Hydrophobic, antioxidant and antimicrobial properties of neem seed oil added soy protein isolate films

Abstract Petroleum based plastics have plagued the market which are non-biodegradable and pose environmental threat. In recent years, plastics derived from starch, cellulose, soy protein are gaining attention due to their biocompatible and eco-friendly nature. Soy-based plastic has many favorable characteristics required to make the film, making it an attractive option for biopolymer production. However, despite its positive attributes, soy-based plastics have poor hydrophobicity, compromised tensile strength and lower shelf-life due to easy microbial attack. To overcome these issues, many plant-derived essential oils such as clove, thyme, rosemary and citronella essential oil have been incorporated into soy protein. This study investigates the effect of neem seed oil on material properties of soy protein isolate (SPI) films. Neem seed oil is recognized for its antibacterial, antifungal and insect repellant properties due to the presence of different bioactive phenolic compounds. In our study, neem seed oil was complexed with SPI by solution casting approach that involves partial denaturation of protein under alkaline conditions followed by surface drying. The concentration of essential oil varied from 7.5%, 12.5%, 15% to 20% with respect to 7% SPI. The prepared films were characterized for structural changes by Fourier Transform Infrared (FTIR) spectroscopy and X-ray diffraction (XRD). A notable decrease in transmittance was recorded for neem seed oil added films in a concentration dependent manner. The addition of oil led to changes in native structure of SPI and made the film more compact and uniform as revealed from XRD. The neem seed oil significantly affected the film’s mechanical properties where, control film had tensile strength of 2.79 MPa that increased to 4.29 MPa for 7.5% of neem seed oil. The water uptake capacity and water vapour transmission rate (WVTR) of control film was recorded to 165% and 15 g/m2/24 h respectively. Addition of neem seed oil made the film hydrophobic as the water uptake decreased to 27.15% for 20% of neem seed oil and WVTR reduced to 9.44 g/m2/24 h for 12.5% of neem seed oil. The DPPH (1,1-diphenyl-2-picryl-hydrazyl) antioxidant activity of modified films increased to 81.3% for 20% of neem seed oil due to the phenolic constituents of essential oil. Furthermore, the addition of essential oil in SPI films demonstrated antibacterial properties against Listeria monocytogenes. Such SPI-neem seed oil films have possible application in food industry due to their improved hydrophobicity, antioxidative and antibacterial properties. Graphical Abstract