Effect of the incorporation of surfactants on the physical properties of corn starch films

The effect of surfactant addition on structural, mechanical, optical and barrier properties of corn starch-glycerol based films was studied. Sorbitan monopalmitate, monostearate or monooleate were incorporated into starch-glycerol (1:0.25) at a surfactant:starch ratio of 0.15:1. The film forming dispersions (FFD) were characterized as to rheology, ζ-potential, particle size distribution and contact angle. Film characterization was carried out at 1 and 5 storage weeks (at 25 °C and 53% relative humidity). Surfactants led to different particle size distribution, zeta potential and viscosity in FFD, and film extensibility, depending on their hydrophobicity and melting properties. Their incorporation to the corn starch-glycerol films produced a coarser film microstructure due to the appearance of free surfactant aggregates or V-amylose inclusion complexes which produce discontinuities in the amorphous continuous matrix. The size of these crystalline complexes was smaller for the surfactant with the lowest hydrophobicity with saturated fatty acid (span 40). This contributed to decrease the WVP values with respect to surfactant-free film. Films containing surfactants were less hard, resistant and extensible, and more permeable to oxygen, than surfactant-free films, but they did not notably affect the film gloss and transparency. Saturated fatty acid compounds with higher melting temperature are recommended to ensure a finer microstructure in the final film which favours water barrier efficiency.