Of all the processes found within winemaking, the most important process of all is fermentation. The fermentation process is responsible for converting grape juices into that seemingly delicious and exquisite alcoholic substance. Fermentation is a very simple and natural process where naturally occurring yeasts convert sugars into alcohol. As with all winemaking processes, the vintner has a variety of choices with what yeasts to use and how to control the yeasts. The proper selection and control of yeasts is directly responsible for the quality found within the final wine.
Fermentation is a natural process with yeasts being the agents responsible for the fermentation process. Various yeasts are found throughout nature. Along with bacteria, these yeasts are responsible for the decay of organic matter. Yeasts themselves are members of the family fungi with many different varieties found in nature. As seen within the genus Vitis and its various species, certain grape species are better-suited fort wine production. The same holds true for yeasts. Certain types of yeasts are better suited for the process of wine fermentation. Yeasts such as S. cerevisiae have the greatest tolerance towards alcohol and can produce up to 23% of volume before dying off. Conversely, the yeast of K. apiculata is very intolerant of alcohol and quickly dies off. Other yeasts can produce undesirable qualities within the wine. For example, the yeast of Brettanomyces is known to produce a mousy smell within the wine. With the various characteristics found within the various yeasts, the vintner must select the yeast that will best ferment the resulting wine.
In the process of yeast selection the vintner has two major choices, either allow fermentation to occur naturally by way of naturally occurring yeasts or by manually introducing yeast cultures. Allowing the yeasts to be introduced naturally is a process of winemaking that was known to the ancients. With respect to modern winemaking, natural fermentation has given ground to the use of yeast cultures. This however is changing in some areas as some vintners realize that natural fermentation techniques produce a wine with a definite character of place.
The use of yeast cultures presents the vintner with the ultimate control of the fermentation process. Through a chemical understanding of the various winemaking processes, fermentation can be retarded to a point where the vintner decides when fermentation should begin. At this point, specific yeast cultures can be introduced to the must. While vintners have a vast array of yeasts to choose from, some enterprising vintners have gone so far as to produce their own cultures of yeast drawing on local conditions. This results in those vintners having their yeast cake and eating it too. Local properties of yeast can be maintained, cultured and introduced into the must. This effectively combines the natural yeast fermentation process into the yeast culture selection process.
After the method of yeast selection has been determined, the vintner must decide upon how to use the various technologies available for the monitoring and control of the fermentation process. In order to control the fermentation process, the temperatures of the fermenting must need to be controlled, the fermentation process must be monitored and logged, and the fermentation process must be stopped at the proper moment. Also, the fermentation process must be closely monitored for the production of sweet wines and the post malolactic fermentation process must be controlled as well.
The temperature of the fermenting must is perhaps one of the most important and easily controllable variables in the fermenting process. Yeasts like all living organisms respond differently to different temperatures. In extreme cold the production of alcohol by yeasts slows down while in extreme heat production of alcohol increases. Other temperature related variables within the fermentation process are the extraction of flavor and color compounds as well as the volatile aromatic substances. With respect to fermentation speed and the release of additional substances, an optimum temperature exists for each wine being produced. Different technologies are also available for temperature control. Technologies include traditional temperature control methods as well as refrigerated wine storage vats. Ultimately, the vintner will have to make a judgment call on the technology and temperature to be used, drawing on their experiences and knowledge of the process.
Once the vintner has determined the technology and temperature to be used, the next step is to start the fermentation process and monitor it. The introduction of the yeast typically starts the fermentation process. Fermentation can then be monitored by measuring the density of the must at regular intervals. As alcohol is less dense than water, the vintner can use a comparison to determine where the must is in the fermentation process and adjust temperature accordingly.
Stopping fermentation is a critical process and timeliness is required in the modern winery. Traditionally, fermentation would come to a stop when all of the sugars found within the must had been converted to alcohol. Today additional processes are used to stop the fermentation process. These processes include the increase of carbon dioxide on the fermented juice, the decrease of the temperature of the fermented juice, killing or removing the yeasts of fermented juice and finally fortifying the fermented juice by the addition of alcohol.
With the fermentation process converting all sugars into alcohol, the production of sweet wines may seem impossible. Yet wines are produced that are sweet and still have alcoholic content. In order to control the sweetness of wines, the rate of fermentation is slowed down with the nutrients of the wine being exhausted. With the slowdown of fermentation and the consumption of nutrients, many of the yeasts die. This results in sugars still present within the wine after fermentation is complete. In addition to fermentation slowdown and nutrient depletion, sweet wines produced from grapes with noble rot have naturally occurring anti-fungal properties that retard fermentation.
With the primary fermentation of the must complete, vintners must focus on a secondary fermentation known as malolactic fermentation. Malolactic fermentation occurs when a bacterium known as Lactobacillus converts the malic acid into a softer lactic acid. This is the process that gives certain wines a buttery quality. As this is a desired effect, the malolactic fermentation process must be properly controlled. An overpowering buttery effect is damaging to a wine while an under powering effect is also not desired. Again balance is the key here.