L. monocytogenes Survival on RTE Meat Products 

Survival of Listeria monocytogenes During Storage of Ready-to-Eat Meat 

Products Processed by Drying, Fermentation, and/or Smoking 

Steven C. Ingham*1

, Dennis R. Buege2, Brenda K. Dropp1, and Jill A. Losinski1 

University of Wisconsin-Madison 

1

 Department of Food Science 

2

 Department of Animal Sciences 

Running Head: L. monocytogenes survival on RTE meat products 

Key words: Listeria monocytogenes, survival, meat 

*Corresponding author 

University of Wisconsin-Madison, Department of Food Science 

1605 Linden Drive, Madison, WI 53706-1565 

Phone 608-265-4801 

Fax 608-262-6872 

e-mail scingham@wisc.edu 

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L. monocytogenes Survival on RTE Meat Products 

ABSTRACT 

The survival of Listeria monocytogenes was evaluated on 15 ready-to-eat 

meat products made using drying, fermentation, and/or smoking. The 

products were obtained from 6 processors and included summer sausage, 

smoked cured beef, beef jerky, snack stick, and pork rind/cracklin products. 

The aw of the products ranged from 0.27 (pork rinds/cracklins) to 0.98 

(smoked cured beef slices). Products were inoculated with a 5-strain cocktail 

of L. monocytogenes, re-packaged under either vacuum or air, and then 

stored at either room temperature (21°C) or under refrigeration (5°C) for 4- 11 

weeks. Numbers of L. monocytogenes fell for all products during storage, 

ranging from a decrease of 0.8 log CFU on smoked cured beef slices during 

11 weeks under vacuum at 5°C to a decrease of 3.3 log CFU on a pork rind 

product stored 5 weeks under air at 21°C. All of the products tested could be 

produced under Alternative 2 of the United States Department of Agriculture 

(USDA) regulations mandating control of L. monocytogenes on ready-to-eat 

meat and poultry products. For many of the products, 1 week of postprocessing storage prior to shipment would act as an effective post-lethality 

treatment and would allow processors to operate under Alternative 1 of these 

regulations. 

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L. monocytogenes Survival on RTE Meat Products 

On June 6, 2003, the United States Department of Agriculture (USDA) published 

an interim final rule addressing the control of Listeria monocytogenes on readyto-eat (RTE) meat and poultry products (2). This rule went into effect October 6, 

2003 and has already had a major effect on processors of these products. The 

rule is intended to encourage processors of RTE products to take one or more 

specific steps to ensure the absence of L. monocytogenes from their products. 

These steps range from focused sanitation steps to adding formulation or 

processing steps designed to kill or inhibit L. monocytogenes growth. The 

processor is also required to perform testing for L. monocytogenes or Listeria 

spp. on food contact surfaces in the area of the plant in which RTE products are 

handled after cooking. The amount of testing is related to the types of RTE 

products made, product ingredients, and how the products are processed and 

handled. In particular, the rule requires processors of RTE meat and poultry 

products to adopt one of three designated “Alternatives” to control L. 

monocytogenes on their products. The Alternatives involve varying levels of 

control and microbiological testing. In Alternative 1, the processor uses a postlethality treatment that reduces or eliminates L. monocytogenes AND an 

antimicrobial agent or process that suppresses or limits L.m. growth throughout 

product shelf-life. In Alternative 2, the processor uses either a post-lethality 

treatment that reduces or eliminates L. monocytogenes OR an antimicrobial 

agent or process that suppresses or limits L. monocytogenes growth throughout 

product shelf-life. Under Alternative 3, only sanitation measures are relied upon 

to control L. monocytogenes. 

