The Lingering Heat over Pasteurized Milk

All the fuss, as well as expense, did not go unnoticed by critics, who argued that the city had neither the money nor the infrastructure to institute mandatory pasteurization, as Straus and Jacobi advocated. Many complained that pasteurized milk featured an unpleasant “cooked” taste and grumbled over the diminishment of its “nutritive qualities.” Others contended that the real menace was lurking in dirty farms and distribution chains and urged a wholesale tightening of inspections. Still others accused the press of grossly exaggerating how often diphtheria and tuberculosis were really introduced into a household by the milkman.

These complaints bear an uncanny resemblance to those still voiced today: pasteurization may kill harmful germs, but it also destroys the properties of milk that resist them. The natural souring of raw milk by harmless bacteria is an important warning about its advancing age. And contamination can easily occur after pasteurization, creating a false sense of security and reducing the attention to cleanliness, rendering the milk all the more dangerous. Most residents agreed that pasteurized milk should be made available to poor children, for safety’s sake, and that excessive sterilization stripped milk of its nutritional value. But by the time The Outlook posed its milk-cooking question in 1907, little other common ground remained.

Nonetheless, the pasteurization movement was gaining steam. In 1909 Chicago became the first American city to enforce a compulsory milk pasteurization law, despite strong opposition at the state level. After vehement back-and-forth editorials, prolonged political maneuvering, and a typhoid epidemic blamed on raw milk, New York’s commissioner of health followed suit in 1914 with the enforcement of a previously adopted ordinance.

Seven years later the city’s infant mortality rate dropped to 71 deaths per every 1,000 births—less than one-third of the rate in 1891. Arguably, other improvements in health and hygiene also played a role in the decline. But the modernization of milk production ultimately settled the matter in favor of pasteurization. By 1938 bulk tanks on farms had begun replacing the ubiquitous milk cans. Pooling milk from multiple sources was becoming increasingly routine, a necessary step to keep pace with rapidly growing demand. But it also greatly increased the risk of contamination and furthered the call—and need—for pasteurization. By the end of 2007 only 4 states still permitted raw milk in their grocery coolers, though another 24 allowed on-the-farm sales.

Today milk-borne diseases are rarely fatal in most developed countries, though many remain on watch lists. Ailments like Brainerd diarrhea and Crohn’s disease have been tentatively linked to raw-milk consumption. Regarding the continuing risk from bacteria in cow’s milk contaminated by urine and especially feces, Cornell University’s Kathryn Boor notes dryly, “What we haven’t been able to do is to get cows to stop defecating.”

The ongoing pasteurization debate, often framed in terms of David versus Goliath, has pitted the Centers for Disease Control and Prevention, the U. S. Food and Drug Administration, the dairy industry, public-health officials, and most scientists against a looser grassroots alliance of farmers, organic-food enthusiasts, and non-profit advocacy organizations like the Weston A. Price Foundation. At its core the battle is all about weighing the potential of heat to kill disease-causing germs against its potential to kill milk’s health benefits and unique taste. Despite their differences, both sides largely agree that pasteurization is no substitute for clean, high-quality milk and that pooling milk from many sources requires heating to thwart the higher risk of contamination. From a 1997 survey of bulk milk tanks in Ontario, researchers calculated a one-in-three chance of finding pathogenic bacteria in raw milk randomly pooled from 10 farms. Nearly 3% of 1,720 samples tested were contaminated with Listeria. A smaller 2006 study found similarly high pathogen counts in bulk milk tanks from Pennsylvania. “I can understand how some people would understand it’s possible to make pathogen-free milk on a very small and very clean farm,” says H. Douglas Goff, a professor of food science at Ontario’s University of Guelph. “But scaling that up, that’s where it falls apart.”

The long-running standard of pasteurizing milk at 145 degrees or more for 30 minutes was set in 1957 to thwart its most heat-resistant pathogen, a highly infectious microbe named Coxiella burnetii that causes Q fever in humans. The dairy industry agreed to tweak the pasteurization process again when, in the aftermath of the 2001 anthrax attacks, a study envisioned the ease with which bioterrorists could taint a portion of the nation’s milk supply with the deadly botulinum toxin. Nonetheless, researchers concede that pasteurization may not kill all pathogenic microorganisms or fully inactivate every toxin.

