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Determination of total acidity
Total acidity includes the complex of fixed acids (tartaric, malic, succinic, lactic, citric) and volatile (which can be removed by boiling, such as acetic acid) present in musts or wines; Acids derived from CO2 and SO2 are not included. Its determination is intended to assess the correct acid content for the stability and shelf life of wines.
Technically, total acidity is the sum of the titratable acids when the pH is raised to 7 by addition of a titrated alkaline solution. Normal values are within a very wide range: usually wines with high alcohol content are less acidic than those with low alcohol content, because alcohol facilitates the precipitation of potassium bitartrate; in addition, acidity decreases with aging. The acidity can be changed as a result of permitted treatments such as acidification, deacidification, refrigeration, etc. .. A certain acidity content prevents disease by bacteria and brings body to the wine.
Expression of the parameter
The acidity can be expressed in meq / L of alkalis or in g / L of H2SO4 or in g / L of tartaric acid. Normally in table wines there are values from 3 to 6 g / L of H2SO4 corresponding to 4.5 ÷ 9 g / L of tartaric acid; lower quality wines have low values, higher quality ones.
Total acidity is determined by titration with NaOH (0.1 N) in the presence of bromothymol blue as an indicator (change in pH 7, colors: yellow in an acid environment, green in a neutral environment, blue in a basic environment) or using a potentiometer with combined glass electrode, previously calibrated, to evaluate the change in pH value. The quantitative determination involves a titration of the wine with NaOH up to pH = 7: in this way all the acids present are titrated which will then be expressed either as tartaric acid or as sulfuric acid. The analysis can be performed only after the elimination of the carbon dioxide present in the wine, which otherwise would increase the acidity rate. Degassing is done by stirring and filtering on filter paper and cotton.
Red, rosé and white wine (photo http://www.assaggiatori.com)
Davanzo P., “Appunti di enologia”, Sixth Edition (available online in pdf)