The respiratory loss of malic acid is more pronounced in warmer climates. As the vine progresses through the ripening stage, malic acid is metabolized in the process of respiration, and by harvest, its concentration could be as low as 1 to 9 g/L. ![]() The levels of malic acid in grape berries are at their peak just before veraison, when they can be found in concentrations as high as 20 g/ L. Its concentration varies depending on the grape variety, with some varieties, such as Barbera, Carignan and Sylvaner, being naturally disposed to high levels. Its chemical structure allows it to participate in enzymatic reactions that transport energy throughout the vine. In the grape vine, malic acid is involved in several processes which are essential for the health and sustainability of the vine. Its name comes from the Latin malum meaning “apple”. It is found in nearly every fruit and berry plant, but is most often associated with green (unripe) apples, the flavor it most readily projects in wine. Malic acid, along with tartaric acid, is one of the principal organic acids found in wine grapes. Malic Riesling from cool climate wine regions, such as the Rheingau in Germany will have more malic acid and green apple notes than wines from warmer regions. Winemakers will often put the wine through cold stabilization, where it is exposed to temperatures below freezing to encourage the tartrates to crystallize and precipitate out of the wine, or electrodialysis which removes the tartrates via a membrane process. The crystallization of these tartrates can happen at unpredictable times, and in a wine bottle may appear like broken glass, though they are in fact harmless. While some variance among grape varieties and wine regions exists, generally about half of the deposits are soluble in the alcoholic mixture of wine. During fermentation, these tartrates bind with the lees, pulp debris and precipitated tannins and pigments. Less than half of the tartaric acid found in grapes is free standing, with the majority of the concentration present as potassium acid salt. As the vine progresses through ripening, tartaric does not get metabolized through respiration like malic acid, so the levels of tartaric acid in the grape vines remain relatively consistent throughout the ripening process. During flowering, high levels of tartaric acid are concentrated in the grape flowers and then young berries. Some varieties, such as Palomino, are naturally disposed to having high levels of tartaric acids, while Malbec and Pinot noir generally have lower levels. The concentration varies depending on grape variety and the soil content of the vineyard. Along with malic acid, and to a lesser extent citric acid, tartaric is one of the fixed acids found in wine grapes. In most plants, this organic acid is rare, but it is found in significant concentrations in grape vines. Tartaric acid is, from a winemaking perspective, the most important in wine due to the prominent role it plays in maintaining the chemical stability of the wine and its color and finally in influencing the taste of the finished wine. ![]() Tartaric While normally clear, tartaric crystals (pictured) can be dyed the color of the wine in which it has been saturated. Sometimes, additional acids, such as ascorbic, sorbic and sulfurous acids, are used in winemaking. Most of the acids involved with wine are fixed acids with the notable exception of acetic acid, mostly found in vinegar, which is volatile and can contribute to the wine fault known as volatile acidity. During the course of winemaking and in the finished wines, acetic, butyric, lactic, and succinic acids can play significant roles. Three primary acids are found in wine grapes: tartaric, malic, and citric acids. In wine tasting, the term “acidity” refers to the fresh, tart and sour attributes of the wine which are evaluated in relation to how well the acidity balances out the sweetness and bitter components of the wine such as tannins. There is no direct connection between total acidity and pH (it is possible to find wines with a high pH for wine and high acidity). Generally, the lower the pH, the higher the acidity in the wine. The measure of the amount of acidity in wine is known as the “ titratable acidity” or “total acidity”, which refers to the test that yields the total of all acids present, while strength of acidity is measured according to pH, with most wines having a pH between 2.9 and 3.9. They are present in both grapes and wine, having direct influences on the color, balance and taste of the wine as well as the growth and vitality of yeast during fermentation and protecting the wine from bacteria. The acids in wine are an important component in both winemaking and the finished product of wine. Malic and tartaric acid are the primary acids in wine grapes.
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