Dairy Chemistry And Biochemistry

3 70 DAIRY CHEMISTRY AND BIOCHEMISTRY

401 P

o!. 8. I. I. I - 1

6.2 6.4 6.6 6.8 7.0 7.2

PH

Figure 9.19 Effect of pH on the heat stability of type A milk (A), type B milk (0) and whey
protein-free casein micelle dispersions (0) (from Fox, 1982).

5. Reducing the level of colloidal calcium phosphate increases stability in
   the region of the HCT maximum.


6. Natural variations in HCT are due mainly to variations in the concen-
   tration of indigenous urea due to changes in the animals’ feed.

The current explanation for the maximum-minimum in the HCT-pH
profile is that on heating, x-casein dissociates from the micelles; at pH values
below about 6.7, 8-1s reduces the dissociation of Ic-casein, but at pH values
above 6.7, it accentuates dissociation. In effect, coagulation in the pH range
of minimum stability involves aggregation of Ic-casein-depleted micelles, in
a manner somewhat analogous to rennet coagulation, although the mech-
anism by which the altered micelles are produced is very different.
As would be expected, heating milk at 140°C for an extended period
causes very significant chemical and physical changes in milk, of which the
following are probably the most significant:

1. Decrease in pH. After heating at 140°C for 20 min, the pH of milk has
   decreased to about 5.8 due to acid production from pyrolysis of lactose,
   precipitation of soluble calcium phosphate as Ca,(PO,),, with the
   release of H +, and dephosphorylation of casein with subsequent precipi-
   tation of the liberated phosphate as Ca,(PO,), with the release of H+.
   The heat-induced precipitation of Ca,(PO,), is partially reversible on
   cooling so that the actual pH of milk at 140°C at the point of
   coagulation is much lower than the measured value and is probably
   below 5.0.