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To make sure your wine ferments properly, you have to make sure your yeast has everything it needs. The yeast used to ferment wine is Saccaromyces cerevisiae, whether it is added or comes from the skin of the grapes. It requires these things:

  • Anaerobic Conditions: No oxygen should reach the main part of the fermentation chamber.. if it does, the yeasts will make water and carbon dioxide instead of alcohol.. not too tasty.

  • Sugar: the yeast needs sugar to use as food. In anaerobic conditions it will turn this into the desired substance, alcohol. Yeast breaks down glucose and fructose, the main sugars found in wine, as well as sucrose, or table sugar.

  • Nitrogen: Like all living things, yeast requires nitrogen. Usually the yeast will find this in the grapes. If for some reason it does not, a nitrogen supplement known as 'DAP' is often added to the mixture

  • Vitamins and Minerals: Yeast requires these. However, you don't need to add them, the yeast can synthesize them or get them from the grapes.

  • Low pH: Yeasts thrive in acid which kills bacteria which would just rot the wine. Luckily, grape juice is naturally acidic

  • Correct Temperature: Yeast can ferment between 50 and 100 degrees F, but fermentation works best around 80-85 degrees. Hotter conditions accelerate fermentation but too much heat can kill the yeasts. Red wine is generally fermented at higher temperatures than white wine, so that tannins can be leached from the skin and seeds.

  • Alcohol % below 16%: Unfortunately, yeast can't survive if there is more than 16% alcohol in the wine. Therefore, it will not ferment past this point. If you want more alcohol in your wine you will have to add it through some other means. But if you just want hard alcohol, drink vodka or something.
  • Addition: introducing oxygen into the unfermented wine ("must") at the start of fermentation, to encourage aerobic fermentation, is a good idea. This is because yeast need oxygen to reproduce. Since very little yeast is actually added to the must by the winemaker (at least at home =) the little yeasties need to spruce up their population before they can go to work effectively.

    Once the population is up and fermentation is on its way, introducing oxygen into the fermenting mix is a bad idea.

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