become vegetative cells, 

multiply, and produce toxin when the foods are stored at temperatures that 

allow growth. It is important to recognize that it is not the spore that produces 

the toxin, but the vegetative cell. If spores are present, but are prevented from 

forming vegetative cells, as in acidified foods, the toxin will not be produced. 

In summary, C. botulinum is of great concern to home and commercial canners 

because: 

V It can produce a potent neurotoxin when it grows. 

V It can be isolated from soil or water practically everywhere in the world. 

V It is the pathogen with the greatest heat resistance due to its ability to
 

produce heat-resistant spores.
 

V Canning foods provides an anaerobic environment favorable to the growth 

of the organism if it has not been destroyed by the canning process. 
 

 

 

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Clostridium perfringens (C. perfringens) 

Transmission oral ingestion of vegetative cells 

Source cooked meat and poultry dishes, beans 

Immediate symptoms explosive diarrhea, abdominal 

cramps, occasionally nausea. A severe 

complication, necrotic enteritis, can 

develop if large numbers of bacteria are 

ingested. 

Long-term consequences Not Applicable 

Onset time 8-16 hours 

MID greater than 1,000,000,000 cells (108

 ) 

Length of disease 1 day 

In food processing, C. perfringens is of concern in ready-to-eat food products. 

There are specific regulatory requirements for the control of C. perfringens in 

RTE foods. 

C. perfringens requires anaerobic or microaerophilic conditions, so it can 

only grow when there is little or no oxygen. It can be present in two forms, 

vegetative cells and spores. Illness is caused when the vegetative cells are 

ingested. Environmental conditions in the body cause the vegetative cells to 

form spores. The heat-sensitive toxin is produced in the gastrointestinal tract 

during the sporulation process (enterotoxin). To get sick, it is necessary to 

consume a large number of vegetative cells. 

Intoxication is often caused by eating temperature-abused leftovers. C. 

perfringens forms spores that can withstand cooking temperatures. When 

the foods are cooled, spores germinate, and the germinated cells multiply. If 

the food is eaten without adequate reheating, C. perfringens enters the small 

intestine and synthesizes enterotoxins. 

The vegetative cells are killed by heat applied during the production of RTE 

foods. However, the spores are heat resistant, so they may survive cooking, and 

the heat applied during the RTE production will cause the spores to germinate. 

The resulting vegetative cells will grow to large numbers during the postcooking cooling period (“stabilization”), particularly if the cooked food is held 

between 40 °F and 140 °F for an extensive period. 
Introduction to the Microbiology of Food Processing

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Molds 

Molds exhibit some of the characteristics of the higher plants. They are multiple 

cell organisms forming tubular filaments. Molds demonstrate branching and 

reproduce by means of fruiting bodies, called spores, which are borne in or on 

aerial structures. Their mycelia, or intertwined filaments, may resemble roots. 

They are many times larger than bacteria and somewhat longer than yeasts. 

Molds are widely distributed in nature, both in the soil and in the dust carried 

by air. Under suitable conditions of moisture, air and temperature, molds will 

grow on almost any food. The black or green discoloration that appears on 

moldy bread is familiar evidence of such growth. Molds also are able to survive 

on a wide variety of substances not normally thought suitable for the support of 

life. These include concentrated solutions of some acids and water containing 

minute quantities of certain salts, as well as on building structures. Molds 

grow readily on the walls and ceilings of buildings where there is high humidity 

and considerable moisture condensation. Mold growth can even occur in 

refrigerators, because molds are much more tolerant to cold than heat. Molds can 

grow at reduced water activities and can be a problem in improperly processed 

dry and semi-dry fermented products. Molds, such as Aspergillus, Rhizopus, and 

Penicillium, are responsible for the spoilage of cured meats. Some molds, in the 

right conditions, produce mycotoxins. Molds can also cause allergic reactions 

and respiratory problems. 

Molds are capable of consuming acids, thereby raising the pH of products. On 

very rare occasions, their growth in foods has removed the acid conditions that 

inhibit the growth of C. botulinum; however, never in meat or poultry products. 

Most molds have little heat resistance and cannot survive the thermal processes 

for low-acid canned foods. Some molds produce a type of spore (ascospore) 

that is more resistant to heat, but these spores are much less resistant than the 

bacterial spores that are the target of processes for low-acid and acidified canned 

foods. These heat-resistant molds have not caused problems in meat and poultry 

products. Therefore, molds are present in canned meat and poultry products only 

as a result of gross under-processing or as a post-processing contaminant. Since 

molds must have oxygen to grow, only slight growth can occur, unless the food 

container has an opening to the outside environment. 
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To date, mold growth in thermally processed, commercially sterile, and shelfstable foods has not been shown to present a public health problem. In fact, 

mold is used in the ripening process of some sausages. 

Although not normally a problem in meat and poultry products, Aspergillus 

flavus (A. flavus) and A. parasiticus are two molds of importance as potential 

foodborne pathogens. At a favorable temperature, around 77-86 °F (25­

30 °C), and moisture level of 13-18 percent, certain strains of these 

molds produce mycotoxins known as aflatoxins, which have been found 

as contaminates in tree nuts, peanuts, other oilseeds (including corn and 

cottonseed), and milk. The major aflatoxins of concern are B1 (the most 

toxic), B2, G1, and G2. Aflatoxins cause aflatoxicosis, characterized by the 

acute death of tissue, cirrhosis, and carcinoma of the liver in a number of 

animal species. Although aflatoxicosis is rarely reported in humans, it is 

believed that the aflatoxins would have the same effects on humans as it does 

in animals. Aflatoxicosis may occur in both an acute and chronic form, and 

it may go unreported because it is not recognized as the underlying cause 

of a patient’s illness. Treatment is supportive (monitoring of liver function, 

dialysis, giving intravenous fluids) and an absorbent substance, such as 

activated charcoal, may help. Aflatoxins are inactivated by reagents such as 

concentrated ammonia and sodium bisulfate. 

Yeasts 

Another microorganism of importance to food preservation/spoilage is yeast. 

Yeasts are single-cell, microscopic living bodies, usually egg-shaped. They are 

smaller than molds, but larger than bacteria. Their greatest thickness is about 

1/2,000 of an inch. Yeasts reproduce mainly by budding. A small bud forms 

on the parent yeast cell, gradually enlarges, and then breaks off into another 

yeast cell. A few varieties reproduce by forming spores within a special cell; 

later, these spores may form new yeast cells. 

Yeasts are widely found in nature and are particularly associated with liquid 

foods containing sugars and acids. They are quite adaptive to adverse 

conditions such as acidity and dehydration. Like molds, yeasts are more 

tolerant to cold than to heat. Compared to bacterial spores, yeasts and their 

spores possess little resistance to heat. Heating to 170 °F (77 °C) destroys 

most yeast forms. In canned food, the presence and growth of yeast may 
Introduction to the Microbiology of Food Processing

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result in spoilage, generally in the form of alcohol production and large 

amounts of carbon dioxide gas, which swells the container. If this happens, 

gross underprocessing, post-processing contamination, or leakage must be 

suspected. 

Yeast growth in processed foods does not present a public health problem. 

Viruses 

Virus particles are so small they cannot be seen by the standard light microscopes 

used in laboratories. A special electron microscope is needed to see these 

microorganisms. A virus particle is composed of either ribonucleic acid (RNA) 

or deoxyribonucleic acid (DNA) enclosed in a coat of protein, sometimes with 

an outer envelope containing lipids (fats). DNA is a nucleic acid which carries 

genetic instructions for the biological development of all cellular forms of life 

and many viruses. RNA transmits genetic information from DNA to proteins 

and carries the genetic instructions for many viruses. Viruses lack the enzymes 

and other components needed to replicate. Thus, viruses cannot multiply in 

food; they can only replicate themselves in suitable living host cells. Viruses 

transmitted by food are produced in the human body and shed in the feces. Of 

particular concern for foods are the hepatitis viruses and noroviruses. Norovirus 

have been mentioned in many news stories in recent years for causing large 

outbreaks of vomiting illnesses at large gatherings and on cruise ships. Viruses 

get into food through contaminated water and infected food handlers with poor 

hygienic practices. 

