Dairy Chemistry And Biochemistry

SALTS OF MILK^25 1

Figure 5.7 Distribution of phosphorus among various classes of compounds in bovine milk.

and sulphate, anions of strong acids, are present as anions at the pH of milk.
The salts of weak acids (phosphates, citrates and carbonates) are distributed
between various ionic forms, the concentration of which can be calculated
approximately from the analytical composition of milk serum and the
dissociation constants of phosphoric, citric and carbonic acid, after allow-
ance has been made for binding of calcium and magnesium to citrate as
anionic complexes and to phosphate as undissociated salts. The distribution
of the various ionic forms can be calculated according to the Henderson-
Hasselbalch equation:

[salt]

[acid]

pH = pK, + log-

Phosphoric acid (H,P04) dissociates as follows:

H3P0,e H+ + H,PO, e H+ + HPOt- e H+ + PO:-

pK,' = 1.96 pK: = 6.83 pK2 = 12.32

H,PO,, HPOZ- and PO:- are referred to as primary, secondary and
tertiary phosphate, respectively.
The titration curve for H3PO4 using NaOH is shown in Figure 5.8.
Citric acid is also triprotic while carbonic acid (H,C03) is diprotic.

H,C-COOH

HOC-COOH

H,C-COOH

Citric acid

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