Chemical Components of Grapes and Wine
Acids in Grapes and Wine
Next to sugars, organic acids are the most abundant solids present in grape juice. They are a very important component of juice and wine. They are responsible for the tart taste and have a marked influence on wine stability, color, and a low pH which results in exclusion of many spoilage organisms. Acids not only produce a refreshing taste (or sourness, if in excess), but also modify the perception of other taste and mouthfeel sensations. This is especially noticeable in a reduction in perceived sweetness. But above all, acidity has a major effect on wine’s tasting balance perception. For the majority of table wines, a range of between 5.5 and 8.5 mg/L total acidity is desirable (Jackson, 2008). White wines are typically preferred at the higher end of the pH scale, whereas red wines are preferred at the lower end.
Acids Found in Wine Grapes
Two primary acids are found in wine grapes are tartaric and malic acids which comprise 90 percent of total acidity. Citric acid is found in grapes too but in much lower concentrations of between 0.1 and 0.5 g/L (Rankine, 2004). Tartaric and malic acids are synthesized in the leaves and grapes, with a majority produced in the grapes prior to véraison. During the early period of berry growth, concentration of both acids increases in the fruit. With the onset of véraison, the total acidity of the juice in the grape decreases with an increase in pH, and this fall in acidity is greater with increasing temperatures.
Vineyard Acid Management
Growers have several vineyard practices they can adopt in an attempt to preserve or decrease the acidity of grapes. Canopy management, irrigation control, and soil fertilization have a significant influence on the acid composition of grapes.
Winery Acid Management
Once the grapes are received at the winery, some cellar operations can influence the titratable acidity (TA) and pH of the wine. Skin contact (or maceration) generally leads to a slight decrease in TA and an increase in pH in the must before the start of fermentation. However, increased extraction of potassium from the skin also coincides with an increase in phenolics.
Acids Produced During Fermentation
As a group, acids are almost as important to the characteristics of wines as alcohols. Acids not only produce a refreshing taste (or sourness, if in excess), but also modify the perception of other taste and mouthfeel sensations. This is especially noticeable in a reduction in perceived sweetness. The role of acids in maintaining a low pH is crucial to the color stability of red wines.
Acidity and pH Relationships
The relative concentration of organic acids is an important factor, because it directly affects wine pH. pH measures the degree to which an acid dissociates or releases its hydrogen (H+) ions into a solution. This degree of dissociation defines the relative strength of the acid. Thus, acidity and pH are related. However, the relationship is neither direct nor predictable due to variation in buffer capacity (i.e., the ability of the juice/wine to resist change). In general, however, higher acid levels in fruit are often associated with lower pH values and vice versa. The pH is a very important quality parameter of a wine.
Potassium
Potassium is an important and essential nutrient element for grapevine growth, yield, and desired composition and quality of grape juice and wine. Potassium plays an important role for example, in the turgor regulation, charge balance, protein synthesis, enzyme activation, and cellular transport processes. Potassium is an inorganic cation present in grapes and is the most abundant of all the cations in grapes and wine.
Tartaric:Malic Acid Ratio
As potassium concentrations increase in the grape, so does the formation of potassium tartaric acid, leading to the subsequent decrease in free tartaric acid. Further, high potassium concentration in the fruit can inhibit the degradation of malic acid. Tartaric acid is associated with the pleasant crisp acidity that is desired in high quality wines, whereas malic acid is associated with sourness.
Vineyard Management
The influence of climate on acidity is probably the most important environmental factor affecting acid in grapes. Vineyard management also plays a role in grape acidity. In the pre-planting stage, if the soil selected for planting the vineyard has high exchangeable potassium levels, an option is to select rootstocks that accumulate low concentration of potassium. Rootstocks, and grapevine varieties in general, differ in their capacity of potassium uptake and translocation.
Winery Treatment
A problem for winemakers is that unless potassium uptake and management is addressed in the vineyard, they will likely have to deal with having high potassium-based fruit for several years.
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