The question of exactly what does—or does not—happen within milk as the temperature rises continues to confound. One chemical process often associated with heating, known as the Maillard reaction, imparts both the caramelized flavor of crème brûlée and the less desirable “cooked” taste of overheated milk. Also blamed for the decreased availability of the essential amino acid lysine, many of the reaction’s numerous compounds and intermediates have eluded chemists, especially within its latter stages. Some researchers are now investigating whether cysteine hydrochloride and green-tea extracts may block some of those flavor-altering steps, of particular relevance to Ultra-High-Temperature (UHT) pasteurized milk that has been heated to between 275 and 302 degrees for 4 to 15 seconds.

Even during regular pasteurization, milk can lose about 20 percent of its vitamin C content, with lesser damage to other nutrients like thymine, vitamin B12, and lysine. Even so, pasteurization supporters contend that the overall effect on nutrition is minimal—a position strongly challenged by opponents.

But what about milk’s countless enzymes? Lactoferrin—also present in human milk, tears, and saliva and sold as an antioxidant—possesses natural antimicrobial properties. The glycoprotein can adhere to bacterial proteins in milk and prevent them from entering human cells, but it doesn’t discriminate between benign and pathogenic microbes. Scientists also say its modest bacteria-fighting abilities can be easily overwhelmed by heat.

Another enzyme, lipase, can degrade milk fat into a particularly pungent free fatty acid—butyric acid—and turn the milk rancid. Heat inactivation of lipase then actually helps preserve milk. But the temperature found to work best for warding off rancidity is slightly above pasteurization, 170 degrees for 16 seconds—and in any case the fat in milk that has not been homogenized is protected within a membrane-bound fat globule from the lipase’s activity.

More confusion has swirled around a third enzyme called xanthine oxidase. One decades-old theory supposed that increased rates of heart disease could be linked to changes in this enzyme caused by pasteurization or homogenization. Despite a subsequent study showing that the underlying mechanism was badly flawed, the myth persists on many Web sites.

Some raw milk advocates contend that pasteurization also kills benign bacteria that aid in digestion. Many researchers believe these lactic acid–producing bacteria have largely disappeared from farms not because of heating but rather from cooling milk immediately after its collection. As a rule all milk is now chilled to 39.2 degrees Fahrenheit, regardless of whether it is pasteurized. The advent of mechanical refrigeration appears to have discouraged some traditional farm-dwelling microbes while benefiting Pseudomonas, which contributes a bitter flavor to raw milk through the production of proteolytic enzymes. Largely missing are the natural Lactococcus, Lactobacillus, and similar cultures that imparted an acidic or sour flavor to milk as it aged. “The same species of bacteria are now being added from a can instead of from the walls and floors of the old farmstead,” Goff says. And in the absence of that raw-milk modification? “Today, it will go bitter before it goes sour.”

Beyond health and nutrition, taste is the paramount feature cited by many raw-milk enthusiasts. Remarkably sensitive to external influences, milk can seem cowy or fishy, flat or metallic, weedy, putrid, or soapy. It can reek of paint or boiled cabbage or simply smell “unclean.” Each odor—and there are dozens of variants beyond the 15 identified on American Dairy Science Association scorecards—comes with its own potential explanation and accompanying molecules of blame. The 1996 second edition of Food Taints and Off-Flavors reads like a how-to manual for spoiling what some purists describe as a fresh, clean, and slightly sweet flavor.

Pasteurization itself can impart a slightly sulfurous taint, whereas higher temperatures can lead to the “cooked” flavor so scorned at the turn of the century. In milk treated by UHT pasteurization, volatile sulfide compounds can produce a temporary essence of boiled cabbage. These chemicals dissipate and a rich or heated flavor emerges after several days of refrigeration. UHT-treated milk’s stronger cooked flavor may derive from sulfur compounds, methyl ketones, and lactones, though the intensity declines rapidly when the milk is stored at room temperature.

Whether chemistry can help resolve the passionate debates and unfinished studies is anyone’s guess. For some backers of raw milk, or as they prefer to call it, real milk, the best science means leaving milk alone. “It’s intact,” says Rachel Lapp Kellogg, owner of the Lapp Farm Dairy in Cassadaga, New York. “It isn’t taken apart and put back together before human beings eat it.”

For Further Reading

Cornell University, Milk Facts. www.milkfacts.info.

Debré, Patrice. Translated by Elborg Forster. Louis Pasteur. Baltimore: Johns Hopkins University Press, 1998.

Saxby, M. J., editor. Food Taints and Off-Flavors, 2nd edition. New York: Springer, 1995.

University of Guelph, Dairy Chemistry and Physics. www.foodsci.uoguelph.ca/dairyedu/chem.html.

Weston A. Price Foundation, Campaign for Real Milk. www.realmilk.com.

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