Kathleen A. Glass

Growth and penetration of Salmonella enteritidis, Salmonella heidelberg and Salmonella typhimurium in eggs

Eggs and egg dishes are important vehicles for Salmonella infections. Salmonella enteritidis, Salmonella typhimurium and Salmonella heidelberg, which can be isolated from chicken ovaries and feces, have been implicated in approximately 50% of the foodborne salmonellosis outbreaks in the United States. In this study, the growth of these three organisms, inoculated into yolks and albumen, was compared at 4, 10 and 25 degrees C. Regardless of whether 10(2) cfu/g or 10(4) cfu/g was inoculated into the yolk or albumen, populations of all strains increased 3 logs or more in number in one day when incubated at 25 degrees C. Maximum numbers of Salmonella ranged from 10(8) to 10(10) cfu/g. All strains grew at 10 degrees C, but peak numbers were lower and occurred later than those at 25 degrees C. Populations of the three Salmonella strains inoculated into eggs stored at 4 degrees C grew sporadically; in some test groups populations declined. The potential for Salmonella in contaminated feces to establish in the interior of eggs was examined by monitoring shell penetration. At 25 degrees C, all three Salmonella strains penetrated the shell in 3 days, but at 4 degrees C, only S. typhimurium was found in one membrane sample. When hatchery conditions were simulated by incubating eggs at 35 degrees C for 30 min followed by storage at 4 degrees C, penetration was enhanced. Penetration was observed by day 1-3 when eggs were exposed to 10(4) cfu Salmonella/g feces. Increasing the inoculum to 10(6) cfu/g feces resulted in 50-75% of the contents of eggs to be contaminated by day 1. All Salmonella-positive samples were detected by enrichment. Results of this study indicate that S. enteritidis, S. typhimurium, or S. heidelberg present in feces can penetrate to the interior of eggs and grow during storage.

Review of Shiga-toxin-producing Escherichia coli (STEC) and their significance in dairy production

The involvement of the pathogenic Shiga-toxin-producing Escherichia coli (STEC; also called verocytotoxic-producing E. coli or VTEC) in sporadic cases and disease outbreaks is presently increasing. Infrequent cases are due to ingestion of milk and dairy products. As ruminants are healthy carriers of STEC and most dairy products may provide these bacteria with favourable conditions for their growth, milk and dairy products are a potential source of STEC. But not all STEC serotypes are pathogens; only relatively small numbers in the entire family of STEC are pathogenic. This review focuses on the recent advances in understanding of STEC and their significance in milk and dairy products. It is intended to gather the information that is needed to understand how these bacteria are described, detected and characterised, how they contaminate milk and grow in dairy products, and how the dairy industry can prevent them from affecting the consumer.

The effects of diacetate with nitrite, lactate, or pediocin on the viability of Listeria monocytegenes in turkey slurries