Viruses are not heat resistant, with most having resistance similar to non-spore 

forming bacteria. Hepatitis A virus is somewhat more resistant, but is still 

inactivated at 185 °F (85 °C). Avian influenza virus, which can infect chickens, 

turkeys, pheasants, quail, ducks, geese, and guinea fowl, as well as a wide variety 

of other birds, has been known to infect humans, but it is not transmitted 

through foods. Nor is the exotic Newcastle disease virus, which also causes a 

highly contagious poultry disease. Heating product to an internal temperature of 

at least to 161.6 °F (72 °C) is considered adequate to inactivate both these viruses. 

Viruses are not a concern in thermally processed, commercially sterile, and shelfstable meat and poultry products handled under good hygienic conditions. 
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Parasites 

The parasites of concern in the production of meat and poultry products include 

worms and protozoa. Some of them are large enough to see with the naked eye, 

whereas others are microscopic. Parasites cannot multiply in food. They can 

only multiply in a host cell, and are not heat resistant. 

Parasitic worms of public health importance are the beef and pork tapeworms 

(Taenia saginata and Taenia solium, respectively) and the roundworm that causes 

trichinosis (Trichinella spiralis, also referred to as trichinae) found in pork. 

The small cysticerci (immature larval stage in a cyst known as a cysticercus when 

there is only one and cysticerci when there is more than one) of Taenia saginata 

and Taenia solium are approximately 6-18 mm wide by 4-6 mm in length when 

found in the muscles or under the skin, the normal sites for the larvae of this 

parasite. The cysticerci may, however, be found in other tissues, such as those 

of the central nervous system, where they may grow to several centimeters in 

diameter. 

Muscle and organs of animals with severe tapeworm infection are usually 

visually detected by Government inspection personnel or by plant employees 

when there is evidence of the cysticerci of tapeworms in the muscles. These 

products cannot be further processed for human consumption. When the 

cysts are less severe or evident, infected meat may enter the human food chain 

and will not cause illness if the meat is properly cooked. Humans consuming 

undercooked meat infected with these tapeworms become ill with taeniasis 

generally after the mature stages of the tapeworms, which develop from the 

cysticercus, invade the intestinal tract. Most cases of infection with adult 

worms are without symptoms. Some persons may experience abdominal 

pain, weight loss, digestive disturbances, and possible intestinal obstruction. 

Taeniasis may last many years without medical treatment. However, people 

can get a more serious illness called cysticercosis by consuming food or water 

contaminated with the eggs of Taenia solium (pork tapeworm). The worm 

eggs hatch, and the larvae then migrate to various parts of the body and form 

cysts (cysticerci). This can be a serious or fatal disease if it involves organs, 

such as heart or eyes, or an organ system, such as the central nervous system. 

Symptoms may vary depending on the organ or organ system involved. For 

example, an individual with cysticercosis involving the central nervous system 
Introduction to the Microbiology of Food Processing

United States Department of Agriculture 29

(neurocysticercosis) may exhibit neurological symptoms such as psychiatric 

problems or epileptic seizures. Death is common.

 Life Cycle of Taenia saginata

Life Cycle of Taenia solium

Adult in small 

intestine

Gravid 

proglottid in 

feces or 

environment

Embryonated 

eggs or 

proglottids 

ingested

Oncosphere 

hatches and 

penetrates 

intestinal wall

Enters 

Circulation

Cysticercus in 

muscle

Undercooked 

or raw beef 

ingested

Scolex 

attaches to 

intestine

Adult in small 

intestine

Gravid 

proglottid in 

feces or 

environment

Embryonated 

eggs or 

proglottids 

ingested

Oncosphere 

hatches and 

penetrates 

intestinal wall

Enters 

Circulation

Cysticercus in 

muscle

Undercooked 

or raw pork 

ingested

Scolex 

attaches to 

intestine

Autoinfection

Egg in feces

Human

Cattle

Egg in feces

Human

Swine

Egg or proglottid ingested

Oncosphere hatches and penetrates 

intestinal wall, then enters circulation

Cysticercus in lungs, brain, eye, and 

connective tissue

 Life Cycle of Taenia solium
Trichinella spiralis is an intestinal worm that produces larvae that migrate to and encyst in 

muscles of a number of animals, particularly swine. Unlike Taenia saginata and Taenia solium,

Trichinella spiralis lesions cannot be observed through visual inspection. Humans consuming

infected pork that is undercooked get ill from the cysts, which then live in the muscles of the

human hosts. The first symptoms are nausea, diarrhea, vomiting, fever, and abdominal pain, 

followed by headaches, eye swelling, aching joints and muscles, weakness, and itchy skin. In 

severe infections, persons may experience difficulty with coordination and have heart and 

breathing problems. Death may occur in severe cases.

Life Cycle of Trichinella spiralis

Parasitic protozoa of concern in meat processing include Cryptosporidium parvum and 

Toxoplasma gondii. Cryptosporidium is typically transmitted to humans from fecal material of

animals, primarily cattle, via contaminated water or occasionally, food. The organism is

destroyed by boiling water. Toxoplasma gondii is carried by cats but can infect many warmblooded animals. A form known as the oocyst is shed and can sporulate and survive in soil and 

other environments for extended times; the sporulated oocyst is infectious to all warm-blooded 

hosts. When ingested, the sporulated oocysts go through several forms, eventually forming cysts

in tissue such as muscle. These cysts are infective if ingested. Toxoplasma can cross the

placenta and affect the fetus, resulting in blindness and more serious effects in the brain. It is

commonly transmitted to humans by eating undercooked meat of an infected animal or by

inhaling the dried feces of cats when cleaning the litter box.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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Trichinella spiralis (Trichinae) is an intestinal worm that produces larvae that 

migrate to and encyst in muscles of a number of animals, particularly swine. 

Humans consuming infested pork that is undercooked get ill from the cysts, 

which then live in the muscles of the human hosts. The first symptoms are 

nausea, diarrhea, vomiting, fever, and abdominal pain, followed by headaches, 

eye swelling, aching joints and muscles, weakness, and itchy skin. In severe 

infestations, persons may experience difficulty with coordination and have 

heart and breathing problems. Death may occur in severe cases. 

Adult worms in 

the intestines 

produce many 

young 

Young worms 

carried through 

blood to muscle 

Young worms 

enter muscle 

and develop 

Worms in the 

muscles 

become 

enclosed in cyst 

Pigs get 

Trichinae by 

eating scraps of 

raw pork, rats, 

or other 

infested animals 

People get 

Trichinae from 

eating 

undercooked or 

raw pork, or 

raw/undercooked 

meat from other 

infested animals, 

like bear. 

Parasitic protozoa of concern in meat processing include Cryptosporidium 

parvum and Toxoplasma gondii. Cryptosporidium is typically transmitted to 

humans from the fecal material of animals, primarily cattle, via contaminated 

water or, occasionally, food that has come in contact with contaminated water. 

The organism is destroyed by boiling water. Toxoplasma gondii is carried 

by cats, but can infect many warm-blooded animals. A form known as the 

oocyst is shed and can sporulate and survive in soil and other environments for 

extended times; the sporulated oocyst is infectious to all warm-blooded hosts. 

When ingested, the sporulated oocysts go through several forms, eventually 

forming cysts in tissue, such as muscles. These cysts are infective if ingested. 

Toxoplasma can cross the placenta and affect the fetus, resulting in blindness 

Life Cycle of Trichinella spiralis 
Introduction to the Microbiology of Food Processing

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and more serious effects in the brain. It is commonly transmitted to humans by 

eating undercooked meat of an infected animal or by inhaling the dried feces of 

cats when cleaning the litter box. 

Given a cool, damp environment, these protozoa may survive in moist foods for 

months; however, they won’t multiply in foods. Protozoa may be introducted 

into food by infected food handlers, from contaminated water used in the 

finished product stage of processing or washing processing equipment, and from 

ingredients contaminated on the farm. [David Dawson/International Journal of 

Food Microbiology 103 (2005)]. 

Control of Foodborne Pathogens 

Primary Source of Microorganisms in Food 

Raw materials and ingredients are the primary source of food contamination. 

Although muscle tissue is generally considered to be sterile, exposed muscle 

quickly becomes contaminated during the slaughter/dressing process and 

supports the growth of spoilage microorganisms, as well as pathogens. 

The bacterial pathogens that are most likely to be found in livestock and 

poultry include Salmonella, Campylobacter, and L. monocytogenes. L. 

monocytogenes also is widespread in the environment. E. coli also are found in 

livestock and poultry, but most strains are not pathogenic. The pathogenic 

E. coli of primary concern is known as E. coli O157:H7 and is found in beef. 

Some strains of non-O157:H7 Shiga-toxin-producing E. coli (non-O157:H7 

STEC) are becoming recognized as an important cause of foodborne illness. Y. 

enterocolitica is a pathogen most commonly associated with pork; only certain 

serotypes are pathogenic. 

