Chemical and Functional Properties of Food Saccharides

© 2004 by CRC Press LLC
as lactose, allolactose, gentiobiose, isomaltose, and melibiose, are substrates for P2O
(see Table 10.3). Their lactones can be converted with aldose reductase (ALR) into
corresponding keto analogs. Some of the disaccharides, for instance, lactulose,
available in this manner are random in nature. Because of documented functional
benefits, they can be tapped as low-calorie bulking sweeteners. Lactulose has been
obtained mainly through the isomerization of lactose, catalyzed by sodium hydroxide,
sodium hydroxide and boric acid, and sodium aluminate. Because of the toxicity
of inorganic catalysts, enzyme-catalyzed lactulose production is riveting.
Tagatose, a ketohexose C-4 fructose epimer, is the lactose-derived noncariogenic
monosaccharide. It can be produced by using either P2O and ALR (see Table 10.3)
or in the sorbitol-dehydrogenase-catalyzed oxidation of D-galactitol, available from
the reduction of D-galactose liberated from lactose by β-galactosidase. D-Galactitol
is relatively inexpensive, and the process can be even more economically feasible
if the oxidation reaction is conducted by whole cells of bacteria such as Gluconobacter
species.
Advances in production of preparations of oxidoreductases and regeneration of
their coenzymes will contribute to their common industrial applications. These
enzymes are valuable biocatalysts, facilitating relatively simple synthesis of rare or
nonnatural carbohydrates and sugar-based precursors (synthons) for further conversions.
10.3 OLIGOSACCHARIDES
Several oligosaccharides appeared to be biologically active compounds, and therefore
their production became one of the most important areas in biotechnology.
Some of them, such as nondigestible oligosaccharides (NDOs) have prebiotic properties
(see Chapter 21). They are produced either by enzymatic transglycosylation
of relatively inexpensive disaccharides such as sucrose and lactose, or by conversion
of polysaccharides such as fructans or starch. Some other NDOs are available by a
limited enzymatic digestion of pectin or hemicelluloses. 19
10.3.1 FRUCTOOLIGOSACCHARIDES
The term fructooligosaccharides (FOs) is usually used for 1 F (1-β-D-frucofuranosyl)
n-sucrose (GF n; n = 2 to 10), though polyfructans and oligofructosides with
another structure also exist (see Chapter 13). 20 1-Kestose is the shortest fructan of
the inulin-type and 6-kestose of the levan-type. The reported FOs producing enzymes
are invertase and inulosucrase (also known as fructosyltransferase), whereas levansucrase
converts sucrose to 2,6-β-D-fructans (Table 10.2). The majority of inulosucrases
show high regiospecificity, and transfer the fructosyl moiety to C-1 hydroxyl
group of terminal fructofuranosides to produce 1-kestose and its homologs, but some
enzymes transfer the fructose to OH-6 F and OH-6 G to form 6-kestose or neokestose,
respectively. Another transferase involved in the fructan formation, β-(2,1)-fructan:
β-(2,1)-fructan 1-fructosyltransferase (FFT), transfers fructosyl groups between
fructan chains. It catalyzes the reaction GF n + GF m = GF n – 1 + GF m + 1, thus contributing
to synthesis of higher fructans in nature.