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L. monocytogenes Survival on RTE Meat Products 

Most small-scale meat processors in Wisconsin have chosen Alternative 3 

for controlling L. monocytogenes. This alternative provides the least assurance of 

safety and requires the most frequent testing of food contact surfaces among the 

three alternatives. For many RTE meat products though, the reduction of water 

activity, accomplished through the addition of salt and cooking or drying, could 

serve as an antimicrobial process by making the finished product unsuitable for 

L. monocytogenes growth. Similarly, the reduction of pH via fermentation or 

addition of an acidulant, or surface-deposition of inhibitory compounds via 

smoking could also be effective antimicrobial processes. Items produced in 

these ways could thus fall under Alternative 2. Compliance guidance from USDA 

(3) has stated that an effective antimicrobial process will allow no more than a 

1.0 log increase in L. monocytogenes on an RTE product throughout its shelf-life. 

This guidance also summarized scientific studies indicating that L. 

monocytogenes will not multiply at a water activity of < 0.92 or a pH of <4.39. 

It is also possible that some RTE products could fall under Alternative 1, 

when the above-mentioned antimicrobial processing techniques are combined 

with short-term storage prior to distribution that effectively serves as a postlethality treatment. The compliance guidance from USDA states that an effective 

post-lethality treatment must reduce numbers of L. monocytogenes by at least 1 

log. 

The regulation requires that post-lethality treatments must be scientifically 

validated and that evidence must be provided to substantiate the effectiveness of 

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L. monocytogenes Survival on RTE Meat Products 

antimicrobial processes. Unfortunately, most small-scale processors are unable 

to provide this evidence. 

The objective of the present study was to evaluate the survival, and 

perhaps death, of inoculated L. monocytogenes during storage (after repackaging) on a variety of RTE meat products made using drying, fermentation 

and/or smoking techniques. Information obtained in the study could thereby 

provide evidence needed by processors to implement Alternatives 1 or 2 in their 

RTE meat product operations. 

MATERIALS AND METHODS 

Packages of RTE meat products were submitted by six processors. Product 

ingredients are summarized in Table 1. Depending on the product, 1 – 3 lots 

of product were tested. A representative sample from each lot of product was 

vacuum packaged and sent to a commercial testing laboratory to be analyzed 

for water activity, pH, and % water-phase salt (forced air oven determination 

of moisture – AOAC method 950.46Bb and potentiometric method for salt – 

AOAC method 980.25; 1). For each product, the lowest % water-phase salt 

value, and the highest water activity and pH values, i.e. conditions least 

restrictive to microbial growth, are reported in Table 2. Because post-lethality 

contamination of RTE products by L. monocytogenes will occur only on the 

product surface, 0.6 cm thick sections of product, with surface dimensions of 

3.7 x 3.7 cm, were cut from each product using a knife [previously sanitized 

using 70% (v/v) ethanol] and placed in a biosafety hood on aluminum foil that 

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L. monocytogenes Survival on RTE Meat Products 

had previously been sanitized with ethanol and ultraviolet light. For beef 

jerky, the 3.7 x 3.7 cm pieces were cut from individual jerky slices. For the 

pork rind/cracklin products, existing individual pieces, each about 2.5 x 2.5 

cm, were individually inoculated.

 The L. monocytogenes strains used in this study were obtained from the 

laboratory of Dr. Eric Johnson at the Food Research Institute, University of 

Wisconsin-Madison, and are listed in Table 3 . Stock cultures were 

maintained at -20°C in Brain Heart Infusion broth (BHIB; Difco, BectonDickinson, Sparks, MD) with 10% (w/v) added glycerol (Fisher Scientific, 

Itasca, IL). Working cultures, maintained at 4°C on Brain Heart Infusion agar 

(BHIA; Difco) were prepared monthly from frozen stock cultures. To obtain a 

working culture, a strain was cultured twice successively at 35°C for 18-24 h 

in BHIB, streaked to a BHIA plate, incubated at 35°C for 18-24 h and 

examined for purity, and then stored at 4°C. Inoculation cultures were 

prepared for each strain by transferring a loopful of growth from the working 

culture plate to 9 ml of BHIB and incubating at 35°C for 20-24 h. To prepare 

the 5-strain inoculum cocktail, the BHIB cultures were combined and 

distributed evenly into two 50-ml sterile plastic centrifuge tubes, and 

centrifuged for 10 minutes at 5,000 x g. The supernatant in both tubes was 

decanted and the pellets were resuspended to the original volume in 

Butterfield’s phosphate diluent (BPD; Nelson Jameson, Marshfield, WI). The 

resulting cocktail was serially diluted in BPD and plated to determine cell 

concentration. To inoculate the meat or pork rind/cracklin pieces, a 0.025 ml 

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L. monocytogenes Survival on RTE Meat Products 