The antilisterial effects of sodium diacetate (0.1, 0.3 and 0.5%) alone or in combination with sodium nitrite (30 ppm), sodium lactate (2.5%) or pediocin (5000 arbitrary units/ml) were evaluated in slurries (25% meat in sterile deionized H2O) prepared from vacuum-packaged, ready-to-eat turkey breast meat and challenged with Listeria monocytogenes. In the absence of food additives, counts of L. monocytogenes increased from 4.5 log10 cfu/ml to ca. 8 log10 cfu/ml within 1 day at 25 degrees C and within 14 days at 4 degrees C. Similarly, the pathogen grew to ca. 8 log10 cfu/ml within 1 d at 25 degrees C and within 28 days at 4 degrees C in slurries containing nitrite or lactate. In the presence of pediocin, after an initial decrease of 0.9 log10 cfu/ml, numbers of the pathogen reached ca. 8 log10 cfu/ml within 5 days at 25 degrees C and within 28 days at 4 degrees C. However, 0.3 and 0.5% diacetate in turkey slurries were listericidal at 4 and 25 degree C, respectively. In the presence of nitrite with diacetate, there was no appreciable difference in growth of L. monocytogenes compared with diacetate alone. Antilisterial activity was potentiated in treatments containing lactate with 0.3% diacetate at 25 degrees C and lactate with 0.1% diacetate at 4 degrees C, compared to similar treatments containing diacetate or lactate alone. A listericidal effect (ca. 7 log10 cfu/ml difference compared to slurries without additives) was observed in treatments containing pediocin with 0.5% diacetate at 25 degrees C and pediocin with 0.3% diacetate at 4 degrees C. The pH of slurries containing 0.3 or 0.5% diacetate was 5.5 and 5.2, respectively, whereas nitrite (pH 6.2), lactate (pH 6.3) or pediocin (pH 6.2) in slurries had a negligible effect on pH compared to the control (pH 6.2). The increased antilisterial activity in slurries with diacetate in combination with other additives was due to synergistic effects and not just pH. Thus, sodium diacetate alone can be used to delay growth of L. monocytogenes in turkey, and an additional level of safety can be achieved using diacetate in combination with sodium lactate or pediocin.

Fate and Thermal Inactivation of Listeria monocytogenes in Beaker Sausage and Pepperoni

Listeria monocytogenes may survive the typical process used to produce sausage such as pepperoni and hard salami that is made from uncooked meat. Studies were done to identify heat treatments that could be applied during sausage-making to inactivate the organism to undetectable levels. Initial studies with beaker sausage revealed that heat treatments of 51.7°C for 8 h or 57.2°C for 4 h reduced L. monocytogenes counts by >2 log10 CFU/g, but for each treatment the organism was detected by enrichment in one of two samples. Heating beaker sausage to an internal temperature of 62.8°C inactivated listeriae to undetectable levels. Studies with pepperoni revealed that heating sausage at 51.7°C for 4 h after fermentation but before the drying cycle killed most L. monocytogenes , but the organism was occasionally detected in samples during drying. Heating pepperoni at 51.7°C for 4 h after the drying cycle completely inactivated L. monocytogenes in all samples. An additional study done to determine the fate of L. monocytogenes in beaker sausage made with and without lactic starter culture revealed that the organism could grow at 32.2°C in sausage batter during the fermentation period if the lactic starter culture was not added. The organism did not grow, but was reduced by about 1 to 2 log10 CFU/g during fermentation, in sausage made with lactic starter culture.

Inhibition of Listeria monocytogenes in turkey and pork-beef bologna by combinations of sorbate, benzoate, and propionate.

The control of Listeria monocytogenes was evaluated with ready-to-eat uncured turkey and cured pork-beef bologna with combinations of benzoate, propionate, and sorbate. Three treatments of each product type were formulated to include control with no antimycotic agents; a combination of 0.05% sodium benzoate and 0.05% sodium propionate; and a combination of 0.05% sodium benzoate and 0.05% potassium sorbate. Ingredients were mixed, stuffed into fibrous, moisture-impermeable casings, cooked to an internal temperature of 73.9 degrees C, chilled, and sliced. The final product was surface inoculated with L. monocytogenes (4 log CFU per package), vacuum packaged, and stored at 4 degrees C for 13 weeks. The antimycotic addition to the second and third uncured turkey treatments initially slowed the pathogen growth rate compared with the control, but populations of L. monocytogenes increased 5 log or more by 6 weeks. In contrast, the addition of antimycotic combinations in the cured bologna prevented growth of L. monocytogenes during the 13-week storage period at 4 degrees C, compared with a more than 3.5-log increase in listerial populations in the control bologna, to which no antimicrobial agents had been added. These data suggest that low concentrations of antimycotic agents can prevent L. monocytogenes growth in certain ready-to-eat meats. Additional research is needed to define the levels needed to prevent growth of L. monocytogenes in high-moisture cured and uncured ready-to-eat meat and poultry and for gaining governmental approval for their use in such formulations.

