Penn State Study Finds Calf Milk Pasteurization Effective, But Variable

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We recently studied calf milk pasteurization on six farms in central Pennsylvania to measure bacteria counts in milk before and after pasteurization and in milk fed to calves. Results of the study were published in the November 2010 issue of the Journal of Dairy Science

Farms in the study milked 500 to 2,000 cows and had each been feeding pasteurized waste milk for more than a year. Two farms used high-temperature, short-time (HTST) pasteurization, where milk is heated to 161°F for 15 seconds. Two other farms used low-temperature, long-time or batch pasteurization, where milk is heated to 145°F for 30 minutes. The final 2 farms employed commercial heating coils and a homemade batch container. None of the farms routinely monitored performance of their pasteurizer. Milk samples were taken before and after pasteurization (147 samples each) and from one bucket, randomly selected from the calves being fed during that particular feeding, before the calf had access to the bucket (129 samples).

The table presents suggested goals for various bacteria counts in milk fed to calves and compares these goals to samples taken during this study. It is important to realize that “safe” levels of bacteria in calf milk are not truly known. The amount of exposure to bacteria that calves can withstand depends a lot on their general health and nutritional status, the presence of environmental stress, as well as the particular bacteria involved and whether there are other infections present. However, we do know that more exposure to pathogens increases the risk of disease, and it is prudent to monitor and limit bacterial contamination of milk fed to calves.

Bacteria counts varied a lot between farms and from one day to the next on the same farm. Although we did not specifically measure them, potential sources of bacteria in milk fed to calves can include shedding from infected cows, inadequate cleaning of cows before milking, and contamination from poorly cleaned or worn milking equipment. In addition, failure to cool milk rapidly after collection can allow bacteria to multiply, and storage of milk in open containers can enable contamination from the environment.

In general, pasteurization was very effective at reducing the number of bacteria in milk. For all of the bacteria counts we measured, more samples met the goal after pasteurization than before, and all counts were lower after pasteurization. Pasteurized milk contained acceptable numbers of bacteria in more than 90% of samples. High levels of bacteria in pasteurized milk primarily stem from failure to heat milk to the proper temperature or hold it at that temperature long enough, which could be due to operator error, system design, or poor cleaning of the machine.

The proportion of samples that met bacteria goals dropped between pasteurization and feeding time, and the amount of all types of bacteria was higher in samples pulled from calf buckets than in pasteurized milk. In the case of environmental staphylococci, bacteria counts in milk fed to calves were as high as those in raw milk before pasteurization.

These results indicate that milk was frequently contaminated between pasteurization and feeding on the farms in this study. When pasteurization is incomplete, bacteria that survive can grow rapidly in warm milk. Other potential causes of high bacteria count in milk fed to calves are poor cleaning of the pasteurizer, prolonged time between pasteurization and feeding that allows bacteria to multiply, lack of refrigerated storage between pasteurization and feeding, uncovered storage of milk between pasteurization and feeding, poor or infrequent cleaning of feeding equipment, and scratched or hard to clean feeding equipment. None of these aspects were specifically measured in this study.

If bacteria counts are reduced by pasteurization, but rise to pre-pasteurization levels before feeding, the costs and labor of pasteurization will be more difficult to justify. The presence of coliforms in milk fed to calves suggests fecal contamination, which could allow re-introduction of pathogens to milk between pasteurization and feeding.

Some of the factors related to increases in bacteria counts observed in this study are unique to pasteurized milk systems. However, many of the factors affecting bacteria counts in milk fed to calves, such as feeding equipment cleanliness, apply equally to other liquid feeding systems. We are not aware of any studies of bacteria count changes between mixing and feeding in milk replacer systems, but it is possible that similar patterns could exist.

Source: Dr. Jud Heinrichs, Professor of Dairy and Animal Science, and Coleen Jones, Research Associate, Penn State Department of Dairy and Animal Science



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