20 Biochemistry of Milk Processing 469electric point is reached, a precipitate (rapid acidifi-
cation) or a gel (slower acidification) is formed; the
latter is cut/broken to initiate syneresis and expel
whey. The mixture of casein and whey is stirred and
cooked to enhance syneresis, and the casein is sepa-
rated from the whey (either centrifugally or by siev-
ing). To improve the purity of the casein, the curds
are repeatedly washed with water to remove residual
salts and lactose, and the final casein is dried, typi-
cally in specialized dryers such as attrition or ring
dryers. For rennet casein, the milk gel is formed by
adding a suitable coagulant to skim milk, but subse-
quent stages are similar to those for acid casein.
Both acid and rennet casein are insoluble prod-
ucts, which are used in the production of cheese
analogs (rennet casein) and convenience food prod-
ucts, as well as in nonfood applications, such as the
manufacture of glues, plastics, and paper glazing
(acid casein).
To extend the range of applications of acid casein
and improve its functionality, it may be converted to
the alkali metal salt form (e.g., Na, K, Ca caseinates)
by mixing a suspension of acid casein with the
appropriate hydroxide and heating, followed by dry-
ing (typically with a low, i.e., 20%, total solids
concentration in the feed, due to high product vis-
cosity, in a spray-dryer). Caseinates, most commonly
sodium caseinate, have a range of useful functional
properties, including emulsification and thickening;
calcium caseinate has a micellar structure (Mulvihill
and Ennis 2003).
Microfiltration (MF) technology can be used to
produce powders enriched in micellar casein (Poul-
iot et al. 1996, Maubois 1997, Kelly et al. 2000,
Garem et al. 2000). Whole casein may be resolved
into - and s/-casein-rich fractions by processes
that exploit the dissociation of-casein at a low tem-
perature (reviewed by Mulvihill and Ennis 2003).
Chromatographic or precipitation processes can also
be used to purify the other caseins, although such
processes are generally not easy to scale up for
industrial-level production (Coolbear et al. 1996,
Farise and Cayot 1998).
EXOGENOUS ENZYMES IN DAIRY
PROCESSING
The dairy industry represents one of the largest mar-
kets for commercial enzyme preparations. The use
of some enzymes in the manufacture of dairy prod-
ucts, for example, rennet for the coagulation of milk
is, in fact, probably the oldest commercial applica-
tion of enzyme biotechnology.
The use of rennets to coagulate milk and in the
ripening of cheese, were discussed in Cheese and
Fermented Milk, above. A further application of pro-
teolytic enzymes in dairy products, that is, the pro-
duction of dairy protein hydrolysates, is discussed in
Protein Hydrolysates, below. The applications of en-
zymes not considered elsewhere, but of significance
for dairy products, are discussed in this section.-GALACTOSIDASEHydrolysis of lactose (4-O--D-galactopyranosyl-
D-glucopyranose), either at a low pH or enzymati-
cally (by -galactosidases), yields two monosaccha-
rises, D-glucose and D-galactose (Mahoney 1997,
2002). A large proportion of the world’s population
suffers from lactose intolerance, leading to vary-
ing degrees of gastrointestinal distress if lactose-
containing dairy products are consumed. However,
glucose and galactose are readily metabolized; they
are also sweeter than lactose. Glucose/galactose
syrups are one of the simplest and most common
products of the industrial lactose hydrolysis pro-
cesses.
-galactosidases are available from several
sources, including E. coli, Aspergillus niger, A.
oryzae,and Kluyveromyces lactis(Whitaker 1992,
Mahoney 2002). -galactosidase for use in dairy
products must be isolated from a safe source and be
acceptable to regulatory authorities; for this reason,
genes for some microbial -galactosidases have
been cloned into safe hosts for expression and re-
covery. Of particular interest are enzymes that are
active either at a low or a very high temperature,
to avoid the microbiological problems associated
with enzymatic treatment at temperatures that en-
courage the growth of mesophilic microorganisms
(Vasiljevic and Jelen 2001). Microorganisms capa-
ble of producing thermophilic -galactosidases in-
clude Lactobacillus delbruckii subsp. bulgaricus
(Vasiljevic and Jelen 2001) and Thermus ther-
mophilus(Maciunska et al. 1998), while cold-active
-galactosidase has been purified from psychrophilic
bacteria (Nakagawa et al. 2003).
Enzymatic hydrolysis of lactose rarely leads to
complete conversion to monosaccharides, due to
feedback inhibition of the reaction by galactose, as