A 2014 multistate listeriosis outbreak was linked to consumption of caramel-coated apples, an unexpected and previously unreported vehicle for Listeria monocytogenes . This outbreak was unanticipated because both the pH of apples (<4.0) and the water activity of the caramel coating (<0.80) are too low to support Listeria growth. In this study, Granny Smith apples were inoculated with approximately 4 log ₁₀ CFU of L. monocytogenes (a cocktail of serotype 4b strains associated with the outbreak) on each apple's skin, stem, and calyx. Half of the apples had sticks inserted into the core, while the remaining apples were left intact. Apples were dipped into hot caramel and stored at either 7°C or 25°C for up to 11 or 28 days, respectively. Data revealed that apples with inserted sticks supported significantly more L. monocytogenes growth than apples without sticks under both storage conditions. Within 3 days at 25°C, L. monocytogenes populations increased >3 log ₁₀ in apples with sticks, whereas only a 1-log ₁₀ increase was observed even after 1 week for caramel-coated apples without sticks. When stored at 7°C, apples with sticks exhibited an approximately 1.5-log ₁₀ increase in L. monocytogenes levels at 28 days, whereas no growth was observed in apples without sticks. We infer that insertion of a stick into the apple accelerates the transfer of juice from the interior of the apple to its surface, creating a microenvironment at the apple-caramel interface where L. monocytogenes can rapidly grow to levels sufficient to cause disease when stored at room temperature. IMPORTANCE Neither caramel nor apples are a food where the pathogenic bacterium Listeria monocytogenes should grow, as caramel does not contain enough free water and apples are too acidic. Caramel-coated apples, however, were recently linked to a deadly outbreak of listeriosis. We hypothesized that inserting a stick into the apple releases juice to the interface between the apple and caramel, providing a more hospitable environment than either component alone. To test this hypothesis, apples were inoculated with L. monocytogenes prior to caramel dipping. Some apples had sticks inserted into them before dipping, while others did not. No growth of L. monocytogenes occurred on refrigerated caramel apples without sticks, whereas slow growth was observed on refrigerated caramel apples with sticks. In contrast, significant pathogen growth was observed within 3 days at room temperature on caramel apples with sticks inserted. Food producers should consider interfaces between components within foods as potential niches for pathogen growth.

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 pro...

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°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°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, ...

High-moisture have been shown to support Listeria monocytogenes growth during refrigerated storage. Prior studies suggest that organic acids vary in their antilisterial activity and that cheeses of lower pH delay growth longer than those of higher pH; however, no standard pH value for Listeria control in cheese exists. The objective of this research was to create a predictive model to include the effects of acid type, pH, and moisture on the growth of L. monocytogenes in a model cheese system. Cream, micellar casein, water, lactose, salt, and acid (citric, lactic, acetic, or propionic) were combined in 32 formulations targeting 4 pH values (5.25, 5.50, 5.75, and 6.00) and 2 moisture levels (50 and 56%). Each was inoculated with 3-log CFU/g L. monocytogenes (5-strain mixture) after which 25-g samples were vacuum-sealed and stored 8 weeks at 4ºC. Triplicate samples were enumerated on Modified Oxford agar weekly in duplicate trials. Model cheeses formulated with acetic and propionic acids inhibited growth (i.e.<0.5 log increase in L. monocytogenes populations over 8 weeks) at pH ≤5.75, while those formulated with lactic acid inhibited growth at pH 5.25 only. In contrast, all model cheeses formulated with citric acid supported growth. Resulting growth curves were fitted for lag phase and growth rate before constructing models for each. pH and acid type were found to significantly affect both growth parameters (p < 0.05), while moisture (50-56%) was not statistically significant in either model (p ≥ 0.05). The effects of acetic and propionic acid were not significantly different. In contrast, citric acid model cheeses had significantly shorter lag phases than the other acids tested, but growth rates after lag were statistically similar to model cheeses made with lactic acid. These data suggest propionic ~ acetic > lactic > citric acids in antilisterial activity within the model cheese system developed and can be used in formulating safe high-moisture cheeses.

Fermentation-derived nitrite (NO2) from vegetable sources is increasingly used as a "clean label" alternative to conventional NaNO2. Previous results suggested that processed meats cured with NO2 derived from a "natural" source had lower antimicrobial activity than did meats produced with chemical NaNO2; however, the differences were likely due to NO2 concentration rather than source. The objective of this study was to compare the antilisterial properties of traditional and clean label alternative curing approaches when combined with antimicrobials in deli-style turkey. Listeria monocytogenes inhibition by NO2 from synthetic and natural sources was validated in deli-style turkey (73 to 74% moisture, 1.8% salt, pH 6.4). Products were prepared with 0, 80, or 120 mg/kg NO2 using purified NaNO2 or cultured celery powder. Additional treatments were supplemented with 3.8% lactate-diacetate blend (LD) or 1% cultured sugar-vinegar blend (DF). Sliced cooked products were surface inoculated with L. monocytogenes at 3 log CFU/g, vacuum packaged, and stored at 4°C for 12 weeks. Results revealed an average 2.4-log increase in L. monocytogenes at 3 weeks in the control without antimicrobials, a 1.3-log increase at 4 weeks for both 80 mg/kg NO2 treatments, and a 1.5-log increase at 6 weeks for the 120 mg/kg NO2 treatments. No significant difference (P > 0.05) in growth inhibition was found between NO2 sources when equivalent concentrations were added. In uncured turkey with 3.8% LD or 1% DF, growth was delayed until 6 weeks, whereas supplementation with LD or DF and 80 mg/kg NO2 from either source delayed listerial growth through 12 weeks. This study confirmed that the concentration of NO2, rather than the source, is a primary factor in enhancing the safety of ready-to-eat meats. Both conventional NO2 treatments and a clean label solution consisting of a fermentation-derived antimicrobial combined with 80 mg/kg naturally derived NO2 inhibited L. monocytogenes through 12 weeks of storage at 4°C.

The effects of fat, type of natural cheese, and adjunct process cheese ingredients were evaluated to determine factors that contribute to the botulinal safety of reduced-fat (RF) process cheese products stored at 30°C. In the first set of experiments, pasteurized process cheese products (PPCPs) were formulated using full-fat (FF) Cheddar, 30% RF Cheddar, or skim milk (SM) cheese as cheese-base types and were standardized to 59% moisture, pH 5.75, 2.8 or 3.2% total salts, and 15 to 19% fat. Subsequent trials evaluated the effect of fat levels and adjunct ingredients in PPCPs made with SM, RF, and FF cheese (final fat levels, less than 1, 13, and 24%, respectively). When fat levels of PPCPs were comparable (15.1, 19.1, and 16.2 for product manufactured with SC, RF, and FF cheese, respectively), botulinal toxin production was delayed for up to 2 days in PPCPs formulated with SM compared with RF or FF cheese; however, the effect was not statistically significant. When fat levels were reduced to less than 1% i...