Galacto-oligosaccharides and Other Products Derived from Lactose - Advanced Dairy Chemistry

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increasing (Ruffing and Chen, 2006). For example, Koizumi et al. (1998)

achieved a yield of 188 g/L of the galacto-oligosaccharide, globotriose

(Gal 1 ! 4Gal 1 ! 4Glc) and no oligosaccharide by-products were observed.

The synthesis simultaneously produced high yields of UDP-galactose from

galactose and orotic acid.

Crout and Vic (1998) favoured the use of glycosidases over glycosyl-

transferases because they are better suited to cheap synthesis methods. They

also point out that exo-glycosidases currently used commercially only allow

glycosyl transfer at the non-reducing terminal monomer of the substrate, and

this restricts the types of oligosaccharide structures that can be produced.

They advocate the use of endo-glycosidases which may allow branched struc-

tures to be formed. To improve the reverse hydrolysis required for this type of

synthesis, organic solvents have been used in a number of studies (Crout and

Vic, 1998). The use of glycosidases with novel approaches may lead to synth-

esis of useful new oligosaccharides.

For the synthesis of a fucose-containing galacto-oligosaccharide, it will

first be necessary to produce the L -fucose required. L -fucose occurs in nature

only at low concentrations and is present in plant species of the Convolvula-

ceae family. On the other hand, D -fucose is found in some seaweeds or can be

produced from D -galactose. Fucose is a hexose deoxy sugar (Figure 5.3) and

CH 2 OH

- D -galactose

- D -glucose

lactose

CH 3

CH 2 OH

D -fucose

C O

CH 3

C O

CH 3

N -acetyl-

- D -glucosamine

CH 3

- N -acetylneuraminic acid

L -fucose

Figure 5.3. Chemical structures (Haworth) of compounds found in human milk (fucose, N-acetyl

glucosamine and N-acetyl neuraminic acid) compared to those of lactose, galactose and glucose.

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