Question

.Milk that has been opened and left in the refrigerator for a long period of time will eventually “sour” from the growth of organisms. Explain why this is possible if the milk has been “pasteurized” and why is the expiration period for cream longer than of milk?  

Answer 1

The milk is pasteurized by treating it under mild heat (generally, less than 100°C) to exclude and eliminate the microbes (pathogen) and extend its shelf life.

However, all the bacteria and pathogens are not eliminated by pasteurization. And, the milk is exposed to more bacteria once it is left open for a longer time. These small pathogenic communities multiply over time and spoil the milk.

Dairy products that have high fat content spoils slowly. Moreover, the "watery" part of the milk is more prone to bacteria and lactose to cause spoilage.

Additionally, the pasteurization of the cream products are done at a higher temperature. Thus, more pathogens are killed during pasteurization in cream than milk.

Hence, cream has a longer expiration period and the shelf life than milk.