Surface octanoylation of high-amylose potato starch films

In this study high-amylose potato (HAP) starch films were prepared by casting from aqueous solution. The relative crystallinity of the HAP films was determined to be 25% using wide angle X-ray diffraction. The films were surface esterified with octanoyl chloride and pyridine using heterogeneous reaction conditions. The chemical composition and the degree of starch substitution of the films were determined with electron spectroscopy for chemical analysis. The esterification reaction at the surface was initially very fast. The HAP films had a degree of substitution of approximately 2 after only 15 min at 80 °C. Attenuated total reflectance FT/IR measurements of the films revealed that the depth at which the esterification took place was at least 1.1 μm. FT/IR transmission analysis through a tablet of the film showed that esterification was not taking place to a high degree in the bulk and was thus concentrated to the surface region. An experimental design was used to optimise the degree of starch substitution by varying concentration, temperature and time. The experiments showed that the degree of starch substitution could be varied from 0.24 to 2.79 depending on treatment conditions, where the concentration of octanoyl chloride was the most important parameter. The morphology of the modified HAP films, as shown by scanning electron microscopy, exhibited a rougher surface structure than the unmodified films. The surface energy of the modified HAP films (∼40 mJ/m2) was lower than for unmodified HAP films (∼50 mJ/m2), as determined by static contact angle measurements. Water vapour transmission and water absorption measurements revealed that the water penetration through the HAP films decreased almost linearly with the degree of starch substitution.