Malic Acid in Grapes: Part 3 The Importance of Acidity

I am back to doing more digging into the role of malic acid. I have blogged about tartaric and malic acids in the following posts:

• Malic Acid in Grapes: Part 1 Our Titratable Acid Conundrum
• Malic Acid in Grapes: Part 2 Chemical Structure and pKa

In the first blogpost, I wrote about the conundrum we faced when our red grapes were harvested at a very nice level of Brix, but the titratable acidity was very high. In the second blogpost, I wrote about the chemical structures of the two most prevalent acids in grapes and how they respond in a juice/wine environment based on pH.
In this blogpost, I will try to further elaborate on the roles that both malic and tartaric acid play in the ripening grapes.
Why is acidity important?:

• it influences the ageing potential or the shelf-life of wine
• it determines the physical, biochemical and microbial stability of wine
• it ultimately determines wine quality in terms of the perceived organoleptic and aesthetic character

Without acidity, a wine can be very flabby and unbalanced. With the appropriate levels of acidity, a wine can become sublime. But knowing what the right level of acidity is, is a tricky business.
The principal organic acids in grapes are L-tartaric and L-malic acid. L-tartaric acid is often found at higher concentrations than L-malic acid and is the stronger acid of the two. I have already blogged about the Two Stages of Grape Berry Development. During the first stage of berry development, there is the rapid storage of L-malic and L-tartaric acid in the vacuoles leading to an increase in size of the grape berry cells. During the lag phase between stage 1 and stage 2 of berry development, berry acidity reaches a maximum concentration. The concentration of L-tartaric acid is relatively constant, while L-malic acid is the only high proportion organic acid that is actively metabolized throughout the ripening of grapes.
It is this metabolism of L-malic acid that is important to us in trying to understand what happened this year when our Brix level was high and our titratable acid was high in our Cabernet Sauvignon and our Cabernet Franc. Stay tuned for more about malic acid metabolism.
References:
1. H. Volschenk , H.J.J. van Vuuren and M. Viljoen-Bloom, "Malic Acid in Wine: Origin, Function and Metabolism during Vinification", South African Journal of Enology & Viticulture, 27(2):123-136, doi:10.21548/27-2-1613.
2. Crystal Sweetman, Laurent G. Deluc, Grant R. Cramer, Christopher M.Ford, and Kathleen L.Soole, "Regulation of malate metabolism in grape berry and other developing fruits", Phytochemistry, Volume 70, Issues 11–12, July–August 2009, Pages 1329-1344.