Acid in wine is as indispensible as salt in food. However, excessively high acid levels lead to “harsh”, unfinished wines; while too little acid produces “flat” wines.

Acid plays a significant role in the vinification process and influences the taste of wine, inter alia. The most important acids are tartaric and malic acid, comprising about 70 to 90 per cent of the total acid content. The “acidity” of a wine is therefore determined by these two acids. It is remarkable that the accumulation of tartaric acid is very scarce in higher plants. There is also much speculation about the physiological role of malic and tartaric acid in the vine.

SYNTHESIS OF ACIDS
There seems to be no connection between the paths leading to the formation of malic and tartaric acid. According to various researchers, tartaric acid is generally accepted to be a secondary metabolyte derived from sugars. There is uncertainty about a possible intermediary, but ascorbic acid is probably the most likely. Malic acid is created by the b-carboxilation of pirovate or phospho-enolpirovate. Malic acid is an intermediary compound which may play an active role in the Krebs cycle.

Accumulation of acids usually occurs at the beginning of berry development and is to a large extent completed at the beginning of ripening. Initially acids were suspected to be formed in the leaves and transported to the berries. Today it is generally accepted, however, that precursors of the acids are produced in the leaves, whereafter they are synthesised into acids in the berries.

DEGRADATION OF ACIDS
There is still some uncertainty with regard to the degradation of acids. Tartaric acid is quite stable, which can possibly be ascribed to salt formation. Malic acid is actively remetabolised and may be respirated, with the resultant release of energy, or it may be reverted to sugar.

FACTORS INFLUENCING ACIDS
Acid concentrations are largely determined by temperature. Higher total acid concentrations were obtained under cooler conditions. The malic acid concentration in particular benefited from a reduction in temperature. Tartaric acid is presumed to be more stable when exposed to temperature increases. Cultivars with a bigger tartrate: malate ratio, such as Semillon and Riesling, will therefore have a higher total acid content in warm climates. Vineyard practices resulting in a cooler bunch zone will also make a contribution to the preservation of acids.

During ripening the rate of salt formation increases. Calcium tartrate has even been observed in crystalline form in grape berries. Large quantities of potassium occur in the skin particularly. Some researchers propose that potassium readily forms malate salts. However, Iland & Coombe (1988) found the malic acid content to be free at all times. It is therefore possible that excessive fertilisation with calcium and potassium may result in increased salt formation, with the subsequent decrease in titrateable acid.

CONCLUSION
Many questions still surround the development and especially the purpose of acids in the grape vine. A better comprehension of these processes will aid our understanding of the complex interactions inside the berry. This may so improve management decisions and practices that eventually we may produce higher quality grapes and even better wines.

ADDITIONAL LITERATURE
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COOMBE, B.G., 1987. Distribution of solutes within the developing grape berry in relation to its morphology. Am. J Enol. Vitic. 38, 120 – 127.
GUTI‚RREZ – GRANDA, M. & MORRISON, J.C., 1992. Solute distribution and malic enzyme activity in developing grape berries. Am. J. Enol. Vitic. 43, 323 – 328.
HARDY, P.J., 1968. Metabolism of sugars and organic acids in immature grape berries. Plant Physiology 43, 224 – 228.
ILAND, P.G. & COOMBE, B.G., 1988. Malate, tartrate, Potassium and Sodium in flesh and skin of Shiraz grapes during ripening: concentration and compartementation. Am. J. Enol. Vitic. 39, 71 – 76.
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SAITO, KL. & KASAI, Z., 1969. Tartaric acid synthesis from L-ascorbic acid- 1- 14C in grape berries. Phytochemistry 8, 2177 – 2182.
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The Author:

HANNO VAN SCHALKWYK
VITICULTURAL CONSULTANT, VINPRO
MAY 2000

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