A Predictive Model to Determine the Effects of pH, Milkfat, and Temperature on Thermal Inactivation of Listeria monocytogenes

Abstract

Listeria monocytogenes is a foodborne pathogen of significance because of its comparatively high heat resistance, zero tolerance in ready-to-eat foods, and growth at refrigeration temperatures. A 3 x 3 x 3 factorial study was done to determine the effects of milkfat (0%, 2.5%, 5.0%), pH (5.0, 6.0, 7.0), and processing temperature (55 degrees C, 60 degrees C, 65 degrees C) on the thermal resistance of L. monocytogenes in a formulated and homogenized milk system. Data were fit to a modified Gompertz equation where parameter estimates characterized three regions of a survival curve: the shoulder, maximum slope, and tail. Statistical analysis was done for each of the 27 individual treatment sets to visualize individual effects on parameter estimates and to evaluate how well the Gompertz equation represented the data. A regression model for the Gompertz equation was generated to predict the logarithmic surviving fraction of L. monocytogenes based on all 27 treatments and their single and interactive effects. The shoulder region of the survival curve was affected by pH; however, the maximum slope was affected by temperature, milkfat, and the interaction of temperature x milkfat. Validation of the model suggests that the predictions are best suited for processing above 62 degrees C. Trends over time for a 4-log reduction in cells (4D values) were evaluated using results from the 27 individual treatment sets, the regression model for the Gompertz equation, and a linear equation. At lower temperatures, 4D values by the three methods varied by twofold. At higher temperatures, all methods gave similar 4D values, suggesting that death became more linear. Based on this study all three factors affect heat resistance for specific regions of a survival curve, and a predictive model was developed that can be used as a preliminary estimate for L. monocytogenes inactivation.

Growth Kinetics of Vibrio parahaemolyticus O3:K6 under Varying Conditions of pH, NaCl Concentration and Temperature

“…The modeling is useful for risk assessment 6) , and has been developed for such food-borne pathogens as Salmonella 7) , Listeria monocytogenes 8) , Escherichia coli O157 : H7 9) and V. parahaemolyticus 10) . In this study, a growth model for V. parahaemolyticus including serotype O3 : K6 was generated from the growth data obtained.…”

INFLUÊNCIA DA GORDURA DO LEITE BOVINO E CAPRINO NA RESISTÊNCIA DOMycobacterium fortuitum À PASTEURIZAÇÃO LENTA

“…No entanto, é possível que isso se deva ao baixo número de ensaios realizados em cada substrato (n=3), o que poderia ser esclarecido com novos estudos. Apesar disso, nota-se uma tendência de maiores níveis de decaimento na população do agente em leite desnatado, de ambas as espécies, sugerindo um efeito termo protetor da gordura sobre o M. fortuitum, o que não surpreende já que essa propriedade da gordura está bem comprovada na literatura para outras bactérias (17)(18)(19)(20)(21) . Ainda na linha de tendências, a gordura teve maior influência no decaimento do M. fortuitum no leite bovino do que no leite caprino, o que, a princípio, poderia estar relacionado ao maior teor de gordura nas amostras de leite de vaca (5,9%) do que nas de cabra (2,8%).…”

“…Nesse particular, há poucos dados na literatura sobre a influência do substrato na curva de morte térmica do Mycobacterium spp. A influência da gordura do leite na cinética de morte térmica não foi avaliada para esta bactéria, embora a literatura indique um efeito protetor para outros microrganismos (17)(18)(19)(20)(21) . Também não foram encontrados estudos dessa natureza em outros tipos leite, além do bovino, mas há relato de que a termorresistência de Listeria monocytogenes é maior em leite ovino que em bovino e caprino, sugerindo que haja algum componente protetor intrínseco ao leite de ovelha para esse agente (19) .…”

Evaluation of the heat resistance of pathogenic <i>Listeria monocytogenes</i> in milk and milk products in Sri Lanka

“…The protective effect of milk fat is important during the maximum death region of the survival curve. Milk foods should not be processed at temperatures below 60 o C because the shoulder regions become more pronounced and thermal processing may be under-estimated (Chhabra 1999). This is due to the buffering action and the poor thermal conductivity exerted by fat in milk.…”