**4. Effect of retrogradation on the property of thermoplastic starch**

Properties of TPS products depend upon its microstructure. Zhang & Han (2010) studied the effect of crystallinity on the properties of the starch film, including moisture content, gas permeability, and tensile properties, etc. When crystallinity of glycerol-starch film increased from 6.0% to 8.0%, moisture content decreased from 11.0% to 8.0%. Similar phenomena were observed in sorbitol-, fructose-, and mannose-films. During the retrogradation process, starch chains aligned to form crystalline lamellas. Water molecules, as well as plasticizer molecules, are pushed out and evaporate resulting in reduced moisture content. Retrogradation of starch was also observed to have negative effect on gas permeability, including oxygen permeability (OP) and water vapor permeability (WVP) (Zhang and Han, 2010). Sorbitol-film had a reduction in OP from 20×10-7 to 0.3×10-7 cc mm h-1 kPa-1 m-2 when its crystallinity increased from 7.0% to 12.0%, while OP in glycerol-film was reduced from 6×10-7 to 4×10-7 cc mm h-1 kPa-1 m-2 when its crystallinity increased from 5.0% to 9.0%. A similar trend was also found in WVP. When crystallinity of glycerol-film increased from 6.0% to 9.0%, its WVP decreased from 1.2 to 0.9 g mm m-2 h-1 kPa-1. Pushpadass & Hanna (2009) explained that the increase in crystallinity with time decreased the free volume in the TPS network and resulted in decrease in WVP. Zhang & Han (2010) proposed that crystallite in the films results in difficulty for gas molecules to diffuse within the starch films, leading to low permeability. Crystallinity also resulted in decreased elongation of the film. Elongation of mannose-film decreased from 4.0% to 0.2% when its crystallinity increased from 6.0% to 19.0%, elongation of fructose-film from 4.0% to 1.5% when its crystallinity increased fom 6.0% to 18.0%. Pushpadass & Hanna (2009) reported similar findings. They reported the relative crystalline content of TPS samples increased from 3 % (after 4 h extrusion) to 7%, 14%, and 17% after 3, 30, and 120 d of storage. As a result, tensile strength of TPS samples increased by 39.3 – 134.1%, elongation decreased by 48.0 – 81.1%, and WVP decreased by 6.1 % – 19.3%. A lower E value means that TPS becomes stiffer, less flexible and more difficult to handle. Crystallites may act as physical crosslinking points which generate internal stresses of TPS, leading to the increase in tensile strength and decrease in elongation (Delville et al. 2003)
