Potassium, the most abundant inorganic element in grapes and wine, forms a multitude of salts (microphotograph of salt crystals) that influence wine’s final pH.
Winemaking is a chemical balancing act. Great wine starts in the vineyard, the experts say, because grapes come to the processing facility with all sorts of variables in need of equilibrium.
Among the variables is pH. Wine pH is determined in a complex dance between the concentration and strength of various acids and the salts formed by inorganic chemicals, including potassium, in reaction with acids.
If pH is too high, the wine becomes an unstable breeding ground for unwanted micro-organisms and. In addition, color fades and the wine may turn brown during aging. If pH is too low, micro-organism growth is inhibited, but the resulting wine may be too sour to enjoy.
Since potassium is the most abundant inorganic element in grapes and wine, understanding its role in pH is crucial to making great wine. In a recent series of cleverly designed and commercially scaled experiments, WSU research enologist Jim Harbertson and his technician, Eric Harwood, shed considerable light on the question of just where potassium comes from and goes during grape crush and fermentation.
Their work, recently published in the American Journal of Enology and Viticulture, suggests ways that winemakers can control potassium – and final pH.
During maceration, in which grape skins remain in contact with juice, the researchers showed that potassium declines during pomace contact time and is later recoverable from the berry skins. (Pomace is the solid remains of the fruit after pressing.)
What’s happening, the researchers suggest, is that the skins of the grapes, although rich in potassium to begin with, are adsorbing potassium during their contact time with the must. Potassium is reacting with complex carbohydrates called polysaccharides in the pomace skin, and acting as an ion-exchange resin. An ion-exchange resin is a highly porous, complex structure that easily traps and retains ions. And ions are key in the formation of the salts that play a significant role in final pH.
Further, this dance of ions can be controlled. The researchers found that potassium extraction from grape skins was limited by the concentration of potassium present in the juice (pulp) and the concentration of ethanol in the must. The main source of potassium in the wine was from the pulp.
Several smaller scale experiments confirmed the results found on a commercial scale by taking grapes harvested from the same vineyard and experimenting with different concentrations of potassium and ethanol.
Results also suggest that treating juice with pectinase while skins are present likely increases the potassium content of the finished wine and potentially increases the pH. Winemakers routinely use pectinase enzymes to improve juice yield and color extraction and, thus, may also be liberating potassium from the pomace.
To better understand this potential twist in the pH dance, Harbertson and his team are conducting further research.
