How is Wine Made? | Yeast and Fermentation
Jane Masters MW is Opimian’s Master of Wine
At harvest time, the influence and skill of the winemaker shapes the flavour and style of wines made. Winemakers decide what type of vessels to ferment in, what yeast to use, fermentation temperatures and a whole host of other things.
Stainless steel and concrete tanks are inert in terms of flavour, whereas fermenting in oak can impart vanilla, coconut and/or toasty characters depending on the size and age of the vessel. The smaller the vessel, the higher the ratio of surface contact with the wine inside it. A large old oak tank gives much less flavour than fermenting in a new small 225-litre oak barrel. In recent years, there has been a trend for clay amphorae – while inert in terms of flavour, the porous clay, like wood, allows for some micro-oxygenation which in turn affects flavour.
Wine is usually fermented by yeast belonging to the Saccharomyces cerevisiae species – the same kind used in baking and beer production. There are many different strains of Saccharomyces cerevisiae and some of these have been selected for their aromatic properties and ability to ferment under certain conditions of high sugar concentration, acidity and pH. Winemakers can use selected yeast strains, which ensure a regular and controlled fermentation, or let wild yeast present in the winery develop. Some great wines are produced from wild ferments; however, the results are less predictable.
As fermentation proceeds, a lot of heat and carbon dioxide gas are created. If the temperature gets too high, yeast die off; too low and the fermentation may be sluggish and stop. Temperature affects yeast metabolism, hence the aromas produced, so winemakers maintain temperature at a desired level according to wine style. Light, fruity white wines may be fermented at 14°C. For richer fuller whites, the temperature may be higher at around 20°C. Red wines are generally fermented at higher temperatures, with light, fruity reds at 20-25°C to maximize fermentation aromas. Fuller bodied and structured reds, made for keeping and often barrel aged, may ferment at 30°C and higher. In sparkling wine production, carbon dioxide produced from the fermentation may be captured to create the sparkle, but even still wines retain a certain amount which adds to the perception of acidity and freshness. At the end of fermentation, yeast drop to the bottom of the fermentation vessel and form “lees”. While no longer viable, lees can add mouthfeel and flavour to a wine by releasing mannoproteins. Some winemakers keep wines on lees for a period and may also carry out “lees stirring” or “bâtonnage”, stirring up the sediment to increase contact and maximize the effect.
Most red wines and some whites undergo “malolactic fermentation”, when malic acid naturally present in grapes (also found in sour green apples) is converted to lactic acid (present in milk) by lactic acid bacteria. This microbiological process reduces wine acidity, making the wine softer and more microbiologically stable. At the same time, other metabolites produced contribute to wine flavour – diacetyl has a creamy buttery aroma. The choice “to do or not to do” can have a big impact on wine style. For Chablis, known for its steely mineral style, malolactic fermentation is usually avoided. In a rich buttery Chardonnay, malolactic contributes to the richness and butterscotch flavour of the final wine – often in tandem with barrel fermentation.
Whether winemakers choose to inoculate selected strains of yeast and lactic acid bacteria or allow wild ferments to develop, microbiology lies at the heart of quality winemaking. To fully maximize the potential in harvested grapes, winemakers must also prevent unwanted micro-organisms that lead to spoilage and off flavours from developing. A judicious use of sulphites during winemaking, maturation and at bottling prevents spoilage of organisms, protects against oxidation and keeps a wine’s aroma fresh so it can be drunk in tip top condition.