Development of appropriate intervention methods to reduce the occurrence of pathogenic bacteria on Countrycured hams 

William Benjy Mikel and Melissa Newman 

Animal and Food Sciences 

University of Kentucky 

205 W.P. Garrigus Bldg 

Lexington, KY 40546-0215 

Abstract 

The purpose of this research was to evaluate the characteristics of market-ready country-cured ham 

products related to product safety and to identify potential intervention methods for use against possible food 

borne pathogens, specifically Listeria monocytogenes. To accomplish these objectives, a market basket survey 

was conducted of country-cured products available at retail followed by inoculation of country ham center slices 

with L. monocytogenes subjected to various chemical and mechanical intervention methods. Market basket results 

provides the following characterization of country-cured ham products; water activity averaged .88, salt content 

6.5%, pH 6.1, and moisture/protein ratio was 0.96. Preliminary data indicated that two chemical methods (Nisin 

and Liquid Smoke) and one mechanical methods (steam) were most promising and had the greatest potential 

synergism. The second experiment focused on reduction of L. monocytogenes on inoculated center cut slices. 

Samples treated with Nisin had the greatest population reductions versus both types of liquid smoke application. 

In addition, the application of steam in conjunction with chemical methods decreased populations by last least 

two-fold. The most effective combination in reducing L. monocytogenes populations was the use of steam and 

nisin at a level of 10 times the minimum inhibitory concentration. These data indicate that steam and nisin when 

used in combination can act as an effective intervention method for country-cured ham center slices. 

Introduction 

The production of country-cured hams has taken place for centuries and is located predominately in the 

southeast region of the United States. Currently, over 10 million hams are commercially country cured each year 

accounting for over five percent of the 180 million hams produced from market hogs. The viability of this 

profitable niche market is an essential aspect of value-added pork products. The United States Department of 

Agriculture (USDA) has standards of production, including salt level, shrinkage and processing 

times/temperatures, which have been used to assure the safe production of this product. The product may be 

smoked but is generally not cooked. The product is also considered shelf-stable and generally displayed at 

ambient temperatures in retail markets. However, certain ethnic groups consider country-cured hams a Ready-ToEat (RTE) product and therefore do not cook the product. Although no food borne illnesses have been directly 

related to country hams there have been outbreaks in similar products such as dried sausages. In addition, earlier 

work in our lab (as early as 1970 until 1997) at UK has suggested that certain pathogens can survive on packaged 

country ham products under refrigeration. This combined with an increase concern over the safety of the meat 

supply has lead to the need to validate the safety of this unique product. However, to date, no work has validated 

the efficacy of processing methods reducing pathogens to safe levels in country-cured hams. The implementation 

of the recent pathogen reduction act (Mega-Reg) has placed a greater amount of responsibility for validating the 

safety of the meat supply on individual companies. The United States Department of Agriculture (USDA) has 

mandated that Ready-To-Eat (RTE) products be produced by a process, which will achieve over a 5-log reduction 

in Salmonella. In addition, the prevalence of Listeria in many RTE pork products as well as consumer concern 

related to recent illnesses due to E. coli O157:H7 leads these researchers to believe that these organisms are of 

equal importance. Recent research conducted in our laboratory (unpublished data) and a similar project funded by 
the National Country Ham Producers Association indicated that the safety of country-cured hams was at question 

if minimum USDA guidelines were not adhered to stringently. The need to strictly follow total time in cure, 

equalization and aging, as well as weight reductions to achieve the desired level of water activity is crucial. This is 

especially true as USDA considers country-cured hams to be a “ready-to-eat” product with no need of heat 

treatment prior to consumption. In addition, most products are displayed as shelf-stable products without 

refrigeration. Our previous data indicate that the minimum required time of drying might vary from USDA 

regulations for larger hams as the industry slaughter weights continue to increase. These data indicate that water 

activity is a much better source of information for determining the safety of the product. In addition, preliminary 

research also indicates that if sufficient numbers of Staph sp. are found on the fresh incoming hams, toxin 

production might be a serious possibility, which needs to be addressed through multiple hurdles or additional 

intervention steps. 

Objectives: 

1) Conduct a market basket survey of country-cured ham products to assess criteria including pH, aw, MPR, 

and Staph. Sp and Listeria to determine the current state of products supplied to the retail grocer. 

2) Examine various methods of pathogen intervention for potential bacterial pathogens, which might occur 

due to lack of cutting edge processing and merchandising. 

