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Handbook of Size Exclusion Chromatography and Related ...

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Rheological investigations <strong>of</strong> stability <strong>of</strong> starch glucans against applied<br />

thermal stress yields information in terms <strong>of</strong> so-called gelatinization temperature:<br />

acritical temperature where disintegration <strong>of</strong> supermolecular glucan structures<br />

starts<strong>and</strong>asignificanttemperaturerangeforthedisintegrationprocess.Obviously<br />

the critical temperature <strong>and</strong> disintegration temperature range strongly depend on<br />

branching pattern characteristics: scb-type starch glucans such as waxy maize are<br />

typically better stabilized <strong>and</strong> thus st<strong>and</strong> applied thermal stress better than starch<br />

glucans with dominant lcb-contributions such as wheat (Fig. 28). Increasing<br />

thermal energy disintegrates wheat starch glucans at significantly lower<br />

temperatures <strong>and</strong> over a smaller temperature range (56–628C; T max ¼588C)<br />

than waxy maize glucans (65–858C; T max ¼788C). Amuch more pronounced<br />

increase in viscosity upon disintegration <strong>of</strong> scb-type waxy maize starch glucans<br />

compared to lcb-type wheat starch indicates acomparably pronounced glucan/<br />

glucanstabilization<strong>of</strong>scb-typestarchescomparedtolcb-typeglucans,whichneed<br />

much more energy for destruction.<br />

For simultaneously applied thermal <strong>and</strong> mechanical stress [Brabender<br />

viscosity for an applied temperature program with three states (heating, holding,<br />

cooling) <strong>and</strong> constant shear deformation] another significant difference between<br />

scb-<strong>and</strong>lcb-typestarchglucansbecomesapparent(Fig.29).Althoughmuchmore<br />

energy is required comparably to disintegrate scb-glucans in the heating period<br />

(20–40 min), once liberated from supermolecular formations, scb-glucans do not<br />

Figure 28 Stability <strong>of</strong> starch glucans on applied thermal stress. Viscosity at increasing<br />

temperature: Wheat (—B—): minor initial stability; disintegration peak, 56–628C. Waxy<br />

maize (—X—): high initial stability; disintegration peak, starting at 658C.<br />

© 2004 by Marcel Dekker, Inc.

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