Optimal synergistic formulation of essential oils: Antimicrobial and physical-mechanical properties of cinnamon-oregano oils-incorporated cassava starch films for fresh Vietnamese bread preservation

The synergistic effects of cinnamon and oregano essential oils (EOs) against Staphylococcus aureus, Bacillus cereus, and Aspergillus flavus were evaluated by using a disc volatilization assay. Minimum inhibitory concentration (MIC) was indicated at 5 μL L−1 air when the ratios of cinnamon and oregano EOs were 8:2 and 9:1 (v/v) for all tested bacteria, which also showed strongly synergistic activities (fractional inhibition concentration index (FIC) ≤ 0.48). However, there was only the combination ratio 6:4 showing a synergistic effect on the inhibition of A. flavus. Therefore, this combination was selected for incorporating with cassava starch films according to the central composite design. Desirability function approach indicated the optimal film formulation was 45.54 % of sorbitol and 0.65 % of EOs. The tensile strength and elongation at break of the optimal film were 8 MPa and 27 %, respectively while the AM film showed 100 % inhibition for all tested microorganisms. In the artificial contamination test, the antimicrobial film could express the inhibition at 85 % and 100 % on the growth of A. flavus in artificially and naturally contaminated Vietnamese bread, respectively.