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Module 2. Microbiology of 

Thermally Processed Foods 

Thermal Processing for Meat and Poultry 

Products Training 
 

 

 

Introduction 

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Microbiology is the study of 

small living organisms seen 

only by using a microscope. 

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The Microbiology of Food Processing 

 Nicolas Appert – began heat 

processing for food preservation 

 Louis Pasteur – found 

microorganisms caused food 

spoilage 

 Food microbiology research that 

began in 1895 linked the two 
•. y 

,, ., 

YEAST BACTERIA 

 

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Characteristics and Behavior of Microorganisms 

 Raw foods contain 

microorganisms 

 Microorganisms of 

concern are molds, 

yeasts, and 

bacteria 

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Classification of Microorganisms 

 Visual appearance in colonies 

 Materials used as nutrients 

 Food spoilage by-products 

 Tolerance to environment 
 

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Useful Functions of Microorganisms 

 Fermented foods 

 Produce useful chemicals 

 Break down organic matter 
 

 

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Some Microorganisms Cause Disease 

 Very few are harmful to humans 

 Only a few can be transmit disease 

through food 
Significant Microorganisms in Food Processing: 

Molds 

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Molds 

Yeasts 

Bacteria 

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Molds 

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 Multicellular, tubular 

filaments 

 Reproduce by fruiting 

bodies 

 Larger than bacteria, 

longer than yeasts 

 Widely distributed in 

nature 

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Molds 

 Survive on many substances 

 More tolerant to cold than heat 

 Some may raise pH of acid or acidified 

low acid foods so that C. botulinum can 

grow and produce toxin 

 Most have low heat resistance 

 Aerobic 
Significant Microorganisms in Food Processing: 

Yeasts 

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Molds 

Yeasts 

Bacteria 

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Yeasts 

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 Unicellular, usually 

egg-shaped 

 Smaller than molds, 

larger than bacteria 

 Reproduce by budding 

 Widely found in nature 

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Yeasts 

 More tolerant to cold than heat 

 Liquid foods with sugar and acid 

 Little resistance to heat 

 Alcohol and carbon dioxide are by-products of 

growth 

 No public health problem 
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Significant Microorganisms in Food Processing: 

Bacteria 

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Molds 

Yeasts 

Bacteria 

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Bacteria 

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 Most important and 

troublesome 

 May excrete enzymes 

or toxins 

E. Coli O157:H7 

 Single cell, microscopic 

 Several shapes (rods, 

cocci, and spiral) and Staph 

forms 
(Q) _ 

ROD 

Shapes of Organisms of Significance in Foods 

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I 

c:, 

 

 

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Reproduction of Bacterial Cells 

Reproduction of a Single Cell Reproduced by division – fission 

1 4 16 1 billion 

0 hours 1hour 2 hours 15 hours 

Bacterial Buildup on a Conveyor Belt 

(count per square inch of belt) 

75,000 300,000 4,800,000 

0 hours 1 hour 3 hours 17 
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Sporeforming and Non-Sporeforming Bacteria 

 Some of the rod-shaped bacteria 

produce spores 

 Spore = dormant stage 
Position of Spores 

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Center 

Terminal 

Subterminal 
 

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Resistance of Spores to Environment 

 Bacterial spores are extremely resistant 

to heat, cold, and chemical agents 

 Vegetative cells less resistant 
 

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Oxygen Requirements 

 Aerobes – require oxygen to live 

 Anaerobes – oxygen prevents growth 

 Facultative anaerobes – tolerate 

presence or absence of oxygen 
 

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Moisture Requirements 

 The amount of moisture and its 

availability in a food are important factors 

for bacterial growth 

 Measured by water activity 

 Influenced by the addition of ingredients 

such as salt and sugar 
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Minimum aw Requirements for 

Microorganism Growth 

Most molds (e.g., Aspergillus) 0.75 

Most yeasts 0.88 

C. botulinum 0.93 

Staphylococcus aureus 0.85 

Salmonella 0.93 

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pH Requirements 

 Microbial growth is generally greater at a 

neutral pH 

 Bacteria, yeast and molds have 

optimum, minimum and maximum pH for 

growth 

pH 

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 

Acidity Neutral Alkalinity 
 

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Approximate pH Range for Certain Foods 