Some bacteria, such as C. perfringens and C. botulinum, also can be found 

in meat and poultry, forming spores that may survive cooking and increase 

in numbers in foods due to temperature abuse, though C. botulinum may be 

found in far fewer numbers than C. perfringens. B. cereus is another sporeformer of concern in meat and poultry products, especially products containing 

dry ingredients or spices, which are a common source of the spores. 
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The following list outlines some of the most common ways in which 

microorganisms enter food products: 

V Soil, water, and in-plant environment 

Many bacteria are carried in soil and water, which may contaminate food. 

These include, but are not limited to, Listeria, Clostridium, Salmonella, and 

Escherichia. The in-plant environment also may be an important source of 

contamination due to daily activities or pest infestation. 

V Animal feeds 

This is a source of Salmonella for poultry and other farm animals. It is 

a known source of L. monocytogenes to dairy and meat animals when 

fed silage. The organisms in dry animal feed are spread throughout the 

animal environment and may occur on animal hides, hair, feathers, etc. 

V Animal hides 

The hide is a source of bacterial contamination, particularly from E. coli 

O157:H7, Salmonella, and Listeria, and some spoilage organisms. The hides 

may contaminate the general environment, hands of plant employees, and 

skinned carcasses through dressing procedures during the slaughter process. 

V Gastrointestinal tract 

The normal intestinal microflora consists of many organisms; notable 

among these are pathogens such as Salmonella, Campylobacter, and E. coli 

O157:H7. Not all of these bacteria are present in every animal species 

slaughtered in federally inspected establishments. 

V Establishment and agency employees 

The microorganisms on the hands and outer garments of handlers generally 

reflect the environment and habits of individuals (hygiene) and may come 

from hides, gastrointestinal tracts, soil, water, dust, and other environmental 

sources. This is generally the source of viruses, primarily Norovirus and 

Hepatitis A. 
Introduction to the Microbiology of Food Processing

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V Food utensils and equipment 

Utensils, such as saws, cutting boards, knives, grinders, and mixers, may 

become contaminated during slaughter and processing. These surfaces 

may retain contaminated product when they are not properly cleaned or 

maintained, and, therefore, serve as a vector for cross-contamination. 

V Food ingredients 

Food ingredients, such as spices or seasonings, may be contaminated with 

pathogens. The spice or seasoning can cause illness if it is added to the food 

product after the lethality treatment. An example of this is Salmonella in 

black pepper used to coat dried sausages. 

V Air and dust 

A variety of bacteria may be found in air and dust in food-processing 

operations. L. monocytogenes is an example of a foodborne pathogen that 

survives in the environment. As air and dust move through the processing 

facility, they can transport pathogens. 

V Condensation 

Water droplets that form on overhead structures because of poor air 

circulation can carry a variety of microorganisms. When the droplets 

contact food or food contact surfaces, the product may be contaminated. 

V Vegetables (plant) and vegetable products 

Vegetables and vegetable products are a concern in the processing of meat, 

poultry, and processed egg products. A good example is the processing 

of frozen entrees, salads, etc., that contain meat and poultry components. 

Dried herbs and spices can be a primary source of bacterial spores since the 

spores will survive for extended times in the dehydrated product. Soy and 

milk protein ingredients also can be sources of spores. 

As you can tell from reading this, microorganisms can enter food products 

through many different routes because they are found virtually everywhere. 
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Conditions Affecting Microbial Growth 

Like all other living organisms, bacteria need a certain environment to live 

and grow. Broadly, the bacteria involved in food processing need very similar 

environmental conditions, although there are some important variations 

between bacterial species. 

There are six basic environmental conditions needed for bacterial growth. One 

easy way to remember these conditions is to use the easy-to-remember acronym 

FAT TOM: Food, Acidity, Time, Temperature, Oxygen, and Moisture. 

V Food (nutrient requirements) 

A suitable food supply is the most important condition affecting the growth 

of bacteria. Every living cell requires certain nutrients to multiply. These 

include solutions of sugars or other carbohydrates, proteins, and small 

amounts of other materials such as phosphates, chlorides, and calcium. If 

the food supply is removed, bacteria will not multiply. 

V Acidity (pH Requirements) 

The term pH refers to the acidity or alkalinity of an aqueous solution. 

The pH scale ranges from 0 to 14, with pH 7 being neutral. Numbers 

smaller than 7 indicate a more acidic condition; numbers greater than 

7 indicate a more basic, or alkaline, condition. All bacteria have an 

optimum pH range for growth — generally around neutral pH — as 

well as a minimum and maximum. The pH of foods can be adjusted to 

help control bacterial growth. 
 

 

 

 

 

 

Introduction to the Microbiology of Food Processing

United States Department of Agriculture 35 

pH Ranges of Some Common Foods 

Food pH Range 

Beef (ground) 5.1–6.2 

Ham 5.9–6.1 

Veal 6.0 

Chicken 6.2–6.4 

Fish (most species) 6.6–6.8 

Corn (sweet) 7.3 

Egg yolks (white) 6.0–6.3 (7.6–9.5) 

Olives (green) 3.6–3.8 

V Time 

Time is a factor in pretty much all of the other five environmental 

conditions. The more amount of time bacteria spend in favorable 

conditions, the higher the rate of reproduction. Remember, the amount of 

time spent in the lag phase of bacterial growth determines how quickly a 

pathogenic bacteria reaches the MID. The length of the phase affects the 

effectiveness of any interventions that your plant has in place to control the 

numbers of pathogenic bacteria. 

V Temperature 

As with pH, all bacteria have an optimum temperature range for growth 

and a minimum and maximum temperature below or above which they 

cannot grow. Bacterial groups bear names that indicate their relationships 

to temperature – psychrophile, psychrotroph, mesophile, and thermophile. 

} Psychrophilic (“cold loving”) and Psychrotrophic group. 

•	 Both grow above the temperature range of subzero to 68 °F (20 

°C). 

•	 Psychrophiles have an optimum temperature of 59 °F (15 °C) and 

cannot grow above 77 °F (25 °C). 

•	 Psychrotrophs grow best between 77 °F and 104 °F (25 °C to 40 

°C), but can grow slowly in or on food at refrigerator temperatures 

(around 40 °F [4 °C]). 
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•	 Both are primarily responsible for the spoilage of refrigerated 

foods. 

} Mesophilic group (meso = middle). 

•	 Grows best at temperatures of 86 °F to 104 °F (30 °C to 40 °C). 

•	 Some grow well at higher temperatures, such as 116 °F (46.7 °C). 

•	 All of the bacteria that affect food safety grow within this 

mesophilic temperature range, although some may be considered 

psychrotrophic as well. 

} Thermophilic group (“heat loving”). 

•	 Grow at high temperatures. 

•	 Found in soil, manure, compost piles, and even hot springs. 

•	 Not pathogenic and do not produce toxins during spoilage of 

foods; therefore, they do not affect food safety. 

The graph below shows the temperature growth ranges: 

V Oxygen. 

Bacteria can be placed into groups based on their need for oxygen. 

} Aerobes require free oxygen to survive. 

•	 Obligate aerobes need a high concentration of oxygen. 

•	 Microaerophiles need a lower concentration of oxygen than 

obligate aerobes. 

} Anaerobes cannot grow if free oxygen is present. 
Introduction to the Microbiology of Food Processing

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•	 Obligate anaerobes must avoid all oxygen. 

•	 Facultative anaerobes use oxygen when it is present but do not 

need it to grow. The majority of bacteria are facultative anaerobes. 

} Aerotolerant bacteria are not affected by oxygen. 

V Moisture. 

Nutrients must enter the bacterial cell through its cell wall, therefore 

nutrients must be soluble so they can be carried into the cell by any free (or 

unbound) water that is available in the environment where the bacteria are 

living (such as in food). The measure of this available free water is known 

as the water activity, or aw. The aw is the ratio of the vapor pressure of a 

substance (p) to the vapor pressure of pure distilled water (po ): aw = p/po

. 

