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Wilbur Brinkmeyer — Taylor, Texas asks,

How does the amount of yeast added to a must or juice affect the wine? A kit has one pack of yeast. What happens if yeast is doubled, tripled, etc.?

There are a few effects on a wine if you add more yeast. Number one, the fermentation might start a little faster and go to completion faster because there are simply more cells to eat the sugar, and each can only eat a certain amount of sugar at a time. This will, in turn, increase the fermentation temperature, which is usually undesirable in winemaking unless you are  fermenting in a very cold environment (for example, in your garage, mid-winter in Anchorage, Alaska) and want to make sure your fermentation really gets going. Temperatures under 50 °F (10 °C) will tend to slow down fermentation, so in a  challenging temperature situation you might want to up the amount by 30–50% only. The same is true for fermenting anything in high-Brix (above 25) situations of overripe fruit or dessert wine production. If you throw a little more yeast at your fermenting wine, there is a better chance of having a complete fermentation. There is no reason to ever double, triple or quadruple the amount of yeast called for in a winemaking recipe.
Response by Alison Crowe.