Hot-water pasteurization of packaged RTE meats 

Evaluation of Small-Scale Hot-Water Post-Packaging Pasteurization 

Treatments for Destruction of Listeria monocytogenes on Ready-To-Eat 

Beef Snack Sticks and Natural Casing Wieners 

Steven C. Ingham*1

, Michael D. DeVita1, Rishi K. Wadhera1, 

Melody A. Fanslau1, and Dennis R. Buege2 

Key words: post-packaging pasteurization, Listeria monocytogenes, RTE meat 

products 

Running Title: Hot-water pasteurization of packaged RTE meats 

University of Wisconsin-Madison 

1

 Department of Food Science 

2

 Department of Animal Sciences 

*Corresponding author

 phone 608-265-4801 

 fax 608-262-6872 

e-mail scingham@wisc.edu 

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Hot-water pasteurization of packaged RTE meats 

ABSTRACT 

This study was conducted to evaluate small-scale hot-water post-packaging 

pasteurization (PPP) as a post-lethality (post-cooking) treatment against Listeria 

monocytogenes on Ready-to-Eat beef snack sticks and natural casing wieners. 

Using a commercially available plastic packaging film specifically designed for 

PPP applications and 2.8 liters of boiling water (100°C) in a sauce pan on a hot 

plate, an average reduction in L. monocytogenes numbers of > 2 logs was 

obtained using heating times of 1.0 minute for individually packaged beef snack 

sticks (three brands), 4.0 minutes for four-per-package beef snack sticks (two 

brands) and seven-per-package beef snack sticks (three brands). Average 

product surface temperatures, measured as soon as possible after PPP and 

opening the package, were 47-51.5°C, 58-61.5°C, and 58.5 – 61°C for the beef 

snack sticks packaged one, four, and seven pieces per package, respectively. A 

treatment of 7.0 minutes for four-per-package natural casing wieners (three 

brands) achieved L. monocytogenes reductions of > 1.0 log and average product 

surface temperature of 60.5 – 63.5°C. Cooked-out fat and moisture resulting 

from tested treatments ranged from 0.2 to 1.1% by weight for beef snack sticks 

and from 0.4 to 1.2% by weight for natural casing wieners. For natural casing 

wieners, PPP had no detrimental effect on overall product desirability to 

consumers; results suggested that PPP may significantly enhance appearance of 

this product. However, for beef snack sticks the cooked-out fat and moisture 

following PPP had a significant negative effect on consumer opinions of product 

appearance. 

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Hot-water pasteurization of packaged RTE meats 

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 (9). This rule went into effect on October 

6, 2003 and was intended to encourage processors of RTE products to take one 

or more specific steps to ensure the absence of L. monocytogenes on their 

products. Possible steps range from using focused sanitation procedures, to 

adding ingredients or using processing treatments designed to kill L. 

monocytogenes or inhibit its growth. Under the regulations, 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 post-lethality treatment that 

reduces or eliminates L. monocytogenes and an antimicrobial agent or process 

that suppresses or limits L. monocytogenes 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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Hot-water pasteurization of packaged RTE meats 

The objective of this work was to evaluate small-scale hot-water postpackaging pasteurization, for use as a post-lethality treatment by small and very 

small processors under Alternatives 1 or 2 of the USDA regulations. This postlethality treatment was studied for use with beef snack sticks and natural casing 

wieners, two products commonly made by small and very small processors. In 

the processing of beef snack sticks, the addition of salt, along with cooking and 

smoking have already been shown to be effective antimicrobial processes by 

making the finished product unsuitable for L. monocytogenes growth (4). Some 

processors also acidify beef snack sticks, either through fermentation or through 

the addition of an acidulant. Either means of acidification would provide further 

inhibition of L. monocytogenes growth. For beef snack sticks then, the adoption 

of hot-water post-packaging pasteurization as an effective post-lethality treatment 

could allow processors to operate under Alternative 1. The second product 

studied, natural casing wieners (finely ground cured sausage contained inside an 

intestine casing, often used to make a “hot dog” sandwich), has traditionally been 

made without the addition of any ingredients that effectively inhibit L. 

monocytogenes growth. Thus, conducting hot-water post-packaging 

pasteurization of this product would allow processors to operate under 

Alternative 2. However, many processors have begun adding an anti-microbial 

agent such as sodium lactate to their natural casing wiener recipes. For these 

processors, the use of hot-water post-packaging pasteurization would allow 

operation under Alternative 1. 

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Hot-water pasteurization of packaged RTE meats 

There are a large number of variables involved in hot-water postpackaging pasteurization. These include packaging film composition and 

thickness, mass of product, volume of water, proportions of water and treated 

product, water temperature, and treatment time. In order to simplify potential 

techniques that small and very small processors might adopt, we focused our 

efforts by using a single type of packaging film, typical masses and packaging 

configurations of products, a single amount of hot water, and a single initial water 

temperature. We varied the treatment time, keeping in mind that processors 

would prefer shorter treatments for reasons of economics and efficiency. 

We also paid considerable attention to whether effective hot-water postpackaging pasteurization treatments affected the sensory properties of beef 

snack sticks and natural casing wieners. Because these products would be 

subjected to a thermal process in a confining package, cook-out (almost entirely 

fat) was determined to be a potential drawback of this procedure. Therefore, we 

measured the amounts of cook-out and also conducted consumer sensory 

panels of treated and untreated products. 

