Vegetarianism and wine
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Wine is sometimes made with animal products. While wine is essentially made from grapes, on occasion animal products are used in small amounts in the production process, and these wines may not be suitable to be part of a vegetarian or vegan diet.
Non-vegetarian/vegan additives 
Wineries might use animal-derived products as finings. To remove proteins, yeast, and other organic particles which are in suspension during the making of the wine, a fining agent is added to the top of the vat. As it sinks down, the particles adhere to the agent, and are carried out of suspension. None of the fining agent remains in the finished product sold in the bottle, and not all wines are fined.
Examples of animal products used as finings are gelatin, isinglass, chitosan, casein and egg albumen. Bull's blood is also used in some Mediterranean countries but is not allowed in the U.S. or Europe. Kosher wines use isinglass derived from fish bladders, though not from the sturgeon, since the kosher status of this fish is in debate.
Of these, casein and albumen (deriving from milk protein and egg white respectively) would be acceptable for vegetarians, but not for vegans.
As an alternative to animal products, Bentonite, a clay mineral, can be used to clarify the wine. Some vintners also let the wine's sediments settle naturally, a time-consuming process. Winemakers are not required to put on their label which clarifier is used, since it is removed from the final product. However, some wine makers will boast on the wine label that their wine is unfiltered, because some wine connoisseurs prefer wine to be unfiltered.
See also 
Further reading