Why is diammonium phosphate added to fermenting wine tanks?

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Dear Dr. Vinny,

Why is diammonium phosphate (DAP) added to fermenting wine tanks?

—Greg R., from the Internet

Dear Greg,

DAP—a water-soluble ammonium phosphate—is a yeast nutrient, resulting in stronger and more viable yeasts that in some cases work faster. Sometimes, fermentations slow down, or even get "stuck," stopping before all the sugar is converted to alcohol. These fermentation problems are often yeast problems—there can be something wrong with the yeast itself (old, weak, or a bad match to the grapes it's working with) or something in the environment (cleanliness, temperature) could be interfering with the yeast and preventing it from performing better. So DAP and similar products are used either to prevent fermentation problems or to fix them.

—Dr. Vinny

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