The Biochemistry of Organic Acids in the Grape

Grape berries are commonly perceived to be composed principally of high concentrations of fermentable sugars, accompanied by a complex suite of polyphenolic compounds responsible for colour and ‘mouthfeel’ properties. The organic acid composition of the berry, which is principally a reflection of the metabolism of tartaric and malic acids during development and ripening, has several important consequences for the use of grapes in winemaking. Early research showed two unusual features of acid metabolism in grapes – the occurrence of significant concentrations of tartaric acid, and a marked decrease in the concentration of malic acid as berries enter the ripening stage. Despite a few ‘false starts’, a synthetic pathway that led to the formation of tartaric acid from ascorbic acid was identified, and in the past few years it has been proved possible to confirm this by biochemical and molecular biological approaches. Evidence for the synthetic route to malate formation in the berry proved simpler to identify, but the greater metabolic versatility of malate compared with tartaric acid, and the post-veraison breakdown of malate by a variety of pathways have ensured a continuing interest in this component of the berry’s acid complement. Modern ‘post-genomic’ approaches have been increasingly used to enable the measurement and analysis of berry metabolism. These approaches, and the advent of a readily transformable ‘model’ grapevine system, will undoubtedly continue to play a major role in the development of the understanding needed for the rational modification of berry acid composition in response to changing environmental and cultural practices.

Keywords: Ascorbic acid, Malate dehydrogenase, Malate synthesis, Malic acid, Organic acid, Oxaloacetate, PEP carboxylase, Tartaric acid, Tartrate synthesis.