Wine is fermented grape juice. The process transforms the sweet, non-alcoholic liquid pressed from grapes, known as “must,” into a complex alcoholic beverage. This transformation is driven by the conversion of sugars into ethanol, which is the alcohol found in wine. Understanding this conversion reveals the science behind the varying styles and strengths of wine available today.
Essential Components for Alcohol Creation
The foundation of wine’s alcoholic nature lies in the presence of three main components: grape must, sugar, and yeast. The grape must provides the water-based environment and nutrients necessary to sustain the conversion process.
The primary fuel source comes from the natural sugars contained within the ripe grape, mainly glucose and fructose. These simple carbohydrates accumulate as the grapes ripen. The concentration of these sugars directly determines the potential alcohol content of the finished wine.
The catalyst for the reaction is yeast, specifically strains of the single-celled fungus Saccharomyces cerevisiae. This yeast may be naturally present on the grape skins or intentionally introduced by the winemaker for controlled fermentation. The yeast consumes the grape sugars to generate energy for its growth and reproduction.
The Chemistry of Alcoholic Fermentation
The process that converts the grape must into wine is alcoholic fermentation, an anaerobic metabolic pathway. This means the reaction occurs in the absence of oxygen, a condition that develops when yeast is submerged in the grape juice. The yeast cells metabolize the simple sugars to create energy for themselves.
The overall chemical reaction converts sugar into ethanol and carbon dioxide, along with the release of heat. For every molecule of sugar consumed, the yeast produces roughly two molecules of ethanol and two molecules of carbon dioxide gas. Ethanol is the alcohol that gives the wine its intoxicating property.
The carbon dioxide created during the reaction bubbles out of the fermenting liquid and is released into the atmosphere. Winemakers trap this gas inside a sealed vessel during a secondary fermentation to create the characteristic bubbles in sparkling wines. The conversion also releases heat, which requires careful monitoring by the winemaker.
Factors Influencing Alcohol Strength
The final alcohol content, measured as alcohol by volume (ABV), is determined by variables affecting the yeast’s ability to complete sugar conversion. The most direct influence is the initial sugar content of the grapes when harvested. Grapes harvested at peak ripeness, having a higher concentration of glucose and fructose, yield a wine with a higher final ABV potential.
The specific strain of Saccharomyces cerevisiae yeast also plays a role, as different strains have varying levels of alcohol tolerance. Most wine yeasts stop functioning when the alcohol level reaches between 14% and 16% ABV, as this concentration becomes toxic. Warmer fermentation temperatures speed up the rate of conversion, but excessive heat can kill the yeast prematurely, leading to a stalled fermentation.
Methods Used to Stop Fermentation
Winemakers often intervene to halt fermentation before the yeast consumes all the sugar, especially when producing sweet wines. One common technique is chilling, where the must temperature is dropped rapidly below 50°F (10°C). This action causes the yeast to become dormant, stopping their metabolic activity.
Another physical method involves filtration, which removes the yeast cells from the wine entirely. Fine-grade filters separate the microscopic yeast from the liquid, eliminating the agent responsible for alcohol production. This method is often paired with chilling to ensure stability.
In fortified wines like Port or Sherry, fermentation is stopped by adding a high-proof distilled spirit, such as brandy. The sudden increase in alcohol concentration raises the ABV to a lethal level for the yeast, instantly killing the microorganisms and leaving behind residual sugar.