Preparation of antioxidant active film based on sesame protein isolate containing ethanolic extract of sesame cake and studying its effect on oxidative stability of soybean oil

Introduction: Oxidation is one of the main factors affecting the shelf-life of food susceptible to lipid deterioration such as vegetable oils (Almasi et al., 2014). To reduce oxidation in sensitive food products, the direct addition of antioxidants to food formulation, dipping food in antioxidant solution and the design of a suitable vacuum or modified atmosphere packaging technology are the three most common alternatives. However, there are many foodstuffs that cannot be protected in these ways, as they are fresh or raw foodstuffs in which the addition of other substances is not permitted. One of the most promising systems to protect the foods against oxidation is to use an antioxidant active packaging. Antioxidant releasing packaging is a type of food preservation system, in which an antioxidant or a mixture of antioxidants is incorporated into the package instead of adding high levels of additives directly into the food. Most of synthetic antioxidants utilized in protection of vegetable oils, such as BHA, BHT and TBHQ have health hazards and for this reason, there is a growing interest in the using of natural antioxidant for shelf life extension of lipid based foods. Sesame (Sesamum indicum L.) is one of the important oilseeds cultivated in many tropical countries. Sesame seed cake is considered to be a by-product of the oil industry, being commonly used as cattle feed in several producing countries. However, this residue can be recovered and value added. Sesame cake extract (SCE) is a rich source of phenolic compounds such as sesamol, sesamin and sesamolin. This extract can be used as natural antioxidant in food formulation and also in preparation of antioxidant active films. Sesame cake also has protein content of about 30 %. Besides its high nutritional value, sesame protein can also be used as food additive. Foaming capacity, whippability, emulsifying activity and fat absorption capacity are some of the functional properties of sesame proteins (Cano-Medina et al., 2011). Sesame protein isolate (SPI) can be produced easily by isoelectric precipitation method. Because of inexpensive raw material (sesame oil extraction by-product), facile extraction process and interesting functional characteristics, sesame protein has become an attractive plant based protein with various applications. High molecular weight and good heat stability make SPI as a good candidate for film forming applications. There are some reports on film forming ability of SPI but the using of this protein for antioxidant active film preparation is not investigated up to now. The aim of this work was to fabricate the antioxidant active film based on sesame protein isolate containing ethanolic extract of sesame cake. The physical and morphological properties of films were investigated and also their effect on oxidative stability of soybean oil during 60 days storage was studied.
Materials and methods: The SCE was extracted by 70 % ethanol solvent and SPI was produced by alkaline method and followed by freeze drying. Antioxidant active films were prepared by addition of SCE in three levels of 3, 5 and 5 % w/w of SPI. Solvent casting method was used for preparation of films. The structural parameters of films were analyzed by FT-IR, XRD and FE-SEM tests. Water vapor permeability and mechanical properties were also studied. Antioxidant activity of films was examined by DPPH scavenging method. At the second step, the films were mounted in contact with antioxidant free soybean oil and the effect of SCE releasing on the preservation of soybean oil was analyzed. Sampling of oil was conducted on days of 1, 20, 40 and 60 and the antioxidant potential and peroxide value of oil samples were analyses. The one-way ANOVA method by using SPSS software was used for statistical analysis of obtained data.
Results and discussion: FT-IR analysis confirmed the formation of new hydrogen bonds between phenolic compounds of SCE and protein chains of SPI. XRD test revealed that up to 5 % concentration, the SCE has no effect on semi-crystalline structure of SPI film. But at the concentration of 7 %, the intensity of peaks related to crystallinity of SPI decreased. According to FE-SEM images, uniformity and ordered design of SPI chains were disrupted by addition of 7 % SCE. The water vapor permeability and extensibility of films decreased but tensile strength and Young’s modulus of films increased by addition of 5 % SCE. When the SCE content was reached to 7 %, an adverse trend was observed in all parameters. The film containing 7 % SCE had the highest DPPH scavenging activity (79.43 %). Evaluation of the properties of soybean oil in contact with antioxidant active films indicated that the DPPH radical scavenging activity of oil samples in contact with active films increased with increasing storage time (p < 0.05). this trend increased by increasing of SCE content in formulation of active films. But this property decreased in TBHQ added oil sample during storage. Antioxidant activity of SPI-SCE contacted oil ad 60th day of storage was similar to 100 ppm TBHQ added sample. Measurement of peroxide value indicated that the all samples had an increasing trend during storage. But the using of active films was able to control the increase of peroxide value in soybean oil samples. TBHQ loaded oil had the lowest peroxide value in all times. This parameter was high for SPI films conducted oil samples but 5 and 7 % SCE containing films were able to control the increasing trend of peroxide value in samples.
Conclusions: The results of this research demonstrated the efficacy of sesame protein isolate films carrying sesame cake extract in slowing the lipid oxidation and increasing the oxidative stability of soybean oil. The SCE had strong antioxidant potential was proposed as potential antioxidant additive to edible oil protection. However, this research indicated that the incorporation of SCE to SPI film increases the efficiency of antioxidant activity by slowed release to oil. Addition of SCE slightly weakened the mechanical, barrier, morphological and structural properties of SPI films. But the good antioxidant activity of films covered these drawbacks. Preparation of these antioxidant active films is proposed for b better using of sesame cake in food industry. This research opens a new horizon on value added utilization of sesame oil extraction by-products.