Yaaaaassss, yeast! We recognise that’s probably a weird thing to say, but without yeast, we wouldn’t have wine. Not so weird now, huh? Maybe there’s an idea for a new religion? ‘Yeastians,’ perhaps?
Single-celled organisms that convert the sugars in wine grapes into alcohol, yeasts come in many different thousands of strains, the most important of which for winemaking is Saccharomyces cerevisiae. Yeasts can ferment grapes in one of two ways — by inoculation or naturally — and they can also contribute to the actual flavours and characteristics of a wine itself.
‘Inoculation’ refers to when a winemaker adds commercial yeasts to a batch of grapes in order to start fermentation. Commercial (or manmade) yeasts might not be particularly exciting — but what they lack in flash, they make up for in speed, reliability, cleanliness, and efficiency. Commercial yeast ferments are easy for winemakers to control and rarely, if ever, present the problems often associated with wild ferments.
Wild (or indigenous) yeasts are already present on the grapes once they’re harvested and brought into the winery. In a wild fermentation, no inoculation occurs; instead, the winemaker simply waits for those naturally occurring yeasts to begin fermentation on their own. Many people believe wild fermentation results in the purest expression of terroir and the most interesting and personable wines, but there’s a risk: wild yeasts are volatile, much less easily controlled, and far more sensitive to factors such as temperature and alcohol level (which can cause the yeast to die prematurely).
Once the yeasts do die (hopefully when fermentation is complete), they form the sediment we sometimes find at the bottom of a wine bottle. The winemaker can choose to filter this sediment out or to release an ‘unfiltered’ wine — which is still perfectly safe to drink. And in the case of many white wines, these ‘lees’ actually contribute beneficial creamy complexity and texture to the finished product.