Bacteria resistant to pasteurization may persist in microfiltered milk

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A new filtration process aimed at extending the shelf life of milk can cause pasteurization-resistant microbacteria to pass into liquid milk if the equipment is not properly cleaned in time, Cornell scientists have found.

Microfiltration – a processing technology that extends shelf life by using semi-permeable membranes to prevent the entry of unwanted microbes – is now used in Europe and soon in American dairies. But without proper cleaning of the equipment early in the process, a tiny microbe called microbacteria can end up in the milk, researchers found.

The results were published September 8 in the Journal of Dairy Science.

“Our work demonstrates the importance of cleaning milk processing equipment before the pasteurization process,” said Nicole H. Martin ’06, MS ’11, Ph.D. ’18, assistant research professor of microbiology of dairy products in the Department of Food Sciences, College of Agriculture and Life Sciences. She is associate director of Cornell’s Milk Quality Improvement Program.

“Fluid milk processors often rely on the pasteurization process to apply the final stage of destroying organisms,” Martin said, “but we show that to achieve a longer shelf life with this new technology, Processors must thoroughly clean inlet equipment for raw milk well before pasteurization. In other words, they must do everything possible to eliminate these microbes before processing.”

Conventional plain dairy products now have a refrigerator shelf life of 14 to 21 days, but adding microfiltration to the process gives grocers and consumers the ability to extend the shelf life to 60 days – and reduce the food waste.

The current technology used to extend the shelf life of liquid milk is high-temperature pasteurization, which can result in off-flavors – such as “cooked” notes that milk drinkers disdain.

Microfiltration, with membrane pores measuring 0.8 to 1.2 microns, offers a gentler alternative to high heat treatment. The latest technology uses less energy, retains the flavor of milk and also extends shelf life by eliminating bacteria via the microfiltration process.

In their research, Cornell scientists examined whole milk and skim milk processed by microfiltration, pasteurized, and then refrigerated at 3 degrees Celsius (38 degrees Fahrenheit), 6.5 degrees C (43 degrees F), and 10 degrees C ( 50 degrees F) for 63 days. The analysis showed significant differences in bacterial concentrations for microfiltered milk held at different temperatures, but no differences for milk with different fat levels.

An unexpected finding was the identification of Microbacterium as a major contributor to the bacterial population in extended shelf-life microfiltered milk, the researchers wrote, suggesting that bacterial harborage sites on the farm and before pasteurization must to be taken into account.

“As the dairy industry moves toward more long-term distribution, people want to drink dairy protein and want a high-quality product,” Martin said. “Dairies are shipping further than ever before and we want consumers to have a great experience. Extended shelf life milk offers this quality product to consumers, but we need to be aware of the barriers and overcome them.

Reference: Lott TT, Martin NH, Dumpler J, Wiedmann M, Moraru CI. The microbacteria represents an emerging microorganism of concern in microfiltered dairy products with extended shelf life. J Dairy Sci. 2023;0(0). doi:10.3168/jds.2023-23734

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