Yeast
Winemaking · Intermediate · 3 min read
Yeast is the single-celled microorganism, mainly Saccharomyces cerevisiae, that converts grape sugar into alcohol, carbon dioxide and a range of flavour compounds during fermentation. Yeasts are essential to winemaking, and the choice of yeast strain, cultured or wild, can have a significant effect on the character of the finished wine. In the UK, both cultured and wild fermentations are used, depending on the producer's philosophy.
How yeast works
Yeast feeds on grape sugar, breaking it down through a series of chemical reactions into ethanol and carbon dioxide. The process releases energy, which the yeast uses to grow and reproduce. The by-products of fermentation, including glycerol, esters, higher alcohols and fatty acids, contribute to the wine's flavour and texture.
A typical wine fermentation will see the yeast population grow from 1 million cells per millilitre at the start to 100 million or more at peak, before crashing back as the alcohol rises and the sugar falls. The whole process takes 7 to 21 days in a still wine, longer in a traditional method sparkling wine's second fermentation.
Cultured vs wild fermentation
Most UK winemakers use cultured yeasts, selected for predictable behaviour, neutral flavour contribution, alcohol tolerance and other traits. Major yeast suppliers, including Lallemand, Anchor and Chr. Hansen, provide a wide range of strains suited to different conditions and styles. The choice of yeast is part of the winemaker's signature, alongside the choice of oak and the choice of malolactic strain.
Wild fermentation, using the yeasts naturally present on the grape skins and in the cellar, is a more adventurous approach. Wild ferments are riskier, because the yeast population is unpredictable, but they can produce wines of greater complexity and a stronger sense of place. Several UK producers, including Davenport, Offbeat, and the more natural-leaning estates, use wild fermentation for at least part of their range.
Yeast in English winemaking
English winemakers have a particular challenge. The cool climate produces musts with high acidity and relatively low sugar, and the yeasts have to work hard to ferment cleanly. A robust cultured strain is the safe choice, and most producers use one for at least the primary fermentation.
For sparkling wine, the choice of yeast for the second fermentation is critical. The yeast must be tolerant of high alcohol and high pressure, and it must produce the right balance of autolytic compounds during the long lees ageing. Champagne-specific yeast strains, including the IOC range, are widely used in the UK.
Yeast nutrients
Yeasts need more than sugar to thrive. They need nitrogen, vitamins, minerals and oxygen, particularly in the early stages of fermentation. UK musts are often low in available nitrogen, so winemakers typically add a nutrient, often a diammonium phosphate (DAP) preparation or a more complex organic nutrient, to ensure a clean fermentation.
Too much nutrient can lead to off-aromas, including hydrogen sulphide, the rotten egg smell that is the winemaker's bane. Too little, and the yeast struggles and may produce volatile acidity. The art is in the middle, with a measured addition at the right time. The winemaker's nose and the lab analysis are the main tools.