Wednesday, May 6, 2015

Sampling Protocol for Bacteria Culturing When Feeding
Pasteurized Milk to Calves

When and where to collect samples when monitoring bacteria contamination of pasteurized milk fed to dairy calves?

To answer this question I like to start with the answers that I want after I have the culture results. They are:
1. Is the dairy starting with quality non-salable raw milk?
2. How well is the pasteurizer working to reduce the bacteria count in the raw milk?
3. How much post-pasteurization contamination is taking place between the pasteurizer and the calves?
4. How much bacteria growth is taking place between the time the milk is pasteurized and the last calf is fed?

I am going to need 4 samples.

Sample #1: Collect a raw milk sample. Results under 100,000cfu/ml (colony forming units per milliliter) are good, results over 1,000,000cfu/ml mean that we are loading the pasteurized milk with a lot of "trash" from the killed bacteria and setting ourselves up for significant post-pasteurization contamination.

Sample #2: Collect a sample directly as the milk comes out of the pasteurizer. We are looking for no growth at 48 hours of incubation. If the raw milk counts are less than or equal to 100,000cfu/ml the pasteurizer is not working well if we get any growth in the pasteurized product. If the raw milk counts are greater than 1,000,000cfu/ml the pasteurized milk may have a modest number of residual bacteria. 

I like to see coliform counts in the pasteurized milk less than 100cfu/ml and total plate counts less than 1,000cfu/ml. If counts are higher than this the fault is probably not with the pasteurizer but rather with excessively high raw milk bacteria counts. Remember, pasteurization is not sterilization!

Sample #3: Collect an "as-fed" sample for the first calf fed. Get the sample as the milk goes into either a bottle or bucket in order feed the first calf. Note that this says, "goes into" and not "from." Do not take the sample from the bottle or bucket. Take the sample as the milk goes into either one. 

These culture results tell me the extent of immediate post-pasteurization contamination of the product. If the milk going into nursing bottles is coming from the source where Sample #2 is obtained, then skip this step. However, if the milk is going into nursing bottles is from any other source then I want a sample as the first bottle is filled. If the milk is going into something to carry to the calves (for example, 5-gallon pail, transport tank) then I want the sample coming out of these as it goes into calf feeding buckets.

If the calves are being fed with mob feeders, then get the sample as the gang or mob feeder is being filled. In this case, skip the fourth sample. 

Sample #4: Collect an "as-fed" sample for the last calf fed. Collect the sample the same way as for the "first-calf-fed" sample.

If the bacteria counts are higher here than the "first calf fed" sample then I can estimate the extent to which bacteria have multiplied while the calves are being fed. Or, if these sample values are lower than the "first-calf-fed" it usually is revealed that systems that include hoses and pumps have a cleaning issue there.

Note that this 4-sample-set sampling process does not capture contamination from nursing bottles, nursing nipples and feeding buckets.

If you have culture results from samples like these you would like to share, please send them to me and I will "blog" about them. Send to

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