Chemistry Reference
In-Depth Information
15.2.
Acid-Base Equilibria
The buffering of milk influences many of its physico-chemical properties (by
controlling pH) during processing, e.g., heat and ethanol stability, rennet
coagulation time, firmness and syneresis of renneted milk gels, rate of pH
change in cheese, the pH of cheese at the end of manufacture and the
subsequent pH changes during ripening. The buffering properties of milk
and milk products have been reviewed thoroughly by Singh et al. (1997),
Sala ¨n et al. (2005) and Lucey and Horne (Chapter 9) and are described
briefly here.
15.2.1.
The pH and Buffering Properties of Milk
The pH of bovine milk at 258C is between 6.5 and 6.7, with 6.6 being the
most common value. Differences in pH and buffering between individual lots
of fresh milk reflect compositional variations. The pH of colostrum is lower,
e.g., pH 6.0 (McIntyre et al., 1952), and that of mastitic milk or end-of-
lactation milk is higher, up to pH 7.5 (Prouty, 1940), than the pH of normal
mid-lactation milk. This increase in pH is presumably due to increased
permeability of the mammary cells, with an increase in [Na
+
] and [Cl
] and
possibly other ions and a reduction in the lactose content, as well as a
reduction in the concentration of soluble inorganic P (White and Davies,
1958).
Milk contains many acidic and basic groups that result in buffering
action over a wide pH range. The principal buffer components in milk are
soluble phosphate, colloidal calcium phosphate (CCP), citrate, bicarbonate
and casein (Jenness and Patton, 1959; Kirchmeier, 1980; Walstra and Jenness,
1984; Srilaorkul et al., 1989; Lucey, 1992; Lucey et al., 1993) (see Table 15.1).
The pK
a
values of many milk constituents are uncertain since the ionization of
a group is affected by adjacent groups and electrostatic effects are long-range
effects (Tanford, 1962).
Quantitative assignment of the buffering of each of these individual
components is rather difficult. Three approaches have been used in attempts
to account for the buffering behaviour of milk in terms of the properties of its
constituents: calculation (Whittier, 1929, 1933), titration of artificial mixtures
(Wiley, 1935a, b) and fractionation (Srilaorkul et al., 1989; Lucey et al., 1993).
Srilaorkul et al. (1989) estimated that the contribution of casein, whey pro-
teins and milk salts to the buffering of skim milk was 36.0, 5.4 and 58.6%,
respectively. Lucey et al. (1993) reported that in the pH range 6.7-4.0, soluble
salts and whey proteins (rennet whey), CCP and casein contributed approxi-
mately 47, 21 and 32%, respectively, to buffering in milk.
