Chemistry Reference
In-Depth Information
McGann, 1960; Knoop et al., 1979; McGann et al., 1983b; Lyster et al., 1984),
although we believe that it is also possible that CCP forms some type of
crystalline structure (Horne et al., 2007). Various researchers have included
casein (serine) phosphate groups in the calculation of the ratio of Ca/P in CCP
and have obtained a value
<
1.3. It is now realized that phosphoserine groups
are not part of CCP but a small number of these peptide residues (around four)
can terminate the growth of CCP crystals (Horne et al., 2007).
The pK
a
values for phosphate reported in chemistry textbooks are 2.1,
6.9 and 12.0, and Walstra and Jenness (1984) suggested that comparing the
pH of milk (
6.7) with the pK
a2
of phosphoric acid, one would expect
CaHPO
4
to be the form of CCP. However, in the presence of Ca, all
phosphates (pK
a2
and pK
a3
) are titrated around pH 6-7 due to the pre-
cipitation of Ca phosphate (Figure 9.3). That observation and the strong
buffering at pH
5.1 during acid titration of milk (Figure 9.2a) caused by
the protonation of the released phosphate ions (from CCP) suggest that the
form of CCP in milk is less likely to be an acidic form (like CaHPO.2H
2
O)
and may be a more basic form (e.g. tricalcium phosphate). Other titration
studies, including those with oxalate (Pyne and Ryan, 1950; Jenness, 1973)
have indicated that most P
i
in CCP is in the form of PO
4
3-
(i.e. tricalcium
phosphate). Holt (1985) suggested that there may be a difficulty in titration
studies if it is assumed that the exposed phosphoserine groups, after the
dissolution of CCP, do not contribute to the titration. This suggestion by
Holt (1985) does not explain the acid-base buffering behaviour shown in
Figure 9.2a as the back-titration with base indicated that CCP does indeed
contribute to the buffering at pH
5. Removal of CCP resulted in the
elimination of the buffering peak at pH
5 (Figure 9.2b). Any calculation
of milk salts equilibria needs to take into account the unexpected pK
a2
and
pK
a3
values of phosphate in milk/serum although this does not appear to
have always been the case. In summary, evidence from the various types of
titration, the real pK
a
values for phosphate in milk and the distinctive
acid-base buffering properties of milk all suggest that the form of CCP is
a basic form (e.g. tricalcium phosphate).
9.7.
Functional Properties of Milk Products
Milk salts greatly influence the functional properties of milk and various
dairy products primarily by influencing the structural integrity of the casein
micelles or the sensitivity to aggregation of caseins. There have been a number
of reviews on the effects of salts on the functionality of milk products (e.g.
Augustin, 2000).
