why does wine ferment when stored?

The Deep Dive

Wine fermentation is a biochemical dance orchestrated by microscopic fungi. When grapes are crushed, sugars like glucose and fructose are released into the juice. Naturally occurring yeasts, primarily Saccharomyces cerevisiae, or added cultured strains, migrate to this sugary environment. Under anaerobic conditions—without oxygen—these yeasts initiate glycolysis, breaking down sugars into pyruvate. Pyruvate is then converted into ethanol and carbon dioxide through alcoholic fermentation, a process that regenerates NAD+ for continued energy production. This transformation not only produces alcohol but also generates heat and various byproducts like esters and higher alcohols, which contribute to wine's complex aroma and flavor profile. Temperature control is crucial; too cold slows fermentation, too hot can kill yeast or create off-flavors. During primary fermentation, which lasts days to weeks, most sugars are consumed, but some may remain. After pressing, the wine is stored in barrels or bottles. If storage conditions allow—such as residual sugars, surviving yeast cells, or contamination from wild yeasts—fermentation can restart, known as refermentation. This is often undesirable as it can cause carbonation in still wines or spoilage. However, a secondary fermentation called malolactic fermentation, conducted by lactic acid bacteria, is often encouraged during aging. It converts sharp malic acid to softer lactic acid, enhancing mouthfeel. Historically, fermentation was a mysterious process until Louis Pasteur identified yeast's role in the 19th century. Today, winemakers carefully manage fermentation through temperature regulation, sulfur dioxide additions to inhibit unwanted microbes, and selecting yeast strains to achieve desired sensory characteristics. Understanding this science allows for consistent wine quality and innovation in styles, from crisp whites to robust reds.

Why It Matters

Fermentation is fundamental to winemaking and many other industries, preserving foods and creating flavors. It enables the production of beer, bread, cheese, and biofuels, with applications in medicine and biotechnology. For winemakers, mastering fermentation ensures wine safety, enhances complexity, and meets consumer preferences. This knowledge also supports sustainable practices by reducing spoilage and optimizing resource use. On a broader scale, studying microbial fermentation advances our understanding of ecology and evolution, with potential impacts on agriculture and health.

Common Misconceptions

One misconception is that fermentation only makes alcohol, but it also produces acids, gases, and flavor compounds that define wine's character. Another is that fermentation stops entirely after primary fermentation; however, dormant yeast can reactivate in storage if sugars remain or temperatures rise, causing refermentation. Winemakers combat this with stabilization techniques like filtration and sulfite addition.

Fun Facts

• The oldest known winery, discovered in Armenia, dates back to around 4100 BCE, showcasing ancient fermentation practices.
• Yeast used in winemaking can produce over 500 different flavor compounds, contributing to the diverse aromas in wine.