Potassium in Grape Berry and Wine

Potassium is a cation that is present in both the grape berry and wine. Potassium in the Grape Berry
Grape berries are very rich in potassium which is an essential macronutrient for grapevine and grape berry growth and development:

• the levels of potassium in grape berries may be affected by numerous factors including potassium level in the soil, grape variety, and viticultural practices
• high permeability of plant membranes to potassium leads to a high concentration in plant tissues
• potassium is absorbed by the roots and distributed to all parts of the vine
• early in the season, when the growth rate is high, much of the potassium accumulates in the leaves
• during an initial rapid phase of berry growth, cells divide and expand at a high rate, during which time potassium may play an important role as an osmoregulator
• after véraison, a sharp increase in berry potassium is observed as a result of potassium redistribution from leaves to berries
• potassium may play an important secondary role in the accumulation of sugars during this phase
• grape skin contains about 9 grams of potassium per liter at harvest
• excessive levels of potassium in berries at harvest may reduce the quality of fruit and have a negative impact on wine quality, particularly on red wines

Potassium plays a key role in the grapevine physiology:

• enzyme activation
• major control on the transmembrane potential difference of the plasma membrane, which to a large extent determines the uptake of many different cations, anions and sugars
• regulation of osmotic potential, thus controlling plant water relations, turgor maintenance and growth
• high potassium levels in the berry may decrease the rate of malate degradation by impairing malate transport from the storage pools in the vacuole to the cytoplasm

Potassium in Wine

• potassium is the main cation in must and wine and can affect the pH which is a critical determinant of wine quality
• exchange of tartaric acid protons with potassium cations results in the formation of largely insoluble potassium bitartrate, leading to a decrease in free acid and tartrate:malate ratio, resulting in an increase of overall pH
• grape juice with a high pH often results in unstable musts and wines that are more susceptible to oxidative and microbiological spoilage
• wines with a high pH and low acidity generally have a flat taste
• high pH of grape juice and wine also leads to a decrease of the colour quality and stability of red wines as a result of reduced anthocyanins ionisation at higher pH
• grape juice contains from 0.5 to 3 grams of potassium per liter
• berry potassium levels are often more important to red than to white wines
• red wine fermentation involves leaving the skin in contact with the must after crushing for the extraction of anthocyanins, even more potassium can leach out of the skins into the juice at this time

References:
1.Carlos Conde, Paulo Silva, Natacha Fontes, Alberto C. P. Dias, Rui M. Tavares, Maria J. Sousa, Alice Agasse, Serge Delrot, Hernâni Gerós, Biochemical changes throughout Grape Berry development and fruit and wine quality, Food, 1(1), 1-22 ©2007 Global Science Books.