| Rehydration: Adding Water to High Sugar Grapes |
After throroughly mixing and gleefully sloshing your grape must, upon taking a hydrometer reading, you realize that the your grape must has a brix reading in excess of 25 degrees. You don't need to worry about producing a high alcohol containing wine or having a wine get stuck with residual sugar. The simple solution is to add water to your must.
Trying to ferment grapes with high sugar / brix levels can cause problems with the primary and secondary fermentations. High brix levels mean a lot of alcohol will be produced. Sometimes this is good, but in most case, it is not what you are looking for. High alcohol in wines can make the wine taste 'hot'. High alcohol will also inhibit most yeast strains and actually kill the yeast a threshold concentration of alcohol. If the yeast die before all the sugar is fermented, you are left with a stuck fermenation and residual sugar in your wine. High alcohol can also inhibit the bacteria that cause Maloactic Fermenation.
What do you do then if the grapes you are fermenting does have too much sugar? Rehydration of the grapes by the addition of acidulated water is the answer. The water used to rehydrate the must be acidulated with tartaric acid, this allows the sugar to be diluted without diluting the acid levels of the grapes. (The exception is if you have a high sugar grape that is also high in acid. In most cases, grapes that are higher than 24.5 brix will have normal to even low levels of total acid.)
The common practice is to add 6 or 7 grams (or 1/4 ounce) of tartaric acid to 1 liter of distilled water to make up your acidulated water dilution solution. (This solution is equivalent to a total acidity of 0.70 g/100 ml or 7 g/L.) This solution of tartaric acid will be used to dilute your high sugar must or juice. (Note: You may need more than 1 liter of acidulated water. See below on how to determine the amount of acidulated water you will need for your volume of wine.)
The most common mistake made is adding acidulated water based on the volume of your must - crushed grapes, not your final volume of the pressed, finished wine. You must first determine how much finished wine you will produce before you dilute your must or juice. The same principle holds when adding sugar to chapitalize must or juice.
For white grape juice, your yield is roughly the same as your starting volume. In general, for red grape musts, the yield is 3-1/3 gallons finished, pressed wine per 5 U.S. gallons of fermented must. This will vary based on the skin to juice ratio. Bordeaux varietals average a little over 3 U.S. gallons finished, pressed wine per 5 U.S gallons of fermented must. Rhone varietals average close to 3-1/2 U.S. gallons of finished, pressed wine per 5 U.S. gallons of fermented must.
There are two ways of approaching the rehydration of the must. California wineries use a rule of thumb of adding 7 gallons (26.5 liters) of water/ton of grapes to lower the brix one (1º) degree. The water addition all goes to the final wine.
Note: 3 U.S. gallons is equivalent to 11.4 liters. Since we are measuring total acidity in metric units, it is important to convert your volume units from the English system to the Metric system. (Note: 28 grams = 1 ounce).
Another method that may be conducted incrementally and checked by hydrometer/refractometer is done algebraically.
In general, the formula is as follows:
Let:
O = Original Brix of must or Juice
L1 = volume (in liters) of finished wine from undiluted must/juice
B = Brix you want to dilute must/juice to
L2 = volume (in liters) of finished wine from diluted must/juice
Y = volume (in liters) of acidulated water to add to must or juice to dilute to desired level, B.
Equation 1: (L1) x (O) / (B) = (L2)
Equation 2: (L2) - (L1) = Y
For Example: We have 5 U.S. gallons of red must at a brix of 26.5°. How much acidulated water do you add to lower the brix to 24.5°?
O = 26.5,
L1 = 11.4 liters (5 U.S. gallons of red must = 3 gallons finished wine)
B = 24.5
L2 = Do equation 1 to determine L2
Y = Do equation 2 to determine Y
(11.4 liters) x (26.5 brix) / (24.5 brix) = L2
L2 = 12.28 liters
(12.28 liters) - (11.4 liters) = Y
Y = 0.88 liters
Therefore, we must add 0.88 liters of acidulated water to our 5 U.S. gallons of red grape must to dilute the brix down to 24.5 brix.
Further refinement:
The Art of Rehydration (now permitted by California law) is evolving. Wineries are adding thousands of gallons of water to their $20-$50 per bottle of wine. Unless the grapes are quite high in acid, the water added to the juice/must should be acidulated with tartaric acid. The normal addition of tartaric acid is 6 grams per liter of non-chlorinated water. This acidulated water is then used to make the sugar dilution in the juice/must as shown above.
It is advised that you add 1/2 the calculated amount of water to your juice or must, then stir extremely well. Check that your calculations are corresponding to the reduction in brix. This will give you the opportunity to make an adjustment in your next acidulated water addition.
White Juice can be "rehydrated" with acidulated water based on the formula above as well.
With Black Must rehydrations, you have further options:
Before making your rehydration, you may bleed off a portion of the grape juice from your must to ferment as a saignée, rosé, blush or oeil de perdrix (it may also need dilution). The removal of juice will allow you to keep the original - or minimize the dilution - relationship of skins to juice after your water addition. It will tend to concentrate the color and flavor of the remaining must at a more 'fermentable' brix level. Some commercial wineries do note bleed off any juice after making their rehydration/dilution, others will bleed off as much as twice the amount of water used for dilution. The choice is yours.
Happy winemaking