Chemistry Reference
In-Depth Information
(to form HPO
4
2
and H
2
PO
4
), resulting in pH buffering. The removal of the
CCP from milk, as in CCP-free milk made by the method of Pyne and
McGann (1960), results in the absence of the buffering peak at pH 5.1 during
acid titration (Figure 9.2b). When the acidified milk sample is back-titrated
with base, buffering is low at pH 5.1, because CCP is already solubilized, but
maximum buffering occurs at pH 6.3, due to the formation of Ca phosphate
(precipitation) with the release of H
+
(from HPO
4
2
and H
2
PO
4
) which can
combine with OH
.
High heat treatments cause an increase in CCP due to the formation of
heat-induced CCP (Figure 9.2c). Some of the heat-induced CCP solubilizes
on cooling (depending on the equilibration time allowed) but there is a
substantial shift in the type of buffering curve observed during the acidifica-
tion of very severely heated milk (e.g. 1208C for 15 min) (Figure 9.2c).
A strong buffering effect in the pH range 6-7 arises from the formation
of Ca phosphate as can be seen in the titration of phosphoric acid in the
presence of Ca (Figure 9.3). This buffering effect due to precipitation of Ca
phosphate has been reported by many investigators, e.g. Visser (1962). Due to
the precipitation of Ca phosphate around pH 6, the titration behaviour of
phosphoric acid in the presence of Ca is completely different from that when
this titration is performed in the absence of Ca (Figure 9.3). In milk, both Ca
and phosphate are present which suggests that this behaviour would occur in
dairy products. As is shown in Figure 9.3, the onset of precipitation of Ca
phosphate results in the release of H
+
. We can speculate that this release of H
+
could also occur during the formation of CCP in the mammary gland and
may contribute to the lower pH of milk compared to that of blood.
13
11
9
7
(a)
(b)
5
(c)
3
1
0
2
4
6
8
10
12
14
16
18
Volume of 0.5 N NaOH (ml)
Figure 9.3.
Potentiometric titrations of 400 mg phosphoric acid with 0.5 N NaOH in the
presence of (a) no calcium, (b) 325 mg CaCl
2
,(c) 650 mg CaCl
2
; the method reported by Visser
(1962) was used for these titrations (Salim and Lucey, unpublished data).
