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 - tiny single-celled micro organisms - were responsible. Wine fermentation occurs when wine yeasts consume the sugar present in grape juice, producing alcohol as a waste product (yes - wine is what yeasts send to the latrine!). Generally speaking fermentation lasts several days for white wines and several weeks for red (so Chablis producers can start thinking about the Christmas shopping before their counterparts in Chianti, for example). 

Why are yeast considered so important by organic winemakers? Well, wine yeasts are the only living organisms - apart from 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 gray-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 winemakers use specially-selected single 'super' strains of yeast (dried yeast or 'packet yeast') which are reliable enough never to give up half way through fermentation - THE winemaking nightmare. But single strains of packet yeast create standardised tastes - compare two similarly-priced Chardonnays from Hungary and Chile fermented with the same brand of packed yeast and you'll be hard pushed to tell the wines apart. As well as natural yeast, and natural yeast in packed 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... no thanks. 

Is it Really Organic?
What are Vegetarian and Vegan Wines?
Organic Wines Versus Wines Made From Organically-Grown Grapes
What is an Organic Vineyard? (A Bug's Eye View)
What is Reconversion?
What are Biodynamic Wines?
GMOS in Wines