Why Does Wine Ferment When Stored?

The Biochemistry of Wine Fermentation: Why Wine Bubbles and Changes Over Time

At the heart of every bottle of wine lies a microscopic workforce: Saccharomyces cerevisiae. This yeast is the primary engine of fermentation, a metabolic process that has been harnessed for millennia. When grapes are crushed, they release a nutrient-rich slurry of glucose and fructose. Without access to oxygen, these yeast cells undergo anaerobic respiration, a process that breaks down simple sugars to generate ATP for the yeast’s survival. The chemical byproduct of this breakdown is ethanol—the alcohol that defines wine—and carbon dioxide gas. This isn't just a simple conversion; it is a complex biochemical cascade that produces secondary metabolites like glycerol, which adds body, and various esters, which provide the fruit-forward aromas we associate with premium vintages.

However, the process is rarely as simple as 'sugar into alcohol.' The environment within a fermentation vessel is highly competitive. During primary fermentation, yeast populations can reach densities of over 100 million cells per milliliter. As the alcohol concentration rises, the environment becomes increasingly hostile. Most commercial strains of yeast are bred to withstand alcohol levels up to 15% or 16%, but they are sensitive to temperature spikes. If the temperature exceeds 30°C (86°F), the yeast may undergo heat stress, leading to the production of volatile sulfur compounds that smell like rotten eggs or burnt rubber. Conversely, if the temperature drops too low, the yeast may go dormant, leaving behind 'residual sugar.' This is the critical juncture where storage issues arise. If a winemaker bottles a wine that is not 'dry'—meaning it still contains residual sugar—and fails to remove the yeast through filtration or chemical stabilization, the yeast can reawaken.

This phenomenon, known as refermentation, is the bane of still-wine producers. Even a tiny amount of residual sugar can provide enough fuel for a dormant yeast colony to begin producing carbon dioxide again. In a sealed glass bottle, that gas has nowhere to go, leading to unexpected fizz, cloudy sediment, or even the explosive popping of corks. To mitigate this, modern enology relies on rigorous stabilization. Winemakers often utilize sulfur dioxide (SO2) to inhibit microbial activity and sterile filtration to physically remove yeast cells before the wine reaches the bottle. Furthermore, the practice of malolactic fermentation (MLF)—a secondary process where Oenococcus oeni bacteria convert harsh malic acid into creamy lactic acid—must be carefully timed. If MLF occurs in the bottle, it can leave the wine with a 'diacetyl' or buttery off-flavor and, again, unwanted carbonation. The science of wine is therefore a delicate balance of timing, temperature, and microbial management.

Managing Stability: How Winemakers Stop Unwanted Fermentation

For the average consumer, an unexpected fizz in a supposedly still wine is a sign of spoilage, often caused by a failure in the stabilization process. You can identify this potential issue by looking for a slight 'prickliness' on the tongue or a faint haze in the glass that shouldn't be there. If you notice these signs in a dry table wine, it is often best to avoid consumption, as secondary fermentation can also introduce undesirable bacteria like Brettanomyces, which produces barnyard or medicinal odors.

Winemakers combat these issues using several key interventions. First, they monitor the 'Brix' levels, which measure sugar content, ensuring the wine is completely dry before bottling. Second, they utilize cold stabilization, chilling the wine to force tartrate crystals to precipitate, which often helps clarify the liquid. Finally, they use sulfur dioxide as a preservative to 'stun' any remaining microbes. If you are a home winemaker, the most important takeaway is to invest in a hydrometer. By measuring the specific gravity of your wine over several days, you can confirm that fermentation has truly ceased before you ever reach for the corker.

Why It Matters

The science of fermentation is the cornerstone of food security and global culinary history. Understanding why wine ferments allows us to preserve perishable fruit juices, turning them into stable, shelf-ready products that can be aged for decades. Beyond the glass, the microbial processes used in winemaking are identical to those used in the production of life-saving pharmaceuticals, biofuels, and essential food staples like sourdough bread and yogurt. By mastering the conditions that control yeast, we improve our ability to prevent food waste and maintain consistent quality in the global food supply chain. Furthermore, the study of wine chemistry drives innovation in biotechnology, as researchers look to these ancient yeast strains to develop more efficient, sustainable methods for producing ethanol-based clean energy, proving that the science of the vineyard has massive implications for our environmental future.

Common Misconceptions

A major myth is that 'natural' or 'unfiltered' wines are inherently prone to exploding in the bottle. While it is true that these wines have more biological activity, skilled winemakers use 'racking'—a process of siphoning wine off its sediment—to naturally remove yeast without harsh chemicals. Another common misconception is that all fermentation is the same. In reality, there is a massive difference between alcoholic fermentation (yeast-driven) and malolactic fermentation (bacteria-driven). Many people believe that if a wine is bubbling, it is a sign of quality, like a fine Champagne. However, if you bought a bottle of still Chardonnay and it is bubbling, it is almost certainly a sign of a technical flaw called 'refermentation in the bottle.' Lastly, many believe that alcohol levels are fixed once the wine is bottled. In reality, if the wine is not sterile, the alcohol content can slightly increase over time as the yeast continues its slow, lingering work on any remaining sugars.

Fun Facts

• Saccharomyces cerevisiae is so vital to human history that it was one of the first organisms to have its entire genome sequenced in 1996.
• The 'pop' of a champagne cork is caused by the release of pressure from the carbon dioxide generated during a secondary, intentional fermentation.
• Some ancient winemakers used 'wild' fermentation, relying on the yeast already present on the grape skins rather than adding lab-grown cultures.
• During the 19th century, Louis Pasteur’s research into why wine turned to vinegar helped him develop the process of pasteurization, which saved the dairy industry.

Related Questions

• Why does some wine taste like vinegar after being stored too long?
• What is the difference between primary and secondary fermentation in wine?
• How do winemakers measure when fermentation is complete?
• Why do some wines develop sediment at the bottom of the bottle?
• What are the health benefits of the microbes found in wine?