Fluid Milk Spoilage by Psychrotolerant Spores and Shelf-Life Improvement

Cornell University researchers created a new predictive model that examines spore-forming bacteria and their impact on pasteurized milk shelf life, published in the August issue of the Journal of Dairy Science entitled “Psychrotolerant spore-former growth characterization for the development of a dairy spoilage predictive model”. The goal of the research was to create a mathematical model to increase the product shelf life and have a more meaningful and accurate way to estimate the shelf life. This can reduce food waste and reduce food spoilage, by a better understanding of the parameters driving spoilage.

The researchers characterized the ability to germinate and grow 14 psychrotolerant spore-formers. The organisms used representing the most common Bacillales subtypes (e.g. Bacillus, Lysinibacillus, Paenibacillus, Psychrobacillus, Viridibacillus) from raw and pasteurized milk at 6°C. Growth curves were generated by counting total bacterial count and spore count every day for 30 days at 6°C. The obtained data d can serve as the foundation for the development of better tools to predict the shelf life of fluid milk.

The researchers developed a predictive model to investigate the growth of psychrotolerant spore-forming bacteria. Such a model allows the identification of parameters that most effectively can increase the shelf life of fluid milk.

They used a Monte Carlo simulation model to predict the spoilage patterns. The model assumed that the spoilage level for fluid milk is >20,000 cfu/mL. Monte Carlo model predicted that 66% of ½ gallons milk pasteurized at high temperature, for short time would reach the critical level of 20,000 cfu/mL after 21 days of storage at 6°C, in agreement with current spoilage rate.

The parameter that influenced most of the shelf life was the maximum growth rate of psychrotolerant spore-formers. The research showed that different psychrotolerant spore-formers have different abilities to germinate and grow in fluid milk.   Better characterization of the ability to germinate and the growth rates of the diverse of psychrotolerant spore-formers will have the best impact on improving the model.

The second parameter that influences the model is the initial concentration of psychrotolerant spore-formers in the bulk raw tanks.  The model shows that microfiltration can reduce the initial numbers of psychrotolerant spore-formers by 2.2 logs, the shelf life can be extended by 4 days.

The research data indicates that if the numbers of spore-forming bacteria are reduced and if their outgrowth can be controlled, the shelf life of milk can be improved from 2 weeks to a month.  The created model shows that if the milk is kept refrigerated at 40 C, instead of 60 C only 9% of the milk will spoil in 21 days as compared with 66% at the currently used temperature.

To produce high-quality milk with extended shelf life, it is essential to control psychrotolerant spore-formers. The study identifies growth parameters that are required to consistently predict the shelf life of fluid milk due to psychrotolerant spore-formers. Processing decisions and storage of the milk through the supply chain can reduce spoilage due to spore-formers.

The authors emphasize that the model developed provides a new tool for the dairy industry to predict the shelf life of pasteurized milk and extend it. This could allow manufacturers to provide a more meaningful “sell-by-date”. The model demonstrates the effectiveness of predictive modeling for the dairy industry and allows a “what if” analysis to estimate the outcome of different processing procedures.


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