Is there any risk to a wine’s quality, or the health of people drinking it, if the winemaker adds too much yeast or nutrients for the fermentation?

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Dear Dr. Vinny,

Is there any risk to a wine’s quality, or the health of people drinking it, if the winemaker adds too much yeast or nutrients for the fermentation?

—Antonis, Greece

Dear Antonis,

Back to basics! Wine is what happens after the sugar in grapes is converted to alcohol with the help of yeast, through the process of fermentation. Some winemakers let nature take its course, allowing native yeasts (also known as “indigenous,” or “wild” yeasts) found on the grapes or in the winery to spontaneously ferment the grape juice without any manmade intervention required. But most wine is made by inoculating the juice with commercial yeasts , in the interest of achieving more control over the profile of the resulting wine.

Sometimes a fermentation gets “stuck,” when the yeasts slow down or stop consuming the sugars prematurely. There could be something in the environment getting in the way, like the temperature is too hot or too cold, or the equipment wasn’t completely clean, or the yeasts are not a good match with the grapes, or they are just too old and weak to continue on. In these cases, a winemaker can add a yeast nutrient, to give the yeasts a boost to keep going. The most common of these nutrients is diammonium phosphate, or DAP.

Back to your question: What happens if a winemaker adds too much yeast? Probably not much—there’s only so much sugar in the grapes for the yeast to convert, and that limits how much work there is for yeast to do. The extra, hungry yeasts without any sugar to consume will end up dying and settling to the bottom along with the rest of the lees and sediment. A winemaker would probably decide to rack the wine off of this extra sediment, so that the wine isn’t hazy and there’s no threat of any unexpected secondary fermentation.

Nutrients are slightly different. There are actually regulations to how much DAP a winemaker can add to a commercial wine in the U.S. and other countries—it’s plenty to allow for a healthy fermentation, but not in excess. These nutrients are useful, but they can increase the risk of volatile acidity and microbial instability (think spoilage organisms).

But the biggest reason it’s regulated is because too much nutrient additive can lead to an organic compound called ethyl carbamate, which is a suspected human carcinogen. Many fermented foods and beverages from soy sauce and kimchi to yogurt, olives, beer and whisky are all known to contain trace amounts of ethyl carbamate.

—Dr. Vinny

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