Microfiltration—an emerging processing technology that extends milk’s shelf life by using semipermeable membranes to keep out undesirable microbes—can introduce bacteria that are resistant to pasteurization into fluid milk if equipment is not cleaned properly, Cornell researchers recently found.

Microfiltration offers an alternative to high-temperature ultra-pasteurization, which is another method for extending the shelf life of milk. Although high-temperature, short-time (HTST) pasteurized milk has a substantially shorter shelf life than ultra-pasteurized milk, HTST pasteurized milk is much more popular in the U.S. because it does not have the undesirable “cooked” flavor associated with ultra-pasteurized milk.

Microfiltration reduces the microbes in raw milk using membrane pores measuring 0.8–1.2 microns. The solution requires less energy than pasteurization and does not compromise the flavor of the milk. Through the removal of bacteria, microfiltration can extend the refrigerated shelf life of fluid milk products from 14–21 days to up to 60 days. The process is already being used in Europe and is soon going to be adopted in the U.S.

While HTST is the standard final killstep in milk production, the Cornell study underlines the heightened importance of thoroughly cleaning intake equipment for raw milk long before the pasteurization process. If equipment is not cleaned adequately and early, microfiltration can pass very small, pasteurization-resistant microbacterium into fluid milk.

For the study, the researchers observed whole milk and skim milk samples that were processed using microfiltration and pasteurization, and were refrigerated at 3 °C, 6.5 °C, and 10 °C for 63 days. The amount of bacteria present in the milk samples increased with refrigeration temperature, but did not vary by fat content. The most significant finding was that of the microbacterium, which were able to permeate microfilter membranes, survive HTST pasteurization, and then grow at refrigeration temperatures.