Wine-making: The art of bacteria
Uncommonly know as oenology, the process of wine-making is an extensive process where grapes go through the process of fermentation by microorganisms; ultimately resulting in the production of the wine we enjoy during holidays and social gatherings.
Above all else that goes into winemaking, the most important part of this process would be assumed to be the quality of the grapes. However, it is actually the microbes that are the keystone to great wine. The yeast and bacteria that go into the creation of wine is the real backbone to its great taste.
Wine generally goes through two fermentation cycles, the first of which is known as alcoholic fermentation. To begin alcoholic fermentation, yeast is added to the must, the freshly pressed fruit juice that contains the skin, seeds, and stems of the fruit. Normally, yeast is already naturally present in grapes. However, due to the unpredictable outcome of allowing this “wild yeast” to ferment the sugars from the grapes, it is rarely used due to the unknown product outcomes. Such outcomes that could occur include the failure of sugar consumption within the must, leaving the wine sweeter than it should be, or the production of an acetic acid that leaves a vinegar-like taste to the wine.
Almost all winemakers use two specific strains of yeasts when it comes to alcoholic fermentation, the first of which is Saccharomyces cerevisiae var. ellipsoideus. Prior to adding the yeast to the must, sulfur dioxide is added. If alcoholic fermentation were to be done without the use of sulfur dioxide, the natural “wild yeasts” would flourish; in most cases asporogenous yeasts such as Kloeckera apiculata would be among the first. However, because these “wild yeasts” have such a low tolerance for alcohol concentrations in excess of 4 percent, they would be killed off, allowing for more resilient yeasts to prosper. This also can produce undesirable products such as acetic acid. That is why the most preferred method is to treat the must with sulfur dioxide.
During alcoholic fermentation, the yeast that was added to the must feeds on the sugars that are present in the juices of the grapes. Through the consumption of these sugars, the yeast produces a byproduct of carbon dioxide gas and alcohol. During this process, the temperature of the must is increased between 22 to 25 degrees Celsius as well. This affects the taste of the wine and speeds up the fermentation process by affecting the enzymes present in the yeast.
Within the wine industry there are a couple of notorious “wild yeasts” that sometimes make it past the sulfur dioxide and into alcoholic fermentation. This can spell disaster for the wine. The most tarnished of which are Zygosaccharomyces and Brettanomyces. These two wines are known to cause faults in the wine. Brettanomyces is also known to produce an assortment of different metabolites which are volatile phenolic compounds. These compounds give off unpleasant aromas.
Once the process of alcoholic fermentation is complete, malolactic fermentation takes over. This process differs from alcoholic fermentation because it uses bacteria. The bacterium that is added to the must converts the malic acid that is present within the must into lactic acid. This bacterium softens the overall taste of the wine by lowering the acidity. Malolactice fermentation continues at a much slower pace than alcoholic fermentation. The wine is also kept in an airlock to prevent oxidation from occurring during this process. During this phase, the proteins from the grapes are broken down while the particulates and yeast begin to settle, allowing the wine to clear.
The process of malolactic fermentation makes use of lactic acid bacteria, which metabolizes malic acid. During this process, the bacteria produce lactic acid and carbon dioxide as a byproduct. This process is carried out because certain wines have a high level of malic acid. This can cause an unpleasantly bitter taste, whereas lactic acid provides a less sour taste. Through malolactic fermentation, the pH of the wine is increased. This is monitored to prevent the pH from rising above 3.80 in red wines. To counter a pH that has risen past this threshold tartaric acid is added to lower the pH.
The key bacterium for malolactic fermentation is Oenococcus oeni. This bacterium is a heterofermenter, which means that it produces multiple products including carbon dioxide, ethanol and acetate. Although some strains of Oenoccus oeni can use fructose to create mannitol, which leads to what is known as mannitol taint, a fault in the wine.
When it comes to selecting a bacterium for fermentation purposes, Oenococcus oeni is the first pick for winemakers due to how compatible it is with Saccharomyces cerevisiae, the main type of yeast that is used in almost all alcoholic fermentation processes for wine. The second beneficial factor that O. oeni has is its ability to tolerate low pH levels that go along with standard alcohol levels for most wines. Lastly, O. oeni is rather resistant when it comes to the sulfur dioxide that is added into the wine in comparison to other bacteria that fall short of completing the fermentation process.
Some preservatives are used to prevent further fermentation from occurring. One of which is sulfur dioxide, which has two main functions in winemaking. It is used as an anti-microbial agent prior to alcoholic fermentation in order to kill any “wild yeast” that is present on the grapes. This prevents unpredictable fermentation as well as malolactic fermentation after a certain point. Without the use of sulfur dioxide, the wine would spoil due to bacteria. Once both fermentation processes have concluded, a dose of sulfur dioxide is added to the wine to prevent the wine from further fermenting while in the bottle.
A great deal goes into the creation of wine. Many of the processes that go on involve the interaction of the produce with the yeast and bacteria that breakdown, as well as the addition of different byproducts to what ultimately becomes tasty wine. Without the use of specific strains of yeast and bacteria, wine would not be what it is today.