Winegrowers Supplies  -  How the wine acids in a grape change during ripening

Ripeness is not simply 'a balance of sweetness and acidity', it is essentially the percentage of tartaric acid in the grape juice. This is not related directly to °Oechsle or titratable Total Acidity.

The acids in a grape are primarily malic ('apple acid') and tartaric acid ('wine acid'), there are tiny amounts of other acids.

Bryce Rankines 'Making Good Wine' has a chapter on Grape Development and Composition', warm-climate research, which records that "as the berry grows both tartaric and malic acid increase in concentration until veraison, then both decrease, with malic acid decreasing at a more rapid rate.".
"as far as the acid taste is concerned", in many tastings at Roseworthy "tartaric emerged as having a marginally more acid taste than malic.".

German research confirms that, "during Growth stages 1 and 2 (after flowering) the titratable acidity and the L-malic acidity grow considerably. With the beginning of veraison (ripening, stage 4) an acid reduction begins, a metabolic synthesis which affects almost exclusively L-malic, while the tartaric acid is hardly changed.

Winkler records "acids are actually formed at low (night-time) temperatures, and metabolised in respiration at high temperatures. The temperature necessary to induce the respiration of malic acid is lower than that for tartaric.". More important than measuring titratable acid is pH, "the gradual rise of pH during ripening reflects the rise in acid salts at the expense of free acid, and this ratio is influenced by the temperature summation during ripening.".

I have found in my own vineyard, which being only 2 miles from the sea has much lower night-time temperatures than inland sites, that the grapes always have high titratable acidity; I recall harvesting Madeleine Angevine at 80 Oeschle and 13 g/l acidity. I regard this as a great advantage as I can leave the grapes to ripen (reduce in malic acid) for as long as possible and then de-acidify the juice (removing equal portions of malic and tartaric acid), leaving me with a high proportion of tartaric.

A long slow ripening, and late harvest, achieves the finest wines. For example, wines from the great German vintage 1975 have a very high percentage of tartaric acid even though the °Oechsle was nowhere near as high as that in the early very ripe vintage 1976. The 1976 wines quickly declined. The 1975 Saar Kabinetts, with only 7.5% alcohol (natural), are still fresh today; they are are some of the best dry wines I've ever tasted.
Expert 'blind' wine tasters can immediately identify 'ripe acidity', and hence tell the vintage year.

Tartaric acid helps to 'preserve' wines, which may be of no interest to producers who aim to sell their wine in the year after it is produced, but to anyone who really loves wine it is only through developing maturity that wines are really worthwhile.

Grapes which are 60 °Oe on 15 September, in England, may be no higher in six weeks time, but the acid will taste much 'riper'. An example of this was in 2003, when a relatively high °Oeschle due to a wonderful sunny dry autumn encouraged some people to pick too soon, compounded by worries about acids becoming too low.
The 'malic' taste of these wines was much less desirable.

In addition to the metabolic reduction of malic acid, the titratable acidity also falls during ripening due to the accumulation of potassium, thus neutralising the acids. Hence vineyard soils that lack potassium should give relatively higher acidity.

The skill of vinegrowing is in helping the 'ripening' process to progress as far as possible within the limits of the vineyard site and micro-climate:- 
Good sites have relatively low (but consistent) rainfall and excellent drainage - because waterlogged soils (and constantly wet leaves) allow vines to take up too much water, which dilutes acid and 'washes out' sugar; however, this is a partly reversible process. After heavy rain it is necessary to allow the soil to dry as much as possible and the grapes recover their acidity. The vineyard micro-climate and interrelated leaf canopy management is also vital. Spraying to protect against mildew damage, de-leafing to avoid congestion (allowing the leaves to dry out quickly) whilst leaving sufficient leaf-wall for maximum photosynthesis.