WINE TECHNOLOGY: WINE MAKING- STEP 2 COLD AND HEAT STABILIZATION - Cold stabilization is a process used in winemaking to reduce tartrate crystals (generally potassium bitartrate) in wine. These tartrate crystals look like grains of clear sand, and are also known as "wine crystals" or "wine diamonds". They are formed by the union of tartaric acid and potassium, and may appear to be sediment in the wine, though they are not. During the cold stabilizing process after fermentation, the temperature of the wine is dropped to close to freezing for 1–2 weeks. This will cause the crystals to separate from the wine and stick to the sides of the holding vessel. When the wine is drained from the vessels, the tartrates are left behind. They may also form in wine bottles that have been stored under very cold conditions. During "heat stabilization", unstable proteins are removed by adsorption onto bentonite, preventing them from precipitating in the bottled wine. SECONDARY (MALOLACTIC) FERMENTATION AND BULK AGING During the secondary fermentation and aging process, which takes three to six months, the fermentation continues very slowly. The wine is kept under an airlock to protect the wine from oxidation. Proteins from the grape are broken down and the remaining yeast cells and other fine particles from the grapes are allowed to settle. Potassium bitartrate will also precipitate, a process which can be enhanced by cold stabilization to prevent the appearance of (harmless) tartrate crystals after bottling. The result of these processes is that the originally cloudy wine becomes clear. The wine can be racked during this process to remove the lees. The secondary fermentation usually takes place in either large stainless steel vessels with a volume of several cubic meters, or oak barrels, depending on the goals of the winemakers. Unoaked wine is fermented in a barrel made of stainless steel or other material having no influence in the final taste of the wine. Depending on the desired taste, it could be fermented mainly in stainless steel to be briefly put in oak, or have the complete fermentation done in stainless steel. Oak could be added as chips used with a non-wooden barrel instead of a fully wooden barrel. This process is mainly used in cheaper wine. Amateur winemakers often use glass carboys in the production of their wine; these vessels (sometimes called demijohns) have a capacity of 4.5 to 54 liters (1.2–14.3 US gallons). The kind of vessel used depends on the amount of wine that is being made, the grapes being used, and the intentions of the winemaker. MALOLACTIC FERMENTATION - Malolactic fermentation occurs when lactic acid bacteria metabolize malic acid and produce lactic acid and carbon dioxide. This is carried out either as an intentional procedure in which specially cultivated strains of such bacteria are introduced into the maturing wine, or it can happen by chance if uncultivated lactic acid bacteria are present. Malolactic fermentation can improve the taste of wine that has high levels of malic acid, because malic acid, in higher concentration, generally causes an unpleasant harsh and bitter taste sensation, whereas lactic acid is more gentle and less sour. Lactic acid is an acid found in dairy products. Malolactic fermentation usually results in an increase on the pH of the wine. This should be monitored and not allowed to rise above a pH of 3.55 for whites or a pH of 3.80 for reds. pH can be reduced roughly at a rate of 0.1 units per 1 gram/litre of tartaric acid addition. The use of lactic acid bacteria is the reason why some chardonnays can taste "buttery" due to the production of diacetyl by the bacteria. All red wines go through complete malolactic fermentation, both to lessen the acid of the wine and to remove the possibility that malolactic fermentation will occur in the bottle. White wines vary in the use of malolactic fermentation during their making. Lighter aromatic wines such as Riesling, generally do not go through malolactic fermentation. The fuller white wines such as barrel fermented chardonnay, are more commonly put through malolactic fermentation. Sometimes a partial fermentation, for example, somewhere less than 50% might be employed. TO BE CONTINUED -
Posted on: Sun, 11 Aug 2013 00:21:37 +0000
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