216 DAIRY CHEMISTRY AND BIOCHEMISTRY
Membraneprocessing. The use of ultrafiltration (UF) for the production of
whey protein concentrates (WPCs) is now well established (p. 223). Obvi-
ously, UF or diafiltration (DF) can be used to prepare products enriched in
total milk protein. Products with protein concentrations up to 85% have
been produced and assessed for a range of functionalities and applications
(Fox and Mulvihill, 1992).
The development of large-pore membranes facilitates the separation of
whey proteins from casein micelles by microfiltration (MF). Membranes
used in MF have cut-offs in the range 0.01-10pm, and therefore casein
micelles may be in the permeate or retentate streams, depending on the pore
size of the MF membranes chosen. MF with large-pore membranes very
effectively removes bacteria and somatic cells from milk and may also be
used to remove lipoprotein complexes from whey prior to the production of
WPCs with improved functionality. The preparation of micellar casein by
MF is still at the exploratory stage.
High-speed centrifugation. The casein micelles may be sedimented by
centrifugation at greater than lOOOOOg for 1 h, which is widely used on a
laboratory scale. A combination of ultrafiltration and ultracentrifugation
has been proposed for the industrial production of ‘native’ phos-
phocaseinate. Almost all the casein in skim milk and UF retentates,
containing 3.0- 17% protein, can be sedimented by centrifugation at greater
than 75 OOOg for 1 h at 50°C.
‘Native’ casein. An exciting new development is the production of ‘native’
casein. Few details on the process are available at present but it involves
electrodialysis of skim milk at 10°C against acidified whey to reduce the pH
to about 5; the acidified milk is centrifuged and the sedimented casein
dispersed in water, concentrated by UF and dried. The product disperses
readily in water and is claimed to have properties approaching those of
native casein micelles.
4.15.3 Fractionation of casein
As discussed in section 4.4, individual caseins may also be isolated on a
laboratory scale by methods based on differences in solubility in urea
solutions at around pH 4.6, by selective precipitation with CaCl, or by
various forms of chromatography, especially ion-exchange or reverse-phase
high performance liquid chromatography (RP-HPLC). Obviously, these
methods are not amenable to scale-up for industrial application.
There is considerable interest in developing techniques for the fractiona-
tion of caseins on an industrial scale for special applications. For example:
0 p-Casein has very high surface activity and may find special applications
as an emulsifier or foaming agent.