THESIS

calorimetry (DSC) (Russell, 1983; Mua and Jackson, 1995), Fourier transform
infrared spectroscopy (FT-IR) (Van et al. 1994) or Raman spectroscopy (Bulkin et al.
1987) and rheological techniques (Doublier and Choplin, 1989; Mita, 1992).
1) Retrogradation measured by X-ray diffraction
Early X-ray diffraction studied on aged starch gels showed that
the B-type diffraction pattern developed slowly (Zobel, 1973). Crystal growth, as
detected by X-ray diffraction, is slower than formation of the gel network and has
been proposed as occurring in the polymer-rich regions of the gel (Miles et al.,
1985a). For both amylose and starch gel, the initial development of crystallinity
occurred at similar rate. However, the crystallization of amylose effectively reached a
limit rates. However, the crystallization of amylose effectively reached a limit after 2
days whereas the crystallinity of the starch gel continued to increase (Miles et al.
1985b). The amylopectin gels show only a slow increase in crystallinity with time and
approach a limiting value after 30-40 days (Ring et al. 1987). Isolated gelatinized
starch granules that have been washed free from all exudates amylose gave no X-ray
diffraction pattern immediately after cooling. After two weeks of storage, the B-type
pattern is obtained. Amylose gels are firm, white and opaque. They can withstand
autoclave at 130 o C. Retrograded amylose gives the B-type starch X-ray diffraction
pattern. Early X-ray diffraction analysis of stretched amylose film in the B-type
structure was interpreted in terms of a six fold single helix structure with a repeat
distance of 10.4 °A. On this basis it could be postulated that the mechanism of cross
linking amylose gels was via junction zones of aggregated single helices. From
studies on Naegeli amylodextrins, however, Kainuma and French (1972) proposed a
model for B-type starch comprising intertwined double helices. This proposal has
recently been confirmed by X-ray diffraction studies (Wu and Sarko, 1978) as showed
in Figure 11. This most recent crystal structure of the B-polymorph of amylose is
based on double stranded helices. The individual strands are in a six fold helical
conformation repeating in 20.8 Å, and are wound parallel around each other. In the
case, however, the X-ray data cannot reliably distinguish between right and lefthanded
models (Cheetham and Tao (1998).
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