Evaluation of sodium diacetate and ALTA 2341 on viability of Listeria monocytogenes in Turkey slurries

The antilisterial activity of sodium diacetate and a commercial shelf-life extender (ALTA™ 2341) were monitored at 25°C in slurries prepared with turkey breast meat. In slurries prepared without either ingredient, populations of Listeria monocytogenes increased about 5-log10 units in 7 d. The addition of 0.3% diacetate extended the generation time (7 h) compared to the control (no food additives; 1.7 h), whereas 0.5% inhibited the pathogen somewhat (0.4-log10 unit decrease in 7 d compared to the control). Slurries containing ALTA (0.25, 0.5, or 0.75%) and 0.3% diacetate extended the lag phase of L. monocytogenes to a greater extent than slurries with 0.3% diacetate alone. In contrast, 0.5% diacetate in combination with all three levels of ALTA tested was listericidal (ca. 2-log10 unit decrease after 7 d compared to the control). These data confirm the efficacy of diacetate for inhibiting L. monocytogenes in turkey meat and indicate that multiple barriers such as diacetate with ALTA may further lessen the likelihood of food-related listeriosis.

Survival of Bacterial Pathogens in Pasteurized Process Cheese Slices Stored at 30°C

Six lots of commercial pasteurized process cheese slices were evaluated for the ability to support the growth of four foodborne pathogens, Listeria monocytogenes, Staphylococcus aureus, Salmonella serotypes, and Escherichia coli O157:H7, during 4 days of storage at 30 degrees C. Individual cheese slices were inoculated separately with each pathogen to yield ca. 10(3) CFU/g. Slices were packaged in sterile plastic sample bags and stored at 30 degrees C for up to 96 h. Population of Salmonella serotypes and Escherichia coli O157:H7 decreased an average of 1.3 and 2.1 log10 CFU/g, respectively, by 36 h and Salmonella serotypes decreased an additional 0.6 log10 CFU/g during the remaining 60 h. Populations of Listeria monocytogenes also decreased, although to a lesser extent, exhibiting approximately a 0.6-log10 CFU/g reduction in 96 h. Staphylococcus aureus levels remained relatively constant during the testing period, and were below levels that support detectable enterotoxin production. The process cheese slices tested allowed survival but did not support rapid growth of S. aureus, whereas populations of L. monocytogenes, E. coli O157:H7, and Salmonella serotypes decreased during the 96-h storage at 30 degrees C.

Controlling Listeria monocytogenes on sliced ham and Turkey products using benzoate, propionate, and sorbate

The objective of this study was to identify concentrations of sorbate, benzoate, and propionate that prevent the growth of Listeria monocytogenes on sliced, cooked, uncured turkey breast and cured ham. Sixteen test formulations plus a control formulation for each product type were manufactured to include potassium sorbate, sodium benzoate, or sodium propionate, used alone and combined (up to 0.3% [wt/wt]), or with sodium lactate-sodium diacetate combinations. Products were inoculated with L. monocytogenes (5 log CFU/100-g package) and stored at 4, 7, or 10 degrees C for up to 12 weeks, and triplicate samples per treatment were assayed biweekly by plating on modified Oxford agar. Data showed that 0.1% benzoate, 0.2% propionate, 0.3% sorbate, or a combination of 1.6% lactate with 0.1% diacetate prevented the growth of L. monocytogenes on ham stored at 4 degrees C for 12 weeks, compared with greater than a 1-log increase at 4 weeks for the control ham without antimicrobials. When no nitrite was included in the formulation, 0.2% propionate used alone, a combination of 0.1% propionate with 0.1% sorbate, or a combination of 3.2% lactate with 0.2% diacetate was required to prevent listerial growth on the product stored at 4 degrees C for 12 weeks. Inhibition was less pronounced when formulations were stored at abuse temperatures. When stored at 7 degrees C, select treatments delayed listerial growth for 4 weeks but supported significant growth at 8 weeks. All treatments supported more than a 1-log increase in listerial populations when stored at 10 degrees C for 4 weeks. These results verify that antimycotic agents inhibit the growth of L. monocytogenes on ready-to-eat meats but aremore effective when used in combination with nitrite.

