Optimisation and Evaluation of the Correlation of Extraction Conditions for Total Flavonoid Content and Antixanthine Oxidase Activity in Psidium guajava

ABSTRACT Introduction Psidium guajava L. ( P guajava ) and its leaf, one of the most popular plants in traditional medicine and cuisine, were demonstrated to have high flavonoid content with many bioactivities, including antigout activity. However, the correlation between total flavonoid content ( TFC ) and antigout activities and the optimal conditions for extracting P guajava have not been reported. Methods This study established the optimisation of ethanol extracts of P guajava leaves for TFC and antigout activities using the single-factor method and response surface methodology. The antigout activities were determined through the xanthine oxidase inhibition ( XOI ) method. Results The four independent variables (ethanol concentration, plant-solvent ratio, extraction time, and temperature) significantly impacted two response variables (TFC and XOI activity). The optimal extracts for XOI activity reach the maximum TFC of 323.86 ± 2.26 mg quercetin/g and XOI activity of 26.37 ± 0.5 μg/ml. The changes in the chemical structure of extracts were analysed by Fourier transform infrared, which indicated that several flavonoids in P guajava do not have XOI activity. The correlation of TFC and XOI is robust with the Pearson correlation of −0.882 and Spearman Rho test of −0.884. Additionally, the biological activities, such as antioxidant and anti-inflammatory, of optimal extracts for XOI and TFC were determined to extend the application capacity to support the treatment of gout diseases. Conclusions The optimised extraction conditions are critical to isolating flavonoids, which can serve as functional food or medicine for managing gout and its complications.