As much pride as our world-famous wineries claim to have in their century-old establishments, the majority of the burden of wine making actually falls to a single-celled organism: yeast. In fact, much of the art of wine making is actually the art of feeding and nurturing the yeast. A famous quote from Dr. Roger Boulton from the University of California illustrates: “Ninety percent of wine making has nothing to do with the winemaker. All a winemaker is doing is preventing spoilage, introducing some unique style to the wine and bottling it.”

So what is yeast, and what is it doing? Yeast is a single-celled organism, a strain of fungus. It moves freely in the environment and when it finds nutrition it replicates quickly to form a colony. The earliest known usage of yeast was in ancient Egypt, where it was often used for bread making. In fact, anyone who has ever had to culture yeast for bread making will understand much of what it takes to be a winemaker.

There are many different kinds of yeast, and the ones generally helpful for wine making are of the genus Saccharomyces (Sakchar meaning sugar and Myces meaning fungus), of which there are over a dozen to pick from, each which adds different characteristics to wine. The yeasts of this genus consume the natural sugar found in fruit juice and transform it into almost equal parts of carbon dioxide and alcohol.And there’s the rub: without yeast, it’s just grape juice.

So then, if wine making is an art, then its primary tool is the ability to control yeast. Fresh pressed grape juice will typically already have dozens of different kinds of yeasts, as well as a number of other microbial invaders. If you left the juice as is, it would develop into wine on its own, though it would likely be over-alcoholic and undrinkable. To create a truly great wine, the winemaker must have a specific yeast in mind, introduce it at the right time, feed it and nurture it, and allowing it to die and be cleaned whilst preserving the fruits of its labor. This involves a rigorous control over temperature, humidity, and quality of fruit.

Over the many centuries since wine makers have bottled their products, they have been returning the old skins and seeds of crushed grapes (the pomace) to the vineyards to be used in fertilization. The grape skins are breeding grounds for yeast, and by returning yeast-rich pomace to the vines, vineyards have managed to subtly alter the natural yeasts that occur in their fields. Over time, the selected strain of yeast begins to dominate so rigidly that a winemaker can simply encourage spontaneous fermentation. Those making homemade wines don’t have this luxury, and even established vineyards may be centuries away from such a yeast domination.

So despite the vigilant claims of expert wine makers across the world, it’s obvious that the real master winemaker is yeast.

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