The Lingering Heat over Pasteurized Milk

milk bottles

Filling milk bottles, Briarcliff Farms, New York

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.”