volume of the undiluted cocktail was pipetted onto the product surface and 

distributed as evenly as possible using a sterile plastic spreader. The product 

pieces were then allowed to dry for at least 15 minutes and then vacuum 

packaged (0.8 atm.; Food Saver bags and packaging machine; Tilia, Inc.; San 

Francisco, CA) and stored at either 5 or 21°C. Pork rind/cracklin pieces were 

allowed to dry and then stored aerobically in zip-lock plastic bags at 21°C. 

At the start of the study and after 1and 4, 5, or 11 weeks of storage, 

samples were analyzed for the number of L. monocytogenes cells per 

sample. The sample bag was aseptically opened, BPD (99 ml) was added, 

and the sample was stomached for 2 minutes at medium speed (Stomacher 

400 lab blender; Fisher). Serial dilutions were made in BPD as needed. For 

the initial dilution, 1.0 ml was distributed for spread-plating among three 

plates (0.3, 0.3, and 0.4 ml) of Listeria Selective Agar (LSA; Oxoid, 

Ogdensburg, NY) with Listeria Selective Supplements (Oxford formulation; 

Oxoid). From the original dilution and each subsequent dilution, 0.1 ml was 

spread on one LSA plate per dilution. Plates were incubated at 35°C for 48 h 

and then examined for typical L. monocytogenes colonies (small-medium, 

grey-brown-to-black; raised, flat, or sunken colonies surrounded by a black 

precipitate zone). Replica plating from Nutrient agar (Difco) to LSA was 

performed for some summer sausage samples but there was little evidence of 

cell injury. Furthermore, injured cells would be unlikely to survive the low-pH 

stress of human gastric juice if products were ingested. Therefore, direct 

plating on LSA was used throughout the study. For each product lot 

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L. monocytogenes Survival on RTE Meat Products 

analyzed, one presumptive L. monocytogenes colony was selected at each 

sampling time for confirmation testing. The colony was transferred to Nutrient 

Agar and after incubation was tested for Gram stain reaction, cellular 

morphology, oxidase activity, and biochemical characteristics (API Listeria kit, 

bioMerieux, Hazelwood, MO). Throughout the study, all presumptive isolates 

were confirmed as L. monocytogenes. 

With the exception of the pork rind/cracklin products, where only a small 

amount of sample was available, three pieces were analyzed at each 

sampling time for each lot of a particular product. For the pork rind/cracklin 

products, only one sample was analyzed per sampling time for each lot. The 

log CFU was calculated for each piece. Then the mean log CFU per piece 

was calculated for each lot and the mean of all lots tested was determined for 

the product at that sampling time. A value of 0.9 log CFU/g was assigned 

when no colonies were present for the least dilute plating. 

RESULTS AND DISCUSSION 

The RTE meat products varied widely in water activity, pH and % 

water-phase salt (Table 2). Given that higher water activity and pH and lower 

% water-phase salt would increase the likelihood of L. monocytogenes 

growth, the product judged most likely to allow growth of this pathogen was 

the smoked cured beef slices and the products judged least likely to support 

growth were the pork rind/cracklin products and beef jerky. Numbers of L. 

monocytogenes recovered from stored products were consistent with these 

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L. monocytogenes Survival on RTE Meat Products 

compositional factors. Levels of L. monocytogenes fell rapidly on the pork 

rind/cracklin products during room temperature storage (Table 4), with 

decreases of 1.2 – 2.1 log CFU in the first week of storage, and subsequent 

decreases to levels at or near the detection limit for samples stored 5 weeks. 

