Wine Making

Along with bread making, the use of the microorganisms called yeasts to produce wine from grapes is one of the oldest uses of microorganisms by man. The origins of wine making date from antiquity. Before 2000 B.C. the Egyptians would store crushed fruit in a warm place in order to produce a liquid whose consumption produced feelings of euphoria. The manufacture and consumption of wine rapidly became a part of daily life in many areas of the Ancient world and eventually became a well-established part of Classical civilization. For centuries, wine making has been an important economic activity. In certain areas of the world, such as France, Italy, and Northern California, wine making on a commercial scale is a vital part of the local economy.

The agent of the formation of wine is yeast. Yeasts are small, single-celled fungi that belong to the genus Ascomycota. Hallmarks of yeast are their ability to reproduce by the methods of fission or budding, and their ability to utilize compounds called carbohydrates (specifically the sugar glucose) with the subsequent production of alcohol and the gas carbon dioxide. This chemical process is called fermentation.

Yeast cells are able to carry out fermentation because of enzymes they possess. The conversion of sugar to alcohol ultimately proves lethal to the yeast cells, which cannot tolerate the increasing alcohol levels. Depending on the type of yeast used, the alcohol content of the finished product can vary from around 5% to over 20%, by volume.

The scientific roots of fermentation experimentation date back to the seventeenth century. In 1680 Anton van Leeuwenhoek used his hand-built light microscopes to detect yeast. Almost one hundred years later the French chemist Antoine Laurent Lavoisier proposed that yeast was the agent of the fermentation of sugar. This was confirmed in 1935 by the examination of yeast vats with the greatly improved microscopes of that day.

In the nineteenth century the role of yeasts as a catalyst (that is, as an agent that accelerates a chemical process without itself being changed in the process) was recognized by the Swedish chemist Jons Berzelius. In the 1860s the renowned microbiologist Louis Pasteur discovered that yeast fermentation could proceed in the absence of oxygen. In 1878 Wilhelm Kuhne recognized that the yeast catalyst was contained inside the cell. He coined the term "enzyme" for the catalyst.

In fact more than two dozen yeast enzymes participate in the degradation of glucose. The degradation is a pathway, with one reaction being dependent on the occurrence of a prior reaction, and itself being required for a subsequent reaction. In total some 30 chemical reactions are involved. These reactions require the function of the various enzymes. The yeast cell is the biological machine that creates the enzymes. Once the enzymes are present, alcoholic fermentation can proceed in the absence of living yeast. Enzymes, however, have only a finite period of activity before they themselves degrade. Hence a continual supply of fresh enzymes requires living yeast.

Many types of yeast exist. The stable types suitable for making wine (and bread and beer) are the seven species of yeast belonging to the genus Saccharomyces. The name comes from the Greek words for sugar (sacchar) and fungus (Mykes). The predominant species in wine making is Saccharomyces cerevisiae. There are multiple strains of this species that produce wine. The selection of yeast type is part of the art of wine making; the yeast is matched to the grape and the fermentation conditions to produce—the wine maker hopes—a finished product of exceptional quality.

The natural source of yeast for wine making is often the population that becomes dominant in the vineyard. Less mature local vineyards, especially those established in North America, rely on yeast strains that are injected into the crushed grape suspension. The growth of the yeast will then occur in the nutrient-rich mixture of the suspension.

The fermentation process begins when the yeast is added to the juice that is obtained following the crushing of the grapes. This process can be stunted or halted by the poor growth of the yeast. This can occur if conditions such as temperature and light are not favorable. Also, contaminating microorganisms can outgrow the yeast and out compete the yeast cells for the nutrients. Selective growth of Sacchromyces cerevisiae can be encouraged by maintaining a temperature of between 158 and 167°F (70 and 75°C). The bacteria that are prone to develop in the fermenting suspension do not tolerate such an elevated temperature. Yeast other than Sacchromyces cerevisiae are not as tolerant of the presence of sulfur dioxide. Thus the addition of compounds containing sulfur dioxide to fermenting wine is a common practice.

The explosion in popularity of home-based wine making has streamlined the production process. Home vintners can purchase so-called starter yeast, which is essentially a powder consisting of a form of the yeast that is dormant. Upon the addition of the yeast powder to a solution of grape essence and sugar, resuscitation of the yeast occurs, growth resumes, and fermentation starts. In another modification to this process, the yeast starter can be added to a liquid growth source for a few days. Then this new culture of yeast can be used to inoculate the grape essence and sugar solution. The advantage of the second approach is that the amount of yeast, which is added, can be better controlled, and the addition of liquid culture encourages a more efficient dispersion of the yeast cells throughout the grape solution.

The many varieties of wine, including champagne, are the results of centuries of trial and error involving the myriad varieties of grape and yeast.

See also Economic uses and benefits of microorganisms; Fermentation