Sustainable valorization of citrus by-products: natural deep eutectic solvents for bioactive extraction and biological applications of Citrus sinensis peel

Orange is one of the most consumed fruits worldwide, generating significant byproducts and wastes. By following the principles of green chemistry, the reuse of orange by-products can be made sustainable. In this context, natural deep eutectic solvents (NADES) have emerged as a promising alternative. The present study aims to analyze the physicochemical characteristics and biological effects of orange peel extracts obtained with choline chloride-based NADES. An investigation was conducted on the extracts, including a chromatographic analysis of phenolic and volatile compounds, a spectroscopic FTIR analysis, as well as the evaluation of their antioxidant capacity, their antimicrobial activity on different pathogens, and their capacity to modulate the activity of digestive enzymes. Both chromatographic approaches and FTIR studies indicated that different NADES had various extraction efficiencies towards phenolic compounds, affecting the antioxidant capacity of extracts. The volatile profile of NADES extracts was primarily composed of alcohols, aldehydes, and terpenes, whereas the ethanolic extract (50% (v/v), used as a control) exhibited a higher abundance of terpenes. All extracts were demonstrated to enhance pepsin enzyme activity without affecting that of chymotrypsin. Only choline chloride: glycerol: citric acid gave an extract capable of inhibiting trypsin and amylase activity, as well as the proliferation of pathogenic microorganisms. In conclusion, choline chloride-based NADES may represent a sustainable method for reusing orange byproducts, as they are more effective in extracting valuable bioactive compounds from orange peel when compared to old-fashioned organic solvents.