Wednesday, March 22, 2006
LET'S MAKE THIS PERFECTLY CLEAR
As wine ages, natural settling and clarification will occur to some degree, although it is inefficient and inconsistent. The public, however, is usually unwilling to accept cloudy wine or wine with crystals or other particles in it, so various methods are used for "cleaning-up" and finishing wine after fermentation, either before, during, or after aging. These processes also insure a level of stability or shelf-life for wines shipped to retail or restaurant outlets where the bottles may spend some time "on the shelf" before purchase and consumption.
Clarification methods are similar for both white and red wines. All methods of clarification remove unsightly particles from wine, but may also strip wine of pleasant aroma and flavor elements, body, and color.
Racking is the oldest technique of clarification that is just one step beyond natural settling. This is simply siphoning off the relatively clear wine after the lees have settled to the bottom, leaving them behind to discard. The lees are the insoluble matter including dirt and dust, cellulose, dead yeast cells, bacteria, tartrates and pectin. Racking may be done only once or several times before a wine is bottled. Red wines, especially those barrel-aged, are sometimes bottled after racking without further processing.
Cold stabilization may be considered an adjunct or enhancement to racking. This process removes excess tartaric acid that, if untreated, might later form potassium bitartrate crystals, which can show up in wine bottles or on corks. Although these tartrates dissolve easily and are edible (cream of tartar, commonly used in cooking) and harmless, they can alarm the uninformed consumer who thinks there is "broken glass" in his wine. Cold stabilization is accomplished by allowing the wine to warm up to "room temperature" and then chilling it down to about 40° F. The tartaric acid crystallizes in the tank and the wine drawn off by racking.
Fining is a method of clarifying or chemically stabilizing wine. The procedure begins by stirring into the container of wine a fining agent that is heavier than both water and alcohol and does not dissolve in either. The agent ultimately settles to the bottom of the vessel (tank or barrel), causing small suspended particles to precipitate out along with the agent. The clarified wine is then separated by siphoning (racking) off the settlings (lees).
Fining can lower high levels of tannin, remove haze, and reduce color. Care needs to be taken to chose the proper fining level that conforms the wine style that winemaker wants to achieve. Over-fining can result in thin wines that lack aroma complexity, flavor depth, viscosity, and aging potential.
Physical agents work by absorbing tiny particles and dragging them. Chemical agents work by forming chemical bonds with hydrogen elements in the undesired particles. Fining agents include egg white, milk, blood, gelatin, carbon, casein (the principal protein constituent of milk and cheese) and isinglass (an extract of sturgeon bladders). Heat stabilization is a fining process that uses bentonite (a clay of hydrated magnesium silicates) to remove protein, which may cloud a wine.
Filtering means passing the wine through a filter small enough to remove undesirable elements. Various filtering technologies allow great flexibility to winemakers to make stable wines of varying styles. As with fining, filtering can also remove elements that contribute to flavors and aromas, so winemakers need to be judicious and conservative with this technique to avoid "collateral damage" that leaves their wine clean but lifeless.
Depth or sheet filtration uses a relatively thick layer of fine material (diatomaceous earth, cellulose powder, perlite, etc.) to trap and remove small particles. Surface or membrane filtration passes wine through a thin film of plastic polymer with uniformly-sized holes that are smaller than the particles.
Sterile filtration uses micropore filters, which are fine enough to remove yeast cells, to prevent further fermentation. This is especially significant when residual sugar is allowed to remain in the wine at low levels. Prior to the advent of modern micropore filtration, slightly sweet wines were endangered by the possibility of revived fermentation in the bottle.