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ABSTRACT:The properties of starch plastic sheets were investigated by stress-strain measurements in relation with starch crystallinity. Granular potato starch was plasticized with different amounts of glycerol and water by extrusion. The materials were amorphous directly after processing. During aging above the glass transition temperature at various humidities single helical (V and E-type) and double helical (B-type) crystallinity was formed. The rate of crystallization is a function of water and glycerol content. The amorphous rubbery materials were soft and weak with high elongations. During aging the materials became less flexible with higher elastic modulus and tensile stress. The changes are related to changes in water content and glass transition temperature and to changes in B-type crystallinity. The changes in stress-strain properties are explained by the formation of helical structures and crystals, which results in a reinforcement of the starch network by physical crosslinking.

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Retrogradation kinetics for a potato starch‐water system (10% w/w gel) was monitored by Fourier Transform Infrared spectroscopy and compared with waxy maize starch. The spectra showed the C‐C and C‐O stretching region (1300‐800 cm−1) to be sensitive to the retrogradation process. A multi‐stage process was observed during the retrogradation of potato starch and characterized as the formation of short‐ and long‐range order. The first stage was characterized as the formation of helices and the fast formation of crystalline amylose regions. The second stage was described as the induction time for amylopectin helix aggregation. Stage three was described as the helix‐helix aggregation and the crystallization of amylopectin. The overall‐first order calculated rate constant of potato starch was (9.6±1.4) 10− 3h−1. The calculated rate constant were in agreement with the known difference in retrogradation kinetics of waxy maize and potato starch. The effects were explained by the differences in retrogradation rate of amylopectin and amylose. Potato starch consists of amylose as well as amylopectin. Whereas amylose crystallization occurs within a few hours, amylopectin crystallization is slow and takes a few weeks.