Production and Investigation of physical and mechanical properties of soy protein isolate - tragacanth based composite film

Introduction : One of the most important problems of synthetic packaging of materials had been the time consuming process of their decomposition. Therefore, they have the potential to contaminate the environment. During the recent years, the above-mentioned problem has paved the way for developing biodegradable biopolymers. One group of these polymers is films and edible coatings. Soy protein isolate is the purest type of soy protein which is available commercially and amount of its protein is more than 90% (based dry weight). These films have better nutritional value. In addition, they have better mechanical features and permeability than films made of carbohydrates and fats. Tragacanth gum is composed of two main ingredients under the name of Tragacantic acid or Basorin and Tragacantin. Basorin is in 60-70% of whole gum and is insoluble in the water. It can swell and make gel. Also, another ingredient or Tragacantin solves in the water and leads to make colloid solution. Hence, the main purpose of this work is studying effect of tragacanth gum as a supplementary and firmness-making material on physical and mechanical features of soy protein isolate film and are determining the best level of tragacanth gum and optimal film for using in food packaging. Materials and methods: A specified amount of soy protein isolate was solved in distilled water. Then, pH of solution was set on 10 by NaoH (0.1N). Solution of soy protein isolate was heated by bain-marie to 70˚c. Afterwards, solution of tragacanth gum with suitable amount of treatment which was solved separately in the water was added to solution of soy protein isolate slowly during agitating by magnetic mixer. Then, glycerol was added to the final solution as plasticizer. Final solution was agitated by magnetic mixer about 15 minutes to become smooth and pure. After degassing, the film forming solutions were casted by pouring the mixture onto polystyrene plate (10 cm diameter) and dried at 25±5˚c for 48 h in a laboratory oven and room relative humidity. Finally, the dried films were peeled off the casting surface. Film solutions were prepared from soy protein isolate with ratio of 0.5:4.5(A), 0.3:4.7(B) and 0.1:4.9(C), tragacanth gum: soy protein isolate and plasticizer of glycerol were prepared in 100gr water with ratio of 90% (w/w) dry weight. According to plasticizer effect of water, to uniform moisture before performing each test, all the films were conditioned inside desiccators containing saturated magnesium- nitrate solution to ensure a relative humidity 0f 50-55% at 25˚c ± 1 for 48 h. Tensile strength (TS) and elongation at break (EB) were conducted by using tensile evaluation machine (7010, Gotec of Taiwan) according to ASTM standard method D882-10 (ASTM, 2010). Films were cut in the dimension of 2×2 cm2 and dried to reach constant weight at 100 ˚C in a laboratory oven. Dried samples weighed to determine the initial dry weight (m1) and placed inside beaker including 50 ml of distilled water with periodic stirring for 24 h at 25˚C. Afterwards, the remained pieces of films were taken out and dried at 100˚C to reach constant weight (m2). After the films reached in moisture balance at 25˚C and a proportional humidity at %55. Their moisture was determined by measuring the loss amount of film weight during drying in a laboratory oven at 105˚c unti constant weight. The specimens of films was provided at 2×2cm2 dimensions and prepared by dried calcium chloride until reached to constant weight (m1). After the initial weight, they were conditioned in a desiccator containing saturated magnesium nitrate solution at 25˚c to provide reaching a relative humidity of %50-55. Then, the specimens were weighed in regular intervals until the equilibrium state was reached (m2). Water vapor permeability tests of films were performed according to ASTM- E95-96 standard (ASTM, 1995).The color of films was evaluated using a colorimeter device (LabscanXE, Hunterlab, USA). Statistical analysis was conducted in a completely randomized design with variance analysis (ANOVA) using SPSS software (Version 21; SPSS Inc., USA). Also, comparing the difference among mean values of film specimens was performed using Duncan,s multiple range tests at the confidence level of 95%. Results and Discussion:The obtained results indicated that tensile test (TS) of film was increased by increasing tragacanth gum and decreasing soy protein isolate but elongation to break (ETB) decreased. This result was agreed with the results of Fazel et al (2013), Tian et al (2011), Chen and Lai (2008). The amount of film moisture increased by increasing tragacanth gum, while amount of solubility in water, water absorption decreased by increasing tragacanth gum. The highest of wvp was observed in SPI:TG (4.9:0.1) but the lowest value was related to the sample SPI:TG (4.7:0.3). In general, the amount of wvp decreased by increasing tragacanth gum. The free space between chains decreases by forming transversal connections between protein filaments. Also, filaments mobility decreases, while penetration and spread of vapor molecules from polymer will decrease and its prevention becomes more in the water. The highest L, a, and whiteness index was observed in SPI:TG (4.5:0.5). In contrary, the highest b, total color difference (ΔE), and yellowness index was observed in SPI:TG (4.9:0.1). Optical properties of films used in food packaging are very important for their effects on acceptance and food quality. Results of Sarmadizadeh et al. (2014) showed that increasing protein has changed films color. Therefore, adding tragacanth gum caused to improve the structure and to improve physicochemical properties of the samples. It could be concluded biodegradable film of SPI+TG has desirable characteristics for the application in food packaging.