MATERIALS AND METHODS 

 Meat products. Three brands each of beef snack sticks and natural 

casing wieners were purchased at local grocery stores and transported within 30 

minutes to the laboratory. Samples were refrigerated (5 + 1°C) until use. A 

sample from a randomly chosen lot of each product was analyzed in the 

laboratory for water activity using a Decagon water activity meter (AquaLab 

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Hot-water pasteurization of packaged RTE meats 

Series 3 TE, Pullman, WA) and pH (sample homogenized in distilled water) using 

an Accumet AB15 pH meter (Fisher Scientific, Itasca, IL) and probe. Samples 

were also sent to a commercial laboratory to be analyzed for % moisture, % salt, 

and % fat using forced air oven determination of moisture – AOAC method 

950.46Bb, potentiometric method for salt – AOAC method 980.25, and Soxhelet 

method for fat – AOAC method 960.39 (2). The Moisture: Protein ratio (MPR) 

and % water-phase salt (WPS) were calculated from these analytical results. 

Representative products were measured to determine average length and 

diameter. Physical and chemical characteristics of the products are shown in 

Table 1. 

Bacterial cultures and preparation of inoculum. The L. 

monocytogenes strains used in this study were obtained from the laboratory of 

Dr. Eric Johnson at the Food Research Institute, University of WisconsinMadison. Strain Scott A was a clinical isolate, strains LM 101 and LM 108 were 

isolated from hard salami, strain LM 310 was isolated from goat cheese, and 

strain V7 was isolated from raw milk. Stock cultures were maintained at -20°C in 

Brain Heart Infusion broth (BHIB; Difco, Becton-Dickinson, Sparks, MD) with 10% 

(w/v) glycerol (Fisher ) added. 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 

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Hot-water pasteurization of packaged RTE meats 

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 into a 50-ml sterile 

plastic centrifuge tube, and centrifuged for 10 minutes at 5,000 x g. The 

supernatant in the tube was decanted, and the pellet was resuspended to the 

original volume in Butterfield’s phosphate diluent (BPD; Nelson Jameson, 

Marshfield, WI) to make an inoculum cocktail. The inoculum cocktail was serially 

diluted in BPD and plated to determine cell concentration (ca. 8 log CFU/ml). 

Inoculation of meat products. Two different methods were used to 

inoculate products. Natural casing wieners, and beef snack sticks to be 

packaged four-per-package or seven-per-package, were inoculated by placing 

110 ml of a 1:10 dilution (in BPD) of inoculum cocktail in a large glass Petri dish 

and rolling an individual product piece through the inoculum for two complete 

revolutions. This procedure was used to ensure that in situations where product 

piece-to-piece contact would occur, all product surfaces were inoculated, thus 

maximizing the opportunity for protection of cells via piece-to-piece contact. 

Individually packaged beef sticks do not experience piece-to-piece contact within 

the package, so each beef snack stick to be packaged individually was 

inoculated by placing 25 μl of the 1:10 dilution (BPD) of inoculum cocktail on 

each of 5 sites along the beef snack stick. The inoculum was then spread across 

the beef snack stick surface using a sterile plastic spreader. After inoculation, 

products were allowed to dry at room temperature (20-22°C) for 90 minutes in a 

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Hot-water pasteurization of packaged RTE meats 

biosafety level 2 hood before packaging. The mean starting inoculum levels in 

initial trials were 4.2 log CFU per sample (see below for description of sampling 

procedure) for beef snack sticks (range of 3.1 – 5.3, standard deviation = 0.7) 

and 5.9 log CFU per sample for natural casing wieners (range of 5.7 – 6.3, 

standard deviation = 0.2). In confirmatory trials, mean starting inoculum levels 

were 3.9 log CFU per sample for beef sticks (range of 2.7 – 5.5, standard 

deviation = 0.7) and 5.7 log CFU per sample for natural casing wieners (range of 

4.7 – 6.1, standard deviation = 0.4). 

Packaging of meat products. A commercial plastic film designed for use 

in hot-water/steam post-packaging pasteurization was obtained from Curwood, 

Inc. (Product SPP94, Material No. CPS302976; New London, WI). Bags were 

custom-sized for products using scissors and a heat-sealer on a commercial-type 

vacuum packaging machine. Dry, inoculated products were placed in 

appropriate bags and vacuum-packaged (approx. 1 atm). Trials were done with 

one, four, and seven beef snack sticks per package and with four natural casing 

wieners per package.

 Hot-water post-packaging pasteurization treatments. Prior to 

pasteurization, 2.8 liters of tap water were placed in an aluminum sauce pan. 

Pan dimensions were 21 cm diameter and 12 cm height. A plastic test tube rack 

was submerged in the pan using a lead “donut” weight. This rack was used to 

hold the packaged product off of the pan bottom during heating and thus allow 

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Hot-water pasteurization of packaged RTE meats 

maximum water contact with the product. Either of two hot plates was used to 

heat the water to boiling, whereupon pasteurization trials were conducted. The 

hot plates had maximum power of 1113 and 1118 watts. A single package of 

inoculated product was placed between levels of the test tube rack in the boiling 

water. Although the water stopped boiling when packaged product was first 

added, it resumed boiling within a short time (< 30 seconds for the largest 

packages). After the prescribed time had elapsed, the package was immediately 

removed from the water and placed in ice-water slush. The next sample was not 

heated until after the water had again boiled. Additional water was added to the 

pan as needed to maintain water volume. An initial trial consisted of three 

packages of product, all inoculated with an inoculum cocktail from the same broth 

cultures, for each of several treatment times. A confirmatory trial control 

consisted of one package of product per treatment. Untreated control packages 

were handled identically to treated packages other than during submersion in the 

boiling water. Initial trials for one-per-package, four-per-package, and seven-perpackage beef snack sticks were done for 0.5, 1.0, and 1.5; 3.0, 4.0, and 5.0; and 

3.0, 4.0, 5.0, and 6.0 minutes, respectively, while initial trials with four-perpackage natural casing wieners were for 5.0, 6.0, and 7.0 minutes. Confirmatory 

trials were done with treatments of 1.0 and 4.0 minutes for one-per-package and 

four- or seven-per-package beef snack sticks, respectively, and 7.0 minutes for 

four-per-package natural casing wieners. 

