Chemistry Reference
In-Depth Information
Other reagents such as basic magnesium salts and sodium hydrosulphite
have also been used (Carobbi et al., 1985). Whilst offering some advantages,
these compounds also created new problems of purification and of disposal of
spent materials in an environmentally friendly way.
Attempts have been made to develop continuous reaction systems, as
opposed to batch reactors. One example is the dual reactor tank system
coupled to a tubular reactor described by Kozempel and Kurantz (1994).
The yield of lactulose from lactose was 70% in this system.
Developing commercially viable processes has proved difficult, but the
use of borate appears to be the preferred approach, mainly because high
yields are obtained. Effective separation of lactulose from the product mix
has proved very difficult to achieve economically. Most pharmaceutical-
grade lactulose syrups contain substantial concentrations of lactose, epi-
lactose and galactose.
Research has been conducted to develop better methods to obtain
lactulose in powder or crystalline forms. As lactulose is highly soluble in
water, it is difficult to obtain in the form of a stable powder and it is generally
produced as a syrup. Nevertheless, complicated processes have been devel-
oped to prepare powdered or crystallized products. Crystallization methods
using alcohols, such as ethanol and methanol, have been developed to obtain
lactulose crystals. Highly pure lactulose crystals can be obtained, but con-
tamination with alcohol remains a problem. Another problem is the presence
of lactose, galactose and other side products which have to be removed also,
and this adds to the cost. Alternative methods that avoid the use of alcohols
have been developed. These are based on seeding crystals into the concen-
trated syrup and then freezing the mixture. The Morinaga Company has
developed a process to form crystalline lactulose trihydrate. This product, it
is claimed, is purer and more stable under atmospheric storage conditions
than anhydrous lactulose or the monohydrate. The process is based on a
crystallization precipitation procedure (Tomito et al., 1994).
Lactulose can also be prepared by an enzymatic route (Lee et al., 2004),
but this route is not used currently for commercial production.
5.4.3.
Lactosucrose (
b
-
D
-Gal-(1!4)-
a
-
D
-Glu-(1!2)-
b
-
D
-Fru)
Lactosucrose is a trisaccharide produced enzymatically by transfer of
the fructosyl moiety of sucrose to lactose as an acceptor molecule. The
structure is shown in Figure 5.4. The enzyme used is a
-fructofuranosidase
(EC 3.2.1.26). It is known to conduct the transfructosylation reaction, as well
as hydrolysis, in a manner analogous to that described for
-galactosidase.
Similarly, the microbial origin of the fructofuranosidase is important.
Enzymes from different sources exhibit different degrees of acceptor types
