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 …