Optimization of ultrasound-assisted extraction of capsicum seed protein isolate

Abstract: Capsicum seed protein isolate is a new type of plant protein resources. From this research, the protein isolate was separated into water-soluble(WS), salt-soluble(SS), alkaline-soluble(AS) and ethanol-soluble(ES) fractions. Water-soluble and ethanol-soluble fractions were the major constituents, about more than 75% of the total protein. This paper focused on ultrasonic-assisted extraction technology of extracting protein isolate from capsicum seed. Response surface methodology was used to determine optimum conditions for protein extraction. The experiments designed 29 groups of different conditions to extract protein from capsicum defatted flours. Independent variables which would affect protein extracting rate were discussed, such as pH (7, 8, 9, 10 and 11), ultrasonic power (200, 300, 350, 400 and 500 W), extraction time (10, 15, 20, 25 and 30 min) and material-solvent ratio (1:10, 1:20, 1:30, 1:40 and 1:50). The model equation was set up. In the model equation, the coefficient of association (R2) was 0.9748, indicating it's a reasonable matching to the experimental data. The optimized conditions were as follows: pH value was 11, ultrasonic power was 336.21 W, extraction time was 13.31 min and the material-solvent ratio was 1:35.58. According to this optimized condition, the extraction rate of protein isolate was 5.90 g/(100 g) defatted red pepper seed. Due to the limitations of experimental facilities, the optimized conditions were modified. The results were as follows: pH value was 11, ultrasonic power was 350 W, extraction time was 13 min and the material-solvent ratio was 1:36. The optimum conditions were verification. The results showed that extraction at the modified conditions gave a protein yield at 6.05 g/(100 g). The experimental values were found to be in agreement with the predicted ones. Analysis of variance (ANOVA) of independent variables was performed. The statistical analysis data revealed that linear, quadratic and interaction terms was significant (P<0.05). The lack of fit test measured the failure of the model to represent data in experimental domain at points which are not included in the regression. There was a non-significant lack of fit that further validates the model (P > 0.05).The significance of each coefficient was determined using the t test and P value. pH value and solvent/meal ratio were the most significant factors (P<0.001). Also, interaction effect of pH and extraction time, interaction effect of extraction time and materials/solvent ratio were both significant (P<0.05). The ultrasonic power was not significant factor (P>0.05). The main factors were analyzed by Design software, which could affect the yield of protein. The results were as follows: pH value>material/solvent ratio>extraction time>ultrasonic power. Compared with traditional extraction methods, the extraction yield of protein increased from 5.24 to 6.05 g/(100g), while the purity of capsicum seed protein isolate enhanced from 78.46% to 83.93% by ultrasonic-assisted method. These results give the advices in processing protein from capsicum seeds according to optimized condition. Furthermore, it provides a theoretical reference for the industrial production and application of capsicum seed protein isolate.