Fom waste to bioactive resources: extraction of phenolics and carotenoids from Arbutus unedo distillers’ grains using alternative solvent systems

The growing interest in sustainable biorefinery strategies has increased attention on the valorisation of agro-industrial residues as sources of high-value bioactive compounds. In this context, Arbutus unedo distillers’ grains, a by-product of the traditional production of Aguardente de medronho in Portugal, represent an underexplored biomass with significant potential for the recovery of phenolic compounds and carotenoids. This study investigates the valorisation of A. unedo distillation residues through the development of innovative extraction processes based on alternative, low-toxicity and biocompatible solvent systems. Extracts were evaluated in terms of total phenolic content (TPC), total flavonoid content (TFC), antioxidant activity (AA), and total carotenoid content (TCC). Two solvent screening studies were conducted. For phenolic compounds, several glycols were assessed and propylene glycol emerged as an effective and safer alternative, showing extraction efficiencies comparable to or exceeding those of conventional solvents such as ethanol and more expensive natural deep eutectic solvents (NaDES). For carotenoid extraction, mixtures of natural hydrophobic compounds, including medium-chain fatty acids, thymol and menthol, were evaluated. The study further explored the use of switchable hydrophilic solvents (SHS) to enable carotenoid precipitation and solvent regeneration through acid–base reactions, avoiding energy-intensive separation steps. The SHS process applied to a hexanoic–octanoic acid mixture (2:1, w/w) enabled the recovery of up to 70% of dissolved carotenoids and the recycling of 76% of the solvent. Finally, the study proposes two different approaches to improve the valorisation process. The first consist in a two-step SHS-based extraction for sequential recovery of phenolics and carotenoids, while the second proposal focused on a ternary biphasic system composed of water, propylene glycol, and hexanoic acid. This biphasic system enabling simultaneous extraction of both target compounds in a single step with improved yields compared to separate extractions. Overall, the results demonstrate that A. unedo distillers’ grains constitute a promising source of bioactive compounds and that the alternative solvent systems studied can support innovative, safer and more efficient extraction processes.