Innovations in the development of antimicrobial edible films made from biopolymers and oregano

Abstract

In the present study, edible films were developed by the casting technique from aqueous solutions containing cassava starch (2.67% w/w) and hydroxypropyl methylcellulose (HPMC, 0.67% w/w), glycerol (1.7% w/w) as plasticizer and potassium sorbate as an antimicrobial agent (KS, 0.3% w/w). Oregano powder was added to the film forming solution (0.5% w/w and 2% w/w) in order to obtain composite films. Mechanical test (tensile) was performed until rupture and the parameters elastic modulus (Ec), stress and strain at rupture were obtained. Also, water solubility (WS), water vapor permeability (WVP) and color, through the Yellow Index (YI) and Hunter L, a and b parameters, were determined. The cristallinity of films was studied by X-Ray diffraction and the microstructure by light, scanning electron (SEM) and atomic force (AFM) microscopies, including the evaluation of the quadratic (Rq) and apparent (Ra) roughness. The results were compared with those obtained for a film without oregano. On the other hand, the effectiveness of films as an antimicrobial barrier was determined by studying the development of the yeast Zygosaccharomyces bailii (yeast) or the bacteria Lactobacillus spp (Gram positive bacteria) inoculated on the films and deposited on a model food with reduced pH and aw (4.5 and 0.98 respectively). The incorporation of oregano into the film formulation increased the Ec and the stress while decreased the deformation at the break point. This tendency was greater as the oregano content increased. The YI and Hunter parameters a and b were significantly higher for the formulations containing oregano while the luminosity L was lower. The WS was similar in systems containing or not oregano whereas that the WVP was significantly lower for composite systems. The diffraction patterns obtained from films showed a characteristic spectrum of amorphous materials. Microscopies revealed that the starch/HPMC films formed a structure with phase separation, with a homogeneous and smooth surface topography together with a dense and compact biopolymeric matrix. The composite systems exhibited agglomeration of oregano particles mainly due to the drying process, observing wellcemented oregano particles and absence of pores. The heterogeneity of the composite systems surface increased with the content of oregano, increasing strongly the Rq and Ra values. The KS presence inhibited the development of Lactobacillus spp. and Z. bailii. The addition of oregano contributed to the reduction of bacterial population without interfering with the antimicrobial activity of KS against yeast during the assay. It can be concluded that the developed composite films containing KS and oregano showed capacity to prevent the development of Lactobacillus spp. and Z. bailii. On the other hand, the films containing oregano showed a dark color and improved mechanical and permeability properties. The microscopy study allowed usto characterize qualitatively and quantitatively the surface of the biopolymeric matrix and, in the same way, permit us to understand the observed trends related to mechanical properties and permeability. Therefore, the films studied represent a novel alternative in the biodegradable composite active materials field, with an enormous potential application in food packaging.