Glass Transition of Starches

Starch is a hydrophilic polymer and the water is its main plasticizer. It is compatible with starch, allows a sliding chain between them, and lowers the temperature of the glass transition. This increased mobility of the chains in the presence of water or other plasticizers has been extensively studied by DSC (Bizot et al., 1997) or dynamic mechanical thermal analysis (DMTA; Lourdin et al., 1997) and also by NMR (Gaudin et al., 1999). This has been confirmed by Saibene and Seetharaman (2006) who studied the complexation of iodine in the starch grains on either side of the Tg.

The other plasticizers of starch are generally polyols: glycerol, sorbitol, propylene glycol, ethylene glycol, and some polyethylene glycol (Lourdin et al., 1997). However, urea, sodium lactate, or sugars and oligosaccharides can also be used as plasticizers of starch. Paradoxically and under certain conditions, these plasticizers can induce the opposite effect on the starch (Lourdin et al., 1997). Thus, depending on the conditions of hydration (Aw≤ 0.22) and/ or of the concentration in plasticizer, they can act as “antiplasticizers” (Lourdin et al., 1997; Chang et al., 2006). For example, a quantity of sorbitol less than 27% (m/m) leads to the formation of starch films more rigid and brittle, while for a higher amount sorbitol has a plasticizing effect “classic” on starch (Gaudin et al., 1999, 2000). The precise determination of Tg values of starch is difficult because the glass transition temperature range is broadened by the presence of crystalline layers between the amorphous layers. Furthermore, crystallinity rate also has an influence on the Tg of starch: more the amorphous phase is interrupted by the crystalline domains, higher the Tg is (Kalichevsky et al., 1992). The glass transition temperature of anhydrous starch is not directly measurable, because the thermal degradation occurs before the glass transition. Therefore, it is extrapolated to zero water content from values obtained at different humidity levels. These Tg measurements can be performed either on native starch granules (Zeleznak and Hoseney, 1987) or on the amorphous destructurized starch by extrusion or casting (Kalichevsky et al., 1992; Bizot et al., 1997). Thus, the Tg values of films of waxy maize starch and of anhydrous potato were measured at 285 and 316°C, respectively (Bizot et al., 1997). With a water content of 12%, the Tg is lowered to 92°C and most of the time, the Tg is less than or equal to room temperature when the water content exceeds 20%.

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