If you multiply the aw by 100, you get the equilibrium relative humidity 

expressed as a percent (so dividing the equilibrium relative humidity by 100 

gives you the aw). The aw is a fraction between 0 and 1.00 (1.00 is the aw of pure water). Most bacteria need an aw greater than or equal to 0.95 for 

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Water Activity of Some Common Foods 

Food aw 

Perishable and canned foods (including 

meats, vegetables, fish and milk) 0.95-1.00 

Liverwurst 0.96 

Some cheese spreads 0.95 

Some cheeses and cured meats 0.91-0.95 

Chorizos 0.92 

Many fermented sausages 0.87-0.91 

Semi-moist pet food 0.83 

Salami 0.82 

Chocolate syrups 0.75-0.83 

Jams 0.75-0.80 

Peanut butter – 15% total moisture 0.70 

Jerky ≤0.80 

Chicharones 0.32-0.40 

Control of Bacterial Growth 

The term “exponential growth” is intended to convey the idea that the logconverted numbers of bacteria increase in a linear fashion over a unit of time. 

The relation of log CFU (colony-forming units) to time is a constant value 

(k). Just as bacterial cells reproduce logarithmically (exponentially), they also 

die logarithmically. Log reduction is a mathematical term used to show the 

relative number of pathogenic microorganisms that are killed or inactivated 

by a particular process or combination of processes, such as sanitizing, 

disinfecting, or cleaning. Thermal reductions in bacterial number are expressed 

as the relation of log CFU to time held at a given temperature. Again, the 

relationship should be linear. The D value is the time (at a given temperature) 

resulting in a 1 log reduction in CFU. 

This is what log reductions look like: 

Log Reduction Number of Bacteria 

1 10 times fewer 

2 100 times fewer 

3 1,000 times fewer 

4 10,000 times fewer 

5 100,000 times fewer 

6 1,000,000 times fewer 

7 10,000,000 times fewer 
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As a plant owner or operator, this is an important concept to understand. 

Bacterial log reduction is required by certain regulatory performance standards, 

specifically for Salmonella and E. coli O157:H7. It also can be part of a 

pathogen modeling program report generated as a response to a critical limit 

deviation from a critical control point in your Hazard Analysis and Critical 

Control Point (HACCP) plan. 

To achieve the necessary log reduction of pathogenic bacteria, their growth 

must be controlled. This control may be achieved through manipulation of the 

bacterial environment, the use of certain chemicals, or an interaction of these 

factors. 

First, let’s look at ways you can control bacteria through the manipulation of 

their environment. 

V Control by pH 

} Bacterial growth can be controlled by reducing the pH below the 

minimum level for growth of the organism. For example, acidification 

can be used to prevent the growth of C. botulinum. However, because 

some bacteria, such as E. coli O157:H7, have low infectious doses 

(only a few cells are needed to cause disease), preventing growth alone 

will not provide a safe product. Reduced pH can be combined with 

other factors to control the pathogens of concern. For example, a 

reduced pH, combined with a mild heat treatment, is used to achieve 

commercial sterility in canned, acidified meat and poultry products, 

such as pasta sauces containing meat. 

} Acidification procedures 

To produce products with a pH of 4.6 or less, acidification must be 

properly done. 

•	 Direct batch acidification-Ingredients are mixed in a kettle with 

an acid or, more commonly, an acid food is added directly to the 

batch. 

•	 Addition of acidified brine, such as with pickled pigs’ feet. 

} Determination of pH 

The most important factor in the production of acidified foods 

is the timely attainment and maintenance of a pH level that will 
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inhibit the growth of C. botulinum spores. To achieve this goal, it is 

necessary to measure pH. Although pH can be measured using dye 

solutions and pH paper, the recommended method for determining 

pH is with a pH meter. FSIS currently requires a pH meter to be 

used any time pH is specified as a critical factor for a scheduled 

process for thermally processed product. 

Approximate pH Requirements for Microorganism Growth 

Microorganism Minimum Optimum Maximum 

Clostridium perfringens 5.5–5.8 7.2 8.0–9.0 

Campylobacter spp. 4.9 6.5–7.5 9.0 

Clostridium botulinum 

toxin 4.6 8.5 

Clostridium botulinum 

growth 4.6 8.5 

Staphylococcus aureus 

growth 4.0 6.0–7.0 10.0 

Staphylococcus aureus 

toxin 4.5 7.0–8.0 9.6 

Enterohemorrhagic 

Escherichia coli 4.4 6.0–7.0 9.0 

Listeria monocytogenes 4.39 7.0 9.4 

Salmonella spp. 4.2 7.0–7.5 9.5 

pH minimum as low as 3.8 has been reported for Salmonella spp. 

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V Control by temperature 

} Bacterial growth can be controlled by keeping food at temperatures 

above or below the temperatures at which bacteria grow. Thus, 

refrigeration, freezing, and hot holding can be used to control growth. 

However, the most effective way to control microorganisms is to kill 

them with heat. The amount of heat needed depends on a number of 

factors, including: 

•	 The specific microorganism (the species and whether or not it is in 

the spore form); 

•	 The number of microorganisms to be inactivated; 

•	 The food product; and 

•	 Factors of the food, such as its pH, aw, the presence of
 

preservatives (such as nitrite), and the amount of fat.
 

} The Danger Zone 

The “danger zone” is a term used to describe the temperature range 

that provides bacteria the optimal growth conditions. This range is 

40 °F to 140 °F (4 °C to 60 °C). 

•	 Design food processes to ensure that food products spend as little 

time as possible in the danger zone. 

•	 Control the entire danger zone temperature. At the high end, 

bacteria will grow slowly, and at the low end, many bacteria will 

survive and some may grow. 

•	 Temperatures above the danger zone begin to destroy most 

microbes, although the time needed for cell destruction is longer 

at lower temperatures. At refrigeration temperatures, some 

spoilage bacteria and a few pathogens can grow very slowly; 

however, for most bacteria, refrigeration temperatures are too cold 

for optimal growth. No significant bacterial growth occurs below 

freezing. 
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Approximate Temperature Requirements for Microorganism 

Growth in °F ( °C) 

Microorganism Minimum Optimum Maximum 

Campylobacter spp. 90 (32) 108–113 

(42–45) 113 (45) 

Clostridium botulinum 

types A & B 

50–54 

(10–12) 

86–104 

(30–40) 122 (50) 

Clostridium botulinum 

type E 

37–38 

(3–3.3) 

77–99 

(25–37) 113 (45) 

Clostridium perfringens 54 (12) 109–117 

(43–47) 122 (50) 

Enterohemorrhagic 

Escherichia coli 45 (7) 95–104 (35–40) 115 (46) 

Listeria monocytogenes 32 (0) 86–99 

(30 –37) 113 (45) 

Salmonella spp. 41 (5) 95–99 

(35–37) 

113–117 

(45–47) 

Staphylococcus aureus 

growth 45 (7) 95–104 (35–40) 118 (48) 

Staphylococcus aureus 

toxin 50 (10) 104–113 (40–45) 115 (46) 

V Control by aw 

} When substances are dissolved, there is a substantial reaction between 

the substance and the water. The water-binding capacity of a 

particular dissolved ingredient influences the amount of water left for 

the growth of bacteria. 

•	 A number of the molecules of the water are bound by the 

molecules of the substances dissolved. 

•	 All of the substances dissolved in the water reduce the number of 

unattached water molecules and, in this way, reduce the amount 

of water available for bacterial growth. 

•	 The extent to which the water activity is lowered depends 

primarily on the total concentration of all dissolved substances. 

The dissolved substances compete with the bacteria for available 

water. 

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} The aw is a fraction between 0 and 1.00, with the aw of pure water 

being 1.00. The measurement of food’s aw provides information as to 

which types of microorganisms are most likely to cause spoilage and 

how close the aw is to the safety limit. 

•	 Most bacteria, yeasts, and molds will grow above a water activity 

of 0.95, and most foods have a water activity above 0.95. 

•	 Spores of C. botulinum are generally inhibited at an aw of about 

0.93 or less. Thus, one food preservation method is to reduce 

the amount of water available to spores to a point where they are 

inhibited and apply a mild heat treatment to destroy the vegetative 

cells. Examples of meat products preserved with mild heat and 

reduced aw are jerky and some dry sausages. 

} Methods for determining aw Several methods exist for determining the aw of a food. 

•	 One commonly used method is an electric hygrometer with a 

sensor to measure equilibrium relative humidity (ERH). The 

sensors are the same as those used to measure relative humidity 

in air. 

•	 A dew point instrument is also commonly used to measure aw. 

