The Determination of Physicomechanical Properties of Nettle Seed (Urtica pilulifera) and Optimization of Its Mucilage Extraction Conditions using Response Surface Methodology

In this study, some geometrical, gravimetrical and frictional properties of nettle seed were evaluated for the first time. The length, width and thickness of seeds were 2.49, 1.98 and 0.78 mm, and the average arithmatic diameter, average geometric diameter, sphericity and surface area of nettle seeds were, respectively, 1.75, 1.29 mm, 0.52 and 5.31 mm2. The true density, bulk density and porosity of nettle seeds were 1168.12, 457.19 kg/m3 and 60.86%, respectively, and the static friction coefficient on the plywood surface, glass, rubber, fiberglass and galvanized iron were 0.28, 0.21, 0.34, 0.22 and 0.27, respectively. Then, using the response surface method, the optimal conditions for extracting nettle mucilage as a new source of hydrocolloids were determined by achieving maximum seed surface ratio, extraction yield, and viscosity. Face central composite design analysis of the effect of three independent variables, including soaking temperature (25-60 °C), soaking time (0.5-4 h) and water to seed ratio (1:20-1:60), was studied. The ANOVA results showed that the quadratic polynomial model was the best model for interpreting the behavior of the three responses. Statistical analysis of the data showed that extraction yield and viscosity were significantly affected by all independent variables (P<0.05), while the effect of temperature on seeds surface ratio was not significant (P>0.05). Based on the numerical optimization method, optimized conditions for extraction of nettle seed gum were determined in terms of soaking temperature of 59 °C, soaking time of 3.4h (204 min) and water to seed ratio of 1:40. Under the optimum conditions, the seeds surface ratio, extraction yield and viscosity values were obtained as 2.39, 9.70% and 6.25 mPa/s, respectively. The results of this study are of practical value for designing post-harvesting and processing equipments for nettle seeds as well as extracting mucilage from the seeds.