Effect of saturated and unsaturated fatty acids on structural and optical properties of corn starch-glycerol based films

Starch is one of the cheapest raw materials which can be used in edible film formation. Due to the fact that it is a natural constituent, its incorporation as a food coating does not involve any health risks, although as a barrier material it has the disadvantage of having poor water vapour barrier properties. This could be improved if different non-polar components are included in the starch matrix. In this sense, edible films were formulated using corn starch and different fatty acids and the impact of these lipids on the optical properties of the films (transparency and gloss) was studied. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) were also used in order to analyse the film microstructure and to gain a better understanding of the films’ optical parameters. Lipid containing films were the least transparent due to the presence of discontinuities in the films’ matrices (lipid aggregates). In this sense, it is remarkable that films containing oleic acid were the least transparent. SEM micrograph allows us to confirm the greater structural heterogeneity of these films, coinciding with the opacity development. Gloss of films (measured at different incidence angles) was also affected by lipid addition and type of fatty acid. Films containing stearic acid showed the highest gloss values, which agrees with that observed in the AFM images where these films also showed the smoothest surface. Films with palmitic acid showed a smoother surface than either the control films (without lipid) or the films containing oleic acid.