There is no direct relationship
between
titratable acidity and pH in apple juice, although generally the pH
goes up as
the acid goes down and vice-versa. The exact relationship differs from
sample
to sample and depends on esoteric concepts like 'buffering capacity'
which will
vary for a whole host of reasons. Nevertheless some general
empirical
relationships may be obtained, and some that I've tabulated are shown
below. It
must be stressed that they only refer to those fruits growing in those
locations at that time. They cannot be held to be exact in other
circumstances.
In general, titratable acid (TA) relates pretty well to the 'acid
taste' of
a juice or cider. If the TA doubles, people will tend to perceive
it as
twice as acidic. The pH relates more to things like microbial
stability
and susceptibility to mould and bacterial spoilage. In particular, the
antimicrobial
effectiveness of sulphur dioxide is very
pH
dependent (see below), and this is the reason that commercial
cidermakers
measure it (see Jarvis
and Lea 2000.) TA is measured by titration, and pH by a pH meter or
by
narrow-range pH test strips*. A pH meter is tricky to set up and
calibrate, and only really worthwhile if it's being used daily in a
laboratory environment. The cheap pH 'dipsticks' do not have
replaceable electrodes and may only have an effective life of a year or
so. For most non-commercial cidermakers, pH test strips are probably a
better bet. If you want to measure titratable acidity, you can
get kits from home winemaking suppliers or from Vigo.
* [eg Merck indicator strips pH 2.5 - 4.5
(Merck product code 109451; VWR catalogue number 31501). A more
readily available alternative may be the 'Vinoferm pH strips 2.8 - 4.6'
available
in the UK from the Home Brew Shop as
http://www.the-home-brew-shop.co.uk/item1860.htm
or elsewhere in Europe
from Brouwland http://www.brouwland.com
as
catalogue no 013.073.2.
(For distributors elsewhere in the world, check out the links on the
Brouwland website)]
Titratable acid is given as % malic. Plots and equations shown
are
'best-fit' to the empirical data (about 50 data points, mostly
bittersweets)
. Both binomial and exponential plots are given.

where Y is
titratable acid in %
malic and X is pH.

where Y is titratable acid in % malic and X is pH
It's often
easier to use a stock solution of sulphur dioxide. To make a 5% stock
solution, dissolve around
10 grams
of sodium or potassium metabisulphite in 100 ml of water. (The
metabisulphite salts contain around 50 - 60% of available SO2
depending
on how they've been stored). Then 1 ml of this per
litre
of juice (5 ml per gallon) corresponds to 50 ppm (parts per million) of
SO2. pH Approx TA For total yeast kill For partial yeast kill SO2 (ppm) Campden tablets SO2 (ppm) Campden tablets 3.0 – 3.3 1.2 – 0.8 50 1 nil nil 3.3 – 3.5 0.8 – 0.6 100 2 50 1 3.5 – 3.8 0.6 – 0.3 150 3 100 2 > 3.8 < 0.3 add more acid! add more acid! 150 3So.......how much sulphite should I
add for the pH?
(% malic)
(when adding cultured yeast)
(for wild yeast fermentation)
per gallon
per gallon