Similar results were obtained for the beef jerky (Table 4). Processing RTE 

products to yield water activity of < 0.75, combined with 1 week of 21°C 

storage, appears to effectively allow the processor to operate under 

Alternative 1, with the processing technique as the antimicrobial process and 

the one-week storage as the post-lethality treatment. It is not known whether 

the decrease in L. monocytogenes numbers during storage occurred at a 

uniform rate or was early in the one-week period. The latter situation would 

result in a shorter pre-shipment storage period serving as the post-lethality 

treatment. 

The sausage products (summer, elk, buffalo) had maximum water activity 

of 0.94 – 0.96, considerably higher than for beef jerky. However, 

fermentation of the summer sausage products during processing resulted in a 

lower finished product pH than for the pork rind/cracklin products and jerky. 

(Table 2). However, neither the water activity nor the pH of these products 

was low enough to predict, based on USDA compliance guidance, that L. 

monocytogenes would not grow. The combination of somewhat reduced 

water activity and pH seemed to effectively inhibit growth and cause death of 

L. monocytogenes. L. monocytogenes decreased in numbers by >1.0 log at 

room temperature (Table 4) or during refrigeration (Table 5). These results 

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L. monocytogenes Survival on RTE Meat Products 

strongly suggest that sausage products, with a mandatory one-week predistribution storage period, could also be produced under Alternative 1. To do 

so, processors would be required to either ensure that their summer sausage 

had water activity and pH at least as low as those used in the present study, 

or have a challenge study conducted to validate the post-lethality treatment 

for their products. 

Somewhat less death of L. monocytogenes occurred during storage of 

snack stick products (Table 5). It is clear that the snack stick products could 

be produced under Alternative 2; case-by-case studies would be necessary to 

determine if Alternative 1 could be chosen for these products. Roomtemperature storage of snack stick products was not evaluated, although 

many products of this type are stored at ambient temperatures. 

The intact and sliced smoked cured beef had the highest water activity 

and lowest % water-phase salt of the products studied (Table 2). As 

expected, L. monocytogenes survival was best on the sliced smoked cured 

beef product (Table 5) with decreases of only 0.8 log CFU during 11 weeks of 

refrigerated storage. Surprisingly, L. monocytogenes numbers decreased 3.0 

log CFU on the surface of intact smoked beef during this same storage 

period. This difference in survival could reflect the somewhat lower water 

activity of the intact product, localized areas on the product surface possibly 

having even lower water activity, and perhaps the presence of anti-microbial 

smoke-derived compounds on the surface of the smoked beef product. The 

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L. monocytogenes Survival on RTE Meat Products 

smoked cured beef results suggest that processing of these products should 

be done under Alternative 2 of the regulations. 

 In conclusion, the smoking, cooking, and drying processes used to make 

the tested products can be considered effective antimicrobial processes 

under the USDA regulations mandating control of L. monocytogenes on 

ready-to-eat meat and poultry products. To verify that their own processing 

techniques allow operation under Alternative 2, processors should either 

verify that their products have water activity, pH, % water-phase salt levels at 

least as restrictive as those in the present study, or have a challenge study 

conducted to validate the lack of L. monocytogenes growth on their products. 

For many of the products studied, a one- week period of post-packaging room 

temperature storage prior to shipment could act as an effective post-lethality 

treatment and would allow processors to operate under Alternative 1 of the 

USDA L. monocytogenes regulations. Again, processors should verify similar 

product composition or conduct a challenge study to validate the post-lethality 

treatment lethality. 

ACKNOWLEDGMENTS The authors acknowledge the cooperation of 

processors providing product. This work was partially supported by the United 

States Department of Agriculture, Food Safety and Inspection Service, contracts 

USDA-FSIS-C-27-2003 and USDA-FSIS-C-37-2003. 

