Organic Acids in Grape Berry and Wine---Malic Acid Metabolism

Malic Acid Metabolism Malic acid is accumulated in the fleshy cells at the end of the first growth phase, reaching its maximal value just prior to véraison.
Both leaves and immature green berries can form malic acid. Photosynthesis in the green berry is responsible for approximately 50% of the accumulating acids.
About 6 to 9 weeks after flowering and midway between bloom and fruit ripening, there is the simultaneous initiation of organic acid breakdown and sugar accumulation at the onset of ripening. When the grape berries begin ripening, the level of malic acid decreases due to malate oxidation.
What is known about the decrease in malic acid in the ripening grape:

• used as carbon and energy source for respiration
• cytosolic NADP-malic enzyme is considered to play a key role in the regulation of malic acid breakdown catalysing the oxidative decarboxylation of malic acid to pyruvate and CO₂
• diffusion of malic acid into the mitochondria and its subsequent degradation through the action of a mitochondrial malate dehydrogenase into oxaloacetate
• malic acid oxidation to pyruvate through the action of a mitochondrial NAD-malic enzyme
• cool regions typically produce grapes with higher concentration of malic acid and, conversely, grapes grown in warmer regions tend to have lower acidity; this negative temperature correlation with malic acid levels is due to the effect of temperature on the balance between malic acid synthesis and catabolism
• accumulation of potassium into the berries results in the formation of tartrate and malate salts, accounting also for a reduction of the total acid concentration
• despite their structural similarity, tartaric and malic acids are synthesized and degraded by markedly different biochemical processes

References:
1. The chemical structure of tartaric acid was drawn by the freely available drawing program from ACD Labs called ACD/ChemSketch Freeware.
2. Carlos Conde, Paulo Silva, Natacha Fontes, Alberto C. P. Dias, Rui M. Tavares, Maria J. Sousa, Alice Agasse, Serge Delrot, Hernâni Gerós, Biochemical changes throughout Grape Berry development and fruit and wine quality, Food, 1(1), 1-22 ©2007 Global Science Books.