by Marina Komarovsky
5-year-old daughter: Can we buy orange juice, is it healthy?
Mother: No, the orange juice is pasteurized.
5-year-old daughter: What’s “pasteurized?”
Mother: It means that they heat it and kill all the vitamins.
5-year-old daughter: [pointing at a Tropicana juice box] What’s that?
Mother: That is an orange with a straw in it to make you think that it’s fresh, but actually it’s pasteurized.
This conversation took place in the checkout line, no joke.
While pushing a toddler to renounce pasteurization may be a bit over the top, it has indeed been shown that heating foods causes denaturation, or breakdown, of their biological components. In other words, vitamins and proteins change shape and may no longer be active. There is less beneficial Vitamin C in pasteurized orange juice, for instance, than in fresh-squeezed orange juice because heating during processing causes a higher rate of breakdown over time. Researchers at Ohio State University noted that pasteurized orange juice lost nearly 20 percent of its vitamin C content, in comparison with less than 10 percent for unpasteurized orange juice, after ten days of refrigerated storage.
But what about milk? Conventionally processed milk undergoes one of several treatments. To comply with standards set by the Food and Drug Administration’s Pasteurized Milk Ordinance, milk must either be heated to 161°F for 15 seconds or to 145°F for 30 minutes, slightly reducing temperature but greatly increasing heating time. The first method, termed HTST (high temperature short time pasteurization), is most common. An alternative method which allows milk to be stored without refrigeration before it is opened involves an “ultra high temperature” of 280°F for just 1-2 seconds. While this is rare in the U.S., you may have seen unrefrigerated 32 oz. cardboard boxes of milk on the shelves in Europe or Latin America.
Like orange juice, milk does contain vitamin C, which may be partially deactivated by pasteurization. However we don’t really view milk as a major vitamin C contributor in our diets. Instead, we go to citrus fruits, kiwi, leafy greens, sweet potatoes … if we don’t get vitamin C from milk, no big deal. When milk fat is removed or reduced, there is a requirement in the U.S. that milk be fortified with Vitamins A and D. These vitamins are actually added before pasteurization and susceptible to its potentially adverse effects. While vitamin A may be damaged by pasteurization, vitamin D – the one that is hard to come by especially in the winter months – seems to be relatively stable.
But vitamins are not the only nutritional components that are affected. One of the principal values of milk is protein. Milk is a particularly important protein source for those who do not have high quantities of meat in their diets, such as vegetarians and young children. In a recent study in France, scientists compared the absorption and retention of protein from raw, pasteurized, and “ultra high temperature” treated milk. The amino acids that compose proteins are made up of carbon, oxygen, hydrogen, and nitrogen. In order to conduct this study, researchers replaced the nitrogen in the milk to be used with the 15N isotope, a heavier version of the element.
Because the goal was to “follow” the very proteins contained in the milk through digestion, absorption, and excretion, using 15N would allow the milk proteins to be distinguished from those already present in the body. Twenty-five study participants were randomly assigned to consume the three different types of milk. Results showed that while the 15N was equally absorbed and incorporated in all three groups, 26% of nitrogen from milk that underwent ultra high temperature treatment was lost to urea, as compared to a lower 18% nitrogen lost in normally pasteurized milk as well as raw milk, eight hours after consumption. This study suggests that with respect to protein benefit, ultra high temperature treatment may be too much, while normal pasteurization may actually be okay.
Of course, there are issues with other components of milk. Raw milk advocates point out that beneficial bacteria are killed by pasteurization, that useful enzymes are inactivated, and that the size of fat globules is reduced, affecting fat uptake and metabolism. While much of the evidence zooms in to the molecular level, epidemiological research has also yielded interesting results. In a multi-center study of nearly 15,000 children in Europe, it was found that those who consumed raw versus pasteurized milk were 25% less likely to experience asthma and were at reduced risk for allergies. However it remains unclear whether this effect was produced by the beneficial compounds in raw milk or whether it was alternately a manifestation of the hygiene hypothesis, the proposal that the necessity to fight off potentially harmful bacteria (as in raw milk) trains the immune system.
As usual, the research agenda is influenced by public health concerns, corporate politics, and economics, but more research on different milk processing methods would be useful. It would help both policymakers and consumers to effectively weigh the risks and benefits of less processed options and to create solutions to optimize safety and nutrition.