"In wine is found the great generalization: all life is fermentation."

-Richard Feynman

In this chapter the general subject of fermentation is addressed with particular focus on the role of yeasts. This is only appropriate as without these unicellular members of the fungal kingdom we would all be drinking grape juice instead of wine, and the world would be a poorer place indeed.

Chapter 17 - Fermentation

Fermentation and maceration

Until the discoveries of Louis Pasteur in the 19th century, the mechanisms of fermentation remained a mystery. It is not surprising that early societies held wine to be sacred. The gods must surely must have been

involved in the seemingly miraculous transformation of fruit juice into something that gladdened the heart while relieving the body of pain. In the 150 years since Pasteur’s time, there has been enormous development, both theoretically and practically, in our understanding of yeast in wine. This knowledge has led to far greater control and optimization of the fermentative process.  

The transformation of the grape sugars to alcohol through the activity of yeast is at the heart of winemaking. Yeast also works on other constituents of the juice, creating flavors during this process. In red winemaking, the heat generated by yeast activity also acts in conjunction with the alcohol to extract flavorants from the skins, seeds, and stems. I will focus on the practical application of current knowledge, ceding the vast territory of the science of yeast to those whose understanding is deeper than my own.

As in much of winemaking, it makes sense to discuss white and red wines separately—after first addressing a basic decision common to both. This is the question of whether to inoculate with selected yeast strains or to allow the wine to ferment with the ambient yeasts originating from vineyard and winery. The uninoculated ferment is often called natural or wild, but these terms falsely suggest that there is something unnatural about inoculating with selected yeast strains. I have worked with both approaches, and one is no more right or wrong than the other. Using selected strains is certainly easier, less risky, and more reliable, but that does not make it preferable per se. Purchasing selected yeasts does add an expense, but it is not prohibitive. Fermenting with ambient strains involves uncertainty and risk, while not necessarily producing a more interesting wine. The fact that ambient yeast fermentation makes for a more romantic story should only be of interest to the marketing department, not the winemaker.

Uninoculated or ambient yeast fermentations

Regardless of whether or not a selected yeast is added to wine, the fermentation will be largely conducted by, and certainly completed by, Saccharomyces. This yeast, whose name means sugar fungus, has adapted evolutionarily, to and is mostly found in high sugar environments such as fruit juices.  It is the only genera of yeast capable of surviving the very high sugars and consequent high alcohols formed by fermentation of wine grapes, which are among the sweetest fruits on earth. However, numerous other genera of yeast will grow and ferment in the early stages of a fermentation, before the alcohol rises. These include, but are not limited to, Kloeckera, Hanseniaspora, Debaryomyces, and Hansenula. They are active until the alcohol level rises to a point they cannot tolerate. Depending on the yeast, this is in the 5–10% alcohol range.