Fining Wine for Clarification
Primarily done during white wine production, fining for clarification (i.e., protein stabilization) helps remove insoluble proteins that slowly precipitate from the solution. Protein molecules are much too small to be visible in the wine. However, under certain conditions, protein molecules can link together (polymerize) and become larger and larger. After much linking together, some protein molecules become so large they are visible in the wine, and they are too large to remain suspended. This protein linking process is very slow at normal cellar temperatures, but protein molecules can grow in size rapidly when wine becomes warm.
Other Types of Hazes
Besides protein hazes, wine clarity is also influenced by polysaccharides, potassium bitartrate, or phenolic polymers.
Bentonite, a negatively charged clay colloid, is principally used to remove positively charged proteins in white and rose wines by forming bentonite-protein complexes that settle to the bottom of the tank. Because bentonite settles out relatively quickly and is easily filtered, it is one of the few fining agents that does not itself potentially create a stability or clarification problem.
Potential Problems with Bentonite Fining
Bentonite fining tends to produce voluminous sediment, which can cause considerable wine loss during racking. Bentonite lees volumes often range from 5 to 10 percent. Some wineries employ centrifugation (followed by filtration) to minimize the problem of wine loss after bentonite fining.
There are two different forms of bentonites commercially available: sodium bentonite and calcium bentonite. Sodium bentonites swell more than calcium bentonites, and thus produce a greater volume of lees for an equivalent dose, but are more efficient at adsorbing proteins and, hence, require a lower dose than calcium bentonites. Sodium bentonite causes a slight increase in the wine's sodium content, but only a negligible decrease in acidity.
Bentonite, independent of type, should be rehydrated with clean, chlorine-free hot (60°C, 140°F) water (Butzke, 2010d). Rehydrating with wine doesn't allow the bentonite to fully swell thereby reducing its fining capacity. Bentonite must be added under immediate, vigorous mixing to the water (not the other way around) and allowed to swell for at least four hours (Butzke, 2010a).
Must and Fermentation Treatment
Some winemakers prefer to add bentonite during the juice stage or during fermentation which aids protein stability and eliminates or reduces the amount of bentonite needed to stabilize the wine when fining. This process reduces the number of treatments required later in the process, and is justified if permanent stability is obtained. It is not compatible with barrel-aging white wines on yeast lees.
Proper mixing is crucial, and it has been shown that mixing speed, time, and temperature affect the efficacy of the treatment. At least 10 to 15 minutes of vigorous mixing is recommended, bentonite acts best at moderate temperature (15°25°C, 59°77°F) for the wine. Pumping breaks up any lumps and homogenizes the mixture. The reaction between protein and bentonite is quick but not instantaneous. Afterwards, the adsorbed proteins slowly settle by gravitation of the bentonite particles to form bentonite lees at the bottom of the tank.
Other Fining Agents
There may be cases where bentonite additions are ineffective in achieving protein stability. This may either be the result of the presence of pectins or certain proteins which do not react with bentonite. In the case of these proteins, the wine can be fined with a gelatin which, together with bentonite, should succeed in removing all the unstable proteins.
Silica gel fining agent (also called Kieselsol) is negatively charged and can interact with positively charged protein molecules, the same as bentonite does. Silica gel is a very clean fining agent in the sense that it can be made at high purity and does not contain other components that might affect the wine aroma and flavor.
Protein Stability Tests
Various laboratory tests have been used for many years to assess the risk of protein turbidity before bottling. These tests are based on the instability of proteins under various conditions: at high temperatures, or in the presence of tannin, trichloroacetic acid, ethanol, or reagents based on phosphomolybdic acid.
Heat tests are the quickest test for determining protein stability by exposing the wine sample to a high temperature for a short period of time, e.g., 49 degrees C (120°F) for 48 hours or 80 degrees C (176°F) for six hours followed by a period of cooling. A minimum of two hours cooling is required for the heated proteins to aggregate and precipitate.
There are a number of chemical tests that have been devised to assess the heat stability of wine proteins. These include the use of ethanol, ammonium sulfate, trichloroacetic acid, phosphomolybdic acid, phosphotungstic acid and tannic acid.
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