Sol–gel transition and gelatinization kinetics of wheat starch

Rheological and optical microscopy experiments were conducted to monitor the sol–gel transition during gelatinization process of starch granules. During the gelatinization phenomenon, the grains grow and dissolve into the plasticizer leading to a drastic increase of the elastic modulus (G′) and the loss modulus (G″), and to their crossover as a consequence of the formation of a physical gel. The kinetics of gelation were studied by oscillatory shear experiments and the influence of parameters such as temperature or volume fraction on the gelation process was investigated. The variation of the gel time with temperature for any composition of glycerol/starch suspension was predicted from the Dickinson model. The relaxation exponent (n = 0.73) was calculated by varying the frequency at constant volume fraction and temperature. The deduced value indicates that the gelation process is described by percolation theory. Apparent activation energies were estimated from the rheological data as well as from the optical microscopy data and were found to be nearly the same. At last the ability of two polyols to act as plasticizer was assessed and compared to that of glycerol.