The fermentation process in winemaking

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Have you ever eaten a grape that was almost rotten on the inside but tasted like a good wine? Or smelled a bag of old grapes and it had that same aroma?

What exactly happens during winemaking to create ‘alcohol’ rather than rotten fruit?

In: Biology
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Yeast consume sugars, and excrete alcohol and carbon dioxide as waste. This type of fermentation does not produce unpleasant flavours or odours, but if the ferment is contaminated with bacteria, it can produce very nasty, even poisonous chemicals. This is why you sanitize your brewing gear.

The first step in winemaking is to press the grapes, then remove what winemakers call the **pomace** – anything which isn’t grape juice. Pomace is up to a quarter of the grape’s weight, and contains a heck of a lot of chemicals (including many vitamins), fibers, fats, and proteins. The presence of these things causes a change in decomposition.

Once the winemaker has just wine juice, they add yeast. Grapes you find in the grocery store (and those you’ve done a good job of washing at home) have only trace amounts of wild yeast on them. Yeast *dramatically* changes the decomposition process, turning it into one of true fermentation, where the yeast eats the sugar in the juice and poops out alcohol. Since wild yeast is on almost every surface, there’s gonna be some on your grapes, so a little fermentation will happen, but not to the degree in winemaking.

All that alcohol that’s now floating around in the juice also has the effect of heavily suppressing bacteria growth (the same way it can be used as a wound disinfectant or mouthwash). Rotting grapes are a big ol bacteria party with lots of highly-flavorful organic chemicals floating around; wine is too alcoholic for bacteria to thrive in and is more selective about what organic chemicals are in it.

The simplified explanation is that there are competing micro organisms. There are molds, yeasts and bacteria etc. and each of these are, in turn, broken down into different types/species. Making wine, beer, cheese, bread, yogurt etc requires making conditions that the “right” kind of microorganism does most of the work that results in “fermentation” rather than what we would term rotting or spoiling.

In bread, wine, beer – yeast is the thing you want mostly. (Sourdough is a mix of yeast and bacterial action). Done correctly, the high yeast concentration inhibits the growth of the non-wanted bacteria and molds while producing the alcohol and/or carbon dioxide. Mess it up and the product spoils rather than ferments.

Yeast normally consumes oxygen, sugar and starch and produces CO2 and water. However when there is insufficient oxygen, yeast uses a different reaction and produces alcohol and CO2 instead. The key is to restrict the oxygen which is why the wine is kept in mostly sealed vessels.