Lemon Juice 2.0 - 2.6 Tuna 5.2 - 6.1 

Apples

Blueberries 

3.1 - 4.0 

3.1 - 3.3 

Sweet Potatoes 

Onions 

5.3 - 5.6 

5.3 - 5.8 

Sauerkraut 3.3 - 3.6 White Potatoes 5.4 - 5.9 

Orange Juice 

Pineapple, canned 

Apricots

Tomatoes, canned 

3.3 - 4.2 

3.4 - 4.1 

3.3 - 4.0 

3.5 - 4.7 

Spinach 

Beans 

Peas, canned 

Corn, canned 

5.5 - 6.8 

5.6 - 6.5 

5.7 - 6.0 

5.9 - 6.5 

Peaches, canned 

Pears, canned 

3.7 - 4.2 

4.0 - 4.1 

Soy Beans

Mushrooms 

6.0 - 6.6 

6.0 - 6.7 

Bananas 4.5 - 5.2 Clams 6.0 - 7.1 

Beets, canned 4.9 - 5.8 Salmon 6.1 - 6.3 

Asparagus, canned

Beef 

5.0 - 6.0 

5.1 - 7.0 

Coconut milk 

Milk 

6.1 - 7.0 

6.4 - 6.8 

Carrots 4.9 - 5.2 Garbanzo Beans 6.4 - 6.8 

Peppers, green

Papaya 

5.2 - 5.9 

5.2 - 6.0 

Chicken 

Eggs, whole 

6.5 - 6.7 

7.1 - 7.9 
 

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pH of Some Canned Meat Products 

Food pH 

Beef stew 5.4-5.9 

Corned beef hash 5.0-5.7 

Ham 6.0-6.5 

Spaghetti sauce with beef 4.0 - 4.2 

Vienna sausage 6.2-6.5 
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Temperature Requirements 

 Bacterial group names are based on 

optimum temperatures for growth 

• Psychrotrophs 

• Mesophiles 

• Thermophiles 
 

 

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The Psychrotrophic Group 

 Grow best at 58°F to 68°F 

 Can grow slowly at 40°F 

 Only C. botulinum Type E and nonproteolytic strains of types B and F are 

of concern in canned food 
 

 

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The Mesophilic Group 

 Grow best 86°F to 98°F 

 Includes all microorganisms that affect 

food safety 

 C. botulinum—a sporeformer—is in this 

group 
 

 

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The Thermophilic Group 

 Optimum growth of obligate 

thermophiles occurs at 122°F to 150°F 

 Spores are very heat resistant 

 No human pathogens 
 

 

 

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Interaction of Conditions Affecting the 

Growth of Bacteria 

 Each growth limiting factor (e.g., 

minimum pH) is determined when all 

other conditions (moisture level, 

temperature, absence or presence of 

oxygen, etc.) are optimal 
 

 

 

 

 

 

 

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Interaction of Conditions Affecting the 

Growth of Bacteria (2) 

 A growth condition that is not optimal can 

affect the optimum for another growth 

condition 

• Lowering the aw raises the minimum pH for 

growth and vice versa 

• Preservatives (e.g., salt or nitrite) can raise 

the minimum pH at which an organism 

grows or increase growth time at lower 

temperatures 
 

 

 

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Clostridium botulinum 

 Pathogen of concern for canned meat 

and poultry products 

 Anaerobic, mesophilic sporeformer 

 The organism produces a deadly toxin 

 Botulism is the disease in humans 
 

 

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Clostridium botulinum 

 The organism is contained in soil and 

water throughout the world 

 Spores found everywhere, but 

vegetative form produces toxin 

 Spores survive unfavorable conditions 

such as acid environments and heat 

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Clostridium botulinum 

 Proteolytic (“putrefactive”) strains 

• Grow best at 86°-98°F; minimum 50°F 

• Metabolize proteins and produce putrid odors 

• High heat resistance 

 Non-proteolytic strains 

• Grow best at 64°-77°F; minimum 38°F 

• Metabolize carbohydrates (sugars and starches) 

• Spores do not survive boiling 

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Clostridium botulinum 

 Certain spores survive 5 to 10 hours in 

boiling water– 212oF (100oC) 

 Although the spores are heat resistant, 

the toxin can be inactivated by boiling 

temperatures 

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Clostridium botulinum 

 Canned foods provide an anaerobic 

environment favorable for spores to germinate, 

become vegetative cells, multiply and produce 

toxin (if the organism has not been destroyed 

by the heat process) 

 Heat processes for canned meat and poultry 

are designed, at a minimum, to produce a 

product safe from C. botulinum 

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Control of C. botulinum with pH 

 Reduced pH designed to prevent growth 

of Clostridium botulinum 

 Spores of C. botulinum will not germinate 

and grow in foods below pH 4.8 

 pH of 4.6 provides a safety margin 

 Acidified low-acid foods – acids or acid 

foods added to reduce the pH to ≤4.6

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Control of C. botulinum with 

Water Activity (aw) 

 It is the free available water, not the % 

moisture, that is important in controlling 

C. botulinum 

 Substances (salt and sugars) dissolved 

in water bind water molecules, reducing 

the water available for microbial growth. 