Fate of Salmonella and Listeria monocytogenes in commercial, reduced-calorie mayonnaise

Two new varieties of commercial low-calorie mayonnaise, i.e., cholesterol-free, reduced-calorie (CFM) and reduced-calorie (RCM), made with different levels of acetic acid, were evaluated to determine the survival characteristics of Salmonella or Listeria monocytogenes . Two formulations of CFM, made with 0.3 or 0.7% acetic acid in the aqueous phase, and four formulations of RCM, made with 0.1, 0.3, 0.5, or 0.7% acetic acid in the aqueous phase, were evaluated. The initial pH of the products after equilibration ranged from 3.9 to 4.3, which was adjusted by addition of HCl. Products were inoculated with an eight- or six-strain mixture of Salmonella sp. or L. monocytogenes , respectively, at ca. 106 CFU per gram and held at 23.9°C for up to 2 weeks. L. monocytogenes survived longer than Salmonella in equivalent preparations of mayonnaise. No Salmonella (per 100 g) was detected at 48 h in either variety of mayonnaise made with 0.7% acetic acid in the aqueous phase. Salmonella levels in mayonnaise made with lower levels of acetic acid decreased during storage, and at 2 weeks the organism was not detectable in samples containing 0.3% acetic acid in the aqueous phase. No L. monocytogenes (per 100 g) was detected at 14 or 10 d postinoculation in CFM or RCM, respectively, made with 0.7% acetic acid in the aqueous phase. Results indicate these new varieties of mayonnaise, when formulated with 0.7% acetic acid in the aqueous phase, will inactivate >107 Salmonella and >104 L. monocytogenes per gram within the 72-h holding time required for regular mayonnaise made with unpasteurized eggs. Hence, properly acidified (pH <4.1) reduced-calorie mayonnaise containing 0.7% acetic acid in the aqueous phase is a microbiologically safe product.

Toxin production by Clostridium botulinum in pasteurized milk treated with carbon dioxide.

The addition of carbon dioxide to milk at levels of <20 mM inhibits the growth of selected spoilage organisms and extends refrigerated shelf life. Our objective was to determine if the addition of CO2 influenced the risk of botulism from milk. Carbon dioxide was added to pasteurized 2% fat milk at approximately 0, 9.1, or 18.2 mM using a commercial gas-injection system. The milk was inoculated with a 10-strain mixture of proteolytic and nonproteolytic Clostridium botulinum spore strains to yield 10(1) to 10(2) spores/ml. Milk was stored at 6.1 or 21 degrees C for 60 or 6 days, respectively, in sealed glass jars or high-density polyethylene plastic bottles. Milk stored at 21 degrees C curdled and exhibited a yogurt-like odor at 2 days and was putrid at 4 days. Botulinal toxin was detected in 9.1 mM CO2 milk at 4 days and in all treatments after 6 days of storage at 21 degrees C. All toxic samples were grossly spoiled based on sensory evaluation at the time toxin was detected. Although botulinal toxin appeared earlier in milk treated with 9.1 mM CO2 compared to both the 18.2 mM and untreated milk, gross spoilage would act as a deterrent to consumption of toxic milk. No botulinal toxin was detected in any treatment stored at 6.1 degrees C for 60 days. At 6.1 degrees C, the standard plate counts (SPCs) were generally lower in the CO2-treated samples than in controls, with 18.2 mM CO2 milk having the lowest SPC. These data indicate that the low-level addition of CO2 retards spoilage of pasteurized milk at refrigeration temperatures and does not increase the risk of botulism from treated milk stored at refrigeration or abuse temperatures.