Development of Tapioca Starch/Decolorized Hsian-Tsao Leaf Gum-Based Antimicrobial Films: Physical Characterization and Evaluation Against Listeria monocytogenes

The physical properties and antimicrobial activities against Listeria monocytogenes of the tapioca starch/decolorized hsian-tsao leaf gum (the polysaccharide gum extracted from hsian-tsao herb followed by removing the color matters, abbreviated as dHG hereafter)-based films incorporated with potassium sorbate (KS) or the ethanolic extract of thyme (TH) were investigated. It was found that the cross-sectional microstructure of starch/dHG films with KS showed some rough texture, and films with TH showed a relatively smooth microstructure with dispersed microparticles. The equilibrium moisture content and water vapor permeability of starch/dHG films with 20 % KS (based on the weight of starch/dHG) was higher than the others tested, possibly due to the plasticizing effect of KS. In general, the tensile strength and tensile modulus decreased with increasing antibacterial concentration. As compared with the KS solutions, the TH solutions showed a pronounced antimicrobial activity against L. monocytogenes as determined by the inhibition zone test. However, the antimicrobial migration of both KS and TH in the starch/dHG matrix was limited to a certain extent, as evidenced by the significantly lower antimicrobial activity in the film system. When applying the starch/dHG films with antimicrobials to the fresh beef slices, all samples showed detectable improvement against the growth of L. monocytogenes, implying that starch/dHG films incorporated with TH or KS were effective against L. monocytogenes in conjunction with some modification of the physical properties due to the interactions between the antimicrobials and the components of film matrix.