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Hot-water pasteurization of packaged RTE meats 

Enumeration of inoculum bacteria. Each chilled package of product 

was removed from the ice-water slush, after which the package surface was 

patted dry with a paper towel and then sprayed with 70% ethanol. Following a 

20-minute drying period, each package was aseptically opened. The ends of 

each beef snack stick or natural casing wiener in the package were removed, 

leaving 2.5 cm pieces. These pieces comprised a sample and were transferred 

to a stomacher filter bag (typical sample weights are shown in Table 1). The 

package, minus remaining product, was also transferred to the stomacher filter 

bag. A standard 198 ml volume of BPD was added to the stomacher bag and the 

contents were then manually massaged for 1 minute and manually shaken for 1 

minute. This procedure ensured thorough contact of BPD with product and 

package interior surfaces. From the initial sample dilution(arbitrarily denoted as 

10-2), further dilutions were made, as appropriate, using BPD. For initial trials, no 

effort was made to allow repair and subsequent enumeration of injured cells. 

From 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, according to the 

manufacturer’s instructions, and then examined for typical L. monocytogenes 

colonies (small-medium, brown-to-black colonies surrounded by a black 

precipitate zone), which were counted. For each product tested in each trial, one 

presumptive L. monocytogenes colony was selected for confirmation testing. 

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Hot-water pasteurization of packaged RTE meats 

The colony was transferred to BHIA and cultured, and then 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. For confirmatory 

trials, spread-plating was done on BHIA, and a one-hour 35°C incubation was 

employed in an attempt (unpublished method) to allow repair by injured cells. 

Following this incubation period, each BHIA plate was then overlaid with about 

10 ml of tempered (46°C) LSA + supplements and then incubated 48 h at 35°C. 

For each trial, the difference in log CFU between treated and untreated product 

was considered the treatment lethality. When no colonies resulted from the least 

dilute plates (arbitrary 10-2 dilution), a value of 9 colonies on a hypothetical 10-1 

dilution plate (effectively one colony less than the detection limit) was assigned. 

The log of 90 CFU, rounded to 1.9, was then used in calculating treatment 

lethality. 

Statistical analysis of microbiological data. The two-sample t-test 

(Minitab, Release 14; Minitab, Inc., State College, PA) with a 5% significance 

level was used to make the following comparisons for log CFU reduction in initial 

trials: 1) processor vs. processor for a given combination of product, number of 

pieces per package, and processing time; 2) heating time vs. heating time for a 

given combination of product, processor, and number of pieces per package; and 

3) heating time vs. heating time for a given combination of product and number of 

pieces per package (data for three processors combined). For confirmatory 

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Hot-water pasteurization of packaged RTE meats 

trials, the same analysis was done to compare processor vs. processor for a 

given combination of product, number of pieces per package, and heating time. 

Determination of product surface temperature. Using conditions 

otherwise identical to those in the initial and confirmatory trials, additional 

experiments were done with uninoculated products to determine product surface 

temperature at various times during hot-water post-packaging pasteurization. At 

the designated time, the package of product was rapidly removed from the hot 

water, opened with a knife, and three product suface temperatures were rapidly 

measured at mid-piece (same location from which samples were obtained for 

microbiological analysis) using an infrared non-contact thermometer (Mini Temp 

FS, Raytek; Santa Cruz, CA). Particularly for multi-piece packages, the product 

size and shape, along with piece-to-piece contact, resulted in a range of surface 

temperatures. Therefore, the temperature was determined at three different midpiece locations and the three temperatures were averaged for that package as 

follows. For individually packaged beef snack sticks, the termperature was 

measured at three places evenly around the beef snack stick circumfrence at 

mid-piece. For four-pieces-per-package beef snack sticks and natural casing 

wieners, the temperature was measured on the upper surface mid-piece for 

pieces 1 and 2 (counting from left to right), and piece 2 was rotated to obtain the 

surface temperature mid-piece where piece 2 had contacted piece 3. For sevenpieces-per-package beef snack sticks, surface temperatures were obtained on 

the upper surface mid-piece for pieces 1 and 3, and piece 3 was rotated to obtain 

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Hot-water pasteurization of packaged RTE meats 

the surface temperature mid-piece where piece 3 had contacted piece 4. For 

each combination of product, processor, number of pieces per package, and 

heating time, duplicate three-temperature averages were obtained and a mean of 

the duplicates was calculated. 

Prediction of process lethality using surface temperature data. The 

lethality of the hot-water post-packaging pasteurization process was estimated by 

inputting average surface temperature and time data into the American Meat 

Institute Foundation process lethality spread sheet (1). Along with the time and 

temperature data, a reference temperature of 62.8°C (145°F) and a Z-value of 

7.9°C (14.2°F), determined by Muriana et al (5) for L. monocytogenes in purge 

from smoked ham, were entered into the spreadsheet. These values were 

chosen because smoked ham, of the three products studied by Muriana et al (5), 

was closest in composition to the beef snack sticks and natural casing wieners. 

The Fo values determined using the spreadsheet were divided by the previously 

determined D62.8-value of 67.6 seconds (5) for L. monocytogenes in purge from 

smoked ham to obtain a predicted log reduction. 

