Clarification and stabilization of wine
This matter may include dead yeast cells (lees), bacteria, tartrates, proteins, pectins, various tannins and other phenolic compounds, as well as pieces of grape skin, pulp, stems and gums.
[1] Before fermentation, pectin-splitting enzymes and, for white wine, fining agents such as bentonite may be added to the must in order to promote the eventual agglomeration and settling of colloids.
[2] After fermentation, the force of gravity may eventually cause the wine to "fall bright" or clarify naturally, as the larger suspended particles gradually settle to the bottom of the storage vessel.
[6] The most common organic compounds used include egg whites, casein derived from milk, gelatin and isinglass obtained from the bladders of fish.
Many white wines require the removal of all potentially active yeast and/or lactic acid bacteria if they are to remain reliably stable in bottle, and this is usually achieved by fine filtration.
[4] In depth filtration, often done after fermentation, the wine is pushed through a thick layer of pads made from cellulose fibers, diatomaceous earth, or perlite.
The finest surface filtration, microfiltration, can sterilize the wine by trapping all yeast and, optionally, bacteria, and so is often done immediately prior to bottling.
An absolute rated filter of 0.45 μm is generally considered to result in a microbially stable wine and is accomplished by the use of membrane cartridges, most commonly polyvinylidene fluoride (PVDF).
While there is some variation according to grape variety and climate, usually about half of the deposits are soluble in the wine, but on exposure to low temperature they may crystallize out unpredictably.
[10] In some white wines there are significant quantities of proteins that, being "heat-unstable", will coagulate if exposed to excessively fluctuating heat; the use of fining agents such as bentonite can prevent the haze this causes.
If both alcoholic and malolactic fermentation have run to completion, and neither excessive oxygen nor Brettanomyces yeast are present, this ought to cause no problems; modern hygiene has largely eliminated spoilage by bacteria such as acetobacter, which turns wine into vinegar.
Similarly, a wine that has not been put through complete malolactic fermentation may undergo it in bottle, reducing its acidity, generating carbon dioxide, and adding a diacetyl butterscotch aroma.
These phenomena may be prevented by sterile filtration, by the addition of relatively large quantities of sulfur dioxide and sometimes sorbic acid,[1] by mixing in alcoholic spirit to give a fortified wine of sufficient strength to kill all yeast and bacteria, or by pasteurization.
[citation needed] Typically, the wine is heated to 185 °F (85 °C) for a minute, then cooled to 122 °F (50 °C), at which temperature it remains for up to three days, killing all yeast and bacteria.
[3] Winemakers deliberately leave more tartrates and phenolics in wines designed for long aging in bottle so that they are able to develop the aromatic compounds that constitute bouquet.
