Evaluation of drying methods and green extraction techniques to enhance the recovery of bioactive compounds from hop leaves: A sustainable approach for the valorisation of agricultural by-products

This study investigated the effects of different drying techniques and green extraction methods on bioactive compounds in hop leaves, typically considered as waste material. Freeze-drying (FD) and oven-drying (OD) were compared for drying the leaves of five hop varieties, while the study focused on the use of microwave (MAE) and ultrasound (UAE) as innovative extraction techniques. The influence of these factors was then evaluated on several bioactive compounds, including polyphenols, flavonoids, pigments, and xanthohumol, as well as the antioxidant capacity and α-glucosidase inhibition of the extracts obtained. MAE yielded higher total polyphenol and flavan content (TPC and FLC, respectively) values than UAE. Similarly, FD samples showed higher TPC and FLC values than OD ones, whereas chlorophyll b was consistently more abundant than chlorophyll a in all samples. HPLC analysis identified catechin, epigallocatechin gallate, and p-hydroxybenzoic acid as the predominant phenolic compounds. Xanthohumol concentrations ranged from 0.04 ± 0.00 to 1.12 ± 0.03 mg g^-1, with MAE yielding higher levels than UAE. Multivariate analysis revealed that the drying process accounted for the largest proportion of variation in the phytochemical profile (37.1 %), followed by the extraction technique (27.3 %) and hop variety (14.3 %). The hop leaf extracts showed α-glucosidase inhibitory activity, with FD samples showing greater inhibition than OD ones. PCA highlighted the significant influence of the extraction method and drying process on the phytochemical composition of hop leaf extracts. This research highlights the potential of hop leaves as a sustainable source of phytochemicals for the food, pharmaceutical, and nutraceutical sectors, and emphasises the importance of optimizing extraction and drying techniques.