Impact of shear force on functional properties of native starch and resulting gel and film

In this study, cornstarch dispersions were physically modified through a high-speed shear homogenizer with various shear-induced rates (56-400/s) and physical, thermal, crystalline and morphological properties of modified starch were investigated. Then, a hydrogel and a soft film were fabricated from the modified starches to evaluate the effect of shear force on their functional properties. Scanning electron microscopy revealed that the shearing treatment altered the shape of granules and changed their surface appearance. This was particularly found to be the case when sufficiently high intensity of shear force was used. X-ray diffraction pattern showed the crystallinity degree of granules decreased after the mechanical treatment. Two endothermic peaks detected on differential scanning calorimetry curves of native sample, whose enthalpy decreased after the treatment. Moreover, swelling power of the starch increased by increasing the shear rate as proven by decreasing the enthalpy of the endothermic peaks. It was found that the shear treatment produced an excellent hydrogel with improved textural parameters and softer structure. The textural analysis revealed a prominent increase in hydrogel hardness with increasing the shear rate, whereas cohesiveness parameter decreased. Atomic force microscopy revealed that the rough hydrogel surface became smooth after the treatment. Regarding the starch-based film, the physico-mechanical results showed that water resistance, water barrier property and tensile strength improved after the shear force treatment.