A shorter version of this article first appeared in Wine and Spirit magazine in 2006, since merged into Harpers Wine and Spirit.
Yeasts are the little gods of wine, fermenting grape juice to that illustrious liquid. Winemakers can exert a little influence by choosing ‘cultured’ yeasts (come in packets) over ‘indigenous’ (a combination of ‘wild’ and ‘wine’, from the vineyard environment) ones, depending on the aim, but the importance of being yeast has moved way beyond the simple conversion of sugar to alcohol. There are around 400 aroma substances in wine influenced by yeasts.
There are a whole bundle of different yeast strains which behave slightly differently during fermentation and they therefore produce wines with different characteristics. The ‘cultured’ ones have been bred in a laboratory specifically for certain traits, both production- and style-related. These include efficiency, so converting less sugar to more alcohol, the ability to resist high pressures for sparkling winemaking, the ability to ferment at very low temperatures to preserve delicate primary fruit characters, or for the delivery of certain aroma profiles.
Cultured yeasts are added to newly harvested juice. A single strain of yeast is usually chosen with a specific character, although some manufacturers also produce packets of blended yeasts with combined properties. In addition to any specific character trait, cultured yeasts are predictable and consistent – the brands of the yeast world.
‘Wild’ yeasts hang out all year round with ‘wine’ yeasts in the vineyard and winery corners, waiting for the juice to become available. A whole gang of different yeast strains is implicated in a wild ferment, each with their own character, sometimes clubbing together to bring flavours that they couldn’t bring individually, for example helping to liberate floral and citrus flavours from flavourless precursors during fermentation. And as the fermentation continues, there is a changing cast of characters having a dominant influence each contributing their own little bit.
But wild ferments are much less predictable, more risky, they can produce funky flavours, they may not ferment to full dryness, they may take longer, and they can build character and individuality into a wine.