Antimicrobial active and smart film with synergical effect of thymol and colour indicator

Abstract

Active and smart packaging is a promising form of food packaging that offers a great economical potential. This is due to consumer demand for a packaging that accommodate a hectic way of life. An antimicrobial packaging with pH colour indicator (AMI) can be made by incorporating suitable antimicrobial (AM) agent and colour indicator into food package matrices whilst applying a bio switch concept to inhibit the pathogenic microorganisms and respond automatically to changes (external stimuli) in the environment. The present work aimed to develop the formulation of hydroxyethylcellulose (HEC)/wheat-starch based AMI film in which the active compound, thymol (0.5, 1, 1.5, 2, and 2.5% w/w) and 50:50% w/w bromothymol blue and methyl red (as the colour indicator) were incorporated into the polymeric material. A solution casting method was used in the film preparation while thymol and colourant were incorporated prior to casting. In order to determine the film characteristics, physical (scanning electron microscopy (SEM) and tensile test), chemical (Fourier transform infrared (FTIR)), and thermal properties (thermogravimetric analysis, differential scanning calorimetry) tests were carried out. Effects of adding thymol into the film base were studied in order to improve the antimicrobial spectrum activity based on measured agar diffusion assay. The distributions of inhibitory effect of the film were determined towards different types of microbial contamination in terms of Gram-positive bacteria, Gram-negative bacteria and fungus. The concentration of thymol as antimicrobial agent added into the film was optimized and modeled in relation to pH changes due to food microbial spoilage activities. The effect of thymol showed a range of microbial inhibition zones of 16.3 - 26.4% and 22.1 - 39.9% towards E. coli and B. subtilis, respectively. Whilst, a lower inhibition zone of 0.4 - 5.1% was demonstrated for fungus A. niger. The mechanical properties of the films were improved by 60.3% with an optimum tensile strength at thymol concentration of 1.5% w/w. Chemical interactions were studied by means of FTIR spectroscopy and SEM was used to verify the surface and cross section of the film matrix. The results demonstrated that thymol could be successfully added into the film without changing the main packaging properties. Direct incorporation of methyl red and bromothymol blue into film was suitable technique for making AMI packaging film. The combination of the indicators showed a distinct colour changes between pH 6 to pH 8 where most food spoilage occur. It can be concluded that the film properties are retained chemically whereas mechanical properties, strength, flexibility and function of the HEC/starch based AMI film are being enhanced remarkably by the incorporation of thymol and pH colour indicator.