Wine Arcana

Concept

Alcoholic fermentation

Alcoholic fermentation is the process by which yeast converts grape sugars into ethanol and carbon dioxide. It turns juice into wine and helps shape its aroma. The main sugars involved are glucose and fructose, and most wine fermentations are completed by yeasts of the genus Saccharomyces.

Inside the cell, enzymes split sugar into smaller molecules and ultimately produce ethanol while allowing the yeast to obtain energy. Some sugar also supports cell growth and other metabolic products. A successful dry-wine fermentation depends on maintaining enough healthy cells to consume the available fermentable sugar as the environment becomes increasingly alcoholic.

Establishing the yeast population

A winemaker can add a selected yeast culture or allow organisms already on the fruit and winery equipment to begin fermentation. In an uninoculated ferment, several species may contribute early before more alcohol-tolerant yeasts take over. Winery populations can include strains introduced commercially in earlier vintages. The organisms present, their relative numbers, and the juice's condition influence how that succession proceeds.

Selected cultures give the winemaker a way to choose fermentation properties, including alcohol tolerance and nutrient requirements. Another approach uses a small, actively fermenting lot to inoculate a larger one. Each approach depends on a viable starting population and continued monitoring.

Temperature, nutrients, and oxygen

Fermentation releases heat. Cooling slows yeast activity and helps retain volatile aroma compounds, while warmth increases extraction from grape skins. Temperature management therefore serves both the yeast and the intended style, especially in red wine, where fermenting juice remains in contact with solids. Large or sudden temperature changes can impair yeast performance.

Yeast also needs nitrogen to build cells and sustain fermentation. Winemakers measure yeast assimilable nitrogen, usually shortened to YAN, which includes ammonium and usable amino acids. A juice can contain abundant sugar yet provide too little nitrogen for a healthy fermentation. Low YAN can contribute to slow or incomplete sugar consumption and unwanted sulfur aromas. Analysis helps guide supplementation, whose timing and amount depend on the juice and yeast.

Controlled aeration during active fermentation can help yeast complete its work. Australian Wine Research Institute trials have found shorter fermentation times in challenging white ferments and changes in aroma and astringency in red ferments.1. Australian Wine Research Institute, “Aeration of ferments,” summary of its research on white- and red-wine fermentation. Effects depend on the timing of aeration and the ferment concerned. Oxygen management changes as the wine moves through production: an actively fermenting population consumes oxygen differently from a finished wine undergoing oxidation during storage.

Following fermentation to completion

Winemakers follow temperature and declining density through repeated measurements, often plotting a fermentation curve. A slowdown gives time to investigate temperature, nutrition, yeast activity, or inhibitory substances. High starting sugar, rising alcohol, excessive sulfur dioxide, and competing microorganisms can each contribute to difficulty.

A stuck fermentation has ceased consuming sugar before reaching the intended endpoint. Restarting it may require a fresh yeast population acclimatized to the wine's alcohol and other conditions.

Sources