Chemistry Reference
In-Depth Information
high temperatures, the stabilizing effects of the high sugar and low water
concentrations (Johansson et al., 1989). The temperature tolerance of the
enzyme also depends on its source. The manufacturing processes for food-
grade oligosaccharide mixtures described in Section 5.4.1 use the equilibrium
principle and the glycosidase,
-galactosidase.
The microbial
-galactosidases (
-
D
-galactoside galactohydrolase, EC
3.2.1.23) have been well studied, but it is often not appreciated that their
catalytic action is best described as a transgalactosylation rather than a hydro-
lysis. The enzyme transfers the galactose moiety of a
-galactoside to an
acceptor containing a hydroxyl group. If the acceptor is water, then galactose
is formed, but if lactose is used as the acceptor molecule, then new oligosac-
charides will be synthesized. The different microbial origins of
-galactosidases
not only determine their temperature tolerance but also the characteristic
mixture of di- to hexa-saccharides formed and whether a
1
!
4ora
1
!
6
linkage is formed. The formation of oligosaccharides reaches a time-course
maximum during a batch reaction. Continuation of the reaction will lead to the
hydrolysis of the formed oligosaccharides to monosaccharides (Smart, 1993).
Allolactose (
-
D
-Gal-(1
!
6)-
D
-Glu) is always formed initially in the
reaction to produce oligosaccharides from lactose (Prenosil et al., 1987a).
Galactobiose (
-
D
-Gal-(1
!
6)-
D
-Gal) is also formed, but allolactose is the
dominant initial product. A range of higher oligosaccharides is then formed,
including galactotriose, [4-
-galactobiosylglucose, 6-
-galactobiosylglucose
and 6-
-galactotriosylglucose (Asp et al., 1980; Rastall and Bucke (1993)].
The products of synthesis from lactose have been thoroughly reviewed by
Prenosil et al. (1987a). Table 5.4 shows the numerous possible reaction
products from lactose resulting from hydrolysis, internal rearrangement
and transgalactosylation.
Methods to enhance the effectiveness and reduce the cost of processing
lactose using
-galactosidase have been examined intensively in recent years.
They include immobilization of the enzyme by encapsulation, by entrapment
in fibre matrices, in gels or on semi-permeable membranes. A variety of
adsorption and covalent attachment techniques have also been described.
5.4.1.2.
Microorganisms Used to Produce -Galactosidases (Lactase)
Historically,
-galactosidase has been an important industrial enzyme
due to its many applications in the dairy industry. These include the hydro-
lysis of lactose to prevent symptoms of lactose intolerance, lactose crystal-
lization problems in processed foods, in cheese manufacture, cheese whey
waste reduction, improvement of whey protein concentrates and in ethanol
production. Since about 1985, the use of lactases for the production of
oligosaccharides
has
added
a
new
dimension,
which
has
required
a
