Chemical and Functional Properties of Food Saccharides

© 2004 by CRC Press LLC
mated. 17 Most probably, variation of branching pattern is the key tool by which
adjustments are achieved at native conditions. Intensity of branching (branching
percentage and location of branching positions) and the ratio of lcb- to scb-glucans
is the dominant control parameter for macroscopic starch qualities. 18,19 nb/lcb-Glucans
are less or poorly soluble in aqueous media, highly tend to retrograde, and
easily gelatinize and form gels and films. scb-Glucans, on the other hand, show
much better solubility in aqueous media, are capable of fixing a high amount of
water, and are less sensitive toward varying environmental conditions. 20–25 Consequently,
characterization of starch qualities includes comprehensive analysis of
branching characteristics: (1) kind of branching pattern; composition of branching
pattern for starch glucan fractions sampled with respect to identical excluded volume,
identical molecular weight (degree of polymerization), identical internal stabilization,
and identical potential for formation of supermolecular characteristics; (2)
number and percentage of branching position within individual glucan molecules
and kind of distribution of number of branching positions in supermolecular
domains; (3) heterogeneity or homogeneity of branching positions within individual
glucan molecules and distinct supermolecular domains; (4) degree of local symmetry
(crystallinity) due to certain kind of branching characteristics, and influence of
increased branching to symmetry and interactive properties.
Experimental approaches to determine branching characteristics are rather laborious
and quite often are estimations rather than quantitative data. Basically,
branching analysis can be achieved by destructive or nondestructive techniques.
Destructive techniques include pure chemical-directed or enzymatically catalyzed
step-by-step fragmentation, reversed-phase HPLC analysis, or quantitative derivatization
and subsequent fragment analysis, for instance, by GC-MS, in both cases
followed by recalculation of mean molecules as a puzzle from fragment data.
Complexing and staining of starch glucans (native glucans, glucan fractions, glucan
fragments) with polyiodide anions in hydrophobic caves of terminal helical starch
glucan branches, and spectroscopy in terms of extinction ratio E 640/E 525 provides
relative information about lcb and scb characteristics of investigated samples. Application
to fractions from semipreparative SEC [Figure 22.8(a) and Figure 22.8(b)]
yields profiles of lcb-to-scb ratio with respect to decreasing excluded volume.
Experimentally, 125 mg freshly sublimated iodine is dissolved in the presence
of 400 mg kI in 1000 ml demineralized water and diluted 1:1 with 0.1 M acidic acid
to a final pH of 4.5 to 5.0 when mixed with the alkaline eluate from SEC. Polyiodide
anions complexed in the helical starch glucan segments shift extinction maximum
from E max at 525 nm of free aqueous iodine to higher wavelengths. 26,27 Actually, the
shift is controlled by both the length of helical segments and the number of available
helical segments; however, correlation of E 640 values are an appropriate indication
for lcb-glucans. Correlation of scb-glucans with E 525 values is supported by corresponding
maxima in ORD/CD spectra for α(1→4)-glucans with DP < 40. 28,29 The
ratio of E 640 to E 525 finally indicates branching characteristics of complexed glucans,
glucan fractions, or glucan fragments as ratio of lcb- to scb-glucans.
Results for semipreparative SEC separation of a wheat and a waxy maize sample,
subsequent complexing and staining of obtained fractions with polyiodide anions,
and monitoring of extinction ratio of complexed (E 640) and noncomplexed (E 525)