Bacterial generation times have a significant impact on the quality of fluid milk and its shelf-life regardless of the source of microbial contamination. While many factors can lower the quality of fluid milk and reduce shelf-life, the most significant is microbial spoilage.
“As the ability to kill, remove, or control microbial growth in pasteurized refrigerated milk continues to improve, the original milk SCC (Somatic Cell Count) will be the factor limiting the time of refrigerated storage before development of an off-flavor in milk.” (Source: Influence of raw milk quality on fluid milk shelf life, D M Barbano, Y Ma, M V Santos, National Library of Medicine).
In previous blogs, I have written about two sources of contamination;
Bacterial generation times affect spoilage of pasteurized milk
Regardless of the source of the contamination, the amount is not the primary concern. It is the growth rates of specific bacteria that most significantly impact pasteurized milk quality and shelf-life.
Bacteria reproduce through a process known as binary fission; one bacterium duplicates its genetic material and becomes two bacteria. The time it takes bacteria to double in number is called the generation time. Variability in generation time is specific to different bacteria and is greatly influenced by temperature and nutrient supply.
The Pasteurized Milk Ordinance (PMO) limits the total bacterial count for Grade-A pasteurized milk to 20,000 per milliliter. However, the total bacterial count is not the whole story.
This chart illustrates how the variability of generation time impacts milk spoilage for three different types of bacteria. Using this example of milk containing bacteria:
- The longest generation time of 15 hours has a shelf-life of 25 days.
- A 12-hour generation time shortens the shelf-life of milk to 15 days.
- The shortest generation time of six hours reduces the milk’s shelf-life to twelve days.
In this illustration, the bacteria were all at the same population level (1 bacterium capable of growth at refrigeration temperatures per milliliter) on Day 5. However, the different bacterial growth rates resulted in significant variability in shelf-life.
“Effective strategies for extending fluid milk product shelf-life by controlling bacterial growth are of economic interest to the dairy industry.” (Source: Evaluation and Improvement of Raw and Pasteurized Fluid Milk Quality, Nicole Martin, Cornell University).
Impact of psychrotrophic bacteria on milk spoilage
Heat-Resistant Psychrotrophic Bacteria or Heat-Resistant Psychrotrophs (HRP) are defined as those that grow at refrigeration temperatures and are the most detrimental to milk quality. Their presence in fluid milk will result in quality defects.
These bacteria originate from cold environments. While they generally have very low contamination levels initially, they significantly impact spoilage due to their fast-generation times at refrigeration temperatures.
Monitoring and controlling microbial spoilage of fluid milk
It is critical to implement continuous process monitoring to quantify bacteria and identify bacterial types. Because their incidence is generally very low, identifying psychrotrophic bacteria in fluid milk requires large volumes of aseptic and representative samples.
Gram-negative psychrotrophic bacteria are highly dependent on oxygen for optimum growth; the use of oxygen-permeable incubation containers is a necessary identification component. Confirmation of the bacteria type should be made through a gram stain.
To learn more, watch our free on-demand webinar, Monitoring Microbial Contamination.
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QualiTru’s representative sampling system is approved for use by the Pasteurized Milk Ordinance (PMO). See Regulatory Approvals.
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