Food Scientist Dr. Stephanie Clark On Dairy Processing

Recently, Northwest Public Radio aired a profile about a family dairy as part of a series on the Northwest economy.It resulted in a flurry of comments from representatives of the dairy industry, and it spurred questions about the practices of pasteurization and homogenization. Mary Hawkins took the opportunity to speak with leading dairy expert, Stephanie Clark.Formerly with Washington State University, Dr. Clark is now an associate professor at Iowa State University, specializing in food science with a focus on dairy foods. She shared her views on common perceptions and misperceptions about the dairy industry and the processing of milk.

Mary Hawkins: The dairy industry is the second most regulated food industry after seafood, and one of the most prolific.Milk and milk products are consumed by millions of people in the US – from the very young to the very old.People get nine essential nutrients from milk: calcium, potassium, phosphorous, protein, vitamins, A, D and B12, riboflavin and niacin.

According to Dr. Stephanie Clark, the processes of pasteurization and homogenization do not significantly diminish these nutrients. She says in the case of protein, conventional pasteurization (heating milk to 161 degrees for 15 seconds) may actually improve the digestibility of milk. During this process, proteins are partly “denatured” or “unraveled” when they are heated…

Dr. Stephanie Clark: So, pasteurization can actually enhance the digestibility of protein, whereas some people have feared, wrongly, that it reduces the digestibility.

Hawkins: Clark says minerals in milk are stable when heated, so they aren’t affected by pasteurization. Vitamin C is heat-sensitive so it’s degraded, but milk is not considered a source of C anyway…

I asked Dr. Clark about a conjecture that pasteurized milk is sterile or dead.

Clark: I’ve heard that terminology before as well … that the milk is dead….well, it’s not completely dead. There are some enzymes that are still functioning to make that milk go bad. Some of them are naturally a part of the milk. Some of them are bacteria that have not been killed by the pasteurization process.

Hawkins: One of the reasons milk is so highly regulated and tested is that there are many opportunities for bacteria to be introduced: in the cow’s teat canal, in the dairy environment, in dairy equipment. Some bacteria are thermal-tolerant. Some are very dangerous; for example listeria can be fatal. Other bacteria are harmless.

Ultra-pasteurization heats milk to a higher temperature, 280 degrees Fahrenheit for at least 2 seconds, and is used to kill the kind of bacteria that spoil milk. That’s why it increases shelf life.

Homogenization is a process whereby milk is forced through a tiny aperture, which makes the fat globules smaller so that they are suspended in the milk.One argument against homogenization is that it contributes to atherosclerosis. The argument is that the smaller fat globules are absorbed into the cardiovascular system, instead of digested.

It was theorized as early as the 1970s in the American Journal of Clinical Research that homogenized milk contributed to unhealthy levels of a substance called xanthine oxidase in humans. Xanthine oxidase is linked to cardiovascular disease. That theory was dispelled in an extensive review by the Federation of American Societies for Experimental Biology in the mid seventies…

Clark: Now since then, more research has also been done to follow up on that, including some research in the 1980s in the American Journal of Clinical Nutrition which concluded that absorption of dietary xanthine oxidase was not demonstrated in the research they conducted and a relationship between homogenized dairy foods and levels of xanthine oxidase activity in the blood could not be established. So, there really was found to be no direct role for xanthine oxidase in atherosclerosis.

Hawkins: Clark says we metabolize homogenized milk just as we metabolize non-homogenized milk. If we want a low fat diet, than skim milk is probably a viable option for us. If we drink whole milk, we will metabolize that milk fat similarly, no matter what size those milk globules are. Homogenized milk, milk with smaller globules that are suspended in the liquid, exists in nature.

Clark: Goat milk and sheep milk are “naturally homogenized”…that’s the term we use for those products…because they have a higher proportion of the smaller milk fat globules. And so it’s funny that people are trying to claim that homogenization is a bad thing because it makes smaller fat globules but at the same time, they want to promote goat milk and sheep milk because they are naturally-homogenized - they have smaller milk fat globules.

Hawkins: In recent years there have been many reports that link cow’s milk and increased hormone levels in people. Clark calls this another myth that needs to be dispelled. Growth hormones are sometimes injected into dairy cows to make them more efficient at converting feed into milk, and cows naturally produce hormones. She says all of the animal foods we eat also contain hormones naturally formed by those animals. But they do not impact hormone levels in people for a very simple reason:

Clark: Because the growth hormones that are naturally present in all milk are not targeting human tissues.

Hawkins: Clark says we simply pass those hormones through our systems.

The use and presence of antibiotics is another area she wants to clarify:

Clark: Antibiotics are strictly forbidden. If there are antibiotics found in the milk, and every single tank or truck is evaluated for the presence of antibiotics – If there are antibiotics found in the milk, that tank is dumped, at the expense of the farm.

Hawkins: Dr. Stephanie Clark.All dairy processors without exception must meet the standards set out by the Pasteurized Milk Ordinance. A link to that document and more information can be found online now at Our Northwest at NWPR dot org. I’m Mary Hawkins.

2007 Pasteurized Milk Ordinance

The American Journal of Clinical Nutrition review:

Homogenized bovine milk xanthine oxidase: a critique of the hypothesis relating to plasmalogen depletion and cardiovascular disease

Dairy Farmers of Washington