Understanding Polymeric Amylose Retrogradation in Presence of Additives

Polymeric aging in starch is an important equilibrating process leading to long-chain amylose recrystallization and anisotropic properties of material made from starch. Investigations into a number of plasticizers and hydrocolloids showed that their water retention/binding capability influence the starch polymer's crystallinity. Addition of up to 8 wt % glycerol and 3 wt % xylitol acted as an antiplasticizer and apparently increased the total crystalline phase, which reduced the degree of elongation of amylase matrix by 15%. Maltodextrin and xanthan gum also reduced matrix elongation capability but X-ray diffraction (XRD) analysis showed samples with varying crystallinity, and the extent of crystallinity did not correlate with respective tensile properties. Additives such as maltodextrin, with similar molecular structure as amylose, were ineffective in increasing degree of elongation even at 15 wt % addition rate, because of formation of pockets of crystalline region, as observed by XRD analysis. Both xylitol and xanthan gum samples showed similar tensile strength and elongation properties, but the water retention capability of xylitol-filled starch samples was about 27% lower than xanthan gum samples, at their respective higher concentrations. A dynamic structural unit is proposed to satisfy the isotropic increase in tensile strength and degree of elongation in oriented starch matrix.