Sensory quality of treated products. The percent of product weight lost 

as cooked-out fat and water was determined for single trials of uninoculated beef 

snack sticks heated for 3.0 and 4.0 minutes (four beef snack sticks per package) 

and for 4 and 5 minutes (seven beef snack sticks per package), and for single 

trials of uninoculated natural casing wieners (four per package) heated 5.0, 6.0, 

and 7.0 minutes. Unscreened, untrained panelists (n ranging from 178 to192) 

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Hot-water pasteurization of packaged RTE meats 

also evaluated treated (4 minutes for beef snack sticks packaged seven-perpackage and 7 minutes for natural casing wieners packaged four-per-package) 

and untreated beef snack sticks or natural casing wieners. The beef snack sticks 

were presented to panelists with cooked-out fat and moisture removed. The 

treated and control natural casing wieners were separately warmed after being 

removed from the packaging by being placed under boiling water for 4.0 minutes 

and then held warm in an insulated container prior to serving. Panelists were 

self-selected from consumers entering a campus facility serving lunch and ice 

cream. Samples were presented in random order in cups coded with a threedigit random number. The panelist ballot contained two structured seven-point 

hedonic scales with possible scores ranging from 1 = dislike very much to 7 = like 

very much. Mean scores for treated and untreated products were calculated and 

an analysis of variance appropriate for a randomized complete block design (8) 

was conducted. Follow-up sensory evaluation panels were done with beef snack 

sticks and natural casing wieners from processor B (rated highest by panelists in 

the initial sensory panels). For the follow-up sensory evaluation panels, the 

packages of treated and untreated products were opened, and the packaging film 

lifted from the product surface so that cooked-out fat and moisture were visible. 

The film surface was then loosely placed back over the product surface and the 

bag re-sealed by stapling. Panelists (n = 198 ) at the same location were asked 

to evaluate the product appearance of control and treated products on a ballot 

with two structured seven-point hedonic scales with possible scores ranging from 

1 = extremely unappealing to 7 = extremely appealing. Mean scores for treated 

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Hot-water pasteurization of packaged RTE meats 

and untreated products were calculated and an analysis of variance appropriate 

for a randomized complete block design (8) was conducted. 

RESULTS AND DISCUSSION 

In initial trials with beef snack sticks packaged one or four pieces per package, 

the average lethality increased with heating time (Table 2), with some statistically 

significant differences between heating treatments. Specifically, increasing the 

heating time from 0.5 to 1.5 minutes for individually packaged beef snack sticks 

and from 3.0 to 4.0 or 5.0 minutes for four-per-package beef snack sticks 

resulted in significantly greater lethality. Lethality differences between treatment 

times for beef snack sticks produced by a single processor were seldom 

significant for individually packaged and four-per-package product. Statistically 

significant differences in lethality for beef snack sticks from different processors 

were also rare. For seven-per-package beef snack sticks, a statistically 

significant increase in lethality was achieved when the heating time was 

increased from 3.0 to 4.0 minutes (Table 2). Further increases in heating time 

did not result in a corresponding increase in lethality because of the plating 

detection limit being reached for PPP treatments of > 4.0 minutes. For sevenper-package beef snack sticks from a single processor, there were no significant 

differences in lethality between heating times, probably because of low trial 

numbers and relatively high inter-trial variation. In initial trials with natural casing 

wieners, increasing the heating time from 5.0 to 7.0 minutes resulted in 

significantly greater lethality (Table 2), while significant differences between 

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Hot-water pasteurization of packaged RTE meats 

wieners from different processors occurred with heating times of 6.0 and 7.0 

minutes. In confirmatory trials (Table 3), there were no significant differences in 

lethality between beef snack sticks or natural casing wieners from different 

processors. Throughout the study, there was noticeable trial-to-trial variation. 

Potential causes of this variability include lot-to-lot variation in product 

composition, surface smoothness, and shape; variation in inoculum level; and 

variation in how closely the packaging film contacted the product surface. 

 Compliance guidelines from USDA state that an effective post-lethality 

treatment must reduce numbers of L. monocytogenes by at least 1.0 log (10). 

However, a higher standard of lethality must be met in order to reduce regulatory 

sampling frequency. In the latter case, L. monocytogenes numbers must be 

reduced by at least 2.0 logs. All of the PPP treatments tested in initial trials had 

some lethality against L. monocytogenes on vacuum-packaged beef snack sticks 

and natural casing wieners (Table 2). With the compliance guidelines in mind, 

hot-water PPP treatments of at least 1.0 minute for individually packaged beef 

snack sticks, at least 4.0 minutes for four-per-package and seven-per-package 

beef snack sticks, and at least 7.0 minutes for four-per-package natural casing 

wieners were viewed as having potential commercial utility. Some beef snack 

stick products have already been shown not to support L. monocytogenes growth 

during storage (4). Applying validated hot-water post-packaging pasteurization 

treatments to such products would allow the processor to operate under 

Alternative 1 of the USDA regulations. 

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Hot-water pasteurization of packaged RTE meats 

Treating individual beef snack sticks for 1.0 minute often eliminated all 

inoculum L. monocytogenes cells. Results from these initial trials would support 

validation of the 1.0 minute treatment for commercial use. Results of 

confirmatory trials using this treatment also support commercialization of this 

treatment (Table 3). Treating four-per-package and seven-per-package beef 

snack sticks for at least 4.0 minutes also resulted in reductions in numbers of 

inoculum L. monocytogenes that met or exceeded the 2.0 log standard (Tables 2 

and 3). Heating times needed to achieve a 2.0 log reduction increased with the 

number of pieces per package. The L. monocytogenes cells located at piece-topiece contact surfaces could have been protected from the heating treatment. 