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L. monocytogenes Survival on RTE Meat Products 

REFERENCES 

1. AOAC International. 1998. pp. 39.1.02, 46.5.02. Official methods of 

analysis of AOAC International, 16th edition, 4th revision, Volume II. p. 

Cunniff (ed.). AOAC International, Gaithersburg, MD. 

2. United States Department of Agriculture, Food Safety & Inspection 

Service. 2003. Control of Listeria monocytogenes in ready-to-eat meat 

and poultry products; final rule. Federal Register: June 6, 2003 (Vol. 68, 

No. 109, pp. 34207-34254). 

3. United States Department of Agriculture, Food Safety & Inspection 

Service. Compliance guidelines to control Listeria monocytogenes in 

post-lethality exposed ready-to-eat meat and poultry products. 

http://www.fsis.usda.gov/OPPDE/rdad?FRPubs/97-

013F/CompGuidelines.doc. Accessed 19 April 2004. 

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L. monocytogenes Survival on RTE Meat Products 

Table 1. Ingredients of ready-to-eat meat products evaluated for survival of 

inoculated Listeria monocytogenes during storage (after re-packaging). 

Product Processor Ingredient Summary 

Description 

Smoked cured A Beef, cured with water, salt, sugar, sodium 

beef erythorbate, sodium nitrite 

piece/slices 

Summer A Beef and pork, salt, dextrose, natural spices, lactic 

sausage acid starter culture, sodium erythorbate, sodium 

nitrite 

Summer B Beef, beef hearts1

, salt, corn syrup solids, dextrose, 

sausage spices, mustard seed, sodium erythorbate, lactic 

acid starter culture, sodium nitrite 

Summer C Beef, pork, water, salt, corn syrup solids, dextrose, 

sausage spices, ground mustard, lactic acid starter culture, 

sodium erythorbate, sodium nitrite 

Buffalo C Buffalo, pork, water, salt, sugar, pepper, mustard 

summer seed, garlic, monosodium glutamate, sodium 

sausage erythorbate, sodium nitrite 

Elk summer C Elk, pork, water, salt, sugar, black pepper, mustard 

sausage seed, garlic, monosodium glutamate, lactic acid 

starter culture, sodium erythorbate, sodium nitrite 

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L. monocytogenes Survival on RTE Meat Products 

Venison F Venison, pork, water, salt, sugar, corn syrup solids, 

snack sticks spices, dextrose, dehydrated garlic, red pepper, 

lactic acid starter culture, sodium nitrite 

Beef snack A Beef, salt, dextrose, natural spices, natural 

sticks flavoring, lactic acid starter culture, sodium 

erythorbate, sodium nitrite 

Pork rinds D Pork rinds, seasoning2 

Pork cracklins D Pork cracklins (fried out pork fat with attached skin), 

salt 

Beef jerky E Beef, water, salt, flavoring, sodium nitrate and 

sodium erythorbate 

1 One other lot also contained pork; another tested lot also contained pork and 

garlic. 

2 Seasonings differed for regular, smoke flavor, and BBQ flavor. 

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L. monocytogenes Survival on RTE Meat Products 

Table 2. Chemical and physical characteristics of ready-to-eat meat products 

evaluated for survival of inoculated Listeria monocytogenes during storage (after 

re-packaging). 

Product Processor aw pH water-phase 

Description salt (%)a 

Smoked cured beef piece A 0.96 5.5 2.9 

Smoked cured beef slices A 0.98 5.6 2.5 

Summer sausage A 0.96 4.7 3.9 

Summer sausage B 0.95 4.9 5.2 

Summer sausage C 0.96 4.8 5.0 

Buffalo sausage C 0.95 5.2 6.5 

Elk sausage C 0.96 5.3 4.5 

Venison snack sticks F 0.91 4.8 7.6 

Beef snack sticks, small A 0.95 5.0 5.6 

Beef snack sticks, large A 0.93 5.0 5.9 

Pork rinds D 0.29 6.0 56.9 

Pork rinds, smoke flavor D 0.27 6.1 60.7 

Pork rinds, BBQ flavor D 0.27 6.1 69.3 

Pork cracklins D 0.28 6.7 69.2 

Beef jerky E 0.75 5.6 14.4 

a

 Also referred to as “brine content”. 

