Yeast is a single-celled fungus that carries out alcoholic fermentation by converting fermentable sugars into ethanol, carbon dioxide and a range of flavour-active compounds. In winemaking, the principal species used to complete fermentation is Saccharomyces cerevisiae, although other yeast species may be active at the beginning of spontaneous fermentation.
Yeast consumes glucose and fructose in grape must and produces alcohol and carbon dioxide. It also creates secondary metabolites that influence aroma and flavour, including esters, higher alcohols and other volatile compounds. Yeast strain, temperature, nutrient availability, oxygen at the beginning of fermentation and must composition all affect fermentation performance and the finished wine.
Selected yeast is a commercial culture chosen for traits such as reliable fermentation, temperature tolerance or a particular aromatic profile. Native or indigenous yeast refers to microorganisms naturally present on the grapes and in the winery. Spontaneous fermentation may involve several species before Saccharomyces becomes dominant and completes the process.
Yeast requires suitable nutrients, including nitrogen, vitamins and minerals. Deficiencies or unsuitable temperatures can lead to sluggish or stuck fermentation and unwanted sulphur aromas. Winemakers monitor temperature and sugar depletion and manage nutrients according to laboratory analysis and the fermentation plan.
Example: a winemaker may inoculate Sauvignon Blanc with a selected yeast strain to support a controlled fermentation and preserve a desired aromatic profile.
Yeast carries out alcoholic fermentation, converting sugar into ethanol and carbon dioxide. Lactic acid bacteria carry out malolactic fermentation, converting malic acid into lactic acid and carbon dioxide. These are separate biological processes.
Chef’s note: yeast is essential to most wine production, but different strains and fermentation conditions can produce markedly different wine styles.
This term is one of hundreds covered in depth across OCS courses, learning paths, and 1-on-1 coaching with Chef Eric.