Some Minimum aw Requirements for Microorganism Growth 

Microorganism Minimum aw for Growth 

Most molds (e.g., Aspergillus) 0.75 

Most yeasts 0.88 

C. botulinum 0.93 

S. aureus 0.85 

Salmonella 0.94 

L. monocytogenes 0.92 

Next, let’s take a look at how you can control bacteria through the use of 

chemicals. Chemicals may be added to foods to inhibit bacterial growth or 

to kill microorganisms. However, at normal levels of use (which must be 

approved by regulatory agencies, such as USDA’s Food Safety and Inspection 

Service and the Food and Drug Administration), most chemicals cause 

inhibition rather than inactivation. Acids and their salts (e.g., lactic acid and 

sodium lactate), nitrites, some phosphates, and sodium chloride (salt) are 
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common chemicals added to meat and poultry. Chemicals are often used in 

combination with other factors, such as heat or reduced aw. 

V Salt 

} Salt (NaCl) is an ingredient which lowers the aw of food. 

} It is often supplemented with other ingredients, such as nitrites, that 

aid in spoilage prevention. 

} If other conditions are not optimum for growth (e.g., low pH or 

temperature), then less salt is required to inhibit growth. For example, 

growth of C. botulinum may occur at a water activity of 0.96 (6.5 

percent NaCl) at pH 7.0, but if the pH is reduced to 5.3, growth will 

be inhibited at a water activity of 0.97 (5 percent NaCl). The salt 

content of a meat product is not as important in inhibiting bacteria as 

the brine concentration (percent of salt in the aqueous portion of the 

meat). 

V Nitrite 

} One important function of nitrite in meat products is that it inhibits C. 

botulinum growth and toxin production. 

} In spite of large amounts of research, there still is not a complete 

understanding of how nitrite controls C. botulinum in meat products. 

} Nevertheless, it is now recognized that nitrite inhibition is due to a 

combination of factors, not nitrite alone. 

Finally, you can control bacteria by utilizing the interaction of various factors 

that were just reviewed. Combinations of inhibitory factors that alone may 

be insufficient to control microorganisms can often be effective when used 

together. This is referred to as the “hurdles,” or “multi-hurdle” concept—if 

enough hurdles or barriers are included, bacteria will not be able to overcome 

the hurdles and grow. For example, when the water activity is lower, the pH 

range at which an organism can grow is more limited. 

Food Preservation Techniques Based on Control Factors 

You’ve probably already figured out that most food preservation techniques 

are based on the control factors mentioned in the previous section. The basic 

principle of all forms of food preservation is to control the growth of bacteria, 
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typically spoilage bacteria. Proper processing of food helps ensure that the 

growth of harmful microorganisms is controlled or eliminated. Popular 

techniques include: 

V Temperature control/refrigeration and freezing 

V Canning/thermal processing 

V Irradiation 

V Chemical preservation 

V Pasteurizing (heat) 

V High-pressure pasteurization (HPP) 

V Acidification/pickling and curing 

V Salting 

V Dehydration 

V Fermentation 

V Carbonation 

V Freeze-drying 

V Modified Atmosphere Processing (MAP) 

Conditions Affecting Molds and Control of Mold Growth 

Remember, molds are widely distributed in nature, both in the soil and in 

the dust carried by air. The only reason they might be present in canned meat 

and poultry products is because of under-processing or contamination after 

processing. 

Molds can grow over a pH range of 1.5 to 9.0 and a minimum aw 0.75. Spores 

can tolerate harsh environmental conditions, but most are sensitive to heat 

treatment. Different mold species have different optimal growth temperatures, 

with some able to grow in refrigerators. The optimum temperature range is 77 

°F to 88 °F (20 °C to 30 °C), but some growth can occur between 32 °F and 95 

°F (0 °C to 35 °C). Molds require oxygen to grow. 

Since molds and their spores are sensitive to heat, they cannot survive the 

thermal processes for low-acid canned foods. Mold spores are not as resistant 

to heat as the bacterial spores that are the target of the processes used for the 
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production of low-acid and acidified canned foods. Since molds must have 

oxygen to grow, only slight growth can occur, unless the food container has an 

opening to the outside environment. 

Conditions Affecting Yeast and Control of Yeasts 

Yeasts are widely found in nature and are usually associated with liquid foods 

containing sugars and acids. Yeast growth in processed foods does not present 

a public health problem. 

Yeasts are able to adapt to conditions such as acidity and dehydration that 

would be too adverse for other types of organisms. Most yeasts require aw 0.87. An aw of 0.85 or less suppresses the growth of organisms of public health 

significance. Like molds, yeasts are more tolerant to cold than heat. Compared 

to bacterial spores, yeasts and their spores possess little resistance to heat, and 

heating to 170 °F (77 °C) destroys most forms of yeast. Yeasts, unlike molds, 

are facultative anaerobes. 

In canned food, the presence and growth of yeast may result in spoilage, 

generally in the form of alcohol production and large amounts of carbon 

dioxide gas, which swells the container. It this happens, gross underprocessing, post-processing contamination, or leakage must be suspected. 

Conditions Affecting Viruses and Control of Viruses 

Since viruses get into food through contaminated water and infected food 

handlers with poor hygienic practices, the best preventative measures involve 

ensuring that only potable water from a trusted (and tested) source is used in 

your processing facility and reinforcing good hygienic practices among your 

employees. You should also not allow employees to work when ill, especially if 

they have symptoms such as diarrhea, nausea, and vomiting. Both Norovirus 

and Hepatitis A have low infectious doses—approximately 10 to 100 viral 

particles for Norovirus, and 10 or fewer virus particles for Hepatitis A. 

Norovirus in food is not inactivated by processes used for preservation 

and storage, such as freezing, acidification, and moderate heat treatments 

(pasteurization). However, it can be effectively inactivated with heat treatments 

used for food preparations, such as baking, cooking, and roasting. 

For the most part, Norovirus retains infectivity at 140 °F (60 °C) for 30 

minutes (Green, 2007). Pasteurization is not sufficient to eliminate the virus. 
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Food containing Norovirus must be heated to at least 158 °F (70 °C). Under 

refrigeration and freezing conditions, the virus remains intact and viable for 

several years. It resists gastric acids at pH 3-4. The virus retains infectivity 

after exposure to pH 2.7 for 3 hours at room temperature. It is believed to 

be sensitive to pH > 9.0, but that has not yet been proven. Based on data for 

other enteric viruses and virus indicators, it is likely that Norovirus persist in 

waters for extended periods (possibly weeks/months) (Carter, 2005; Rzezutka 

and Cook 2004). Norovirus has caused many waterborne outbreaks and is 

often detected in environmental waters. Norovirus is also resistant to drying. 

Infectious Norovirus has been detected on environmental surfaces, including 

carpets, for up to 12 days after outbreaks (Carter, 2005; Greening and Wolf, 

2010). [First published November 2009, doi: 10.1128/AEM.01797-09 Appl. 

Environ. Microbiol. January 2010 vol. 76 no. 2 536-545] 

The Hepatitis A virus is very stable; shows high resistance to chemical and 

physical agents such as heat, acid, and solvents; and has been shown to survive 

in the environment for over 3 months. 

Hepatitis A integrity and infectivity were retained after incubating laboratoryprepared solutions containing the virus for 60 minutes at 140 °F (60 °C), and 

infectivity was retained after 10 minutes at 176 °F (80 °C). The Hepatitis A 

virus is even more heat resistant when present in foods and shellfish. Food 

containing the Hepatitis A virus must be heated to 185 °F (85 °C) or above. 

Under refrigeration and freezing conditions, the virus remains intact and 

infectious for several years. The virus is stable in acidic conditions. It retained 

high infectivity after 2 hours and was still infectious after 5 hours at pH 1.0 

and 77 °F (25 °C). The virus is also resistant to drying. It remained infectious 

for over 1 month at 77 °F (25 °C) and a humidity of 42 percent. There is 

greater resistance to inactivation at low humidity and temperatures. 

Conditions Affecting Parasites and Control of Parasites 

Parasites are readily destroyed by cooking. They are not a major concern in 

thermally processed, commercially sterile meat and poultry products since 

they are subjected to temperatures well in excess of what is needed to destroy 

parasites. Parasites are a concern in shelf-stable products that are not cooked. 

