Pasteurization Reduces Risk of Disease Transfer to Calves

If you are feeding waste milk or bulk tank milk to calves, pasteurization is a process you should consider. Milk is a perfect medium for bacterial growth, and many pathogens from cows and from the environment can be transferred to calves drinking raw (not pasteurized) milk. Some organisms that can be spread through milk include Mycobacterium avium subspecies paratuberculosis (MAP), Salmonella species, Mycoplasma species, Escherichia coli, Mannheimia haemolytica, Pasteurella species, Staphylococcus species, Enterobacter species, Mycobacterium bovis, BVD virus, and bovine leukosis virus. Less common pathogens such as Listeria monocytogenes and Camphylobacter species also can be transmitted.

As you would expect, waste milk is more likely than other calf feeds to contain large populations of pathogenic organisms. The most obvious reason for higher microbial loads in waste milk is that cows contributing the milk are either recently fresh or sick and therefore very likely to be shedding large numbers of any organisms they are carrying. However, there are several other important factors that directly affect the number of organisms in waste milk. Clean and dry the udder and teats before attaching the milking unit. Bucket milkers and other containers used to collect, store, and feed milk can serve as a reservoir of bacteria if they are not properly cleaned and sanitized after every use. Clean this equipment with the same care as equipment used for milking healthy cows. Additionally, once milk is collected it must be fed or cooled immediately. The longer milk sits around at ambient temperature, the more opportunity bacteria have to multiply. Collect and store milk in closed containers to prevent contamination from manure, flies, and other environmental sources.

Pasteurization can be used to reduce the number of infectious organisms in waste milk before feeding to calves. It is important to understand that pasteurization is not sterilization and that waste milk with a high microbial load before pasteurization will still contain a large number of organisms after pasteurization. There are basically two types of milk pasteurization: HTST and batch. High temperature, short time, or HTST, pasteurization is a continuous flow system; milk is directed through a long, coiled tube, heated to 161°F, and kept at that temperature for 15 seconds. Batch pasteurization requires heating a vat of milk to 145°F and holding it there for 30 minutes. Research shows that batch pasteurization is more effective when the milk is agitated during the process. This has led to the development of “turbulent batch” pasteurizers, which circulate milk during processing. Both types of pasteurizers are available in various sizes, depending on the amount of milk to be pasteurized.

Controlled research designed to test the effectiveness of pasteurization on specific bacteria has shown that both batch and HTST systems destroy common pathogens, including MAP, Salmonella species, Mycoplasma species, E. coli, and Staphylococcus aureus. This research was conducted with commercially available pasteurizers designed for milk fed to calves. Surveys of pasteurizers in use on dairy farms have also shown that pasteurization reduces the number of bacteria in milk. A recent survey of 31 Wisconsin farms showed that milk samples contained and average of 10 million cfu/mL of E. coli before pasteurization. After pasteurization, the count was reduced to 21,000 cfu/mL. Averages can be deceptive though; the range of E. coli in the raw milk samples was from less than 10,000 cfu/mL to 80 million cfu/mL. In pasteurized milk the range was from less than 10,000 cfu/mL to 330,000 cfu/mL. These data reinforce that if milk is highly contaminated before pasteurization, it will probably still be contaminated after pasteurization. Another survey found similar results on farms in North Carolina. In that study, milk had an average standard plate count of 17.3 million cfu/mL before pasteurization. After pasteurization, the count dropped to 450,000 cfu/mL. We'd like to see a standard plate count of 20,000 cfu/mL or less in milk fed to calves. This shows that improper waste milk handling before pasteurizing can lead to a poor quality product after pasteurization. Again, pasteurization is not making a sterile product at the end.

Another practical issue highlighted by these two studies is that pasteurization was not always effective on the farm. Researchers measured effectiveness of pasteurization by the presence of alkaline phosphatase in milk. If alkaline phosphatase was present, pasteurization was considered incomplete. In the Wisconsin study 13 percent of the farms had incomplete pasteurization. On 2 of the 3 North Carolina farms studied, incomplete pasteurization occurred in 18 and 15 percent of milk samples. Not surprisingly, bacteria counts in this milk were quite high in both studies. Pasteurization fails if milk is not heated to the correct temperature or if it is not held at that temperature long enough. Rushing the process is a common cause of this failure.

Inadequate cleaning and resulting build-up are another possible cause of pasteurization failure and a potential reservoir of bacteria. Cleaning procedures from the manufacturer must be strictly followed to maintain the equipment and prevent this problem. Training of all employees who use the equipment can help them understand the process and the importance of following pasteurization and cleaning procedures exactly. To ensure that a pasteurizer is doing the job you expect it to do, some regular monitoring is needed. Record the temperature and holding time for every batch of milk, and periodically test bacteria counts in milk before and after pasteurization. The alkaline phosphatase test also can be used to check your procedures.

Remember, warm milk is a perfect medium for bacterial growth. After pasteurization milk must again be handled with care; either feed it right away or cool it for storage. Storage and feeding equipment need to be cleaned and sanitized after every use. Also, be aware that feeding methods can affect bacterial growth. If milk must sit at ambient temperature while being fed to a large number of calves, bacteria can multiply rapidly between the first and last calf. This is more likely to happen in summer months.

In summary, pasteurization can be extremely helpful in reducing the microbial load of waste milk and reducing the risk of disease transfer from cows to calves. However, it does not sterilize milk and it cannot overcome poor sanitation before or after pasteurization. Careful attention to heating and cleaning protocols and to handling and storage of milk are essential for success.

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Coleen Jones, Dairy and Animal Science Research Associate, and Jud Heinrichs, Professor, Dairy and Animal Science Extension

Department of Dairy and Animal Science