Yeast in Wine Making


Dear Wine Master,

In the making of wine, don’t the yeast spores come directly from the grape skins themselves as well as from the air? Someone is telling me that ”just squeezed” grape juice—the must, I think it is called—doesn’t contain any yeast spores. This person believes that freshly squeezed grape juice must ”sit for several days” prior to having the yeast spores develop so that only then can it begin to ferment to become wine. Could you please tell me what is correct?

Thank you,

Dear Kire,

Thanks for taking the time to write us about your conundrum! Those yeast spores are pretty much everywhere around the winery, but you are absolutely right about the grape skins. Yeasts abound in the powdery substance called ‘floor’ which is visible on the skins, and it’s this which forms the lion’s share of yeasts needed for fermentation.

In days of old, winemakers harvested the grapes, crushed them and allowed the sugary juices to come into contact with yeast straight away, with fermentation beginning at its own pace. Nowadays the skill of the winemaker is to control the fermentation, preferably at cool temperatures, which makes for a slower start to the proceedings, but a much improved end product!

You can probably imagine that unfermented grape juice has to be extracted in temperature-controlled conditions to prevent a reaction from any lurking spores, before adding whatever is required to stabilise it. Without these safeguards, nature would take its course and the inevitable process would begin.


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