Wine, beer, cider and fruit juices often contain floating matter in them which gives them a haze. It is not desirable to sell them like this, as consumers expect such beverages to be clear. (Even when drinking home brewed wine or beer, we still expect a crystal clear drink.) More importantly, the particles which cause the haze — proteins, phenols, and/or yeast — can also affect the storage life and taste of the product, sometimes even making it bitter.
Fine powders called “finings” are used to clarify the liquids. Most finings used are produced from animal matter. While it’s true that the finings used have been chosen precisely because they leave no discernible trace of themselves behind, sadly, the fact remains that because dead animal matter was in the wine, it is no longer vegetarian.
Apologies to those “don’t ask, don’t tell” vegetarians who would have preferred at least to finish their drink before they heard this.
Many wineries are now experimenting with not using animal-based finings, and passing the wine through clay (bentonite) filters instead, but the process still needs some work, and is mostly being used with white wines only.
The process of clarifying beverages has been done at least since the Romans, who clarified their wines with gelatin.