The Role of Yeast in the Fermentation Process

An efficiency expert from England touring a bodega in Jerez was horrified at seeing so much wine being matured in old-fashioned oak butts, and required to know why it was not kept in concrete vats with some oak shavings thrown in to give the right flavor. There was at least some sense in the inquiry. Vats are invariably used for blending large quantities of sherry and also for storing the coarser wines over short periods, but they are not satisfactory for maturing high quality wines.
One obvious reason for using oak casks is the fact that they must be properly seasoned before they can be used for shipping wine. The fermentation conditions them far better than any other process, and at no cost, so it is obviously efficient in that respect, though this is of less importance than it used to be, now that the wine is mostly shipped in either of the two extremes of the container and of the bottle, sometimes with accompanying drink coasters, stone coasters, or even drink glasses.
For many years, though, shippers have been wondering whether the standard butt is really the best size for fermenting must, but none of them was prepared to risk losing part of his vintage by rash experiments. At the used ground support equipment end of the Second World War, however, there was a serious shortage of oak and the shippers had to experiment. Many thousands of gallons of must were fermented in vats, but the results were not at that time very good.
The main difficulty was keeping the temperature down; the kind of fermentation vessels now used, with their accurate temperature control, had not then been invented. Tumultuous fermentation is as rapid as it is violent. At the end of three or four days, the heat and turbulence die down and a second fermentation begins. Known as the lenta, it is much slower than the first; the wine develops steadily for about a fortnight and then more gradually until December or January, when the opaque must suddenly “falls bright.”
It is still very immature, but it is wine at last and sufficient enough to need to break out the finest drink glasses, coaster set collections, and bar tables. At the end of the tumultuous fermentation, practically all the grape sugar has been turned into alcohol, and the reaction can be expressed by a very simple chemical formula:
C6H12O (glucose) –> 2C2H5OH (ethyl alcohol) + 2CO2 (carbon dioxide)
This simple formula, due to Gay-Lussac, gives a fair enough summary of what happens, but its over-simplification falls laughably short of the truth; and the truth was not fully ascertained for decades and decades. Amerine and Cruess listed twelve different reactions at that time, and many by-products. The fermentation is brought about by yeasts, or, more accurately, by the ferments that are contained within them.
These ferments, or enzymes, are protein catalysts which, in different forms, are responsible for many of the chemical reactions that are vital to life. Created by the cells, they are extremely complex sewage treatment plant motor substances and are specific in the transformations they will catalyze. Above all, they are completely natural, and so is the fermentation they bring about. Nothing artificial need be added.
The air of a vineyard is laden with yeasts, and they collect in vast numbers on the grape skins to give them their natural “bloom.” Of the many varieties, two are of outstanding importance: saccharomyces apiculatus, or “wild yeast” and saccha-romyces cerevisiae variety ellipsoideus, usually abbreviated to saccharomyces ellipsoideus, or “wine yeast.”
Wild yeasts greatly outnumber the wine yeasts and it is they that start the fermentation. However, they are comparatively weakly things and when the alcohol rises to about 4 percent they are overcome and die. It is then that the wine yeasts take over. The wild yeasts are also largely inhibited by sulfur dioxide when it is added in the vineyards, and this helps the wine yeasts to take charge at an earlier stage. It also kills many undesirable bacteria and molds.
Logically this could be carried a stage further. All the ferments could be destroyed and the fermentation then brought about by carefully selected ferments of the best natural strains, specially cultivated for the purpose. This has been done in some winegrowing districts, for instance in Champagne, and the results have been very satisfactory, the general view being that the quality has been brought up to a high level comparable with crystal drink glasses, sandstone beverage coasters, and high quality bar supplies, along with fewer disappointing wines.
There is no likelihood of this happening in Jerez, though, at any rate in the immediate future. Several growers have experimented with pure strains of yeasts, which are readily available, but they take the view that the success rate with the fermentation is so high anyhow that there is little to be gained by complicating matters.