Colorimetric method for the determination of total acidity in wine
Ascorbic acid (vitamin C) is a powerful antioxidant that is used during the manufacte process to quickly eliminate any presence of dissolved O2 that could oxidize phenolic compounds, especially those derived from cinnamic acid, which produces a darkening of the wine. Ascorbic acid very efficiently converts said oxygen into hydrogen peroxide that is subsequently neutralized by the SO2 present in the wine (thus, it is recommended that the wine contains between 30 and 50 mg/L of SO2 before the addition of ascorbic acid to be effective in the elimination of hydrogen peroxide formed). It also prevents oxidation of iron ions as prevention of iron case. An excess of ascorbic can negatively affect the color of the wine and its subsequent evolution.
The majority acids in the grape are tartaric, L-malic, which represent about 90% of the total, and citric acid. In addition, during the fermentation process, other acids such as L-lactic, succinic or acetic appear that also contribute to total acidity. The total content of acids is important because it gives the wine its organoleptic characteristics, resulting in harsh wines when it is excessively high or flat, when it is too low. In addition, the total content of acids has relevance in terms of the conservation of wine, inhibiting the development of microorganisms.
Enzymatic method for sucrose, glucose and fructose measurement
In addition to monomeric hexoses (glucose and fructose), the must contains small amounts of sucrose disaccharide, which is hydrolyzed into fructose and glucose by the enzyme β-fructosidase (β-F). The natural sucrose content in the must is relatively low (and zero in the finished wine), and its addition is specifically prohibited in some countries. However, the addition of sucrose is a specific practice of the process of the production of sparkling wines (second fermentation) and in the captalization (to increase the alcoholic strength artificially, in specifically authorized areas). The determination of the total sugar content, including the one derived from the hydrolysis of sucrose, improves the control of the fermentation process both at its beginning and at the end (residual sugars), improving its general control.
Most of the lactic acid present in the wine is produced during the malolactic fermentation with the transformation of L-malic acid into L-lactic acid, so that more than 75% of the lactic acid present in the wine is the L isomer. D-lactic acid, on the other hand, is associated with the metabolism of glucose (and other hexose sugars) by the same lactic bacteria (mainly leuconostoc and lactobacillus). A presence of D-lactic acid greater than 0.3 g/L is an indication of bacterial contamination, since these bacteria compete with the yeasts for sugars, until inhibiting alcoholic fermentation.
D-Glucose and D-Fructose are the main reducing sugars present in grapes and other fruits. The content of D-glucose and D-fructose in grapes is similar (in a ratio between 0.74 and 1.12), with small variations depending on the ripeness of the grape and its variety. Since D-glucose is fermented more rapidly by yeasts, monitoring the relationship between D-glucose and D-fructose, in addition to its total sum, provides information on both the fermentation process and the final degree of expected sweetness. Fructose levels are calculated from the total glucose and fructose content (code SY2404) by directly subtracting the glucose content.