For example, trichinae are a concern in shelf-stable products containing pork, 

such as dried sausages. 
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FSIS has regulations regarding the disposition of carcasses affected with Taenia 

saginata and Taenia solium: Title 9 of the Code of Federal Regulations, Section 

311.23 (9 CFR 311.23), “Tapeworm cysts (cysticercus bovis) in cattle,” and 

9 CFR 311.24, “Hogs affected with tapeworm cysts.” According to 9 CFR 

311.23, if the cattle carcass shows signs of severe infestation (meaning that the 

lesions are “extensive” or “if the musculature is edematous or discolored”), the 

carcass must be condemned. Carcasses without extensive infestation may be 

passed for human food only after: 

V the removal and condemnation of the lesions and surrounding tissues (done 

after careful postmortem examination), 

V the carcasses are held in cold storage at a temperture of 15 °F or below
 

continuously for a period of 10 days or more, or
 

V the carcasses are heated throughout to a temperature of 140 °F or more. 

In the case of boned meat derived from carcasses without extensive infestation, 

the meat, when in boxes, tierces, or other containers, must be treated by: 

V holding them at a temperature of 15 °F or below continuously for a period of 

20 days or more, or 

V heating the meat throughout to a temperature of 140 °F or more. 

The carcasses and meat are under retention and the control of FSIS inspection 

program personnel until treatment is complete. Edible viscera and offal from 

the carcass are disposed of in the same manner as the carcass unless any lesion 

of cysticercus bovis is found in the byproducts. If so, they are condemned. 

For tapeworm cysts in swine, 9 CFR 311.24, “Hogs affected with tapeworm 

cysts,” states “Carcasses of hogs affected with tapeworm cysts (Cysticercus 

cellulosae) may be passed for cooking, unless the infestation is excessive, in 

which case the carcass shall be condemned.” The carcasses or meat should be 

heated to 140 °F or more. 

FSIS also has regulations regarding the treatment of pork and pork products for 

Trichinella spiralis – 9 CFR 318.10, “Prescribed treatment of pork and products 

containing pork to destroy trichinae.” The regulation outlines the requirements 

for heating, refrigerating, and curing of pork products to kill trichinae. 

Essentially, all forms of fresh pork (including fresh unsmoked sausage 

containing pork muscle tissue and pork, such as bacon and jowls [except as 
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described in paragraph (b) of the regulation]), are considered products that 

are usually well cooked before they are consumed, so they do not have to be 

treated. In addition, pork from carcasses or carcass parts that have been found 

free of trichinae, as described under paragraph (e) or (f) of the regulation, is not 

required to be treated for the destruction of trichinae. 

All of the products listed in paragraph (b) of the regulation, as well as products 

having the same character as products listed in paragraph (b), must be treated 

to kill trichinae. For detailed information, please review 9 CFR 318.10. 

Time/temperature table for heating pork 

Minimum Internal Temperature Minimum Time 

Degrees Fahrenheit Degrees Centigrade 

120 49.0 21 hours 

122 50.0 9.5 hours 

124 51.1 4.5 hours 

126 52.2 2 hours 

128 53.4 1 hour 

130 54.5 30 minutes 

132 55.6 15 minutes 

134 56.7 6 minutes 

136 57.8 3 minutes 

138 58.9 2 minutes 

140 60.0 1 minute 

142 61.1 1 minute 

144 62.2 instant 

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Time/temperature table for freezing pork 

Temperature °F Group 1 (Days) Group 2 (Days)

 5 20 30 

-10 10 20 

-20 6 12 

Group 1 comprises product in separate pieces (not exceeding 6 inches in 

thickness), or arranged on separate racks (with the layers not exceeding 6 inches 

in depth), or stored in crates or boxes (not exceeding 6 inches in depth), or 

stored as solidly frozen blocks (not exceeding 6 inches in thickness). Group 

2 comprises product in pieces, layers, or within containers, (the thickness of 

which exceeds 6 inches but not 27 inches), and product in containers including 

tierces, barrels, kegs, and cartons (having a thickness not exceeding 27 inches). 

Alternative time/temperature table for commercial freeze-drying or 

controlled freezing 

Minimum Internal Temperature Minimum Time 

Degrees Fahrenheit Degrees Centigrade 

0 -17.8 106 hours 

-5 -20.6 82 hours 

-10 -23.3 63 hours 

-15 -26.1 48 hours 

-20 -28.9 35 hours 

-25 -31.7 22 hours 

-30 -34.5 8 hours 

-35 -37.2 1/2 hour 

The parasitic protozoa Cryptosporidium parvum can be destroyed by heating 

water to a rolling boil (212 °F or 100 °C) for at least 1 minute. Cryptosporidium 

parvum is resistant to inactivation by chlorine or iodine. Some of the factors 

affecting the survival of Cryptosporidium are [David Dawson/International 

Journal of Food Microbiology 103 (2005): 
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V Environment – The oocysts are resistant to cool, damp conditions. Oocysts 

have survived for 1 year in sea water (Tamburrini and Pozio, 1999) and 6 

months in river water and cow feces (Robertson et al., 1992). 

V Heat – The oocysts are not infective when water or milk is heated to 161.1 

°F (71.7 °C) for 5, 10, and 15 seconds (Harp et al., 1996); or, when water is 

heated to 147.6 °F (64.2 °C) for 2 minutes (Fayer, 1994). 

V pH – The oocysts lose viability in orange juice, carbonated beer, or cola 

at a pH 3.9 and a temperature of either 39.2 °F (4 °C) or 71.6 °F (22 °C), 

(Friedman et al., 1997). Infectivity is reduced by malic acid, citric acid, 

tartaric acid (Kniel et al., 2003), ethanol, and low water activity (Dawson et 

al., 2004). 

V Freezing – The oocysts are not infective after freezing at -94 °F (-70 °C). 

However, some oocysts remain viable after 7 days at 5 °F (-15 °C) and -4 °F 

(-20 °C) (Fayer and Nerad, 1996). 

V Disinfectant – The oocysts are very resistant to disinfectants. Although 

routine chlorination of water is not effective, the use of ozone disinfection is 

highly effective (Casemore, 1995). 

V Drying – The oocysts are very sensitive to drying. In one study, 95 percent 

of oocysts died within 4 hours at room temperature (Robertson et al., 1992). 

V UV Light – The oocysts are sensitive to UV light used in water treatment 

(Hargy et al., 2000). 

The cysts of Toxoplasma gondii, the other parasitic protozoa, can be destroyed 

through heating product to an internal temperature of about 153 °F (67 °C) 

and freezing to a temperature of 10.4 °F (-12 °C). More specifically, the 

internal temperature of the product must be heated to 131 °F (55 °C) or higher 

and held at that temperature for at least 20 minutes, 141.8 °F (61 °C) for 3.6 

minutes, or 152.8 °F (67 °C) for 7 seconds to inactivate Toxoplasma gondii tissue 

cysts. 

Insect control is very important for the control of both protozoa because insects 

may actually transfer oocysts to uncovered food. 
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Microbiological Sampling Programs 

Microbiological sampling is not a magic bullet. Sampling by itself will 

not render an adulterated or contaminated product safe. Rather, it is 

the antimicrobial interventions that reduce or prevent contamination or 

decontaminate the product. However, sampling does provide some assurance 

(evidenced by negative test results) that products are not adulterated, which is 

why it is used as a “verification” procedure. Therefore, sampling is a means of 

verifying that the antimicrobial interventions are working in controlling the 

hazard. Federally inspected establishments are not required to conduct their 

own microbiological sampling of their meat or poultry products, but many do 

for various reasons (e.g., verifying effectiveness of antimicrobial interventions, 

customer request, etc.). 

As a small plant owner and operator, it’s important for you to be aware of the 

fact that pathogens in food products are usually present at very low levels, if 

at all, and are unevenly distributed. Also, bacteria are not always predictable. 

Keep in mind that the food environment is complex. Some ingredients of foods 

will actually provide a protective environment for bacteria (e.g., fat), while other 

food ingredients are injurious to bacteria (e.g., acid, preservatives, etc.). 

Design of a Sampling Plan 

When designing a sampling plan, you should seek what you hope you won’t 

find. Anyone can design a plan that, when followed, will result in no or negative 

findings. However, it is in your best interest to find the pathogens before the 

contaminated product enters commerce. Otherwise, the impact on public 

health and the associated costs to your business could be devastating. Therefore, 

detecting the pathogen is the desirable outcome of a well-designed and executed 

sampling program. 

