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The Vine

Juice Composition

  • Grape juice is composed of roughly water and carbohydrates. Only 1% of the volume is composed of substances other than water and sugar. This tiny fraction includes the different types of acid (mostly tartaric, but also malic, citric, succinic and lactic), the minerals (including potassium, magnesium, phosphorus, aluminum, boron, copper and iron), the phenolic compounds (tannin and color or anthocyanins), amino acids and other proteins, and trace quantities of vitamins.

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  • The acidity of grapes is very important to the balance of wine. Malic acid predominates in the early stages of ripening, but it is respired during growth, and by harvest the majority of acid left is tartaric. If all of the malic acid disappears, however, the fruit becomes over-ripe.

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  • Other important types of compounds in the grape are polyphenols, including the coloring agents (anthocyanins) and tannins of the grape. Tannins are important because they help a wine to age and they act as preservatives, combining with oxygen that would otherwise ruin the flavor of the wine.

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  • Proteins and related compounds are also essential to wine because they are composed of amino acids that act as enzymes or catalysts for the various reactions that take place within the grape, such as the transformation of carbon dioxide during photosynthesis. It is also protein-type compounds that give wine its aroma. Some exist on their own in the juice, but many more are bound to sugar. When this sugar is consumed by the yeast, the aromas are released, and this is why wine is so aromatic while unfermented grape juice is not.

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  • 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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