Purpose: Modeling spoilage of fluid milk due to growth of psychrotolerant sporeforming bacteria will allow for improved predictions of fluid milk shelf life and will facilitate assessment of different approaches to control psychrotolerant sporeforming bacteria in the fluid milk production and processing continuum.
Methods: Spore suspensions of psychrotolerant sporeformers, representing the most common Bacillales subtypes, isolated from raw and pasteurized milk were used to test germination and growth in skim milk broth at 6°C. Complete growth curves were obtained by following total and spore count every 24 h. Based on growth curves at 6°C, probability distributions of initial spore counts in bulk tank raw milk and subtype frequency in bulk tank raw milk, a Monte Carlo simulation model was created to predict spoilage patterns in HTST-pasteurized fluid milk.
Results: Growth parameters lag phase (λ) and maximum growth rate (µmax) were significantly different among psychrotolerant sporeformer subtypes. Lag phase ranged from 1.9 to 18.2 days and maximum growth rate ranged from 0.64 to 1.5 log CFU/mL/day. Given current raw milk sporeformer contamination patterns, Monte Carlo simulations indicated that 91% of samples of HTST fluid milk will have a cell density greater than 20,000 CFU/mL by day 21 of storage at 6°C, consistent with current spoilage patterns observed in commercial products.
Significance: This study provided a baseline understanding of the growth rates of psychrotolerant sporeformers in fluid milk and provided a stochastic model of spoilage by these organisms over the shelf-life of fluid milk, which will allow for assessment of different approaches to reduce fluid milk spoilage.