Results of trials to determine average surface temperature (Table 4), support this 

hypothesis. 

Lethality of hot-water PPP against L. monocytogenes on natural casing 

wieners was lower than that seen for beef snack sticks (Tables 2 and 3). This 

difference is likely caused by the wieners having a larger mass than the beef 

snack sticks and thus taking longer to heat (Table 4). As with the beef snack 

sticks, it is likely that piece-to-piece contact protected L. monocytogenes on the 

contact surfaces. A treatment time of 7.0 minutes appeared to result in sufficient 

lethality to meet the guideline for effectiveness, but did not reach the lethality 

level needed to reduce regulatory sampling frequency. 

Comparison of Tables 2 and 3 would suggest that greater reductions in 

numbers of L. monocytogenes occurred in confirmatory trials than in initial trials, 

and that the overlay plating procedure, intended to allow enumeration of injured 

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Hot-water pasteurization of packaged RTE meats 

cells, failed to achieve its objective. However, the overlay method always 

resulted in greater time-zero values that direct plating did in confirmatory trials. 

This finding indicates that some L. monocytogenes cells were initially injured 

upon exposure to the product surface. Numbers of cells recovered after the hotwater post-packaging pasteurization treatments were similar for overlay and 

direct plating methods, suggesting that few injured cells survived heating. The 

combination of greater time-zero cell numbers and comparable post-heating cell 

numbers was responsible for the apparent difference in lethality between the 

initial and confirmatory trials. 

Variability in results for wieners and beef snack sticks was probably due to 

slight differences in product size, product surface topography, heat transfer 

through adjoining pieces, and tightness of vacuum-packaging. Increasing the 

PPP treatment time beyond those times studied here would undoubtedly 

decrease the variability of results and increase lethality, but would probably not 

be economically feasible for processors. 

The process lethality values calculated using surface temperature data are 

shown in Table 5. Calculated process lethality values were generally much lower 

than what was observed in initial (Table 2) and confirmatory (Table 3) trials, with 

the exception of seven-per-package beef snack sticks heated for 6.0 minutes and 

four-per-packaged natural casing wieners heated 7.0 minutes. For these 

product/treatment combination, calculated lethality was relatively closer to 

observed lethality. Possible reasons for the difference between observed and 

calculated lethality include compositional characteristics of beef snack sticks and 

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Hot-water pasteurization of packaged RTE meats 

natural casing wieners enhancing thermal destruction of L. monocytogenes 

relative to that occurring in the purge from smoked ham (the medium in which L. 

monocytogenes D- and Z-values were determined), differences in 

thermotolerance between the strains used in the present study and those used 

by Muriana et al (5), and lower-than-actual surface temperature measurements 

resulting from product cooling during the 5 – 10 seconds elapsing while the 

package was opened and the surface temperature measured. 

Several related studies on post-packaging pasteurization of ready-to-eat 

meat products have already been conducted. Collectively, these studies differ 

from the present one in three important ways: 1) the desired target for reducing 

numbers of L. monocytogenes cells was often greater than that required in the 

USDA compliance guideline, 2) different products were tested, 3) the volume of 

hot water used for treatments, along with the degree of circulation and source of 

heat, ensured that addition of cold packaged products did not cause any 

reduction in water temperature. Thus, direct comparisons between the present 

study and earlier work are difficult. Chen et al (3) studied post-packaging 

pasteurization of peeled (no casing) frankfurters, packaged five per package, 

using 96°C water. The frankfurters also contained pediocin, which may have 

enhanced thermal lethality relative to that observed in our study. Reported 

reductions in L. monocytogenes were approximately 2.5 – 3.5 log CFU in 30-120 

seconds. Murphy et al (7) reported that a 5-minute exposure of 4 kg inoculated 

turkey breast to 96°C water in a cooker with a heat transfer coefficient of 800 

W/m2

k resulted in a 2.0 log reduction in L. monocytogenes numbers. Similarly, 

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Hot-water pasteurization of packaged RTE meats 

Muriana et al (6) found that a 2-minute exposure of 1.8 – 5.0 kg deli turkey 

product in 189 l of 93.3°C water always resulted in a > 2.0 log CFU reduction in 

L. monocytogenes numbers. 

Although process conditions in the present study were fairly specific, 

certain adjustments could be legitimately made to increase processing versatility 

without losing lethality. For example, use of larger volumes of boiling water, 

adding salt to the water to elevate boiling point temperature, and/or increasing 

the heat source power could be done with the same product and packaging 

material. However, changing the product composition, number of product pieces 

per package, or type of packaging film would invalidate application of the present 

study results. 

In addition to the challenges inherent in achieving consistently adequate 

lethality in an economical manner, processors adopting PPP would face the 

challenge of maintaining desirable sensory properties in the treated product. The 

cook-out loss in beef snack sticks treated by PPP ranged from 0.2 to 1.1% by 

weight for beef snack sticks and from 0.4 to 1.2% by weight for natural casing 

wieners (Table 6). Cook-out loss could potentially be reduced by the addition of 

phosphates to the products. Processors seeking to use hot-water postpackaging pasteurization treatments may wish to investigate such formulation 

changes. When PPP-treated beef snack sticks (with cooked-out fat and moisture 

removed) were served alongside untreated beef snack sticks to untrained 

consumer panelists, no negative effect of the PPP treatment was detected (Table 

7). The hot-water PPP-treated beef snack sticks from processor B were actually 

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Hot-water pasteurization of packaged RTE meats 

evaluated as significantly better than the untreated control. Similar results were 

obtained for the natural casing wieners (Table 8). However, when cooked-out fat 

and moisture from beef snack sticks were clearly visible to panelists in follow-up 

sensory evaluation trials, the PPP-treated product was rated significantly less 

desirable in appearance than untreated beef snack sticks (Table 9). In contrast, 

the appearance of PPP-treated natural casing wieners (with no product heating 

after the package was opened) was rated significantly better than for untreated 

wieners. Panelist comments suggested that product color and shape (resulting 

from compression as the packaging film shrank during PPP) were important 

factors resulting in higher appearance scores. None of the comments indicated a 

problem with the amount of cooked-out fat and moisture. 

