Dairy Chemistry And Biochemistry

MILK PROTEINS 153

lactose will interact with casein on heating via the Maillard browning

reaction, with undesirable consequences.

The procedure used for the industrial production of acid (isoelectric)

casein is essentially the same as that used on a laboratory scale, except for

many technological differences (section 4.15.1).The whey proteins may be

recovered from the whey by salting out, dialysis or ultrafiltration.

4.3.2

Because they occur as large aggregates, micelles, most (90-95%) of the

casein in milk is sedimented by centrifugation at 100 000 g for 1 h. Sedimen-

tation is more complete at higher (30-37°C) than at low (2°C) temperature,

at which some of the casein components dissociate from the micelles and are

non-sedimentable. Casein prepared by centrifugation contains its original

level of colloidal calcium phosphate and can be redispersed as micelles with

properties essentially similar to the original micelles.

Cen tr ifuga t ion

4.3.3 Centrifugation of calcium-supplemented milk

Addition of CaCI, to about 0.2 M causes aggregation of the casein such that

it can be readily removed by low-speed centrifugation. If calcium is added

at 90"C, the casein forms coarse aggregates which precipitate readily. This

principle is used in the commercial production of some 'casein co-precipi-

tates' in which the whey proteins, denatured on heating milk at 90°C for

lOmin, co-precipitate with the casein. Such products have a very high ash

content.

4.3.4 Salting-out methods
Casein can be precipitated from solution by any of several salts. Addition of
(NH,),SO, to milk to a concentration of 260 g 1- causes complete precipi-
tation of the casein together with some whey proteins (immunoglobulins,
Ig). MgSO, may also be used. Saturation of milk with NaCl at 37°C
precipitates the casein and Igs while the major whey proteins remain
soluble, provided they are undenatured. This characteristic is the basis of a
commercial test used for the heat classification of milk powders which
contain variable levels of denatured whey proteins.

4.3.5 Ultrajiltration

The casein micelles are retained by fine-pore filters. Filtration through
large-pore ceramic membranes is used to purify and concentrate casein on
a laboratory scale. Ultrafiltration (UF) membranes retain both the caseins