How does yeast turn grape juice into wine?

Yeast does it. Tiny yeasts feed on the sugar in grape juice and turn it into ethanol, the alcohol in wine, with carbon dioxide as a by-product. That is all fermentation is: during fermentation, yeasts transform the sugars in the juice into ethanol and carbon dioxide.

For a long time nobody understood this. The very word comes from the juice's behaviour: the earliest uses of "fermentation" described the apparent boiling of the juice as carbon dioxide bubbled out, and the Latin fervere means to boil. Two famous chemists, Berzelius and Liebig, had proposed that fermentation was caused by decomposition. Pasteur showed that this was wrong and that yeast turns the sugar into alcohol.

His proof was elegant. Pasteur showed that the skin of grapes is the natural source of yeasts, and that sterilised grapes and grape juice never fermented. He drew juice from under the skin with sterilised needles, and he covered grapes with sterilised cloth. Neither experiment produced wine in sterilised containers. No yeast, no wine.

So where do the yeasts come from today? Winemakers distinguish ambient yeasts, which are naturally present on the grapes and in vineyards and cellars, from cultured yeasts that are isolated and added on purpose. The cultured yeasts most commonly used belong to one species, Saccharomyces cerevisiae, also known as sugar yeast.

Fermentation runs hot, and heat can push the juice out of its ideal temperature range, so winemakers have always cooled their vats. The old Bordeaux method was to put the vat on blocks of ice.