Dairy Chemistry And Biochemistry

SALTS OF MILK 253

For the second dissociation, i.e. H,PO, e HPOi- + H’; pK: = 6.83

[salt]

[acid]

6.6 = 6.83 + log-

[salt]

[acid]

log- = -0.23

[salt] HPOi- - 0.59

i.e. ~ - -

[acid]’ H,PO, 1 ’

For the third dissociation, i.e. HPOi- *PO:- + H’; pKi = 12.32

[salt]

[acid]

6.6 = 12.32 + log-

[salt]

[acid]

log- = -5.72

[salt] PO:- - - 1.9 x

[acid]’ HPOi- 1’

Dihydrogenphosphate (primary) and monohydrogenphosphate (secondary)

are the predominant forms, in the ratio of 1.0:0.59, i.e. 63% H,PO, and

37% HPOi-.

Citric acid. Using pK,s of 3.08, 4.74 and 5.4:

H,Citrate- 3300

H,Citric acid - 1

HCitrate2- 72

H,Citrate- 1

Citrate3- 16

HCitrate2- 1

Therefore, tertiary (Citrate, -) and secondary (HCitrate’ -) citrate, in the
ratio 16: 1, are the predominant forms.

Carbonic acid.
as the bicarbonate anion, HCO;.

--

-_ -

-_ -

The small amount of carbonic acid present occurs mainly

Calcium and magnesium. Some calcium and magnesium in milk exist as
complex undissociated ions with citrate, phosphate and bicarboante,
e.g. Ca Citr-, CaPO,, Ca HCO;. Calculations by Smeets (1955) suggest
the following distribution for the various ionic forms in the soluble phase:

0 Calcium + magnesium: 35% as ions, 55% bound to citrate and 10%
bound to phosphate.