The amount and frequency of sampling depends on the amount of risk inherent 

to the product and on a number of variables. These include, but are not limited 

to, the number of antimicrobial interventions, the number of suppliers, whether 

you follow best practices, and how product lots are determined. Another variable 

that will impact risk is the level of contamination. Products with low levels of 

contamination must be sampled more frequently than heavily contaminated 

products to obtain the same level of confidence from the results. 
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Essential Components of a Sampling Plan 

To be effective, your sampling plans need to include certain components. Those 

components should be clearly identified and include: 

V Purpose of Sampling – Why is sampling being done? 

There are many reasons to sample product. The purpose of the bacterial 

sampling will impact what product is to be sampled and how. For 

example, is the sampling being conducted to determine if product is 

adulterated? Is it to determine if the food contact surfaces or product are 

contaminated? Is it to determine the level of contamination, as when 

establishing a baseline (e.g., Salmonella baseline study)? Is the sampling 

being conducted to determine the effectiveness of a procedure (e.g., 

sanitary dressing), or is it done to satisfy a customer requirement? 

V Product To Be Sampled – What is to be sampled? 

It’s most effective to sample both source materials as well as finished 

product. Not all source materials carry the same risk of contamination. 

For example, beef from advanced meat recovery systems (AMR) is more 

likely to be contaminated with E. coli O157:H7 than beef heart meat. 

Likewise, beef from bench trim is more likely to be contaminated with E. 

coli O157:H7 than beef manufacturing trim. 

Remember, a well-designed sampling plan is one that is designed to find the 

pathogen, if it is present. Since contamination is usually more prevalent on 

the surface of the carcass, it’s logical to collect samples from the surface as 

opposed to collecting core samples, which would decrease the likelihood of 

finding the pathogen. 

V Frequency of Sampling – When will the sampling occur? 

Since pathogens, such as E. coli O157:H7, are not evenly distributed 

throughout the lot, the portions should be collected randomly, at different 

sites throughout the lot, and/or at different times throughout production. 

FSIS’ N-60 sampling is based on this type of sampling design. 

V Sample Size – How much will be sampled?
 

An effective sample is one that fully represents the lot.
 

V Analytical Method – Which test will be used? 
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There are many microbiological sampling methods. The method used 

should be appropriate for the purpose. The sampling method used 

should be one capable of detecting the lowest possible levels of pathogens, 

specifically in meat or poultry products. 

V Effectiveness of Analytical Method – Will the test find the target organism? 

The detection method should be able to identify intact pathogens, as well 

as pathogen cells that may have been damaged by antimicrobial treatments 

or preservation methods, such as refrigeration or freezing. Not all sampling 

methods used by all laboratories are capable of achieving this level of 

detection. 

Samples should be properly and completely identified. Be sure to minimize 

opportunities for spoilage while transporting samples to the laboratory because 

growth of competing spoilage bacteria could mask the presence of pathogens 

in the sample and make them harder to detect. Avoid freezing samples during 

storage and transit because pathogens can die off during freeze/thaw cycles. 

Remember that the best sampling plans provide the opportunity for detection. 

However, there is no guarantee of detection. 

C. botulinum in a petri dish. 
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Product Versus Environmental 

and Food-Contact Surface Sampling 

Environmental testing refers to testing of the surrounding environment in your plant, 

not to food contact surfaces. Food contact surface testing refers to the testing of 

all surfaces, including tables, belts, equipment, etc., that come in contact with your 

product. Environmental and food-contact surface testing assesses the control of the 

in-plant environment, not whether a specific lot of product has been contaminated. 

You use environmental and food-contact surface testing to verify the effectiveness of 

your sanitation procedures. Although a statistically designed program can be used, 

environmental and food-contact surface testing programs are often not statistically 

designed. Instead, they are based on prior experience and familiarity with your 

processes and equipment. 

Environmental and food-contact surface testing is a better and more cost-effective 

method than product testing for assessing process control. It is used to detect trends 

that indicate a potential loss of sanitation control in the environment and enable 

timely corrective actions. Testing that focuses on controlling sanitation in the food 

processing environment and exposure to the risks associated with food-contact 

surface contamination is actually more informative than end-product sampling. For 

example, unlike food-contact surface sampling, end-product samples found to be 

positive do not indicate how the products became contaminated or how to prevent 

recurrences. 

Niches are often involved (e.g., hollow rollers or conveyors) when a foodborne 

pathogen, such as L. monocytogenes, gets established in a RTE food processing facility. 

A niche is a site within the food processing establishment where bacteria become 

entrenched and multiply. Niches are often reservoirs where the pathogens are spread 

during processing and, subsequently, contaminate your product and product-contact 

surfaces. 

Construction in areas where RTE products are exposed can increase the risk of 

product contamination with L. monocytogenes. This may be due to dust being 

dispersed throughout the area and can lead to the introduction of new, more virulent 

strains of L. monocytogenes into the environment. 

If a construction event increased the rate of product, food contact surface, or environmental contamination, the routine sampling plan would become more effective as 

verification. Higher rates are more likely to be detected as positive results. It would be 

prudent to enhance sampling because of the overall probability of a false negative. 
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Under these conditions, you would need a plan to segregate the production areas 

from the areas under construction and to continue to verify the effectiveness of the 

sanitation/L. monocytogenes control program with testing. 

Environmental and Food-Contact Surface Sampling Programs 

The design of your program and your response to positive findings determines the 

effectiveness of the program. Routine environmental sampling and food-contact 

surface sampling programs are essential for maintaining an assessment of control. 

Positive findings should be addressed with corrective actions necessary to identify 

and control the source of contamination. Ideally, samples should be analyzed 

individually. However, compositing of samples may be done to reduce costs 

associated with sampling. 

Sample sites should include areas found to have been good indicators of control, 

for example, equipment surfaces that are exposed to food, brines for chilling, or 

floors near packaging machines. 

Sampling Frequency 

A good sampling frequency is to sample from each processing line at least once 

a week. The number of samples should be enough to assess control (e.g., 2-10 

samples). The specific number should reflect your plant’s history of control and 

the complexity of the system. The sampling program should identify appropriate 

sample sites. 

Analyzing Results 

You should review results as soon as they become available. It’s very important 

to keep in mind that positive results will occur. We know that L. monocytogenes 

is in the food-processing environment, but it’s often difficult to find its specific 

source or location. Even with all this sampling, there is often difficulty in 

detecting the source. Ideally, sampling programs will reveal the extent of a 

problem so that your resources should be directed where risk is highest. 

Positives from an environmental testing program will help you identify where the 

contamination is occurring in the process. Once identified, the problem can be 

corrected. The findings should be viewed as a success because they indicate your 

monitoring program has been effective in finding and identifying the pathogen. 
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Aseptic Sampling Techniques 

An aseptic technique implies that you do not add any organisms to the sample 

when it is collected. It does not imply that the sample is aseptic or free of 

microorganisms. Extraneous microorganisms from the environment, hands, 

clothing, sample containers, and sampling devices may lead to erroneous 

analytical results. Stringent requirements for microbiological analysis are 

necessary; therefore, the use of aseptic sampling techniques and clean and 

sanitized equipment is of utmost importance. 

The purpose of aseptically collecting a sample is to prevent contamination of 

the sample or the surrounding product/product contact area. 
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Glossary of Useful Terms 

Acute – Sudden onset and repaid progression (when used in reference to a 

disease or condition). 

Aerobic – Bacteria that require oxygen to grow or will grow in the presence of 

oxygen. 

Anaerobic – Bacteria that do not utilize oxygen to grow, or will not grow in 

the presence of oxygen. 

Aqueous – Relating to water. 

Autoinfection – Infection caused by bacteria, viruses, or parasites persisting on 

or in the body. 

aw (Water activity) – The measure of free water in the environment that is 

available for use by bacterial cells. 

Bacteriocin – A substance that is produced by specific bacteria that is toxic to 

closely related strains of the same specific bacteria and either kills or slows the 

growth of those other specific bacteria. 

Coliform – Bacteria that most often inhabit the intestine of animals and do 

not utilize oxygen, but can grow in its presence. Bacteria that are classified as 

coliforms have the same shape and many of the same characteristics. These 

bacteria are used as indicators of sanitary quality in many food products. 

Colony – A visible growth of microorganisms (bacteria) on a solid nutrient 

medium. Members of the colony are identical to the original or parent cell. 

Colony-forming units (CFUs) – Visible units counted in a plate count, which 

may be formed from a group of cells rather than from one cell. CFU is used to 

measure the number of living cells present. 

