Yeast is an essential ingredient in wine making. Wine makers add yeast during the process to promote fermentation, which makes the alcohol in wine. Once the yeast is "spent"---which means it's no longer active--it becomes a sediment known as lees at the bottom of the wine making vessel. Some styles of wine are aged with this yeasty sediment in place, a process known as "sur lie," or "on the lees." Other wine makers will quickly remove the lees, which then become a waste product. But often that is not the end of their usefulness; lees can be used for several other functions.
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Spent yeast is most frequently used to supplement animal feed and some kinds of fish meal. It contains protein, carbohydrate and vitamins, so it's of considerable nutritional value. Because of its active ingredients, it's considered a probiotic supplement. After processing, it's fed to domesticated farm animals, such as sheep and beef cattle. There is an international trade in wine lees for this purpose.
Spent yeast is particularly high in substances called beta glucans, found in the cell walls of the yeast. These can be used in dietary supplements. Manufacturers use either hot water and organic solvents, or an enzyme treatment to remove the beta glucans from the wine lees. When added to a supplement, these substances are said to stimulate immune response against infections and even cancer.
Lees is just one of the waste products from wine making that can be used to recover calcium tartrate, a salt of tartaric acid. This substance is also extracted from spent grape skins after they have been pressed for wine. Tartrates are used as an additive in processed food, as they can have antioxidant properties, helping keep food fresh longer. They can also act as acidity regulators and as emulsifiers.
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