Yeast is a single-celled fungus-like organism (scientifically called Saccharomyces, meaning "sugar fungus") which ferments sugar to alcohol under deoxygenated, or anaerobic conditions. There are several types of yeast, some of which are much more suitable for winemaking than others.
To start with, brewer's or baker's yeast (Sacch. cerevisiae) should be avoided. In its fresh form, this type has an unpleasant flavor which can be imparted to a wine. In its dried form it works reasonably well during fermentation, but it consists of much smaller cells than the genuine wine yeast (Saccharomyces cerevisiae ellipsoideus) and so does not readily form a sediment when fermentation is complete. Clearing the wine can then be a problem. Genuine wine yeasts are derived from the yeasts which occur naturally on the skin of grapes as bloom. There are both general-purpose and specific varieties of wine yeast available in dried granulated form.
GENERAL PURPOSE YEASTS
These yeasts are a mixture of strains selected to give the best all-round performance. This means, really, that they have four desirable characteristics:
1) They ferment rapidly and vigorously. No one wants to wait months for the finished product! However, it is worth remembering that rapid fermentation also depends on a good supply of yeast nutrient and vitamins (see below), and a stable, consistent temperature at the correct level.
2) They have a tolerance to high levels of alcohol. Although alcohol is exactly what the winemaker wants in his wine, to the yeast alcohol is nothing more than a poisonous waste product of its own metabolism. And at a certain level of alcohol, which varies between different types of yeast, the yeast is simply inhibited or killed. General-purpose yeasts can ferment up to about 16% or 17% alcohol by volume - more than enough for table wines.
3) They rapidly form a firm sediment when fermentation is complete. As I mentioned, one of the problems with baker's yeast is that the cells are small and do not drop rapidly out of suspension when the ferment has finished. This problem shouldn't really arise with wine yeasts, although the "dropping out" of the yeast can speeded up by moving the wine into a cool place. These yeasts form a firm sediment which remains undisturbed when the wine is siphoned off (racked).
4) They do not impart off-flavors to wine. Once, winemakers had to siphon the wine off the yeast sediment as soon possible when fermentation had finished - this in case the yeast began to decay and imparted a bad smell or taste to the wine. Although one should still rack as soon as possible, the of a good general-purpose yeast tainting a wine during or after fermentation seems quite small.
All in all, therefore, the requirements of the average wine are fulfilled by the general-purpose yeasts; the two most well known are C.W.E.'s Formula 67 and Unican's Super Yeast. However, any good-quality brand will successfully carry out any task asked of it - except dry sherry and sparkling wine production. (See later.)
Specific Yeast Varieties
As well as the general-purpose yeasts, you can get a range of specific yeasts labeled Bordeaux, Burgundy, Port, Graves, Champagne and so on. These are cultures of different strains of S. cerevisiae. You may wonder why so many different types of wine yeast are available if the general-purpose wine yeasts are so satisfactory. This is an interesting question.
Forts of all, the use of a specific wine yeast does not produce a wine of the corresponding type. In fact the idea underlying the use of different strains of yeasts is that different musts produce an environment which is more suitable to certain strains of yeasts than to others. Thus, for example, a strong peach-based must is said to favour a Sauternes yeast, and so on. The reasoning is that the composition of the must resembles the composition of the corresponding grape and hence the type of wine from which the yeast was originally derived. In addition, it is claimed that each type of yeast has a slightly different metabolism and therefore produces a slightly different flavor and bouquet in the finished wine. But would anyone except an expert notice these differences?
One way of finding out for yourself would be to ferment two similar musts with different yeasts and to compare the finished wines. My own opinion is that a correctly balanced and constituted must is far more important than the variety of yeast used. If you have this, then using specific varieties of yeast is nothing more than fine-tuning. However, two cases where it is essential that specific varieties of yeast are used is in the making of dry sherry and sparkling wine. This is explained later.
Perhaps the only other time when the amateur need use a specific type of yeast is in the production of strong sweet dessert wines: Sauternes and Port yeasts are best for white and red wines, respectively, of this type.
Note that if you wanted to produce a wine of the highest possible alcoholic strength, for example to make a mock port or sherry without having to add any extra alcohol, you would have to use a Madeira, Tokay or Sherry yeast, since those have the highest alcohol tolerance of all. To recap, then, I would suggest the following scheme:
Type of wine Type of yeast
White dessert wines Sauternes
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Original posting date
01 January 2008
Original posting date
01 January 2008