Choices, choices, choices… Like all things in life winemakers also have a number of choices when producing wine. One of these is if yeast should be added or not when making wine.
Yeast plays a very important role in our lives. Yeast are responsible for the transformation of sugar in grape juice (consisting of glucose and fructose) into ethanol, carbon dioxide and heat. People have been using these little metabolic factories to produce beer, wine and bread for thousands of years.
When one picks grapes, squeeze the juice out and leave it be one will end up with wine eventually (or different versions of it). This is due to a number of different types of yeasts that occur naturally on grapes and potentially contributing to the alcoholic fermentation. In our department we have found that the diversity of yeast are much higher on grapes grown organically than on those produced conventionally. This might be due to the pesticides used on the latter, to prevent diseases from destroying the crop, which might also influence the yeast population.
The Latin name of the yeast that completes alcoholic fermentation is Saccharomyces cerevisiae. However, their number on grapes are normally quite low and other yeast strains such as Kloeckera, Hanseniaspora, Candida and a number of other yeast types normally occur on the grapes at higher numbers. During alcoholic fermentation these so called non-Saccharomyces yeasts starts the alcoholic fermentation if no yeast is added by the winemaker. However, their numbers start to decrease as the alcohol level increases and after the middle of fermentation Saccharomyces cerevisiae normally dominates and eventually complete the alcoholic fermentation. The reason for this is that the latter is more alcohol tolerant. I suppose the same happens in people, the more you drink, the more alcohol tolerant you become…
Non-Saccharomyces yeast under the microscope
In any case, winemakers nowadays have the advantage of buying the Saccharomyces cerevisiae and adding it to their grape juice or must. The reason for doing this is that the fermentation is then much more controlled, as a large number of a known yeast strain is added, that can complete the fermentation effectively. These commercially available yeast strains have also been selected to not produce off flavours in large quantities. When a natural fermentation is conducted the winemaker does not know which type of yeast occurs on the grapes. A wonderful wine can thus be made, as more yeast strains contribute to the complexity of the wine. However, a salad dressing can also be made, as some of these non-Saccharomyces yeast can produce off aromas very quickly, such as acetic acid, or does not complete the fermentation effectively. A natural fermentation is therefore a bit more risky than a so called inoculated fermentation, where a commercial yeast is added.
Small scale white wine fermentations where yeast was added
Research has also shown that natural fermentations are are also not always that “natural”, as it has been found, unbeknown to the winemaker, that during some natural fermentations the fermentation is sometimes completed by a commercial yeast strain, even if not added to the must. This is due to the fact that yeast floats around us and a commercial strain from an inoculated tank could end up in a naturally fermented tank. One should also keep in mind that most commercially available yeast strains have initially been selected in a vineyard, tested and propagated, so they were not bred in a laboratory.
A winemaker must thus decide on an inoculated or natural fermentation, like a number of decisions in the process. This makes wine making not always easy, but definitely interesting!