According to howstuffworks.com, reverse osmosis takes place when pressure applied to a highly concentrated solution causes the solvent to pass through a membrane to the lower concentrated solution, leaving a higher concentration of solute on one side, and only solvent on the other. In the case of high-alcohol wine, the wine is filtered to separate alcohol, water, and volatile compounds from the color and flavor components. The filtered material (permeate in the figure below) is then subjected to a distillation process wherein some portion of the alcohol is removed. At the completion of this process the remaining permeate material is re-combined with the flavor and color particles that had been filtered out previously and the end result is a lower-alcohol wine than at the beginning.
|Reverse osmosis (Source: memstar.com.au)|
The spinning cone is comprised of two sets of inverted cones, one fixed against the wall of the steel container, the other set on a rotating rod and parallel to the fixed cones. The wine is fed into the assembly from the top and flows from fixed cone to parallel rotating cone where the centrifugal force spins it out towards the center as a thin film which then falls on to the next lower fixed cone. This pattern is repeated until the liquid reaches the bottom. Simultaneously, a low temperature vapor is introduced into the assembly from the bottom and flows up the core stripping volatiles from the thin film on its way up. This volatile-impregnated vapor flows out of the top of the assemblage and into a condensing system where the volatiles are captured in a condensed liquid form. The cones provide a large surface area for evaporation of the volatiles into the vapor.
In most cases, the wine is run through the system a number of times in order to obtain the required alcohol levels. On the first pass the vapor may extract the volatilized aromas and flavors and the remaining liquid is run through the cone again at higher temperatures to remove some portion of the alcohol. The aromas and flavors are then added back into the liquid at the end of the process to produce a lower alcohol wine. Producers will use this method to reduce the alcohol by 1 to 2%.
The third option is hydration of the grape must using water acidulated with tartaric acid. By using acidulated water, the sugar can be diluted without a concurrent dilution of the acids. In California wineries, according to Brehm Vineyards, the practice is to use 7 gallons (26.5 liters) of water per ton of grapes in order to lower Brix by one degree.
The best-case situation for a winemaker is that growing conditions yield grapes that produce a balanced wine. In the event, however, that the wine is unbalanced due to high levels of alcohol, vis a vis acid levels, the winemaker has the above tools available to assist in amelioration of the problem.
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