Investigating the effect of carvacrol on physical, mechanical and antimicrobial properties of bio-hydrogel film based on filter flour obtained from wheat milling process

Hydrogels are a three-dimensional network of polymeric matrices with the ability to absorb water through chemical or physical cross-linking. Recently, the development of bio-based hydrogel with the aim of reducing the use of fossil fuel is becoming interested. Wheat filter flour (WFF) is a by-product obtained from air-classification in the modern wheat milling industry. It contains a high level of non-endosperm materials with the ability of water absorption and bio-film making capacity. Nevertheless, hydrogel-based films usually display weak water resistance, flexibility problems, and gas barrier properties. Carvacrol as a phenolic component is used to improve the functional properties of film and reduce the growth of pathogenic and spoilage microorganisms.

Hydrogel based films were prepared by casting technique. Six grams of WFF was added to 100 ml of distilled water, the pH of the solution was adjusted near to 10.7 with NaOH solution (1 N). The dispersion was heated up to 85°C,  with gentle stirring at 250 rpm for 30 min.Glycerol (35 g/100g of dry polymer) was then added and stirred for another 15 min. Carvacrol, as an active agent incorporated at two concentrations (5 and 10% (g/100g of dry polymer)) and stirred for another 15 min at 40°C and dried in a forced-air oven at 35± 5°C for 24 h. Physical (density, water-solubility, moisture content), mechanical, barrier and antimicrobial properties of active hydrogel-based film were determined. Antimicrobial properties of active hydrogel-based were evaluated in the vapor phase by using the micro- atmosphere method and liquid medium (immersion in broth) as a food model system. Statistical analyses were performed on a completely randomized design with the analysis of variance procedure using SAS software.

Increasing the carvacrol concentration in the film making solution led to decreasing the water solubility, moisture content, and hydrophobicity properties of WFF based films. The SEM observations confirmed a porous structure of the active hydrogel-based film. The highest water vapor permeability (1.32×10-10 gm-1s-1Pa-1) and the minimum water solubility (37.01± 0.63%) were observed in hydrogel film with 10% carvacrol. An increase in the concentration of carvacrol produced a greater growth inhibition zone for all microorganisms. The results in vapor phase showed that A.niger exhibited greater sensitivity to carvacrol than other studied bacteria. Hydrogel based films with 5% carvacrol in liquid food model system produced 1.16 and 1.34 log reduction against E. coli and L. monocytogenes, respectively. The greatest antibacterial activity was observed with films containing 10% carvacrol against L. monocytogenes (2.71 log reduction). This work suggested that the WFF hydrogel base film with 10% carvacrol can be used as an active packaging for improving the safety and shelf-life of food products.