MILK PROTEINS 187wheys by dialysis and drying the retentate. The products prepared by these
various methods differ: acid whey contains some y-casein and proteose-
peptones; immunoglobulins are co-precipitated with the caseins by
saturated NaCl; rennet whey contains the ic-CN macropeptides produced by
rennet action, plus, perhaps, very small amounts of other caseins; small
casein micelles remain in the ultracentrifugal supernatant, especially if Ca is
not added. The salt composition of the serum differs very considerably in
wheys produced by various methods.
On a commercial scale, whey protein-rich products are prepared by:- Ultrafiltration/diafiltration of acid or rennet whey to remove varying
amounts of lactose, and spray-drying to produce whey protein concen-
trates (30-80% protein). - Ion-exchange chromatography: proteins are adsorbed on an ion ex-
changer, washed free of lactose and salts and then eluted by pH
adjustment. The eluate is freed of salts by ultrafiltration and spray-dried
to yield whey protein isolate, containing about 95% protein. - Demineralization by electrodialysis and/or ion exchange, thermal evap-
oration of water and crystallization of lactose. - Thermal denaturation, recovery of precipitated protein by filtration/
centrifugation and spray-drying, to yield lactalbumin which has very low
solubility and limited functionality.
Several other methods are available for the removal of whey proteins
from whey but are not used commercially. Several methods for the purifica-
tion of the major and minor whey proteins on a commercial scale have
also been developed and will be discussed briefly in sections 4.15.6 and 4.16.4.6.2 Heterogeneity of whey proteins
It was recognized 60years ago that whey prepared by any of the above
methods contained two well-defined groups of proteins which could be
fractionated by saturated MgSO, or half saturated (NH,),SO,; the precipi-
tate (roughly 20% of total N) was referred to as lactoglobulin and the soluble
protein as lactalbumin.
The lactoglobulin fraction consists mainly of immunoglobulins (Ig),
especially IgG,, with lesser amounts of IgG,, IgA and IgM (section 4.10).
The lactalbumin fraction of bovine milk contains three main proteins, p-
lactoglobulin (p-lg), a-lactalbumin (a-la) and blood serum albumin (BSA),
which represent approximately 50, 20 and 10% of total whey protein,
respectively, and trace amounts of several other proteins, notably lactotrans-
ferrin, serotransferrin and several enzymes. The whey proteins of sheep, goat
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