In summary, we believe that it is possible to meet USDA compliance 

guidelines for an effective post-lethality treatment by treating beef snack sticks 

and natural casing wieners with the relatively simple technique of small-scale 

hot-water PPP. Before PPP can be widely adopted by small and very small 

processors of these products, however, further validation studies are necessary. 

In addition, processors must determine whether hot-water PPP treatments would 

result in product characteristics that are acceptable to their customers. 

ACKNOWLEDGEMENT This project was funded in part by the United States 

Department of Agriculture, Food Safety & Inspection Service, contract USDAFSIS-C-26-2003. Author DeVita was supported in part by the University of 

Wisconsin-Madison’s Medical Scholars Summer Research Program. The 

21 
Hot-water pasteurization of packaged RTE meats 

authors gratefully acknowledge Sungjoon Jang for assistance with sensory 

evaluation panels. 

References 

1. American Meat Institute Foundation. 2004. Process lethality 

spreadsheet. http://amif.org/FactsandFIgures/AMIF-ProcessProcessLethality.htm. Accessed 19 April 2005. 

2. AOAC International. 1998. Official methods of analysis of AOAC 

International. 16th edition, 4th revision, Volume II. P. Cunniff (ed.). AOAC 

International, Gaithersburg, MD. 

3. Chen, C.-M., J.G. Sebranek, J.S. Dickson, and A.F. Mendonca. 2004. 

Combining pediocin (ALTA 2341) with postpackaging thermal 

pasteurization for control of Listeria monocytogenes on frankfurters. J. 

Food Prot. 67: 1855-1865. 

4. Ingham, S.C., D.R. Buege, B.K. Dropp, and J.A. Losinski. 2004. Survival 

of Listeria monocytogenes during storage of ready-to-eat meat products 

processed by drying, fermentation, and/or smoking. J. Food Prot. 67: 

2698-2702. 

5. Muriana, P.M., W. Quimby, C.A. Davidson, and J. Grooms. 2002. 

Postpackage pasteurization of ready-to-eat deli meats by submersion 

heating for reduction of Listeria monocytogenes. J. Food Prot. 65: 963­

969. 

22 
Hot-water pasteurization of packaged RTE meats 

6. Muriana, P., N. Gande, W. Robertson, B. Jordan, and S. Mitra. 2004. 

Effect of prepackage and postpackage pasteurization on postprocess 

elimination of Listeria monocytogenes on deli turkey products. J. Food 

Prot. 67: 2472-2479. 

7. Murphy, R.Y., L.K. Duncan, K.H. Driscoll, J.A. Marcy, and B.L. Beard. 

2003. Thermal inactivation of Listeria monocytogenes on ready-to-eat 

turkey breast meat products during postcook in-package pasteurization 

with hot water. J. Food Prot. 66: 1618-1622. 

8. Steel, R.G.D. and J.H. Torrie. 1960. Principles and procedures of 

statistics. New York: McGraw-Hill. 

9. United States Department of Agriculture, Food Safety and Inspection 

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

and poultry products; final rule. Fed. Regist. 68: 34207-34254. 

10.United States Department of Agriculture, Food Safety and Inspection 

Service. 2004. Compliance guidelines to control Listeria monocytogenes 

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

Available at http://www.fsis.usda.gov/OPDE/rdad/FRPubs/97-

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

23 
Hot-water pasteurization of packaged RTE meats 

Table 1. Physical and chemical characteristics of commercial beef snack sticks and natural casing wiewater post-packaging pasteurization. Product Beef Snack Sticks Natural Casing WienProcessor A B C A B Average Pre-heat Length (cm) 20 12.6 15.1 12 13.2 Average pre-heat Diameter (cm) 1.3 1.6 1.4 2.1 2.4 pH 4.5 5.0 5.0 5.9 6.1 Water activity (aw) 0.91 0.86 0.89 0.95 0.97 Moisture (%) 36.0 29.1 34.6 53.6 52.3 Protein (%) 20.6 17.2 20.0 12.3 12.8 Moisture:Protein 1.7 1.7 1.7 4.3 4.1 Water-phase 9.6 8.8 10.1 3.6 3.8 Salt (%) Fat (%) 35.6 46.4 36.6 27.1 31.6 

Typical sample wta(g) 
Hot-water pasteurization of packaged RTE meats 

 1 per package 4.5 6.0 4.5 ---- --- 

4 per package 18.0 23.0 18.0 44.0 43.0 

 7 per package 31.0 40.0 31.0 --- ---- 

aSample consisted of one 2.5 cm longitudinal segment from the middle of each piece of product, obtain

cuts in the plane of the product diameter. 
Hot-water pasteurization of packaged RTE meats 

Table 2. Initial trials (no attempt to allow repair and enumeration of injured cells) to determine lethality water post-packaging pasteurization treatments against Listeria monocytogenes on commercial beef s

natural casing wieners. A single package of product was submerged in 2.8 liters of boiling water for ththen chilled immediately thereafter in ice-water slush. Product Treatment Decrease (log CFU/sample) in Listeria monocyt

