Natural deep eutectic solvents extraction of anthocyanins – effective method for valorisation of black raspberry (Rubus occidentalis L.) pomace

This study aimed to establish NaDES-based ultrasound-assisted extraction as a novel, sustainable, and ecologically suitable method for valorisation of black raspberry pomace remaining after juice production. Accordingly, a range of NaDESs, composed of hydrogen bond donor choline chloride and different hydrogen bond acceptors were screened to select the most appropriate for cyanidin-3-O-rutinoside and total anthocyanins extraction from black raspberry pomace, and to compare with conventional solvents. In addition, the impact of hydroxypropyl-β-cyclodextrin (HP-β-CD) on anthocyanins extraction capacity of NaDES and water was evaluated, as well as the influence of NaDES on anthocyanins storage stability at 4, 25 and 40°C. On the basis of cyanidin-3-O-rutinoside and total anthocyanin content (TAC), along with antioxidant activity, citric acid/choline chloride NaDES was selected for extraction conditions optimization by Box-Behnken design coupled with response surface methodology. The optimal conditions were found to be an extraction time of 52.9 min, a temperature of 65°C, and 15.6 % (w/w) water in NaDES, resulting in maximized extraction yields of target compounds, cyanidin-3-O-rutinoside (7.60 mg/g DW) and TAC (6.88 mg CGE/g DW). By incorporating different concentrations of HP-β-CD (1.5, 3 and 6 % w/w) in citric acid/choline chloride NaDES, downward trend was observed, while in the case of water anthocyanins extraction was improved. A 30-day stability study revealed that NaDES provided the best anthocyanins preservation at a defined temperature range compared to extracts prepared with water, 70 % ethanol and 70 % methanol. Black raspberry pomace is a promising raw material rich in anthocyanins that can be used in green technology-oriented industries.