Sustainable extraction of phenolics from underutilised onion and garlic field residues: subcritical water approach and bioactivity evaluation

Increasing production of agri-food waste is a global challenge, considering the huge amounts of waste materials that are generated, and often not used or disposed adequately. During the agricultural production and further use and processing of onion and garlic, a large amount of waste is generated globally. Up to now, scientific research was mainly focused on the waste/by-products generated in the processing phase – by the food industry. However, a significant part of the waste originated from these crops comes from the agricultural production and harvesting phase. This material, agricultural waste from onion (OAW) and garlic (GAW), is recognised as the cut dried upper part - dried stalks, husks and leaves, which remains in the arable fields, and is ploughed or in some cases burned causing harmful effects on the environment. The aim of this study is to investigate the potential of this kind of waste by applying high-efficiency subcritical water extraction (SWE), and focusing on the influence of temperature as a key parameter in the extraction process. The targeted compounds of investigation were polyphenols. The dominant compounds identified and quantified by HPLC-DAD analysis were phenolic acids (especially protocatechuic and neochlorogenic) and flavonoids (quercetin and quercetin 3-β-D-glucoside in the case of OAW, and epigallocatechin, epicatechin and catechin in the case of GAW). Based on the chemical profile, an in silico study was conducted where the effects of the compounds of OAW and GAW extracts were specified with a focus on cardiovascular diseases. Furthermore, the anticancer activity of the selected extracts was evaluated, contributing to sustainable strategies for agricultural waste use.