Wine, beer, cider and fruit juices often contain floating matter in them which gives them a haze. It’s 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.
The step in which the beverages are clarified is called “Fining”, and the material that is used to do this is called “Finings.” Finings are protein matter that is mixed into the liquid. They do not dissolve in the liquid, but rather they float in it and attract to themselves like magnets the haze-making matter, which is stuff such as proteins and phenols in the liquid.
When the proteins stick to them, the Finings get heavy and sink to the bottom of the jug or tank. At that point, the liquid is siphoned out, leaving the Finings sediment at the bottom, which is then discarded.
A great many studies have shown that the Finings leave a very negligible trace of themselves behind. This makes sense, as the reason they’ve been chosen over the millennia (though by trial and error, of course) is on the basis of their not affecting the taste or storage life of the product being made.
Without Fining agents, not only would the proteins floating in the liquid make the beverages hazy, they would also make some products go bitter over time and not store and age as well.
Vegetarian synthetic Fining agents have been worked on, but no satisfactory ones have really been found.
Some of the Finings traditionally used are isinglass, egg white, gelatin, modified casein (protein from milk), chitin and Irish Moss (actually dried sea algae.) In some parts of France, particularly the Rhone Valley, blood albumen or dried oxblood has been used, though this practice is declining.
Some Fining agents have a positive charge to them (such as gelatin and Isinglass), and so attract negatively-charged proteins to them; others, such as Irish moss have a negative charge, attracting the positive ones to them. Sometimes a combination of both Fining agents are used.
The process of clarifying beverages has been done at least since the Romans, who clarified their wines with gelatin. Ground oyster shells have also been used in the past.