Chemical and Functional Properties of Food Saccharides

© 2004 by CRC Press LLC
Wheat consists of an lcb population (ca. 22%), which may be precipitate with nbutanol
from aqueous solution and contains significantly less A-chains, and an scb
population (ca. 78%), which is very similar to that of waxy maize starch.
22.4.3 SUPERMOLECULAR STRUCTURES
As no obvious differences of molecular dimensions in terms of occupied volume (V e
→ R e) will be found for different starch glucans such as wheat and waxy maize, and
molecular weight at least appears somewhat ambiguous, strongly depending on applied
approach, branching pattern characteristics are strongly suspected to control macroscopic
material qualities by promoting formation of more or less stable supramolecular
structures. Investigation of segment mobility in starch glucan solutions by noninvasive
techniques such as photon correlation spectroscopy is a perfect tool to check such
supermolecular structures. Dynamic light scattering (DLS) and photon correlation
spectroscopy (PCS) experiments provide data on diffusive mobility of individual glucan
molecules and mobility of supramolecular segments, in particular, on translational
diffusion of glucans and glucan aggregates. If laser light is applied to starch glucan
solutions, mobility of the segments in such solutions cause Doppler shifts, which may
be monitored as the autocorrelation function, G 2(τ), (Equation 22.3a):
1
G2 () ? = I( t) ⋅ I( t + ?) dt
T
(22.3a)
where T is the temperature (K), t the time, I the intensity of scattered laser light,
and τ the correlation period.
Indirect Laplace transformation of G 2(τ) yields G 2(t), which contains the translational
diffusion coefficient, D T, 44 (Equation 22.3b):
where h is the scattering vector, and A and C are coefficients.
According to Stokes and Einstein (Equation 22.3c), D T of observed glucans and
glucan aggregates can be correlated with radius, R H, or diameter, d, of moving
equivalent spheres. As in particular supramolecular structures are dynamically
formed by individual glucan molecules, d is rather the length of coherently moving
segments, and thus, for practical purposes, coherence length, l coh, of supramolecular
segments replaces the equivalent sphere diameter d:
(22.3c)
where k B is the Boltzmann constant, T the temperature (K), and η the viscosity of
solution.
?
∫
0
2
⎡ ⎛ τmax
t ⎞ ⎤ −
⎢ DT()
?
G2()
t = A⋅ + C⎜ τ
⎢ i ⋅e
d ⎟
⎥
1
1 τ with (22.3b)
⎜ ∫ 2
⎟
⎥ τ = 2
τ
D
⎢
⎣
⎝ τmin
⎠ ⎥
T h
⎦
D
T
kBT = →
6π ηR
H
l
coh