Procedures 

This research was conducted in two phases. Phase one included a market basket survey of various countrycured products presently produced evaluate the safety of products currently in the marketplace. In addition, 

previous research has led to questions of the safety of hams produced, which are processed only following the 

minimum guidelines set forth by the United States Department of Agriculture. The two most opportunistic 

bacteria, Staph. Sp and Listeria, which has shown possible emergence in country-cured hams will be focused on. 

Hams produced under minimum processing steps with added intervention strategies were evaluated for microbial 

reductions to safe levels. The goal of the proposed research is to identify the state of the products presently in the 

marketplace and to evaluate various intervention methods such as hot washes or organic acid sprays either pre- or 

post-processing. 

The first objective will be achieved by purchasing a variety of products (whole hams, slices, bits, etc.) from 

numerous retail markets, which carry a wide variety of producer products. Products will be examined for water 

activity (aw), pH, salt, nitrite, moisture protein ratio (MPR), and for populations of Staph sp. (As well as toxin) and 

Listeria. Code dates from packages will be checked with processors to determine product age at the time of 

purchase. 

The second objective will be accomplished by slicing hams processed under the minimum USDA 

requirements with or with out added intervention methods. Ham slices will be inoculated with six logs of Listeria 

monocytogenes and the following interventions applied separately or in conjunction with each other. These 

methods included the following treatments. 

Treatment I: Control ham slices 

Treatment II: Steamed ham slices 

Treatment III: Ham slices with liquid smoke 

Treatment IV: Ham slices with Nisin 

 Treatment V: Ham slices steamed with either liquid smoke or nisin added. 

Cured ham slices will be obtained from a commercial processor and transported to the University of 

Kentucky Meat Science Laboratory. Ham slices will be surface inoculated with at least 106

 CFU/cm2 of L. 

monocytogenes. The inoculum level will permit evaluation that the cure treatment is capable of resulting in a 6 log 
reduction in population of the test pathogen in the finished product. Six different ham slices per treatment group 

will be analyzed after inoculation on days 0, 3, and 7 in order to determine the population of the inoculated test 

species on the hams. The swab rinse procedure will be used to determine the populations of the inoculated hams. 

Two 25 cm2 areas per ham slice will be swabbed and the swab rinse solution used for directing plating of the test 

species. Enrichment procedures will be used to determine the presence of test species on control hams and on 

inoculated hams after the storage procedure. Scrapings, less than 2 mm, will be aseptically obtained from each 

ham’s surface. The scrapings will be composited and a 50-g sample from the composited sample will be added to 

the enrichment broth. Hams will be analyzed for water activity (aw), pH, salt, nitrite, moisture protein ratio 

(MPR), and for populations of Staph sp. and Listeria. 

Results 

Market basket results were obtained from a wide range of country-cured ham products; center slices, bits 

and pieces, end pieces, biscuit slices, seasoning meats, hocks, etc. Water activity ranged from .74 to .93 with an 

average of .88) indicating various types of products had more available moisture for bacterial growth. Salt levels 

ranged from 4.0 (the minimum required level by USDA standards) to 9.7% (with an average of 6.5%). The 

moisture:protein ratio ranged from .54 to 1.56 (with an average of 0.96) again indicating a wide range in product 

characteristics. The evaluation of of both Listeria monocytogenes and Salmonella spp. indicated no positives 

were found in any of the products tested. Staph determinations found populations ranged from non-detectable to 

six log populations in the various products. However, no toxin production was determined. These data indicate 

that although ham slices produced from ham produced using the minimum USDA guidelines had characteristics 

which favored greater bacterial growth, the combination of salt content and the reduced water activity led to the 

production of a microbiologically safe product. 

Preliminary evaluation of various intervention methods (water, steam, organic acids, nisin, liquid smoke, 

flame) indicated that two chemical methods (Nisin and Liquid Smoke) and one mechanical methods (steam) were 

most promising and had the greatest potential synergism. Therefore, the second experiment focused on reduction 

of L. monocytogenes on inoculated center cut slices through the use of steam, nisin, and/or liquid smoke. Steam 

application times were tested at both 5 and 10 seconds of exposure with the 10 second time chosen for its greater 

effectiveness. Both the nisin and liquid smoke interventions were first tested on inoculated broth to determine the 

minimum inhibitory concentrations. Then ham slices were treated with either the minimum concentration or 5 or 

