Successful operation of on-farm pasteurizers

Bob James, Department of Dairy Science, Virginia Tech University

Two items are measures of success, one is minimizing bacterial growth and the other is preserving the nutritional quality of milk. This article will focus on critical control point areas to minimize bacterial growth. Many of these items involve a great deal of common sense but are sometimes overlooked.

Milking of treated or fresh cows

Are fresh cows milked in a hospital string with the same facilities as the milking string, or a separate facility?  Milking practices and sanitation must be identical to those used for the milking string, or bacterial growth can accelerate to levels beyond which pasteurization is   effective.  Generally, there are greater challenges when milking treated or fresh cows into “pot” milkers. This equipment must be hand-rinsed with lukewarm water, washed with hot (120oF) detergent water, then treated with an acid sanitizer, allowed to drain, and dry completely before the next milking.  

Transfer of USM (unsaleable milk) to appropriate storage containers prior to and after pasteurization

How is USM moved from the udder to pre-pasteurization storage, through the pasteurizer and then to a post-pasteurization tank? What practices are in place for cleaning hoses, valves, gaskets and nozzles which are potential sources of contamination? Ideally, milk moves to the pasteurizer as soon as possible after harvest.


How is milk stored?  Is refrigeration adequate, and are these vessels adequately cleaned and sanitized?  One problem commonly found in our field studies with pasteurizers was the slow cooling or no cooling of milk after pasteurization and feeding.  In some instances, bacteria counts were higher after pasteurization than before.  Grade A PMO states that milk should be cooled to <40oF after completion of milking.

Pasteurization and feeding to calves

Is the pasteurizer operating as desired?  There are several ways to measure this, but the best is probably to culture the post-pasteurization milk for standard plate count. Our field studies in North Carolina and California found that well-managed systems achieved the desired reduction of bacteria counts to <20,000 cfu/mL.  Batch pasteurizers need to reach 145oF and hold it for 30 minutes.  HTST pasteurizers need to reach 161oF for 15 seconds. In one field study we measured bacteria counts 5 days weekly  on USM immediately after treatment with an HTST pasteurizer and found average counts of >400,000 cfu/mL.  Counts ranged from <20,000 to well over 10,000,000 cfu/mL.  Herds we have worked with have utilized testing laboratories operated by milk processors, cooperatives or state department of agriculture labs to provide bacterial count analysis.

When to test USM?  

Ideally, sample and test milk immediately after pasteurization and after the last calf is fed. The first test will indicate if the pasteurizer is operating correctly. The second measure will be a reflection of sanitation at multiple points in the process.  A pasteurizer represents a significant investment in equipment and labor. Develop protocols for:

·         Sanitation of all surfaces in contact with USM.

·         Routines where USM is pasteurized and fed as soon as possible. This may require an adjustment in labor management of the calf feeding operation, but it may reduce energy costs and need for refrigerated storage.

·         Test pasteurized USM routinely.  The frequency depends upon one’s risk tolerance.   Certainly weekly testing is not too frequent, given the likelihood that something may prevent successful reduction of bacterial counts.

To read more of James’ comments on his pasteurized waste milk blog, visit