Wine begins as simple fruit juice, transforming into an alcoholic beverage through fermentation. This microscopic biological conversion causes grape juice to evolve into a product containing alcohol by volume (ABV), typically ranging from 5% up to 20% or more in fortified varieties. Understanding how wine becomes alcoholic involves examining the ingredients, the biology of the reaction, and the controls winemakers use to guide the process.
Preparing the Must: Grapes, Sugar, and Juice Extraction
The grape is the foundation of wine, serving as the primary source of sugar required for fermentation. As grapes ripen, they accumulate simple sugars, mainly glucose and fructose, through a process called veraison. By harvest, the grape pulp is composed of 15% to 25% simple sugars, water, and organic acids.
The initial step is to crush or press the harvested grapes to release this sugary liquid, known as the “must.” The sugar content of the must is measured in degrees Brix, a scale indicating the percentage of sugar by weight. This measurement directly predicts the finished wine’s potential alcohol level. For instance, a must measured at 24 Brix will yield a wine of approximately 14.4% alcohol if fermented completely.
The Chemistry of Conversion: Yeast and Anaerobic Respiration
The chemical conversion is performed by yeast, a single-celled fungus, with the species Saccharomyces cerevisiae being the most common choice for winemaking. These microorganisms consume the glucose and fructose in the must to generate the energy they need to survive. This energy-generating process is known as alcoholic or ethanol fermentation.
The reaction is anaerobic, meaning it takes place without oxygen. Yeast breaks down sugar into ethanol and carbon dioxide (C6H12O6 \(\rightarrow\) 2C2H5OH + 2CO2). Ethanol remains in the wine, while carbon dioxide escapes as gas, requiring fermentation tanks to be vented.
The alcohol and carbon dioxide are the primary byproducts of the yeast’s metabolism. This biological process also releases heat, which winemakers must manage. The chemical exchange continues until the yeast either runs out of sugar or the alcohol concentration becomes too toxic for the cells, typically around 15% to 18% ABV.
Factors That Determine the Final Alcohol Content
The final alcohol content of a wine is carefully controlled by the winemaker through several factors. The most direct influence is the initial sugar concentration of the grape must, as more sugar means a higher potential for alcohol. Winemakers adjust this by harvesting grapes at different ripeness levels or by adding sugar to the must, a process known as chaptalization, in regions where it is permitted.
Temperature management is another important control, as cooler fermentation temperatures slow the yeast’s activity. Fermenting at a lower temperature helps to preserve desirable volatile aromatic compounds, but it also extends the time required for the yeast to consume the sugar. The specific strain of Saccharomyces cerevisiae used is also a factor, as different strains have varying tolerances to high alcohol levels.
To produce wines with residual sweetness, winemakers intentionally stop the fermentation process before all the sugar is converted into alcohol. This can be achieved by chilling the must to a temperature that stuns the yeast, or by adding a neutral grape spirit to increase the alcohol content to a level that kills the yeast. This method is used to create fortified wines like Port or Sherry.