Optimization and comparison of maceration and microwave extraction systems for the production of phenolic compounds from Juglans regia L. for the valorization of walnut leaves

This study compares maceration (ME) and microwave assisted (MAE) extractions aiming to maximize the extraction of valuable compounds from Juglans regia L. leaves. An experimental design, assisted by response surface methodology, was developed to optimize the variables of the ME and MAE systems (time, temperature and ethanol-water proportion). The responses used as criteria were the quantification by HPLC-DAD (3-O-caffeoylquinic acid, quercetin 3-O-glucoside and quercetin O-pentoside), the extraction yield and the spectrophotometric results of total phenolics and flavonols. The global optimum conditions were: 112.5 min, 61.3 °C and 50.4% of ethanol for ME; and 3.0 min, 107.5 °C and 67.9% of ethanol for MAE. The solid/liquid ratio effect was studied at the optimal values showing a decreasing linear relation until 140 g/L for all criteria assessed. This study contributes to the valorization of walnut leaves as source of valuable compounds (e.g., quercetin) to be used as ingredients for the development of functional foods.