Wine is essentially the great result of a meeting between grape and mushroom … well, actually, fungi. Fermentation cannot occur without the presence of yeast, and yeast is a member of the fungi family which includes mushrooms and toadstools. Yeasts are classified as ascomycetes, fungi that take the form of a sac or capsule. Most cultured wine yeasts are in the genus Saccharomyces (sugar loving) with the most common species being cerevisiae.
So what exactly do yeast do? Yeast feed on natural grape sugars and break them down into alcohol, carbon dioxide gas and heat. In the actual chemical equation one molecule of sugar (glucose or fructose) will yield two molecules of ethanol and two molecules of carbon dioxide.
Sugar → alcohol + CO2
For “dry” wines, winemakers allow the yeast to consume all of the sugar in the grape juice, thus ending fermentation. For sweet wines, fermentation must be stopped prematurely by removing the yeast through filtration, adding alcohol (or “fortifying” the wine) or by killing the yeast through the addition of sulphur dioxide or through pasteurization.
So yeast are good? Right? The ones that ferment sugar to alcohol, absolutely!
However, there are other yeasts that are found in wine that can produce undesirable aromas and flavors. One of the most common of these yeasts is Bretannomyces. Brett, as it is known in the industry, is a species of yeast used to ferment the lambic beers of Belguim. Some winemakers feel that the flavors and aromas produced by Brett add to the complexity of certain red wines. Other winemakers consider Brett to be spoilage yeast that creates “barnyard” or “animal like” aromas. Scientists have discovered that there are a number of chemical compounds produced by Brettannomyces, some of which have pleasant, spicy, clove aromas and others which have an offensive barnyard smell. Spoilage, therefore, is a matter of opinion and quantity. People have different sensitivities to the Brett aromas and what some may find acceptable, others cannot tolerate.
The photo above is a photo of Bretannomyces .
So what else is living in your wine? Bacteria. Not the kind of bacteria that can cause disease but rather alcohol-tolerant bacteria that can convert certain natural wine chemicals into others. The two most important bacteria for winemakers are lactic acid and acetobactor. Lactic acid bacteria perform a useful role in wine by converting malic acid to lactic acid. In a process known as Malo-lactic (ML) fermentation, the conversion of malic to lactic acid softens the sensation of acid in the wine. Almost all red wines undergo this fermentation but in white wines, it is optional. An important by-product of ML fermentation is diacetyl, the chemical that makes butter smell like butter. In wines such as Chardonnay, this flavor addition can be a benefit.
Photo of Lactic acid bacteria
Acetobactor are much less desirable. They convert alcohol to acetic acid, which is essentially vinegar. When acetic acid reacts with alcohol, it forms ethyl acetate which smells like fingernail polish or paint. Needless to say winemakers would like to avoid the presence of acetobactor. This is achieved by reducing oxygen contact with wine and by adding sulphur dioxide to inhibit the growth of these bacteria.
Winemaking is a process that depends on micro-organisms. Most of these micro-organisms have completed their tasks and died by the time the wine is bottled. If a wine is dry, i.e. without sugar, the yeast have no food source and they typically cannot survive. If all of the malic acid present is converted to lactic acid, the lactic acid bacteria have nothing to do. Many wines, therefore, can be safely bottled without filtration. Because sweet wines and wines with incomplete fermentations still have residual sugar, they are usually filtered. The addition of small quantities of sulphur dioxide gives winemakers added insurance that their wines are microbiologically stable. Even without the addition, no wine is “sulfite free”, as yeast themselves actually produce small quantities of sulphites.
So as you enjoy your next glass of wine take a moment to remember the “mushrooms” and bacteria that have made wine possible. Cheers!