10 times that level. Samples treated with Nisin had the greatest population reductions versus both types of liquid 

smoke application in non-steamed samples (3.8 vs 4.8 log population). Slices had a one log greater reduction with 

nisin regardless of concentration used. In steamed slices, the minimum level of nisin had a 1.5 log greater 

reduction ( 2.6 vs 4.1 population) than liquid smoke treated samples, with the 10 X minimum concentration level 

having a 2.5 log greater reduction in populations (1.4 vs 4.1). In addition, the application of steam in conjunction 

with chemical methods decreased populations by last least two-fold. Furthermore, the application of steam and 

nisin at the highest level resulted in a six log reduction when evaluated at 7 days post treatment (0.5 log population 

vs innoculum level of approximately 6.3). These data indicate that steam and nisin when used in combination can 

act as an effective intervention method for country-cured ham center slices. 
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Table 1. Country Ham Market Basket Survey Results 

Sample 

ID 

Aw pH Salt 

% 

Moisture 

% 

Protein 

% 

M/P 

ratio 

Listeria Salmonella Staph. 

(Log CFU) 

1 .911 5.73 5.8 64.35 66.31 0.97 - - 10 X 2 

2 .904 6.24 6.2 60.68 67.66 0.90 - - 10 X 5 

3 .929 6.56 5.9 60.35 55.66 1.08 - - 10 X 5 

4 .912 6.44 6.1 64.67 41.50 1.56 - - 10 X 4 

.839 6.22 6.3 30.81 33.41 0.92 - - 10 X 4 

6 .741 6.05 4.9 18.27 34.09 0.54 - - 10 X 0 

7 .900 6.23 7.4 60.73 58.53 1.04 - - 10 X 2 

8 .888 6.23 6.9 58.14 62.72 0.93 - - 10 X 2 

9 .882 6.15 5.6 43.06 26.94 1.60 - - 10 X 0 

.837 6.00 4.3 12.68 26.41 0.48 - - 10 X 0 

11 .884 6.42 5.4 50.34 55.41 0.91 - - 10 X 4 

12 .891 6.64 5.0 54.58 65.34 0.84 - - 10 X 5 

13 .889 6.42 5.4 57.48 49.41 1.16 - - 10 X 3 

14 .882 5.98 6.5 60.14 57.31 1.05 - - 10 X 0 

.833 5.73 9.0 44.66 52.63 0.85 - - 10 X 3 

16 .837 5.72 9.0 52.27 56.28 0.93 - - 10 X 0 

17 .877 6.33 7.4 46.42 65.63 0.71 - - 10 X 6 

18 .823 6.12 9.7 54.16 53.38 1.02 - - 10 X 3 

19 .754 6.17 8.0 26.77 40.25 0.67 - - 10 X 3 

.810 5.99 7.9 31.22 31.56 0.99 - - 10 X 2 

21 .880 6.14 6.1 51.74 63.13 0.82 - - 10 X 5 

22 .838 6.10 5.1 38.77 56.97 0.68 - - 10 X 3 

23 .902 5.88 5.2 57.99 65.13 0.89 - - 10 X 4 

24 .927 6.61 4.4 66.41 69.53 0.96 - - 10 X 5 

.841 5.61 6.5 57.33 51.06 1.12 - - 10 X 4 

26 .850 5.95 5.8 48.32 57.47 0.84 - - 10 X 5 

27 .909 6.02 4.5 62.04 66.41 0.93 - - 10 X 5 

28 .908 5.92 4.0 60.69 65.69 0.92 - - 10 X 5 

29 .902 5.73 4.5 55.24 63.63 0.87 - - 10 X 4 

.881 5.59 5.0 58.70 62.84 0.89 - - 10 X 3 

31 .913 6.51 5.5 53.95 66.41 0.81 - - 10 X 5 

32 .902 6.79 6.1 64.24 65.00 0.99 - - 10 X 5 

33 .888 6.01 8.1 61.82 62.13 1.00 - - 10 X 0 

34 .902 5.08 7.7 64.45 60.34 1.07 - - 10 X 0 

.879 5.92 9.6 58.69 56.06 1.05 - - 10 X 2 

36 .890 5.10 9.5 60.17 53.94 1.12 - - 10 X 2 

37 .897 6.08 7.8 51.56 43.38 1.19 - - 10 X 2 

38 .907 5.85 7.3 60.53 56.13 1.08 - - 10 X 0 

Average .875 6.06 6.5 52.22 54.87 0.96 - - 10 X 3