Melhoria de propriedades físicas de filmes e revestimentos comestíveis de zeína

Abstract

The production of biodegradable materials has been increasingly studied and encouraged, due to the increased awareness about environmental risks caused by the extensive disposal of petroleum-derived plastics, especially those used in food packaging. Zein, a protein obtained from corn gluten, is characterized by a lower hydrophilicity than other proteins, which implies a higher resistance to humidity. The general objective of this work was to study different modifications and a combination of different technologies on a zein matrix in order to obtain films with potential of use for food packaging or coating, contributing to increase of food shelf life. For preliminary tests, the solvents ethanol (from 70 to 95%) with and without acetic acid 5% and the plasticizers glycerol and oleic acid (1:0, 2:1, 1:1, 1:2 and 0:1, total of 20%) were tested, and physical (solubility, WVP, contact angle, mechanical properties) and morphological properties (OM, SEM) were evaluated. Ethanol 80% without acetic acid was chosen as the solvent, and a ratio of 2:1 (13.3% and 6.7% of zein mass, respectively) of glycerol and oleic acid was defined for the plasticizers. In the crosslinking study with non-oxidized or oxidized tannic acid, crosslinker concentration (from 0 to 8%) and dispersion pH (from 4 to 9) were varied and film properties were analyzed. The results showed that higher amounts of tannic acid and pH values resulted in films with better general properties, notably lower water solubility and better mechanical properties. Oxidized tannic acid provided a more expressive improvement in tensile strength, water vapor permeability and water solubility. 4% tannic acid and pH 9 were chosen as the best conditions for crosslinking. In the study of the treatment with UV-ozone (UVO) for functionalization with nisin, films were evaluated after UVO treatment at different times and then after immersion in nisin (FTIR, XPS, MEV, AFM and inhibition zone test). It was shown that UVO treatment is effective for the formation of negative charges on the film surface, with increased hydrophilicity, maintaining mechanical and structural properties. It was verified by the XPS technique that nisin was adsorbed on the surface of the films, especially after previous treatment of UVO for 120 s, indicating that the combination between UVO and immersion in nisin is viable for functionalization of zein films with nisin. However, further studies are needed to verify the antimicrobial activity of these films after surface adsorption of nisin. In the guava coating study, fruits coated with zein (Z) and zein crosslinked with tannic acid (CZ) were submitted to survival analysis, physical analyzes (mass loss, color, firmness), respiration rate (CO2) and ethylene production, pH, total titratable acidity, total soluble solids, vitamin C, chlorophyll, carotenoids, hydrogen peroxide content, lipid peroxidation and superoxide dismutase (SOD) and catalase (CAT) enzymes. The coatings showed to be effective in delaying the ripening of guavas, with delay of fruit color change, decrease of respiratory rate and ethylene production, maintenance of firmness, soluble solids, chlorophyll and H2O2 content, with more expressive effect of the crosslinked coating. The results indicated that the zein coatings increased the shelf life of guavas, especially the ZR coating.