Bioactive Andean sweet potato starch-based foam incorporated with oregano or thyme essential oil

In this research, sweet potato starch and oregano (OEO) or thyme (TEO) essential oil at two concentrations (7.5 and 10 %) were used to produce bioactive foams by thermopressing. The foams were characterized according to microstructure, mechanical properties, antimicrobial properties, and structural properties by X-ray diffraction, scanning electron microscopy, and Fourier-transform-infrared spectroscopy (FT-IR). In all cases, essential oil addition affected the foam color, yielding reddish/yellowish foams, but not the foam thickness. FT-IR spectrum and X-ray diffraction revealed starch-lipid interactions. According to the micrographs, the lipids were localized in the first layer. Thus, formation of amylose-essential oil complexes in the foam may have prevented the essential oil from degrading under the thermoforming temperature. Essential oil addition yielded starch foams with low water solubility and mechanical resistance, especially for 10 % OEO. Meanwhile, these foams were more effective against Salmonella (Gram-negative bacteria) and L. monocytogenes (Gram-positive bacteria). The antimicrobial activity of the foams containing essential oil makes them beneficial for application as bioactive materials. Therefore, bioactive sweet potato starch-based foams can be prepared by thermopressing and be applied as food container.