 Time (min.) Processor Trial Values Mean Group Beef Snack Sticks 1 per pkg. 0.5 A 0.7 2.3 b 0.3 1.1 

1.8C,1 B 2.4 1.7 1.3 1.9 10

 C 2.5 3.5 b 2.9 3.0 

1.0 A 1.0 2.3 b 2.7b 2.0 

B 3.7 b 2.5 2.9 b 3.0 2.8 C 3.6 b 3.2 3.6 3.5 

 1.5 A 4.4 b 2.3 b 2.7 b 3.1 

B 3.7 b 3.5 b 2.9 b 3.4 1 3.4 10

 C 3.6 b 3.5 b 3.9 b 3.7 

Beef snack sticks 4 per pkg. 3 B 1.4 1.5 0.2 1.0 
Hot-water pasteurization of packaged RTE meats 

C 1.4 1.4 0.9 1.2 2 1.1 11,12 

4 A 3.9 3.9 b 4.0 b 3.9 3

 B 2.7 3.9 b 1.4 b 2.7 2.9 11,13

 C 2.1 1.9 2.5 2.2 2,3

 5 A 4.0 b 3.9 b 4.0 b 4.0 4.0 12,13 

Beef snack sticks, 7 per pkg. 3 B 2.5 2.7 1.5 2.2 1.8 14

 C 2.1 1.8 0.4 1.4 

4 B 4.0 b 2.8 b 4.3 b 3.7 3.1 14

 C 2.2 b 2.8 b 2.8 b 2.6 

5 A 2.3 b 2.1 b 4.1 b 2.8 2.7 

C 2.2 b 2.8 b 2.8 b 2.6 

6 A 2.3 b 2.1 b 4.1 b 2.8 2.8 

Natural Casing Wieners 4 per pkg. 

5 A 1.3 0.9 1.1 

B 1.0 1.0 1.1 1.0 1.1 15

 D 1.3 1.3 
Hot-water pasteurization of packaged RTE meats 

1.6 4, 5 6 A 2.0 1.2 1.5 

1.2 4, 6

B 

1.1 1.1 1.4 1.3 

1.3 5, 6 D 1.1 1.4 

2.4 7, 8

7 

A 4.3 1.1 1.9 

2.4 7, 9 2.2 15

B 

1.4 3.4 

1.7 8, 9

D 

1.6 1.8 

a Each value is the result from an individual trial. Three treated samples were analyzed for each trial. b Indicates that no surviving cells were detected. For such a trial, a value of [Log (zero time) - 1.9 was 

decrease in cells (see Materials and Methods for details). 

cPairs of means in a column with the same superscript number are significantly (P < 0.05) different. 
Hot-water pasteurization of packaged RTE meats 

Table 3. Confirmatory trials (procedure involved attempt to allow repair and enumeration of injured celethality of small-scale hot-water post-packaging pasteurization treatments against Listeria monocytogbeef snack sticks and natural casing wieners. A single package of product was submerged in 2.8 liters the time indicated, then chilled immediately thereafter in ice-water slush. Product Treatment Decrease (log CFU/sample) in Listeria monocyt

 Time (min.) Processor Trial Values Mean Std. DeBeef Snack Sticks 1 per pkg.  1.0 A 3.6b 3.5 b 3.4b 3.5 0.1 

B 3.6 b 3.0b 3.1 b 3.2 0.3 C 2.7 b 4.4b 4.5b 3.9 1.0 

Beef snack sticks 4 per pkg. 

4 A 3.6 b 3.8 b 3.7 ---

C 3.4b 3.1b 3.0 3.2 0.2 

Beef snack sticks, 7 per pkg. 

4 A 2.9b 3.2b 3.1 ---
Hot-water pasteurization of packaged RTE meats 

B 3.3 1.9 3.1 b 2.8 0.7 

C 2.0 2.2 b 2.1 2.1 0.1 

Natural Casing Wieners 4 per pkg. 

7 A 2.0 1.6 1.8 1.8 0.2 

B 3.4 2.9 2.1 2.8 0.7 

D 1.8 2.0 1.9 ---

a Each value is the result from an individual trial. One treated sample was analyzed for each trial. b Indicates that no surviving cells were detected. For such a trial, a value of [Log (zero time) - 1.9 was 

decrease in cells (see Materials and Methods for details). 
Hot-water pasteurization of packaged RTE meats 

Table 4. Surface temperature of beef snack sticks and natural casing wieners during hot-water post-ppasteurization. Values are mean (n=2) for average of three surface mid-piece temperatures. Product Surface temperature (°C)  after hot-water post-packaging pasteurization for given time (m

Processor 0 0.5 1.0 1.5 2.0 3.0 4.0 5.0 6.0 

Beef Snack Sticks 1 per pkg. 

A B 

C 

5 5 

5 

43 42 43 

47 51.5 

49.5 

56.5 

56 

51.5 

------

---

------

---

------

---

------

---

------

---

Beef snack sticks 4 per pkg. 

A 

B 

C 

5 

5 5 

---

------

43.5 

39.5 44 

---

---

---

51 

51.5 

52 

56.5 

55.5 

59.5 

61 

58 

61.5 

65 

61 

66 

---

---

---

Beef snack sticks, 7 per pkg. 

A B C 

5 5 5 

---------

39 40.5 

42 

---

------

51 

46.5 49 

59 

52.5 

55 

61 

58.5 

62 

62.5 

62.5 

66 

67 

66 

67.5 
Hot-water pasteurization of packaged RTE meats 

Natural Casing Wieners 4 per pkg. 

