It
has taken all the years since Granny was a girl for us to learn that,
in winemaking, we really have to pay attention to specific gravity and
total titratable acidity. Behind us a long trail of horrid wines marks
the progress of our learning.
In
recent years we have developed a positively friendly attitude towards
hydrometers and titration.
Now,
POW! We are hit upon the head with the pH thing. It is expressed to us
in complexities that instill fear and boggle our minds. Small wonder
that some of the more conservative amongst us are crying STOP! Enough
is enough! Don’t even tell me about pH!
Be
of good cheer. The only thing wrong with pH is that we have been
trying to understand it.
Like
a lot of other things in life, pH is what you make of it. It is easy
to learn to use pH, and you don't really need to understand it. Most
of us don't understand the cars we drive so easily, or the TV sets we
so blithely manipulate.
Start by simply recognizing pH as a very important number that can
help you to make better wines. True, it doesn't work like most numbers
we grew up with, because what it measures grows greater as the pH
number gets smaller. Those of us who have lived long enough to watch
what has happened to the value of the dollar bill over the past fifty
years can grasp that. As we receive more and more dollar bills they
become worth less and less. Not to worry.
It
does help to have some understanding of what pH measures. It has to do
with acids. Musts and wines are complex solutions of weak Acids mixed
with a variety of other materials. It is essential that we start a
wine with its acids just so.
"Total titratable acidity", which is what you measure with your
titration kit, is not a measure of total acid, but of the acid
that is available to react with the NaOH [Sodium Hydroxide]
solution with which we titrate.
It
measures that available acid in grams per
litre
or in percentage. It does not tell us how strong that acid is,
and acids vary greatly in strength.
pH
is a measure of the acid strength in the must or wine. The "p"
in pH is an abbreviation for the Swedish word for power.
It
is pH, rather than total titratable acidity, which indicates the
ability of a must to resist oxidation and invasion by bacteria, and
which determines how much SO² is needed.
pH
has its primary importance in checking grapes for purchase and in
preparing musts.
All
wine musts that are to produce satisfactory wines have to start with
their pH in the range of pH 3.1 to pH 3.55 at the commencement of
fermentation. What their pH becomes later is considerably less
important.
More
particularly, white wine grapes and musts should have a pH close to
3.2 (3.1 to 3.3). Red wine grapes or musts should have a pH of 3.3 to
3.4 with pH 3.55 as tops.
Anything
started outside the range of pH 3.1 to 3.55 is headed for trouble.
Below pH 3.1 the wine will be highly acidic. Above pH 3.55 the must
will be gravely at risk of oxidation and of invasion by bacteria, and
it will be difficult or impossible to control with metabisulphite.
There are, at the least, likely to be fermentation by-products that
will detract from the wine.
In
high quality grapes, picked at the right point of maturity, the pH,
total titratable acidity, and specific gravity will all be in their
correct ranges, and no adjustment is required. Fermentation should
produce a quality wine that fully shows the potential of the grape
variety.
In
grapes which are under ripe, overripe, or too heavily cropped, and in
non-grape musts the pH, total titratable acid and specific gravity
will deviate from their ideal ranges. They may be adjusted to those
ideal ranges. They may then produce pleasant and useful wines, but
they will not regain the ability to produce wines of high quality.
You
may usually expect to be able to raise or lower the pH of a must, but
in many musts, and in varying degree, the pH is reluctant to change.
This
article was published in the October 1989 Grapevine.