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L. monocytogenes Survival on RTE Meat Products 

Table 3. Strains of Listeria monocytogenes used to inoculate ready-to-eat 

meat products prior to re-packaging and storage. 

Genus and species Strain Designation Original Source 

Listeria monocytogenes 

Listeria monocytogenes 

Listeria monocytogenes 

Listeria monocytogenes 

Listeria monocytogenes 

Scott A 

LM 101 

LM 108 

LM 310 

V7

Human outbreak isolate 

Hard salami 

Hard salami 

Goat cheese 

 Raw milk 

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L. monocytogenes Survival on RTE Meat Products 

Table 4. Survival of inoculated L. monocytogenes on ready-to-eat meat 

products stored after re-packaging under air or vacuum at 21°C. 

Product Log CFU/sample (mean) 

Description with n and range in parentheses 

(processor) Start 1 week 5 weeks 

Summer sausage (B) 4.2 (1,0) 1.0 (1,0) 0.9 (1,0)ab 

Summer sausage (C) 3.4 (1,0) 1.5 (1,0) 0.9 (1, 0) 

Elk sausage (C) 4.0 (1,0) 2.4 (1,0) 0.9 (1,0) 

Buffalo sausage (C) 3.1 (1,0) 0.9 (1,0) 0.9 (1,0) 

Pork rinds (D) 4.3 (2, 0.1) 2.2 (2, 0.6) 1.1 (2,0.4) 

Pork rinds, smoke flavor (D) 4.4 (1, 0) 2.7 (1, 0) 1.7 (1,0) 

Pork rinds, BBQ flavor (D) 4.2 (1,0) 3.0 (1,0) 0.9 (1,0) 

Pork cracklins (D) 4.2 (2,0.4) 2.6 (2,1.6) 1.1 (2, 0.4) 

Beef jerky (E) 3.6 (3, 1.2) 1.2 (3, 0.2) 0.9 (3,0) 

a 0.9 is value assigned when no cells were detected. 

b

Value obtained after 4 weeks of 21°C storage. 

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L. monocytogenes Survival on RTE Meat Products 

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L. monocytogenes Survival on RTE Meat Products 

Table 5. Survival of inoculated L. monocytogenes on ready-to-eat meat products stored after re-pa

vacuum at 5°C. Product Log CFU/sample (mean) Description with number of lots and range in parentheses 

(processor) Start 1 week 4 weeks 5 weeks 11 weeks Smoked beef piece (A) 4.5 (2, 0.2) 3.7 (2, 0.9) NTa NT 1.5 (2,1.1) 

Smoked beef slices (A) 4.3 (2, 0.1) 4.2 (2, 0.2) NT NT 3.5 (2,0.5) Summer sausage (A) 3.6 (3, 0.6) 2.3 (3, 0.4) 0.9 (3, 0) NT NT 

Summer sausage (B) 3.9 (3, 0.3) 1.5 (3, 1.8) NT NT 0.9 (3, 0) Summer sausage (C) 3.4 (1,0) 1.5 (1,0) NT NT 0.9 (1,0) 

Buffalo sausage (C) 3.7 (1,0) 2.6 (1,0) NT NT 1.5 (1,0) 

Elk sausage (C) 3.8 (1,0) 3.2 (1,0) NT NT 2.4 (1,0) Venison snack sticks (F) 4.0 (1, 0) 3.6 (1, 0) NT 1.1 (1, 0) NT 

Beef snack sticks, small (A) 3.5 (2, 0.2) 2.5 (2, 0.3) NT 2.1 (2, 1.0) NT 

Beef snack sticks, large (A) 3.7 (1, 0) 3.0 (1,0) NT 2.0 (1, 0) NT 
L. monocytogenes Survival on RTE Meat Products 

aNot tested.