Chemistry Reference
In-Depth Information
pH
5.1 (during acidification of milk) and at pH 6.3 (during back-titration of
acidified milk) (Lucey et al., 1993). Acidification of milk to a low pH value
(5.0-4.6) and neutralization resulted in a reduction in the buffering maximum
at pH 5.1. This suggests that little reformation of CCP occurs on neutraliza-
tion; presumably, other calcium phosphates are formed that have different
buffering properties compared with normal milk.
The pH and buffering properties of milk are influenced by a number of
compositional and processing factors, including temperature, pH, heat treat-
ments, concentration, presence of CO
2
and concentrations of proteins and
salts. These factors are discussed fully in Chapter 9.
15.2.2.
Titratable Acidity
Titratable acidity is determined in the dairy industry mainly for two
reasons: (a) to check the freshness of milk and milk products and (b) to
control the manufacture of cultured (fermented) dairy products. The main
advantage of using titratable acidity as a quality index is its simplicity and
speed of measurement. Titratable acidity is a measure of the buffering of milk
between pH 6.6 and 8.3 (phenolphthalein end-point) and is due mainly to
phosphates, proteins, citrate and CO
2
(van der Have et al., 1979).
The titratable acidity of milk is usually expressed as percentage lactic
acid, although lactic acid (pK
a
3.95) does not buffer in the pH range 6.6-8.3.
The acidity of milk and dairy products is usually determined by titration with
NaOH to the phenolphthalein end-point (pH
8.3). Titration of fresh milk
from pH 6.6 to 8.3 requires between 13 and 20 ml of 0.1 M NaOH per 100 ml
(1.3-2.0 meq/l00 ml). Various methods exist, prescribing titration with NaOH
of various strengths: M/10, M/9 (Dornic) or M/4 (Soxhlet-Henkel) (Jenness
and Patton, 1959; Walstra and Jenness, 1984).
A high initial acidity (in the absence of lactic acid development)
suggests a milk that is rich in proteins and other indigenous buffering
constituents. The initial acidity of milks from individual cows varies within
the range 0.08-0.25% lactic acid but the titratable acidity of fresh bulk milk
seldom falls outside the range of 0.14-0.16%. The titratable acidity of milks
varies only slightly with the breed of cow (Herrington et al., 1972). The
liberation of fatty acids from milk fat by lipase action can result in some
acidity being developed in cream and high-fat products (Walstra and
Jenness, 1984).
The titratable acidity of milk is affected by the speed of titration, mainly
due to the precipitation of calcium phosphate which causes the release of H
+
and therefore a decrease in pH. This phenomenon is described as 'fading of
the phenolphthalein end-point'.
