Acidity in Grapes
The Role and Development of Acidity in Wine Grapes
Acids are critical for the survival and function of grapes. Their internal environment needs to be maintained at certain acidity levels for optimal metabolic activity and cellular function. Over time, certain acids have developed that provide this function.
These are known as primary acids; they include tartaric acid, malic acid, citric acid, gluconic acid, mucic acid, and coumaric acid. The relative and absolute concentrations of these acids vary by varietal, and are influenced by the terrain, climate and vineyard practices.
It is important to remember that concentrations of these acids change as the berries develop. The winemaking process alters acidity as well; the acid in wine is very different from the acid in grapes, even at maturity.
Acidity and Vineyard Conditions
The influence of climate on acidity is probably the most important environmental factor affecting acid in grapes. As vineyard temperature increases, the vines and grapes will rely on alcoholic respiration more than glycolysis (the breakdown of sugar). This is an evolutionary adaptation to save sugar for the seeds.
Vineyard management also plays a role in grape acidity. The type of trellis system used and canopy management techniques can directly affect sun exposure, which is intimately associated with temperature and grape metabolism.
The influence of terrain on acidity is not as pronounced. A west facing hillside vineyard will have longer days and more sun exposure than flat-land or east facing vineyard. Viticulturalists take this all into account when they determine the varietal and clone to plant on their land.
Important Acids in Grapes
The most important acid in grapes is tartaric acid. Its concentrations get as high as 15g/L in unripe grapes in cooler climates. Compare this to around 2g/L in ripe grapes from warm regions. Tartaric acid is rare in nature; for this reason, it has garnered the name “wine acid.”
Malic acid is found in almost every living organism. It is responsible for the crisp, tart taste of green apples. Its concentrations in unripe white varietals can reach 25g/L. At maturity, this number can drop to 1g/L in warmer regions.
Metabolically, the most important acid is citric acid. Citric acid is an intermediary in the TCA cycle. This is cycle is one of the fundamental energy deriving process in nature, and citric acid is an integral component. Concentrations in must range around 0.5g/L.
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