Bacteria fair poorly at lower pasteurization temperatures

Hoard's Dairyman: 

Bacteria fare poorly at lower pasteurization temperatures

Date: 
Mon, 01/09/2012

A 16° drop slows bacterial regrowth.

A temperature reduction of 16° F may not seem like a big deal. But, such a reduction, as researchers with Cornell University’s Milk Quality Improvement Program found, could reduce bacterial counts during storage. They shared their results in the January 2011 Journal of Dairy Science.

According to the 2009 Pasteurized Milk Ordinance (PMO), to achieve HTST pasteurization, milk must be heated to a minimum of 161° F for 15 seconds. The goal of pasteurization is to eliminate all nonspore-forming human pathogens associated with raw milk.

In the absence of contamination after pasteurization, the spore-forming bacteria Bacillus ssp. and Paenibacillus ssp. limit fluid milk shelf life. These organisms can enter milk at various points. While several spore-forming bacteria can survive HTST in spore form, not all can reproduce at refrigeration temperatures in fluid milk. A shift in the microbe population of pasteurized milk from Bacillus ssp. to Paenibacillus ssp. after 14 days indicates the latter is an important cause of fluid milk spoilage when contamination doesn’t occur.

In an attempt to bolster food safety, some processors raised pasteurization temperatures well above the minimum. However, some logged an elevated number of consumer complaints regarding spoiled milk.

Bacterial numbers were measured in high-temperature, short-time pasteurized fluid milk for both the 15 months prior to and after a temperature reduction from 185° F to 169° F. Milk was pasteurized for 18.25 seconds. Counts were measured in whole fat, 2 percent, and fat-free milk on the day of processing and subsequently while in refrigerated storage on days 7, 14, and 21 postprocessing.

Mean total counts on the 604 samples were significantly lower after processing as well as 21 days postprocessing in samples pasteurized at 169°. Bacterial growth was also reduced during storage when milk was pasteurized at a lower temperature.

On the day of pasteurization, none of the samples were above the 20,000 cfu/mL limit.

On days 7,14, and 21 postprocessing, 98.5, 81.3, and 46.5 percent of samples, respectively, did not exceed this limit when pasteurized at 169°. At 185°, these values were 98.3, 74.3, and 32.6 percent, respectively.

The research’s take home: Lowering the pasteurization temperature for unflavored fluid milk processed in a commercial facility significantly reduced bacterial growth during refrigerated storage. A balance between pasteurizing to ensure inactivation of pathogens and optimizing processing conditions that limit the growth of bacteria can be reached nearer to the minimum temperature specified by the PMO.

While the data came from only one commercial processing plant, the results are applicable to other plants working with a high-quality raw product.