The grape juice ready to ferment with or without the skins is known as the must. The must should contain 10–13% potential alcohol, or be approximately 18–22% sugar by weight. The acidity of the must should be 7–10 g/l (expressed as tartaric acid), although some of this will be lost during winemaking. This will give wine between 4.5 g/l (the legal minimum in Europe) and 8 g/l. The outside limits of acidity are approximately 3–16 g/l.
Must weight is the measure of the sugar in the ripe grape. It is measured in most of Europe (especially France) and Australia in degrees Baumé (1° Baumé = 1% potential alcohol). Brix is another measure, used in the U.S. 1° Baumé=5/9 of 1° Brix. Brix is a measure of the percentage of solids by weight. In Germany, a degree Oechsle (°Oe) measures the density of the juice (14.7° Brix=60° Oe). Usual must weight at harvest is between 11.1-13.3° Baume, 20–24° Brix, and 83-104 ° Oe). The outside limits are roughly between 50° Oe (in some English vineyards) and 326° Oe (some German eiswein). Sugar means little without acidity to balance it.
All of these considerations of ripeness will depend on the type of wine that is being made, for example, Pinot Noir for Champagne is picked earlier than it is in Burgundy for still wine. Other factors also play a role, such as tradition and the general opinion of other growers in the area and what they consider ripe. Ultimately, ripeness is subjective.
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