Fermentation Explained

When the ancients discovered how to make wine, probably around 6,000 B.C. somewhere in the Caucasus region of modern Georgia, they must have been in awe of the liquid that the English wine writer Hugh Johnson says made them feel like “gods…Their anxieties disappeared, their fears receded [and] lovers became more loving when they drank the magic juice.”

Most likely they attributed the magic of wine to their own local gods. They hardly could have understood that wine was the result of an entirely natural, earthly process we now call “fermentation” [derived from a Latin word meaning “to boil”].

It was the celebrated French chemist, Louis Pasteur, who proved, in the 1850s and 1860s, that fermentation was caused by living organisms. When the grape juice was exposed to these organisms, it underwent a chemical transformation in which the sugar in the juice turned into alcohol. Because Pasteur was struck by the fact that a living organism created wine, as opposed to the process of “spontaneous generation” people had supposed responsible for wine, he referred to the Latin phrase “omne vivum ex vivo,” or “all life is from life.” This also is known as the “germ theory of fermentation.” “Although this view had been previously surmised, it remained for Pasteur to bring this truth to its full realization,” in the words of an historian of science.

We now know that the organism responsible for fermentation is yeast. Technically, yeast is a fungus, like mushrooms, algae and lichens. There are many types of yeast; the type most commonly found in the fermentation of wine is called “Saccharomyces,” or sugar fungus. Commercial yeasts of great purity are available to winemakers, but some prefer to allow “natural,” “wild” or “native” yeasts (found in the air and on grape skins) to spark the fermentation process—although they probably have a bag of the manufactured stuff in the backroom “just in case,” for sometimes, if the yeast doesn’t work properly, the fermentation will become “stuck.”

The “pathway” by which grape sugar becomes ethanol—the alcohol of wine—is complex, but in general, it can be said that the yeasts digest the sugars (mainly glucose and fructose) and the result is the production of alcohol. As the alcohol level on the fermenting wines rises, the yeasts die off. Dead yeast cells, called “lees,” in wine sound unpleasant—but many wines, such as Chardonnay, benefit from “resting” on them, with the winemaker occasionally stirring up the mixture of lees and wines, in a process called “batonnage.” (Of course, prior to bottling, the wine is usually filtered clean.) Wine itself is a mixture of alcohol, water and hundreds if not thousands of other compounds.

Fermentation is central to some of humanity’s finest foodstuffs: wine, beer, bread, soy sauce and cheese. So next time you enjoy a great glass of wine with some fresh-baked bread and a hunk of cheese, give thanks to our tiny neighbors, the yeasts of the world!

Steve Heimoff is one of America’s most respected and well-known wine writers. The former West Coast Editor for Wine Enthusiast Magazine and a contributor to Wine Spectator, he has also authored two books on the subject of California wine, including “New Classic Winemakers of California: Conversations with Steve Heimoff,” published in the fall of 2007.