Valorization of residual walnut biomass from forest management and wood processing for the production of bioactive compounds

The aim of this work was the valorization of various biomass residues (shavings, edged cuts, and pruning wastes) from walnut (Juglans major 209 × Juglans regia) obtained from forest maintenance and wood processing, as sources of antioxidant compounds by means of a green extraction process. A Plackett-Burmann experimental design was applied to analyze the influence of temperature, liquid/solid ratio, time, and aqueous ethanol concentration on extraction yield (EY) and extract properties (total phenols content (TPC) and FRAP antioxidant activity). Under the operational conditions selected, experiments were carried out at a larger scale, and the extracts were analyzed to determine the TPC and the antioxidant activity (FRAP, DPPH, and ABTS assays). In addition, to identify and quantify the compounds with potential antioxidant activity, the extracts were analyzed by ultra-performance liquid chromatography coupled with electrospray ionization and time-of-flight mass spectrometry. Temperature and ethanol concentration were the most significant variables for all the fractions studied, and in general, increasing both variables, the extraction yield and extract properties improved. Extraction of walnut shavings with 50% EtOH at T = 80 °C, L/S = 10/1, and t = 30 min led to the extracts with the greater TPC and antioxidant activities. All walnut extracts showed the same phenolic profile. Gallic, ellagic, and ferulic acids were the major phenolic acids, and the flavonol quercetin-3-β-D glucoside was the main flavonoid. The greater antioxidant activity of shaving extracts was related to their greater contents in gallic and ellagic acids. This study contributed to the valorization of various walnut biomass residues as sources of valuable compounds with bioactive functions and various potential industrial applications.