Evaluation of the solvent effect on the extraction and antioxidant activity of phenolic compounds from the nettle (Urtica dioica L.) seeds: application of PCA and regression analyses

The solubility of phenolics is not only determined by the chemical composition of the plant sample, but also by the polarity of the solvents used. In this paper, the influence of solvents with different polarity indices, such as methanol, ethanol, and their 70% alcohol–water mixtures, on the content and antioxidant activity of phenolics in nettle seeds was analyzed. A regression analysis was conducted to evaluate the relationship between the content and antioxidant activity of phenolics, as well as between the different antioxidant assays used. Subsequently, the correlation coefficient (R) and the coefficient of determination (R²) were determined. A PCA analysis was also performed to evaluate the comprehensive effects of the solvents on the content and antioxidant activity of phenolics. The results indicate that the alcohol–water mixtures are more effective at extracting phenolic compounds compared to solvents with analytical purity. Among these mixtures, aqueous methanol was found to be the most effective solvent. Regression analyses revealed highly positive correlations between phenolic content and antioxidant activity, suggesting that a higher phenolic content leads to better antioxidant activity. Furthermore, all three methods used for the evaluation of the overall antioxidant activity of nettle seed alcoholic extracts proved to be suitable and reliable, with the reducing power test being particularly recommended. These findings were further supported by a statistical analysis—PCA, which showed a clear separation between extracts with high and low phenolic content.