Development and characterization of edible films based on corn starch and eggplant powder with Mentha longifolia L. essential oil

Introduction: Recently, the increase in synthetic polymers has been pushing researchers toward the development of new biodegradable and natural polymers, suitable for food packaging. To extend the shelf-life of foods and the preservation them from microbial spoilage and oxidation while reducing packaging waste, the tendency is to use natural materials such as edible films and coatings. (Khazaei et al., 2014). These films, as the carrier of the functional components like antioxidants, the antimicrobial agents, colorants, flavors and spices can improve both the capability and the functionality of the packaging materials (Ramos et al., 2012). Biodegradable and edible films can be prepared using proteins, carbohydrates, lipids or mixture of them. Among the natural and renewable resources, starch is one of the most important ingredients of films. It is commonly used in the packaging industry due to its abundance, low price, ability to form the edible films and biodegradability. The films based on starch are odorless, tasteless, colorless and transparent, while enjoying good gas barrier properties (Acosta et al., 2016; Šuput et al., 2015). In recent years, there is an increasing attention in developing edible films by agriculture crop flours and powders due to their availability, easy obtaining and low cost compared to pure components such as starch and proteins (Nouraddini et al., 2018). Among the agriculture crop, eggplant (Solanum melongena L.) is recognized for its high dietary fiber content, antioxidant capacity, oxygen radical scavenging capacity and minerals. Also, it has flavonoids and high amount of anthocyanin in peel (Niño-Medina et al., 2017). Eggplant powder has significant amount of protein, carbohydrates, fat and crude fiber (Nouraddini et al., 2018). In the last decades, the essential oils of  plants and spices have got attention to use as the antimicrobial and antioxidant agents, which can be added to the edible films (Jahed et al., 2017). Mentha longifolia essential oil and extract showed the antioxidant and antimicrobial activity (Farzaei et al., 2017; Gulluce et al., 2007). The antioxidant activity of Mentha longifolia EO and the extract might be related to its phenolic content such as phenolic acid, rosmarinic acid and polyphenols (Gulluce et al., 2007).
According to our survey, there are no studies on the development of edible films using eggplant powder and Mentha longifolia essential oil. Therefore, the object of this study was to prepare the edible active films based on corn starch and eggplant powder incorporation with Mentha longifolia essential oil (0, 1, 3 and 5% w/w), in order to examine their physical properties.
Material and methods: For the extraction of Mentha longifolia essential oil, the dried samples were submitted to the hydro-distillation for approximately 3 h by a Clevenger. The obtained essential oil was kept in dark glass at 5 ºC. For the preparation of eggplant powder, the eggplants were sliced and dried in hot air oven at 50ºC for 24 h without peeling off the skin. Then the dried samples were grinded into powder and it stored in glass dishes at 4 ºC. The corn starch and eggplant powder based films were prepared at ratio of 1:1 by the casting method. The pH of solution was adjusted at 7.5 by NaOH and then it was stirred and heated at 85 ºC in a water bath. After that, the glycerol was added at 36% w/w/ and the solution was mixed for 30 min. Then the Mentha longifolia essential oil was added at different ratio (1, 3 and 5% w/w) to film solutions. The emulsion was homogenized with a Rotor-stator homogenizer and degassed using an ultrasonic homogenizer. The film samples solution was dried at room temperature for 48 h. Physical properties such as thickness, moisture content, density, mechanical properties, color, opacity and light transmittance values, solubility and swelling index of films were evaluated. All the tests were carried out in triplicate. The analysis of variance (ANOVA) was applied to the data and the means were compared by Duncan’s test using SPSS statistical software.
Results and discussion: The thickness, swelling index of films incorporated with essential oil were higher than control films (P<0.05). A decrease was observed in the density and moisture content of the films that incorporated with Mentha longifolia essential oil. The solubility in water of control films was the highest (67.61%), while the addition of Mentha longifolia essential oil, the solubility values significantly (P<0.05) decreased. The results showed that the lowest level of water vapor permeability was found for the control films (0.0023 ± 0.0009 g.mm/m².h.Pa) and the highest was related to films containing 5% Mentha longifolia essential oil (0.0028 ± 0.0006). Results showed significant differences between color parameters (P<0.05). Color measurement of the edible films indicated that increasing the concentration of Mentha longifolia essential oil, increased the lightness (L*), redness (a*) and yellowness (b*) and decreased the ΔE of films. The addition of essential oil decreased the tensile strength, Young’s modulus and elongation at break of films. Light transmittance value at UV region was negligible and for control films were higher than other films and with an increase in the wavelength (visible light region), the light transmittance of the films increased. According to the results, the maximum light transmittance was related to control films. The evaluation of the opacity values revealed thatthe control films showed higher opacity compared to the films incorporated with essential oil.
Conclusion: The edible active films based on corn starch and eggplant powder with Mentha longifolia essential oil were successfully prepared. The addition of Mentha longifolia essential oil to edible films led to the formation of a film with good transparency and lightness characteristics. According to results the mechanical properties of films were decreased with the addition of essential oil. In general, addition of Mentha longifolia essential oil to edible films based on corn starch and eggplant powder improved the water solubility and transparency of films, furthermore it has negative effect on mechanical properties and water vapor permeability of films. According to the results obtained in this research, eggplant powder and corn starch based films with Mentha longifolia essential oil can be used in food packaging.