Chemistry Reference
In-Depth Information
compound is sold as a much purer product for food-grade use than galacto-
oligosaccharides, lactosucrose and lactulose. The food grades of these other
products all contain substantial amounts of other reactant and product
carbohydrates. Lactitol can also be prepared by reducing lactose using
NaBH
4
(Scholnick et al., 1975; Saijonmaa et al., 1978), but industrially,
lactitol is prepared by the catalytic hydrogenation process (Van Velthuijsen,
1979). The reaction is carried out in an autoclave at over 40 bar and over
1008C. A lactose solution of 30-40% is used. The ratio of Raney-nickel
catalyst to lactose is critical for efficient conversion. After hydrogenation is
completed, the catalyst is sedimented and filtered. The lactitol solution is
treated with an ion-exchange resin and activated carbon, and the purified
solution is concentrated. Crystals of lactitol are removed from the mother
liquor by centrifugation and the process repeated. After repeated crystal-
lization, the mother liquor can be used as a 64% syrup of lactitol (Van
Velthuijsen, 1979; Booy, 1987).
Preparation of derivatives of lactitol has also been studied. Polyalcohol
esters can be used as non-ionic emulsifying agents. For example, sorbitan
esters and sucrose esters of fatty acids are well known. Van Velthuijsen (1979)
described the preparation of esters like lactitol palmitate and their properties
and possible applications as laundry detergents.
Research has also demonstrated the formation of a range of oligosac-
charides from lactitol using
-galactosidase (Yanahira et al., 1992); they
were able to form six different oligosaccharides - all as a trisaccharide
containing a lactitol unit. The general chemistry and properties of the
sugar alcohols are described by Benson (1978). Comprehensive reviews on
lactitol are by Van Velthuijsen (1979), Booy (1987) and Mesters and Brokx
(2000).
5.4.5.
Lactobionic Acid
Lactobionic acid is formed by oxidation of lactose and is 4-O-
D
-galac-
topyranosyl-
D
-gluconic acid (Figure 5.5). Heterogeneous catalytic oxidation
and microbiological/enzymatic oxidation of lactose have been researched.
The facile dehydrogenation of lactose at high pH over a noble metal catalyst
is used commercially (Figure 5.1).
The aldehyde group of the glucose in the lactose molecule is oxidized to
the carboxyl group by either (a) chemical oxidation or (b) biochemical
oxidation. Electrolytic methods of oxidation are also possible. For chemical
oxidation, a mild treatment with hypobromite or hypoiodite produces an
equilibrium mixture of lactobionic acid and its
-lactone. Biochemical oxida-
tion can be achieved with enzymes isolated from microorganisms or by using
the microorganisms themselves for the bioconversion. For the latter process,