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Control of C. botulinum with 

Water Activity (aw) 

 Spores of C. botulinum will not 

germinate and grow in foods with an aw 

less than 0.93 

 If the aw is 0.85 or less, product is not 

subject to the canning regulations. This 

value was chosen because S. aureus 

can grow above an aw of 0.85 

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Control of C. botulinum with 

Chemicals 

 Acids (lowers pH) and preservatives such 

as nitrite and sodium chloride (salt) 

 Most inhibit growth rather than kill 

 When used to produce commercially 

sterile products, chemicals often 

combined with other factors such as heat 

or reduced aw 

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Control of C. botulinum with Salt 

 Salt lowers aw 

 Inhibits germination and growth 

 Most resistant C. botulinum inhibited by 

10 percent salt (aw = 0.93) 

 Less salt is needed when the pH has 

been reduced 

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Control of C. botulinum with Nitrite 

 Inhibits growth and toxin production 

 Reduces the heat resistance of the spore 

 Still not a complete understanding of how 

nitrite controls C. botulinum 

 Nitrite inhibition is due to a combination of 

factors, not nitrite alone 

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Control of Microorganisms 

with Temperature 

 Use heat to kill microorganisms 

• Amount depends on target microorganism 

(e.g., C. sporogenes spore, C. botulinum 

spore, vegetative pathogen) number of 

microorganisms, and the food product 

• pH, aw, and preservatives such as nitrite 

can have an impact on death 

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Thermal Destruction (Death) 

of Microorganisms 

 Low-acid canned foods-high temperatures 

are needed 

• Heat processes designed to destroy C. 

sporogenes and similar anaerobes 

• C. sporogenes spores have greater heat 

resistance than C. botulinum spores 

• Targeting C. sporogenes spores will also 

destroy spores of C. botulinum 

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Thermal Destruction (Death) 

of Microorganisms 

 Acidified low-acid canned foods can be 

processed at lower temperatures 

• Mild heat kills, mold, yeast, Salmonella, E. 

coli O157:H7 and other vegetative cells of 

pathogens 

• pH of 4.6 or less prevents germination and 

outgrowth of C. bot and C. sporogenes 

spores 

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MIU) HEAT ~ TO bES'ffeOY VE,~Anv-e aeus. 

- HIGH HEAT 

MILD HEAT 

1pH3,0 pH4.6 pH7.0 

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Thermal Treatment of Acidified LowAcid Versus Low-Acid Foods 

Acidified Low-Acid foods Low-Acid Foods 47 
 

 

 

 

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Thermal Destruction (Death) 

of Microorganisms 

 Canned cured products (hams and 

luncheon meats) can be processed at 

lower temperatures 

• Presence of nitrite and salt makes C. bot 

and C. sporogenes spores more sensitive 

to heat 

• Salt and nitrite prevents germination and 

outgrowth of spores 

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CS versus Absolute Sterility 

 Commercially sterile foods may contain viable 

microorganisms 

 Spores of thermophilic bacteria survive 

commercial sterility processes 

• Not harmful 

• Do not grow under normal storage conditions 

• May need to be destroyed for hot-vended items 

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Bacterial Spoilage 

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 Indications of 

bacterial spoilage 

• Most bacteria 

produce gas 

• A few produce 

acid and no gas 

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Causes of Bacterial Spoilage 

 Incipient spoilage 

 Post-process contamination 

 Inadequate heat processing 

 Thermophilic bacterial growth 
 

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Incipient Spoilage Before Processing 

 Incipient spoilage is microbial spoilage 

caused by too long of a delay between 

container closing and retorting 
Contamination After Processing 

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 Leaker spoilage 

 Generally due to 

inadequately 

formed seams, 

container damage, 

or cooling water 

contamination 

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Inadequate Heat Processing 

 Heat processes are designed to destroy 

public health organisms 

 Inadequate heat process may lead to 

public health hazard 
 

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Thermophilic Spoilage 

 Spores are very heat resistant 

 Processes for low-acid foods do not 

destroy thermophilic spores 

 Cool product to below 105°F 
 

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Non-Microbial Food Spoilage 

 Chemical action causing hydrogen 

swells or pin-holes 

 Overfilling of container gives the 

appearance of spoilage 

 Zero or low vacuum cans may appear to 

be spoiled 
Questions? 

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Questions?

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