The Lingering Heat over Pasteurized Milk

milk bottles

Filling milk bottles, Briarcliff Farms, New York

In the 16 March 1907 issue, The Outlook asked its New York City readers, “Should the city cook its milk?” Forty-five years had passed since a French chemist named Louis Pasteur tested the heating process that would eventually bear his name. More than a century later the uncertainty of this question still reverberates through farmers’ markets and online forums as public health officials and consumers battle over safety, nutrition, and taste.

Perhaps no other food has been subject to as much modification as cow’s milk. Milk contains more than 100,000 molecular species: a breathtaking chemical complexity that has led to endless caveats and countless myths. It’s little wonder that the debate over whether and how to alter “nature’s perfect food” is still a heated topic of conversation.

Has the heating process, as its critics suggest, destroyed milk’s natural benefits? Are families playing Russian roulette by spurning the bacteria-killing power of pasteurization, as some public health officials report? The sharp-edged assessments are exceptional not only for their longevity but also for the kernels of truth and hyperbole in each. More remarkable still, the process alternately lauded and demonized for its life-and-death implications germinated within a far more bookish question about the chemical properties of a common crystal.

As a graduate lab assistant at the École normale supérieure in Paris, Pasteur noticed that a crystal named tartaric acid directed polarized beams of light to the right. The organic acid was commonly found in wine barrels and grapes and sold in a milky-white crystalline form. Its alter ego, eventually named paratartaric acid, likewise had been discovered in wine vats and boasted an identical atomic weight and molecular composition. Mysteriously, it was unable to rotate light at all.

In a famous experiment made possible in part by his poorly heated lab, Pasteur used a microscope to hand-separate paratartaric acid crystals and show that they were composed of two mirror-image, asymmetrical compounds (the asymmetry would have been much harder to distinguish under warmer conditions). In solution the ones with a right-handed arrangement rotated polarized light to the right, while left-handed ones rotated light to the left. When mixed, their activities canceled each other out and polarized light appeared unaffected. The observation not only established the field of stereochemistry, but also suggested that right-handed molecules—so designated because of their effect on light—might have left-handed counterparts. Pasteur also suspected that only living organisms could produce such asymmetrical compounds.

Upon his appointment as professor of chemistry and dean of the new Faculté des Sciences at the University of Lille some years later, Pasteur embraced the emerging ideal of applying science to practical problem-solving in industry. Among his new clients in the bustling regional capital was a local distiller struggling to keep his fermented beetroot alcohol from turning sour. The chemist eventually returned to tartaric acid and its easily obtainable crystals for guidance. Soon Pasteur found that the growth of a fermenting agent effectively isolated the left-handed isomer of paratartaric acid, providing a biological basis for what was supposed to be the chemical process of fermentation. To prove that this “ferment” was a living cell and central to alcohol production, Pasteur turned to lactic acid, produced by the defective fermentation of beetroot as an alcohol by-product. In short order he had isolated lactic yeast as the agent responsible for lactic acid production and demonstrated that the yeast cells grow like plants if given the proper food—beetroot sugar, for instance.

Pasteur’s experiments helped refute the notion that fermentation was proceeded by spontaneous generation and laid the groundwork for the germ theory of disease. For Lille’s beetroot distillers the implications were much more immediate. Heating beet juice destroyed the contaminating lactic yeast, allowing it to be reseeded with the desired alcoholic yeast.

A few years later Pasteur was commissioned by Napoleon III to study diseases of wine. With the aid of a microscope, Pasteur observed parasites, fungi, and other microbes. The idea of heating wine to preserve it wasn’t strictly new, but his “pasteurization” method, which he patented and defended aggressively from critics, was the first to provide an explanation for its effectiveness. Heating wine to between 140 and 212 degrees Fahrenheit for a few minutes in the absence of air killed microorganisms that turned wine sour.

Although Pasteur was applauded for his findings, the gifted self-promoter wasn’t immune to charges of opportunism. According to one critic, “he is no more its practical inventor than a man who invents a new theory of the plough, however ingenious, would be the inventor of plowing.” Another criticism began to take hold when some distillers complained that the new heating method sullied the taste of wine and wilted its bouquet. A panel of wine experts, hastily assembled by Pasteur, settled the matter in his favor well enough that his technique soon spread to other vintners. “Pasteur is as popular with the vintners of California as the president of the United States,” one declared.