A 5 --- 42 --- 45.5 53.5 58 58.5 62.5 

B 5 --- 40 --- 44 51.5 53 56.5 60 D 5 --- 39.5 --- 45 47.5 54 58 60.5 
Hot-water pasteurization of packaged RTE meats 

Table 5. Calculated reduction in log CFU of Listeria monocytogenes during hot-water post-packaging beef snack sticks and natural casing wieners. Reductions based on measured surface temperatures (American Meat Institute Foundation process lethality spreadsheet using D62.8 = 67.6 seconds and Z = Product Pieces per Heating Processor Fo Reduction in L. monocytoge

 Package Time (min) (min) (log CFU) 

Beef Snack 1 0.5 A 0 0 Sticks B 0 0 

C 0 0 

1.0 A 0 0 

B 0.01 0.01 C 0.01 0.01 

1.5 A 0.05 0.04 

B 0.05 0.04 C 0.02 0.02 

4 3.0 A 0.12 0.11 

B 0.10 0.09 
Hot-water pasteurization of packaged RTE meats 

C 0.24 0.21 

4.0 A 0.49 0.43 

B 0.28 0.25 

C 0.78 0.69 

5.0 A 1.75 1.55 

B 0.70 0.62 

C 2.40 2.13 

7 3.0 A 0.20 0.18 

B 0.04 0.03 C 0.07 0.06 

4.0 A 0.66 0.59 

B 0.20 0.18 

C 0.52 0.46 

5.0 A 1.42 1.26 

B 0.81 0.72 

C 2.20 1.95 
Hot-water pasteurization of packaged RTE meats 

6.0 A 5.74 5.09 

B 2.55 2.26 C 5.47 4.85 

Natural 4 5.0 A 0.47 0.42 Casing B 0.18 0.16 

Wieners C 0.22 0.19 

6.0 A 1.07 0.95 

B 0.48 0.43 C 0.60 0.53 

7.0 A 1.99 1.77 

B 0.96 0.85 

C 1.48 1.31 
Hot-water pasteurization of packaged RTE meats 

Table 6. Cook-out loss during hot-water post-packaging pasteurization of beef snack sticks and natur

Product Treatment Time Processor % weight lost in cook-outa

 (min.) 

Beef Snack Sticks 4 per package 3 A 

B

 C

 0.9 

 0.2 0.8

 4 A

 B C

 0.9 0.6 0.6 

7 per package 4 A 

B

 C

 1.1 

 0.4 0.7

 5 A

 B C

 0.8 0.7 0.7 

Natural Casing Wieners 4 per package 5 A 

D

 0.4 

 0.4

 6 A

 D

 0.8 0.5 
Hot-water pasteurization of packaged RTE meats 

7 A 1.2

 B 1.0 D 0.6 

aEach value is for a single package of product. 
Hot-water pasteurization of packaged RTE meats 

Table 7. Consumer overall acceptance of beef snack sticks (seven per package) treated for 4.0 minu

post-packaging pasteurization. Cooked-out fat and moisture was removed before presentation. “Trtm“Ctrl” = untreated product. Processor A B C

 Trtmt. Ctrl Trtmt Ctrl Trtmt Assigned Descriptor Numerical score Like very much = 7 Like moderately = 6 Like slightly = 5 Neither like nor dislike = 4 

 Number of responses 

64 65 

71 69 33 35 

6 7 

52 

84 32 

8 

49 

66 33 

11 

76 

61 41 

7 

Dislike slightly = 3 Dislike moderately = 2 Dislike very much = 1 Mean score 

5 

2 

1 

5.95 

4 

2 

0 

5.98 

7 

2 

1 

5.84 

18 

7 

2 

5.47 

4 

2 

1 

5.98 

Trtmt significantly different? NO YES NO 
Hot-water pasteurization of packaged RTE meats 

Table 8. Consumer overall acceptance of natural casing wieners (four per package) treated for 7.0 mi

post-packaging pasteurization. Product cooked in boiling water before presentation. “Trtmt” = treated untreated product. Processor A B D

 Trtmt. Ctrl Trtmt Ctrl Trtmt Assigned Descriptor Numerical score Number of responses Like very much = 7 53 60 66 45 56 Like moderately = 6 79 81 68 79 71 Like slightly = 5 35 24 29 26 33 Neither like nor dislike = 4 5 9 12 14 7 Dislike slightly = 3 6 6 6 10 9 Dislike moderately = 2 3 3 2 6 2 Dislike very much = 1 2 0 1 4 0 Mean score 5.83 5.93 5.90 5.55 5.85 
Hot-water pasteurization of packaged RTE meats 

Trtmt significantly different? NO YES NOTab

acceptance of the in-package appearance of beef snack sticks (seven per package) treated for 4.0 min

casing wieners (four per package) treated for 7.0 minutes with hot-water post-packaging pasteurizationproduct, “Ctrl” = untreated product. Product Beef Snack Sticks Natural Casing Wieners Processor B B Trtmt. Ctrl Trtmt Ctrl 

Assigned Descriptor Numerical score Number of responses Extremely appealing = 7 13 36 34 23 Moderately appealing = 6 36 79 92 63 Slightly appealing = 5 39 44 39 45 Neither appealing nor unappealing = 4 31 16 15 24 

Slightly unappealing = 3 51 12 15 33 

Moderately unappealing = 2 19 7 1 6 

Extremely unappealing = 1 9 4 2 4 
Hot-water pasteurization of packaged RTE meats 

Mean score 4.17 5.37 5.52 4.92 Trtmt significantly different? YES YES