Modeling the Biphasic and Monophasic Microbial Growth in Pasteurized Cow Milk Under Isothermal Temperature Abuse

This study first reports the microbial growth dynamics and shelf life of pasteurized milk at extreme temperature abuse (37°C and 45°C), that occurs frequently in tropical countries. We determined total plate count (TPC) in pasteurized milk: double-toned milk (DTM), toned milk (TM), and full-cream milk (FCM) with a fat content of 1.5%, 3%, and 6%, respectively, at 4°C, 10°C, 20°C, 37°C, and 45°C (±1°C). Milk was considered to be expired when TPC exceeded 4.47 log CFU/mL (Food Safety and Standards Authority of India). Our study revealed a biphasic microbial growth pattern (two sequential sigmoid curves) at 4°C and was explained by the biphasic Baranyi model. At higher storage temperatures (10°C–45°C), monophasic microbial growth was observed and was described by the Baranyi and Roberts model. The temperature dependency of the growth rate was described by the Huang square root model. Our analysis indicated that the milk's shelf life decreased exponentially with increasing storage temperatures. At 4°C, we found a maximum shelf life of 168, 350, and 120 h for DTM, TM, and FCM, respectively. The biochemical properties of milk, including pH, proteolysis, lipolysis, and titratable acidity, significantly (p < 0.05) correlated with the microbial load in pasteurized milk at different storage temperatures studied. The results and the developed models can be used for efficient cold chain management and estimating the shelf life of pasteurized milk subjected to temperature abuse.