No wine fining agent is 100% specific in what it does. As well as fining out phenolics (tannins), wine fining agents typically take out some aroma and flavor. Whichever wine fining agent is selected, winemakers will use as little as necessary to produce the quality of red or white wine that is required.
Free run juice typically contains very low levels of phenolics. Phenolics originate in the grape berry and affect taste, bitterness, astringency and color. They are the key wine preservative and basis of long aging. Free run juice is not in contact with skins or seeds for any length of time and, therefore, does not pick up any of the undesirable phenolic substances.
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"Press fraction" of juice is phenolic rich and typically harder, sometimes more bitter, will age far more quickly, and usually makes a lower quality wine when compared to wine made from free run juice. The wines are simply combined; the two juices are mixed and fermented together.
The finished wines are subjected to processes that strip out the phenolics. This process is called wine fining.
Wine is supposed to be very clear and completely free of any suspended particles, so wine makers go to great lengths to clarify it. Wine fining agents typically are natural proteins or substances that have been synthesized (the process of producing a chemical compound) to mimic the action of proteins.
Through winemaking history, a variety of agents have been used for wine fining: ox blood, egg whites, milk casein, isinglass which is prepared from the swim bladders of certain fish, horse gelatins, seaweed, and clay, to name a few. Almost any protein will work at least somewhat, by binding to other proteins and forming solid deposits.
Wineries use industrial filtration machines and wine fining agent chemical additives both organic and inorganic to precipitate out suspended solids, no matter how small, to clarify wine before bottling or long term storage.
Probably the best known commercial wine fining agents are mixtures of pure inorganic bentonite, aluminum silicate clay from Wyoming, quite popular among Californian wineries, and polyvinyl-polypyrrolidone or PVPP, used to fine white juices. Activated carbon is sometimes used to strip the color out of Pinot noir juices for sparkling wine production.
All the particles clouding up wine have an electrical charge. Positively charged wine fining agents like gelatin, Claro K.C. and Sparkolloid, attract negatively charged particles, binding with them, making them too heavy to float. They then sink to the bottom, leaving the wine free of any cloudiness and brilliantly clear. The wine may also be allowed to sit still for long periods so the solids that have fallen to the bottom can be racked out. Clear wine is then siphoned off from the top, leaving the sediment at the bottom of the tank or barrel.