How wine is made

By definition, wine is a drink made by the partial or complete fermentation of the juice of fresh grapes. Grapes are the only fruit with a high enough level of sugar and with the proper balance of acid and nutrients to sustain a natural fermentation to dryness with stable results. Other fruits or berries may be fermented, but without additions of sugar, acid, or various yeast nutrients, they are soon to spoil. Although the biochemistry of fermentation was mysterious until the late nineteenth century, the results of the process were known to man for over 5000 years. Fermentation was thought to be a spontaneous act of Nature, merely set up by man. The grapes were crushed to release the juice into a fermentation vessel. When the fermentation was complete, the wine was pressed by some mechanical means to separate the liquid from the stems, skins, pips and pulp. It was then stored to age and clarify until it was drunk. The bloom, that hazy coating on ripe grapes, is actually a collection of single-celled plants called yeast. There are approximately 6000 yeast cells per ounce of fermenting must. When yeast comes in contact with the grape juice, it begins to feed on it. An enzyme within the yeast converts sugar in the grape juice into roughly equal parts of alcohol and carbon dioxide and also releases energy in the form of heat.

 

This process will continue naturally until the sugar is used up or, more likely, until the yeast cells are no longer able to tolerate the level of their waste products: alcohol, carbon dioxide and/or heat. Like all living things, yeast cells have a primary drive to reproduce. In the first and most vigorous stage of fermentation (2 to 4 days), the yeast action mainly produces more yeast. This is the aerobic (contact with air) fermentation. The anaerobic (without air) fermentation follows and produces most of the alcohol. Under optimum conditions, a wine fermentation will last approximately three weeks, but this may take as much as several months, sometimes for no apparent reason. Fermentation can continue until the wine is dry (without residual sugar), or be stopped at some mid-point to make wines at levels of sweetness ranging from the barest hint to extremely sweet. This can be accomplished by killing or removing the yeast cells by one of several methods: adding alcohol to raise the leve 1.to 15% or more (as in port or sherry), adding sulfur dioxide or sorbate (sorbic acid), chilling the must and filtering out the yeast cells, or by simply filtering out the yeast cells using a sterile filter.