The great French scientist Louis Pasteur was the first to discover the secrets of the alcoholic fermentation, which transforms grape juice into wine. He discovered that yeasts, the tiny single-celled microorganisms were responsible. Wine fermentation occurs when wine yeasts consume the sugar present in grape juice, producing alcohol as a waste product. Generally speaking fermentation lasts several days for white wines and several weeks for red.

Why are yeasts considered so important by organic winemakers? Well, wine yeasts are the only living organisms apart from the wine producer of course to play a role in the winemaking process. The taste of the wine is determined to a considerable degree by the type of yeast present in the grape juice. Organic growers produce grapes with thriving populations of natural yeast. The yeasts show themselves on the grapes as a grey waxy film known as the 'bloom'. Each vineyard has several strains of natural ('indigeneous') yeast, and together these strains produce a complex cocktail of flavours in the wine during fermentation.

Some winemakers use specially selected single 'super' strains of yeast (dried yeast or 'packet yeast') which are reliable enough never to give up halfway through fermentation which is the winemaking nightmare (and may lead to spoiled wine). But single strains of packet yeast create standardised tastes, compare two similarly priced Chardonnays from Hungary and Chile fermented with the same brand of packet yeast and you'll be hard pushed to tell the wines apart. As well as natural yeast, and natural yeast in packet form, we now have genetically modified yeast, too. These yeasts produce mathematically predictable fermentations useful if you want to sell millions of bottles of wine to a supermarket, which is looking for a specific flavour profile.