Mechanical and anti-microbial properties of edible films based on sesame meal protein obtained by alkaline and saline extractions

Introduction: In today's world, contaminants from synthetic polymers derived from oil derivatives have attracted public attention to the use of degradable materials such as macromolecules. Films and food packaging come from a variety of natural sources including proteins, polysaccharides, lipids, or a combination of these materials (Saremnejad et al. 2009). The characteristics of edible films, including biodegradability, have caused researchers to study the properties of film and various edible packaging as a suitable alternative to plastic packaging (Gonzard and Giulbert 1994). Edible films have appeared as a substitute for synthetic plastic for food applications and have gained substantial attention in recent years due to their benefits over synthetic films. Casings, pouches, bags can be developed from edible films and also be utilized as wraps or covers. At low relative humidity protein films are expected to be good oxygen barriers. Edible films from different protein sources have been formulated. These include casein, gelatin, whey protein, corn zein, soy protein, wheat gluten, peanut protein and mung bean protein (Saglam et al. 2013). Sesame (Sesamum indicum L.) is an important oilseed which is grown in many tropical countries. Sesame meal contains 35 to 40 % protein, is a by-product after oil extraction and it is major protein source used to fed animals in India (Onsaard et al. 2010). Isoelectric precipitation is normally used to prepare sesame protein isolates or concentrates. Sesame protein is very stable to heat and contains large quantities of methionine. So far, there is no work reported on using sesame protein isolate for the formation of edible films for packaging applications.There are several methods to incorporate antimicrobial agents in food, but the most effective of which is the addition of active compounds to packaging materials. Since the surface of the food is a critical region easily exposed to contaminants and packaging material, the slow release of antimicrobial substance to surface of food and, subsequently, diffusion across the food during preservation are predictable. Recently, several nanoparticles such as nanosilver, gold, copper, chitin, and essential oils have been known for their antimicrobial properties. Generally, antimicrobial nanoparticles are classified to organic and inorganic materials, each of which has its particular advantages. For example, inorganic agents are more stable against processing and storage conditions, but organic materials are more compatible, accessible, and inexpensive ( Sahraee et al. 2017). Zinc oxide (ZnO) is one of the most widely accessible metal nanoparticles with low cost, introducing more UV barrier property in polymers in comparison to silver and gold nanoparticles (Llorens et al. 2012).
Materials and methods: Sesame meal protein was extracted by two methods of alkaline and saline extractions according to Oonsard et al. (2010) method. In order to increase the protein extraction efficiency, the sesame meal was thoroughly crushed and powdered before extraction, then the sesame powder was mixed with water from 1 to 10 w / v, then in the first method, the pH was achieved to 11 with 2 molar NaOH. In the second method, pH was achieved to 7 with 2 molar NaCl. In both methods, the mixture was stirred continuously with a magnetic stirrer for one hour and then centrifuged at 2822g in 15 minutes and the dissolved phases were separated. The soluble phases were achieved by using 0.1 or 1 M HCl to pH 4.5. The suspension was centrifuged at 2822 g in 15 minutes, then the upper part was discarded and the sediments were weighed and the protein content was measured according to the AOAC 2000 standard. To preparation of films, the sesame meal protein was dissolved in 3 grams in 100 ml distilled water at 25 °C temperature. Glycerol was used in the making of films at 40%, 45%, and 50%; according to a pre-test. Glycerol percentages of less than 40% caused cracking of films, and increased glycerol content by more than 50% caused a high adhesion of films. To prepare nanocomposite films, different amounts of 1%, 3% and 5% of ZnO nanoparticles were added to 100 ml of distilled water and stirred with a magnetic stirrer at 30 ° C for 1 h, followed by ultrasonication for 15 min. Then 3 gr of sesame meal protein and glycerol were added to the solution of ZnO nanoparticles and then stirred for 1 hour. The solution was then heated in a water bath for 30 minutes at 90 °C and then cooled for 20 minutes to remove the air bubbles until it reached 25° C temperature. Then, 100 ml of the film forming solution was poured on Teflon dishes with a diameter of 16 and then dried at 25 °C for 24 h to obtain a uniform thickness. The films were kept in a desiccator with a relative humidity of 50 ± 5% and temperature 23 ± 2 °C (Lee et al.  2014).
Results and discussion: In this study, the biodegradable edible films of sesame meal proteins were produced by two different protein extraction methods including alkaline and saline extractions. In these films, glycerol was used as a plasticizer in three concentrations of 40%, 45% and 50% and zinc oxide nanoparticles were used in three concentrations of 1%, 3%, and 5%. According to the results, it was shown that in both samples, the tensile strength and Young's modulus significantly decreased by increasing glycerol, the percentage of length to break and water vapor permeability increased, but the oxygen permeability did not increase by increasing glycerol content. The results showed that Increase of 3% in content nanoparticles, the tensile strength, and Young's modulus increased, but the percentage of elongation to break point and water vapor permeability decreased of the films. With increasing zinc oxide nanoparticles to 5%, Tensile strenght, Young's modulus and oxygen permeability decreased and the percentage of elongation to break and water vapor permeability increased. The results of this research showed that the antimicrobial properties in both samples increased by increasing zinc oxide nanoparticles, and the glycerol level was unaffected. Finally, considering these parameters the tensile strength and the percentage increase in elongation to break, Young's modulus in the protein films were more for the alkaline method than the saline method. But the rate of water vapor and oxygen permeability in the protein films was higher by the saline method.
Conclusion: It was concluded that the films prepared from the sesame meal protein produced by alkaline extraction have stronger inhibitory and mechanical properties than films based on protein extracted by saline solution, which has negative effect salt ions on peptide bonds and positive role alkali is attributed to preventing solubility and better extracting of proteins in these conditions