This is copy and pasted from Chapter 9 of Lum Eisenman's Home
WInemakers manual.

"Tartaric and malic acids make up about 90 percent of the total
organic acid content in grapes. Malic acid is found in many different
varieties of fruit, but only grapes contain significant amounts of
tartaric acid. The amounts of tartaric acid and malic acid are about
equal when grapes start to change color and soften (veraison), and the
quantity of tartaric acid remains roughly constant throughout the
ripening period. On the other hand, the quantity of malic acid
decreases as the grapes ripen, and the loss of malic acid results in a
gradual decrease in the total acid content during the ripening period.
In hot growing regions, much of the malic acid is gone by harvest
time. Depending upon the grape variety and the growing conditions,
grape juice contains 1.5 to 5 times more tartaric acid than malic
acid.

In cold viticulture regions, ripe grapes have an acid content ranging
from 0.7 to 1.3 percent. Such high acidity often results in
excessively tart wine. To avoid excess tartness, winemakers in cold
growing regions use special procedures to reduce wine acidity. The
situation is quite different when grapes are grown in warm regions
like the interior valleys of California. Here, the acid content of
ripe grapes often falls between 0.4 and 0.8 percent. Grapes grown in
such warm climates often have too little acidity, and wine produced
from grapes low in acidity is often bland and flat tasting. Besides
producing bland wines, fermenting grapes low in acidity often results
in other problems, and adding acid to grapes grown in warm regions is
a standard winemaking practice. Large acid additions are best made
before fermentation is started.

The acid content of grapes (or wine) is usually measured using a
procedure called titration. Sometimes winemakers call the acid content
"total acid," but the preferred term is "titratable acid." Titration
measures the sum of all the different types of acids in the wine, but
the results are expressed as if all of the acids were tartaric acid.
French winemakers use a different convention. They express titratable
acid as if all the acids present were sulfuric acid. These different
customs result in different numbers, and sometimes novice winemakers
are confused by the different conventions. "

On Oct 28, 11:34 am, wrote:
> They were very ripe, as a matter of fact. I didn't know that had
> anything to do with the amount of malic acid. To answer you other
> question, no, I didn't put anymore SO2 in at press. I'm going to give
> it another week and rack off the lees. Last year I have equally ripe
> grapes and didn't add MLF. We racked off the lees at about 4 weeks
> after press - that worked out fine.
>
> So what is the connection of ripeness to Malic acid?