The Use of Yeast in Wine Making
Yeast may be described as the power or driving force that makes wine. There are many kinds of yeast and, as the subject is complex, those who are both interested and able to understand a little chemistry are advised to consult specialist publications (see Bibliography). But in general terms, yeasts do not themselves cause fermentation, but excrete enzymes causing it to take place. Because, obviously, one cannot deal in detail with this aspect of the subject, the part yeast plays in winemaking is here described in general and lay terms.
WHAT ARE YEASTS?
Yellowish substances produced by the propagation of a fungus (Saccharvmyces cerevisae) which, in vineyards, is simply floating in the air. It is a part of the natural life of the district, and is spread by the action of insects. As far as wine is concerned, it is caught and retained in the bloom on the skin of the grapes (which is why, if it rains hard during the time when the grapes are to be gathered and wine to be made, fermentation can be difficult, because the bloom may have been at least partly washed off).
HOW YEASTS WORK:
Wine yeasts are those that enable wine to be made (in regions where new vineyards are created, they are absent and may have to be imported from other wine regions). Wild yeasts of which the principal species is Saccharomyces apiculatus, may start up the fermentation in the must. But this is a primitive method of winemaking and these yeasts die at 4% of alcohol by volume. Saccharomyces ellipsoideus can ferment up to 12 or 14% of alcohol by volume. Depending on the amount of sugar in the must. They can go a little higher, but usually cease to work completely at 16%.
This is why the alcoholic strength of table or natural wine is fixed at 14% of alcohol by volume. Wines from very hot vineyards, where the sunshine gives a high degree of sugar in the must, occasionally come into conflict with the Customs, because they may be higher in alcohol and. Therefore, may be charged ‘heavy wine’ duty. Yeasts of such wines are vigorous compared with others, which is why they can sometimes be used in vineyards where the original strain has become weak.
Yeast is affected by extremes of temperature and will not work if the atmosphere becomes too hot or too cold (or if the alcoholic level of the wine rises too high), exactly like baking. Also, if the dead yeasts are left in a made wine, the vinegar bacteria will feed on them – and the wine will turn to acetic acid or vinegar. This is why newly made wine is run off the vat or cask in which all the debris of pips, bits of skin and yeasts remain, so that it can pursue its life without being endangered. Some badly-made wines do, in fact, smell yeasty – this smell is easily recognised by anyone who has ever had yeast tablets as a type of tonic. The wines may not be harmful or even unpalatable, but the yeasty smell, if it remains noticeable, can indicate that they have not been properly supervised and cared for. But in general the presence of yeast in wine is something the taster and drinker need not be concerned with.
The specific action of certain yeasts of wine is exemplified by some of the Jura wines, with which Pasteur conducted his investigations and which resulted in his discovery of bacterial action. It is also seen in the way in which sherry is made and also in wines made in a similar way to sherry. Sometimes these use native yeasts, sometimes a strain developed from imported yeasts.