Bentonite and barrel leaks
A small amount of bentonite can be directly added to any wine and used to seal small leaks in a barrel that was previously dry-stored or has a more leakage- prone, e.g. fortified, wine in it.
Questions about bentonite
application Should I use sodium or calcium bentonite?
It doesn’t really matter, as long as the enologist performs an ethanol titration or one of the arbitrary heat stability bench tests on each wine and determines the smallest effective dose to satisfy the test.
Bentonite clay is the most widely used fining agent against heat-instable grape proteins in white wines. In red wines from Vitis vinifera, the inherent tannins usually denature these proteins enough to cause precipitation during aging. However, anthocyanin-rich but tannin-deficient red European-American varieties, as well as blush/rosé wines and red viniferas from very cool climates, should be tested for protein- stability.
There are two different forms of bentonites commercially available: sodium-rich ones and calcium-rich ones. Suppliers of sodium bentonites argue that this form has a protein fining capacity twice as high as its calcium cousin. Suppliers of calcium bentonites argue that their form swells less in water, and it creates fewer lees and a smaller loss of wine when racking.
Excess additions of sodium to wine are undesirable, as sodium consumption may contribute to high blood pressure and heart disease. For the same reason, the use of sodium metabisulfite for sulfur dioxide additions or sodium bicarbonate (baking soda) for deacidification purposes is not permitted by the U.S. Tax and Trade Bureau (CFR Title 27 Part 24 § 24.246 Materials authorized for the treatment of wine and juice). The quantities added to wine by a heavy bentonite treatment can double the amount of sodium naturally present in grape juice (10 to 20 mg/L), but even then wine is still considered a “very low sodium” beverage.
On the other hand, excess release of calcium into a wine from bentonite via exchange with grape proteins may increase the risk of calcium tartrate instability.
For example, an addition of 1,920 mg/L (16 lbs/1,000 gal) calcium bentonite — equivalent to 960 mg/L (8 lbs/1,000 gal) sodium bentonite — to a batch of protein-rich Gewürztraminer would result in an additional potential for 114 mg/L calcium tartrate. Since calcium tartrate does not respond as readily to cold stabilization as potassium bitatrate, this may mean the difference between a stable wine and a wine throwing a glass-like precipitate that may worry consumers.
Should I rehydrate my bentonite in water or in wine?
Water. Bentonite, independent of type, should be rehydrated with clean, chlorine-free hot (140°F, 60°C) water. It must be added under immediate, vigorous mixing to the water (not the other way around) and allowed to swell for at least four hours. The lump-free slurry shouldn’t sit longer than overnight, as this may encourage microbial growth. A maximum of 16.7 L of water may be used to dissolve each kilogram of bentonite (2 gallons of water per pound). Note that the total amount of water introduced from all processing sources during the winemaking should not exceed 1 percent of the wine. For bench trials in the winery lab, a mixing ratio of water to bentonite of 16 to 1 (60 g per 1 L) results in an easily pipettable 6 percent w/v slurry (Table 1).
Table 1: Bentonite (6%) slurry additions for bench trials
Rehydrating with wine doesn’t allow the bentonite to fully swell, thereby reducing its fining capacity. In addition, it is a waste of wine that cannot be recovered.
However, if one would use 6 percent slurry at additions above 60 g/hl (5 lbs/1,000 gal), the amount of water added to the wine exceeds 1 percent. Thus in practice, bentonite is typically dissolved at ratios of about 8 to 1 (1 kg bentonite per 8 L of water; 1 lb/gal) which allows for bentonite additions of up to 120 g/hl
Fining with Bentonite