The Components of Wine
Gluconic acid Can be the predominant acid in wines made from overripe grapes infected with Botrytis cinerea – noble rot. It appears with the oxidation of glucose. Succinic acid Produced by yeast during fermentation up to the value of 1g per litre. Its salts are succinates. Ascorbic acid or vitamin C. Oxalic acid Appears mainly in the form of oxalate salts. D-Galacturonic acid Is a component of pectin, which is the building material of the cells of grapes.
Phenolic compounds Phenols are compounds consisting of a benzol ring, which can in nature only be found in plants. They are divided into two large groups: • phenolic acids with a single benzol ring or linked into two rings maximum; • antocyanins and tannins, compounds in a molecule with three rings. In grapes there are numerous phenolic acids, which give a bitter taste. They can bind with organic acids into new compounds and with alcohols into aromatic substances. They can be found mainly in grape skins and pips. The best known compound with one ring is catechin, and with two rings resveratrol. The latter gives the skin rot resistance and is beneficial to man as it prevents the blocking of veins, i.e. cardiovascular diseases. The researcher Urška Vrhovšek from the Agricultural Institute of Slovenia has ascertained that Modra Frankinja26 is particularly rich in resveratrol. Antocyanins give red wines their colour. Each variety of grapes contains different quantities of individual antocyanins and thus a different shade of red. Their quantity depends on the variety of grapes, as well as on their maturity and health condition and the extraction techniques used. Antocyanins are in nature bound to one molecule of glucose in Vitis vinifera and with two molecules of glucose in the American grapevine and its hybrids. Thus with respect to red wines we can chemically differentiate between the noble European varieties and hybrids with American varieties, which are more resistant to disease. Acids can bind to the glucose in antocyanins, creating new compounds called acylated forms. Antocyanins, which give colour to red wine, can be coloured or colourless. At lower acidity and a higher redox value, i.e. oxidation-reduction potential, they have a stronger colour as their level in the coloured version is higher. In the skin and later in the must antocyanins appear in free form. They quickly travel from the skin to the must as they are both water and alcohol soluble, especially at higher temperatures. In must they bind to oxygen and oxidise, but they also quickly bind to 26
Vrhovšek, Urška: Bioaktivne polifenolne spojine grozdja in vina (Bioactive polyphenolic compounds in grapes and wine). In: Vino-hrana, zdravje 2000. Strokovni posvet. Zbornik referatov. Poslovna skupnost za vinogradništvo in vinarstvo Slovenije, Celje 2000, pp. 42-56.
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