One of the most onerous aspects of wine making is figuring out how to cold and heat stabilize wine. Frequently the problems that might arise from heat and cold instability are not seen by winemakers who take care to store their cases properly; not too hot, not too cold. However, what appears to be stable to a winemaker may become cloudy or riddled with small flakes or crystals in the bottom of the bottle when it leaves the winery. All of these visible features do little to the taste of the wine in and of themselves, with a few obvious caveats.
Wine stored in a warm place can become cloudy due to protein instability. Sometimes these wines demonstrate “ropiness”, a stringy ethereal essence in wines that can appear quite unappetizing. These proteins bond together in clusters, much like the albumin in egg whites, to create these ghostlike structures. Although it is harmless, ropiness can be an indicator of wine that may have heat damage in other ways; maderization is the presence of a sherry aroma in wines that have been overheated.
Heat stability is usually achieved through the application of bentonite. Bentonite is a clay that is protein-philic. Through a series of bench trials, winemakers try to use the least amount of bentonite to strip out proteins that cause heat instability without stripping out many of the proteins that contribute to color and aroma. The amount of protein available to bond with the bentonite is determined by the isoelectric point of the proteins, which in turn are dependent on the pH of the wine.
Cold stability is the point at which a wine precipitates no acids in the form of tartrate crystals. Wines stored in regular refrigerators (at 42 degrees) often cause acids to form into crystals that fall in flakes or tiny balls in the bottom of the bottle or cluster on the bottom of the cork, looking a good deal like glass. These crystals are harmless, but they do change the pH structure of wine. When acids fall out of solution, the pH rises and wines that may have been stable become unstable, especially for microbes and molecular SO2 levels.
Wines that have sediment can often be wines that are simply unfiltered. No amount of racking in the winery can prevent the formation of colloidal that fall to the bottom of the bottle over time.