Malic Acid in Grapes: Part 1 Our Titratable Acid Conundrum

This year has presented us with a bit of a conundrum when we began to check on the Brix, pH and titratable acidity (TA) of our various grapes. We have always encountered high TA in our Chenin Blanc, but this year, we encountered high TA in our Cabernet Franc as well as our Cabernet Sauvignon. The puzzle was that the Brix were also high. In looking back at this year's growing season, we felt that we had good weather throughout the summer which would explain the high Brix, but what was puzzling was the high TA which never seemed to come down even though we let the grapes hang on the vines as long as possible.
Here are our numbers in the order in which the varieites were harvested:

Variety	Harvest Date	Brix	pH	TA
Auxerrois	September 23, 2019	19.6	3.26	7.35 g/L
Chardonnay	September 28, 2019	21.5	3.17	9.4 g/L
Chenin Blanc	October 14, 2019	21.6	3.15	13.5 g/L
Cabernet Franc	October 26, 2019	23	3.32	11.3 g/L
Cabernet Sauvignon	November 3, 2019	21	3.00	14.9 g/L

Looking at the high titratable acids, we wanted to know more about the role of malic acid in grapes. There is a beautiful diagram that shows grape berry ripening and the changes in grape metabolites during ripening.¹

I used the above diagram in a previous blogpost called Two Stages of Grape Berry Development, but in that blogpost, I focussed on the changes that occured in the grape berry in stage one and then in stage two.
In this and subsequent blogposts, I'm going to focus on the fate of malic acid in the ripening grape. Stay tuned!
Reference:
1. Moschou, Panagiotis & Aziz, Aziz & Roubelakis-Angelakis, Kalliopi, Chapter 7, Polyamines and grape berry development, The Biochemistry of the Grape Berry, (2012), 137-159.