Chemistry Reference
In-Depth Information
Galacto-oligosaccharides
α-D-Glu-(1→
Lactulose
β-D-Gal-(1→4)-β-D-Fru
4)-[β-D-Gal-(1
→
6)-]
n
where n = 2 to 5
Transgalactosylation by
β-galactosidase (lactase) Alkaline isomerisation
LACTOSE
β-D-Gal-(1→4)-α-D-Glu
Lactobionic Acid
4-
O
-β-D-galacto
pyranosyl-D-gluconic
acid
Catalytic hydrogenation (Ni, 100°C,
Dehydrogenation at
high pH over noble metal
catalyst
elevated pressure)
Lactitol
4-
O
-β-D-galactopyranosyl-D-glucitol
+ SUCROSE
Hydrolysis by
Transfructosylation ß-galactosidase (lactase)
by β-fructofuranosidase
D- Tagatose
C4 epimer of D-fructose
D- Galactose
Isomerization
+
by L-arabinose isomerase
Lactosucrose
β-D-Gal-(1→ 4)-α-D-Glu-(1→2)-β-D-Fru
D-Glucose
Remove or recycle
Figure 5.1.
The preparation of galacto-oligosaccharides, lacto-sucrose, lactulose, lactitol,
lactobionic acid and tagatose from lactose.
initially from galactose by alkaline isomerization in a manner parallel to the
preparation of the disaccharide, lactulose from lactose, but it is now manu-
factured by an enzymatic route from lactose.
The basis of the preparation of galacto-oligosaccharides, lactulose,
lactitol and lactosucrose from lactose is shown in Figure 5.1.
5.4.1.
b
-Galacto-oligosaccharides
A typical flow chart for the manufacture of galacto-oligosaccharides
is shown in Figure 5.2. Similar methods are used by the main galacto-
oligosaccharide manufacturers (Yakult, Friesland Foods Domo and Nissin
Sugar). The process depends on reversing the normal degradative hydro-
lytic action of
-galactosidase. Instead of producing glucose and galactose
from the feedstock lactose, the enzyme is ''pushed'' into synthesis of a
mixture of tri, tetra and penta galacto-oligosaccharides. This is achieved
by having a high concentration of lactose in solution (e.g., 400 g/L or
higher). This is achievable only if the temperature is elevated to between
50 and 808Candtheenzymechosentocatalysethereactionmustbeactive
at these temperatures. The enzyme chosen must also have minimal hydro-
lytic activity to avoid the simultaneous formation of excessive amounts of
glucose and galactose. The
-galactosidases used may produce either
1
!
4
or
1
!
6 linkages in the oligosaccharide chain. The ratio of products
