Spontaneous fermentation is the fermentation that naturally occurs when the wild yeast and microorganisms that the grapes bring in with them from the vineyard are encouraged to propagate. In days of old, this was the only type of fermentation that was known.

However, in this day and age grape juice is often clarified several times before adding any of several cultured yeast, which are available on the market. Cultured yeast fermentation generally runs its course without causing any problems. This reduces the winemaker’s risks, but it also reduces the complexity of the wines. Many wine connoisseurs are now raving about the effects that spontaneous fermentation and terrior can have on wine. Does wild yeast fermentation really make the wines richer and more elaborate? Or is it just a matter of personal preference, or is it even charlatanism? For quite some time now committed winegrowers have been coaxing the terrior out of their wines. This means they concentrate on the individuality of each wine, drawing on the influences of the vineyard microclimate, the top soil, the minerals deep under the ground’s surface, the moisture content and slope angle of the vineyard, and so on. This has prompted curious winegrowers to give the wild yeasts, which are brought in naturally on the grapes from the vineyards, the chance to also express themselves in the finished wines. But the experts’ opinions differ greatly.

Looking at it more closely, the fermenting of sugars into alcohol is a rather complicated process. After all, yeast, the heart and soul of fermentation, are living beings. There is always the potential of a surprise in store. Wild yeast fermentation has its advantages, but also more surprises, so it can be riskier. To go into depth about the science of fermentation would be taking things a bit too far, but there is one important fact which should be mentioned: there are many types of yeast and other microorganisms which produce a multitude of substances and aromas other than alcohol. In technical terms these are referred to as by-products of fermentation. Winemakers, and wine drinkers, are thrilled to encounter some of these by-products, but by all means not all of them are praiseworthy. Acetic acid, a volatile acid which imparts a vinegary flavor on the wine, is particularly undesirable, as are many other flaws in flavor and aroma.

All of these microorganisms are omnipresent in nature, come in from the fields on the grapes and then go on into the juice and the barrels in the cellar. Among these, and unseen by the naked eye, is an abundance of yeast cells, which in turn influence the style of wine any winery creates. So, left to its own resources, grape juice will automatically begin to ferment. For centuries winemaking was done just so. Today around 700 different types of yeasts, with 5000 different origins, have been identified. The most important and most desirable of these yeasts, one which yields fruity wines, is called Saccharomyces cerevisiae. In turn, there are many different varieties of this yeast strain, some for wines, others for beer and also for baking. Recent studies show that over 16 types of yeast can be tracked into the cellar from the vineyards, and it makes no difference where the vineyard is located. The unfortunate news is that only 3% of these wild yeasts are Saccharomyces cerevisiae, whereas 50 to 90% are of the undesirable Hanseniaspoara uvarum type. The latter of these yeast strains is the one which causes the majority of flavor and aroma taints, such as acetic acid.

So how can the winemaker ensure that the wine will be good? The answer is, yeast reproduce very rapidly and quickly claim all of the sugars in the juice for themselves. Yeast use oxygen but can also continue to live without air, using the fermentation process to survive. Yeast break down the sugars, eventually converting them into alcohol and CO2. In terms of winemaking, the “good” yeast can survive much better under lower oxygen conditions and have fewer difficulties with the accumulating alcohol than the “bad” yeast. When the winemaker provides a low oxygen atmosphere at the start of fermentation, the sought-after yeast will soon win out over the undesirable ones. For even greater insurance that the desired yeast will win out over the flaw-producing varieties, it has made sense to clear the juice before fermentation and then add cultured yeast. A wide variety of cultured yeast has been available for over 30 years and can be selected according to what properties are wished for in a particular wine. This practice of adding cultured yeast before fermentation begins ensures a controlled fermentation process and, in turn, clean, fruity, pleasant wines which can be very enjoyable when they are young. It’s easy to understand why this method is still practiced so often today. However, these wines also seem to show less individuality and seem to age faster. This observation has led to a new and lively discussion about spontaneous fermentation, the positive effects on flavor and on the ageing potential of wines. Some of these considerations have long since been left by the wayside in winemaking.

For everyday enjoyment, certainly there’s nothing to be said against fresh and fruity wines which have acquired their brightness through cultured yeast fermentation. However, white wines of this type should be enjoyed before the next vintage comes around. When an occasion that asks for something special arises, then let a spontaneously fermented wine unfold its terrior and complexities in your glass. It’s such a shame that, with the exception of some very expensive red wines, the potential for richness in wines has almost been forgotten.