Chemical and Functional Properties of Food Saccharides

© 2004 by CRC Press LLC
In particular, for midrange- and high-DP fructans, calibrated size-exclusion chromatography
(SEC) with mass detection by refractive index provides quantitative
information about molar mass and DP distribution. 11,12
Application of preparative and semipreparative soft-gel systems such as Biogel
P-2 or P-4 provides fructan fractions and pools for subsequent detailed structural
analyses such as reductive methylation, acetylation, and GC/MS or GC/FID detection.
The resulting 1,5-D-anhydrosorbits and 2,5,-D-anhydromannits provide quantitative
information about the kind of glycosidic linkages and enable computation
of molecules with mean structural conformations which typically are checked by
simultaneously accomplished NMR spectra. 13–15
13.3 FRUCTANS IN CROPS: BIOSYNTHESIS AND
PROPERTIES
Biosynthesis of fructans in crops differs significantly from pathways in bacteria or
fungi. The only key enzyme for the formation of high-DP fructans — exopolysaccharides
(EPS) — starting from sucrose in bacteria, such as in Pseudomonas
species, is levansucrase (EC 2.4.1.10). These readily aqueous-soluble EPSs are
nonreducing levans, 16–18 (2Æ6)-linked b-D-fructans with branching option at O-1
and molecular weights up to 10 6 (g/mol).
Fructosyltransferases of fungi Aureobasidium pullulans and Aspergillus niger
form fructooligosaccharides with DPs of 3 to 6 at high initial sucrose concentrations;
however, the final mixture still contains sucrose and fructosyl-transfer-equivalent
concentrations of glucose. 19,20
The key compound in the formation of fructans is the disaccharide sucrose
(Figure13.1). Polymerization, and thus the formation of low-molecular-weight fructans
(fructooligosaccharides, FOSs) and high-molecular-weight fructans runs via
transfer of fructofuranosyl units from a donor sucrose to an acceptor sucrose or to
the already formed acceptor fructan.
In contrast to glucans, fructans contain no reducing terminal hemiacetal but have
a single heteromonomer glucopyranosyl residue. Those amounts of fructans without
terminal glucose are the result of internal rearrangements or depolymerization reactions
in fructan metabolism, which are amplified by cell-specific propagation processes.
For optimization of fructan metabolism, biosynthesis of fructan in crops is most
probably a two-step process (Figure13.2). In the initial steps, polymerization runs
without a precursor, catalyzed by sucrose:sucrose 1-fructosyltransferase (1-SST: EC
2.3.1.99), with donor sucrose and acceptor sucrose forming trisaccharide 1-kestose.
The remaining glycosyl residue immediately gets processed in the cytosolic pathway
by sucrose synthase. If necessary, the glucosyl residue may even be transferred to
the starch metabolism pathway or to pathways for the formation of cell wall structurizing
materials (Figure13.1).
The second step is the formation of fructans, starting from kestose and catalyzed
by several crop-specific fructosyltransferases (fructan:fructan fructosyltransferases,