The conversion of sugar to alcohol by yeasts is the most essential reaction during winemaking, without which wine cannot be produced. The sugar concentration not only plays a role in the eventual alcohol concentration of the wine, but also in the balance and style of the wine.
The sugar concentration of grapes can be determined directly or indirectly. Indirect methods to determine the total dissolved matter include refractometry and hydrometrics. These methods measure the density and correlate it with an index, which expresses it as Brix, Beaumé, Oechsle or other terms. Indirect methods are plain and cheap, but not necessarily a good reflection of the sugar concentration. Non-fermentable compounds in grape juice, like tannins and pectin, reflect light in the same way as fructose and glucose and can consequently increase the reading artificially. The correlation with an index can also be increased by non-fermentable, dissolved substances in grape juice. At 18 degrees Brix the correlation with the actual sugar concentration is at its best. Readings below that indicate a too high sugar reading, while those above it, present a too low sugar concentration.
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