Curing – The addition of salt, sodium, or potassium nitrate (or saltpeter), 

nitrites, and sometimes sugar, seasonings, phosphates, and cure accelerators 

(e.g., sodium ascorbate), to pork for preservation, color development, and flavor 

enhancement. 
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Detection limit – The lowest threshold amount of bacteria that must be 

present in a sample to be found. Detection level depends upon methods used. 

Direct plating – The application of a sample, or dilution thereof, to solid 

media, usually containing agar and other material used to grow and enumerate 

bacteria. 

D-value – The amount of time needed to destroy one log unit of a specific 

bacteria at a specific temperature in a specific medium. 

Enrichment – The addition of nutrient-rich broth so that certain bacteria or 

types of bacteria increase in number to result in a bacterial cell count that is 

higher than the detection limit. This is used to detect only the presence or 

absence of the bacteria, not the amount present. 

Enterobacteriaceae – Large group of bacteria that are closely related and are 

commonly found in fecal material of warm-blooded animals. They include 

coliforms and pathogens such as Salmonella. 

F-value – Measured in minutes, the D-value of a specific organism at 250  ̊F 

(121  ̊C) multiplied by the desired log reduction. 

Facultative aerobes – Microorganisms that grow best when oxygen is present 

but do not need it to grow. 

Facultative anaerobe – Microorganisms that do not need oxygen to grow, but 

will use it when it is present. 

g (Generation time) – The time it takes for bacterial cell numbers to double. 

Germination – The process of a spore becoming a vegetative cell. 

Gravid – Pregnant or containing eggs. 

Heat labile – Destroyed or altered by heat. 

Hemolytic anemia – A condition in which red blood cells are destroyed and 

removed from the bloodstream before their normal lifespan is over. This leads 

to the blood having a lower than normal number of red blood cells. Hemolytic 

anemia can lead to many health problems, such as fatigue (tiredness), pain, 

irregular heartbeats, an enlarged heart, and heart failure. 
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Hemorrhagic colitis – Abdominal cramps and bloody diarrhea, without fever. 

Inactivation – The destruction of the activity of a pathogenic microorganisms 

so the microorganism is no longer harmful. 

Infection – The invasion by, and growth of, pathogenic microorganisms in a 

host. 

Infectivity – The ability to produce infection. 

Infestation – The presence of large numbers of organisms on or in a host 

causing illness or damage. 

Inhibition – The slowing or stopping of bacterial growth. 

k (Growth rate) – The rate at which bacterial cells reproduce. 

Lag time – The time that bacteria take to become acclimated to a new 

environment before starting to multiply. Bacteria divide and their numbers 

grow exponentially: 1 becomes 2 becomes 4 becomes 8, etc. 

Lethality – The effectiveness of a treatment to destroy or kill bacteria. 

Log unit – An exponential (multiplicative) relationship between units in a 

numerical scale. The exact relationship is conveyed by the base. Assuming 

that “log” is “log base 10,” every unit is expressed as the exponential of 10. For 

example, 1 log10 is equal to 101

 (10 to the 1st power) or 10; 2 log10 is equal to 

102 (10 x 10) or 100, and so on. Moving 1 unit in the log10 scale is equivalent 

to multiplying or dividing the preceding number by 10 (multiply if increasing 

the log number, divide if decreasing the log number). 

Scientists convert bacterial counts to log scales (or plot on log-log or semilog graphs) because it allows them to see the large changes apart from the 

irrelevant data inherent in the process of measuring bacteria populations. If two 

populations of bacteria differ by less than 10 fold (1 log10 unit), the distinction 

is not likely to be significant. But differences of 10, 100, or 1,000 fold (1, 2 or 3 

log10 units) are more likely to be significant and scientifically important. 

Mesophiles – Bacteria that have optimum growing temperatures between 

77 °F (25 °C) and 104 °F (40 °C). 
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Microaerophiles – Microorganisms that require oxygen at a lower level than is 

found in normal air to survive. 

Microflora – Bacteria, molds, and yeasts. 

Obligate aerobe – Microorganisms that require a high concentration of 

oxygen to survive. 

Obligate anaerobe – Microorganisms that must avoid all oxygen to survive. 

Oncosphere – The larva of the tapeworm contained within the external 

embryonic envelope and armed with six hooks. 

Pathogen – Organisms that cause illness. These organisms include bacteria, 

protozoa, or viruses. 

pH – Level of acidity or alkalinity in a product. The pH scale ranges from 1 

to 14, with 7 considered neutral, 1 the most acidic, and 14 the most alkaline. 

Fresh meat usually has a pH near 5.6. 

Proglottid – An individual segment of a tapeworm. 

Psychrotrophs – Bacteria that have optimum growing temperatures between 

68 °F (20 °C) and 86 °F (30 °C), but can grow at temperatures as low as 32 °F 

(0 °C). 

Scolex – The head of a tapeworm that usually has suckers and/or hooks. 

Shocked (heat shocked) – Occurs when a product is heated, but the 

temperature is not high enough to destroy the bacteria. It results in bacteria 

that are injured for a while, but in most cases can repair itself and becomes 

more resistant to heat the next time the product is heated. Heat shocked also 

can refer to the process by which a spore is induced into germination. When a 

product is heated thoroughly, the vegetative cells are destroyed, but the spores 

are undamaged by the heat. The spores then germinate into vegetative cells 

once the temperature has decreased to an optimum level. 

Significant difference – Statistical difference in results. 

Spore – A highly resistant, dormant form that some bacteria can change into. 

Spores are usually very resistant to heat, long periods of dryness, and other 

adverse conditions that normal vegetative cells cannot survive. Most must be 
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adverse conditions that normal vegetative cells cannot survive. Most must be 

heat-shocked to germinate into normal, vegetative cells. Most of the time, spores 

have a toxin associated with them, either within the spore covering, or released at 

the time of germination, or when becoming a spore (sporulation). 

Strain – A specific subset of bacteria. For example, Escherichia is the genus, 

coli is the species, and O157:H7 is the serotype (strain). 

Thermotolerant – Bacteria that can withstand higher-than-normal 

temperatures. 

Thermobocytopenia – An abnormal drop in the number of blood cells, called 

platelets, that are involved in forming blood clots. 

Toxin (enterotoxin, mycotoxin, neurotoxin) – A compound produced by 

a bacterium or fungi (molds and yeasts) that can cause illness in other living 

organisms. Specific examples include enterotoxins which affect the intestine, 

mycotoxins (toxins produced by fungi), and neurotoxins that attack the nervous 

system. 

Transdermal synergists – Compounds that work with other compounds 

against bacteria when applied to the surface of a carcass. 

Treatment – The method of processing that is being tested. A good research 

study will compare various treatments, such as levels of salt in a product, to a 

control. In this example, the control may be no salt added. All other conditions 

should remain the same for all samples tested, except the specific treatment. 

Vegetative cell – The normal bacteria cell. This is in contrast to a spore. 

Vegetative cells are susceptible to destruction or damage from heat, additives, 

and other factors that can damage and destroy them relatively easily. 
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The Small Plant Help Desk

A resource for small and very small plants 

Call toll-free 1-877-FSISHELP (1-877-374-7435). 

Knowledgeable USDA-FSIS specialists from the Outreach and Partnership 

Division are available weekdays, 8:00 a.m. to 4:00 p.m. ET: to give you 

personal assistance on matters relating to the regulation of meat, poultry, and 

processed egg products. Or email questions to InfoSource@fsis.usda.gov. 
United States Department of Agriculture

Food Safety and Inspection Service

The U.S. Department of Agriculture (USDA) prohibits 

discrimination in all its programs and activities on the basis of 

race, color, national origin, age, disability, and where applicable, 

sex, marital status, familial status, parental status, religion, sexual 

orientation, genetic information, political beliefs, reprisal, or 

because all or part of an individual’s income is derived from any 

public assistance program. (Not all prohibited bases apply to 

all programs.) Persons with disabilities who require alternative 

means for communication of program information (Braille, 

large print, audiotape, etc.) should contact USDA’s TARGET 

Center at (202) 720-2600 (voice and TDD). To file a complaint 

of discrimination, write to USDA, Director, Office of Civil Rights, 

1400 Independence Avenue, S.W., Washington, DC 20250-9410 

or call (800) 795-3272 (voice) or (202) 720-6382 (TDD). USDA is 

an equal opportunity provider and employer. 

Office of Outreach, Employee Education and Training 

August 2012