1 - Quality of the
raw milk received
All of these can affect
the quality of the final product and as a consequence the shelf life.
There are different groups of microorganisms capable of growing and spoiling the milk, for our study we consider that the mesophilic (growing between 68° and 113°F), and psychrophilic (growing between 20° and 68°F) are the most important groups, so we focus our effort on the mesophilic's which can spoil the milk when it is abused, and the psychrophilic's which can spoil the milk even at temperatures below 45°F. Most psychrophilic microorganisms are a result of post - pasteurization contamination, due to the fact that they usually die with pasteurization. It is known that milk is an excellent media for the growth of many microorganisms, it is also known that if the milk is kept under 45°F many microorganisms stop or decrease their growth.
The legal analysis for pasteurized bottled milk uses one milliliter of milk in a solid media for total aerobic counts and coliform counts. It is not a requirement to analyze psychrophilics, but the most widely used analysis takes 7 to 10 days to complete. The solid media employed is very different from liquid milk, so much so that it takes 48 hrs to see colonies growing in the solid media plates, incubated at 113°F, while it takes only a few hours for the milk to spoil. This means that for most of the microorganisms it is easier to grow in a liquid than in a solid. The reason for the use of solid media is to be able to count the amount of colonies per milliliter of milk.
We have developed a new method using liquid milk in a volume 10 times higher than normal in special equipment using new software. This allows us to detect microorganisms in very low concentrations. We also employ two different medias for psychrophilic bacteria, along with media for pseudomonas, and mesophilic bacteria. This allows us to have a wide spectrum of detectable milk inhabiting microorganisms.
We created a database
with our results and compared them against the results of the plated fresh
samples, and the same samples preincubated at 68°F for 16 hrs. Finally,
the milk was flavored over a period of time until it was determined to
be no longer acceptable for consumption.
We will issue a second report in 48hrs stating the amount of psychrophilic, pseudomonas, coliform, and mesophilic bacteria in the milk sample, with a prediction, in number of days, with which the milk will be in good condition if stored at 45°F. Also, the number of days of shelf life if the milk is kept between 50° and 55°F will be supplied.
During our study we found a type of bacteria capable of growing at 68°F. We identified this as Bacillus megaterium (mesophilic). As its presence occurred frequently we studied it and determined that even though it is capable of growing at room temperatures it is not capable of growing below 45°F, however it grows rapidly if the milk is abused.
As the milk is a biological product, and there are millions of different microorganisms capable of growing and making changes in the milk, we will continue our study. In the future we will be able to provide more information to help production managers make better decisions regarding the shelf life of their products.