Optimization of extraction yield and physicochemical and functional properties of Roman nettle

Introduction: Urtica is a genus of flowering plants in the family Urticaceae. They have a long history of use as an medicinal plants. The main varieties of the Urtica genus are Urtica dioica L., Urtica urens L., Urtica pilulifera L., Urtica cannabina L., Urtica membranacea Poiret, and Urtica kiovensis Rogoff). Roman nettle (Urtica pilulifera) is widely used in folk remedy to treat hypertension, hyperglycemia and inflammation of some organs while the seeds of this plant can be considered as a potential source for mucilage extraction. Given that in recent years, many studies have focused on finding new hydrocolloids and investigation of their physicochemical and functional properties, the aim of the present study was to optimize the mucilage extraction conditions from Roman nettle seeds in order to achieve maximum extraction yield and optimal physicochemical and functional properties of this hydrocolloid.
Materials and methods: The U. pilulifera seeds were purchased from a local market in Chaharmahal and
Bakhtiari Province, Shahrekord, Iran. The seeds were identified at Medical Plants Research Center. Then the seeds were manually cleaned to remove all foreign matter and stored in a plastic bag at refrigerator before to analysis. At first, proximate analysis of seeds including moisture content, crude oil, crude protein, crude fiber and ash content were determined using the AOAC official methods. Total of carbohydrate was determined by difference. Then, using the response surface method (RSM), the optimal conditions for extracting Roman nettle mucilage were determined. D- Optimal design analysis of the effects of four independent variables, including water to seed ratio (1:5-1:30), temperature (30-85 °C), soaking time (30-240 min) and pH (3-10) was studied on mucilage extraction yield, protein content, emulsion stability and color indices of extracted mucilage. For emulsion stability determination at first, 0.5% solution of mucilage was prepared at 70 ° C. The solution was then cooled and kept at 4 ° C for 24 hours to completely hydration. Oil-in-water emulsion (20% by weight) was prepared and then the emulsion was placed in a hot water bath at 80 ° C for 30 minutes, centrifuged at 1200 g for 10 minutes and finally stability of emulsion was calculated. Color indices including lightness (L*), redness (a*) and yellowness (b*) were measured by Hunterlab model Color flex (US).
Results: According to the obtained results, increase in temperature to a certain extent, water to seed ratio and pH had a significant effect (p<0.05) on mucilage extraction yield. Increasing the water-to-seed ratio allows more polysaccharide compounds to be hydrated due to the availability of sufficient water, which increases the thrust force to remove mucilage from the seed. Mucilage extraction yield increased at higher pH which can be due to the release of protein materials and easier release of mucilage. Ttemperature showed a quadratic effect on mucilage extraction yield, so that with increasing temperature to a certain extent, first the amount of mucilage extraction increased, which can be due to the decrease in the viscosity of mucilage attached to the seed and also higher mass transfer rates of soluble polysaccharides at higher temperature. However, a further increase in the extraction temperature showed the negative effect and reduced the extraction yield. This may be due to breaking and hydrolysis of the mucilage structure due to thermal stress in the aqueous medium. The most important and significant factors (p <0.05) on protein content were linear effect of pH, linear effect of time, effect of pH-time interaction, effect of water to seed ratio- time interaction and, quadratic effect of extraction temperature. At lower pH, the protein content of mucilage was significantly (p <0.05) higher than alkaline pH. In general, extraction of mucilage at higher pH and shorter time will lead to extraction of mucilage with minimal protein content may be due to the solubility of proteins in the alkaline pH range. Increasing the water-to-seed ratio also reduced the amount of protein in mucilage, which is probably due to the possibility of more solubility of the protein in the aqueous medium. Among the studied variables, the linear effect of pH, temperature and water-to-seed ratio showed a significant effect on the stability of emulsion containing mucilage (p <0.05). In addition, the interaction of pH and water to seed ratio as well as the interaction of extraction temperature and time, the quadratic effect of pH and the quadratic effect of water to grain ratio were also significant on the changes of this response. Since the presence of proteins with hydrocolloid can show a positive effect on the stability of the emulsion, therefore, the obvious decrease in the stability of emulsions prepared with mucilage extracted at higher pH is probably due to the lower protein content in these samples. What is clear is that when higher water-to-seed ratio and higher pH are used to extract mucilage, it will lead to the weakest emulsion stability. Increasing the water to grain ratio in the extraction stage can lead to the removal of more impurities from the grain and their entry into the mucilage, and thus have a negative effect on the stability of the emulsion. In the study of changes in L* and a* indices, it was found that pH was the only effective variable and with increasing pH, a significant decrease and increase (p<0.05) in lightness and redness indices were observed, respectively. Based on the numerical optimization method, optimized conditions for extraction of Roman nettle seeds mucilage were determined in terms of pH of 4.23, soaking time of 239.9 min, soaking temperature of 45.79 °C, and water to seed ratio of 1:5. Under the optimum conditions, extraction yield, protein content, the highest emulsion stability and L* and a* indices were 11.08%, 14.55%, 50%, 47.59 and 15.05, respectively, which was in good agreement with the predicted values.