Equilibrium and fixed-bed kinetics study on hazelnut skin polyphenols extraction using choline chloride –lactic acid-based NADES and water as solvents

This paper focuses on the potential of Hazelnut Skin, a waste product from the food industry, as a source of polyphenols in line with circular economy principles. Two environmentally friendly solvents, water and Natural Deep Eutectic Solvent (NADES), were utilized for the extraction process. First of all, the extraction potential of both solvents was evaluated and an amount of 172.2 mg/g and 470.2 mg/g of free polyphenols per unit mass of solid were obtained using Water and NADES respectively. The solid–liquid equilibrium was also deeply investigated observing that two main phenomena are involved during the process: solubilisation and adsorption. An innovative model based on the combination of Langmuir isotherm and the introduction of a partitioning coefficient to describe a progressive approach to the solubility limit was then proposed to characterize the thermodynamics of the system. While NADES showed promising results compared to water in terms of amount of polyphenols extracted, there are several challenges that need to be addressed before implementing NADES in large-scale extraction processes. Hence, in this study continuous tests were conducted to observe the behavior of both solvents in a fixed-bed column that emulates a section of an industrial belt extractor. The experimental data collected were fitted by means of the Broken Intact Cells (BIC) Model. It was found out that the internal mass transfer resistance is the controlling one and the correspondent mass transfer coefficient estimated was 1.3·10-3 min−1 for Water and 6.67·10-4 min−1 for NADES. The results obtained from this study can be considered valuable resources for improving the existing methods of extracting polyphenols from waste matrices using water and for developing innovative approaches that utilize NADES as a solvent