Dairy Chemistry And Biochemistry

MILK PROTEINS 217

0 Human milk contains p- and K-caseins but only a low level of a,,-casein;

hence ,!?-casein should be an attractive ingredient for bovine milk-based

infant formulae.

0 K-Casein, which is responsible for the stability of casein micelles, might be

a useful additive for certain milk products.

0 As discussed in section 4.16.7, all the principal milk proteins contain

sequences which have biological properties when released by proteolysis;

the best studied of these are the /I-caseinomorphins. The preparation of

biologically active peptides requires purified proteins.

Methods with the potential for the isolation of p-casein on a large scale,

leaving a residue enriched in rsl-, xs2- and K-caseins, have been published.

The methods exploit the temperature-dependent association characteristics

of /?-casein, the most hydrophobic of the caseins. Up to 80% of the /I-casein

may be recovered from sodium caseinate by UF at 2°C; the ,!?-casein may

be recovered from the permeate by UF at 40°C (Figure 4.39). MF at 2°C

has been used to isolate casein from milk or sodium caseinate. It is not

known whether these methods are being used commercially.

Casein dissociates from the micelles when milk or a dispersion of casein

micelles at pH 2 6.7 is heated to at least 90°C; in the former, the dissociated

K-casein is complexed with whey proteins. The functional properties of

K-casein-/I-lg complexes isolated by centrifugation of heated milk have been

reported by Singh, Fox and Cuddigan (1993).

Dilute sodium caseinate Solution

I2OC

Ultrafiltration (10 kDa cut-off membranes)

Retentate * Permeate

a,, -/ CQ/K -enriched caseinate pcasein enriched

Freeze dry

1 40"c

Ultrafiltration

Permeate t7 Retentate

t t

Discard P-enriched caseinate

Freize dry
Figure 4.39 Method for preparing usl-/us2-/K- and P-casein-enriched fractions by ultrafiltration
(from Murphy and Fox, 1991).