One study, done five years ago, estimated fluid milk losses accounted for more than $6 billion in food waste in the United States, in part due to premature spoilage, says Nicole Martin, a research support specialist in the Department of Food Science at Cornell University.
“Raw milk Standard Plate Count (SPC) has been recognized as one of the primary indicators of both hygienic production as well as influencers of finished product quality,” she says. The vast majority of raw milk comes into processing plants well below the Food and Drug Administration’s (FDA) limit of 100,000 cfu/mL, with an average SPC of 42,000 cells/mL.
But Martin notes “under certain conditions such as lack of proper cooling or prolonged storage time, psychrotolerant bacteria, such as Pseudomonas, may grow to levels whereby they express heat-stable enzymes.” It’s these enzymes, and not the bacteria themselves (which are killed by pasteurization), which can cause product deterioration.
A bigger problem are sporeforming bacteria. “Typically, spores are found at low levels (less than 10 cfu/mL) in raw milk,” says Martin. “Yet because of their resistance to environmental stressors such as high temperature, desiccation, sanitizers and other pressures, spores are able to survive processing hurdles that are used in the dairy industry, and then subsequently grow in processed finished product.”
These spore formers grow in refrigerated fluid milk, and they are responsible for about 50% of fluid milk products that reach FDA’s limit of 20,000 cfu/mL during shelf life, says Martin. “It is these organisms that ultimately prevent the extension of fluid milk shelf life beyond 17 to 21 days, restricting the ability of fluid milk processors to enter new distribution channels and markets,” she says. Spore formers also can produce defects in ultra-high temperature milk, cheese and whey products.
Controlling sporeformers is more challenging than controlling somatic cell counts or other bacteria. Because sporeforming bacteria are found in soil, they are virtually everywhere in natural environments. As a result, cleanliness of bedding, cows and housing are key to the prevention of the contamination of milk. “A recent study has shown that spore levels in bulk tank raw milk can be significantly reduced through worker training on teat-end cleaning and implementing changes in towel preparation procedures,” Martin says.
Processors also play a role, she says, in maintaining fluid milk integrity. Careful attention to pasteurization temperature and time can affect bacterial growth. Storage temperatures after pasteurization are also critical. For example, fluid milk held at 39°F has significantly longer shelf life than milk held at 43°F, she says.
In the end, farmers and processors have to work together to control bacteria that affect milk quality and stability. “This comprehensive and collaborative approach will ultimately result in higher quality products for consumers,” she says.