A Sustainable Alternative for Cosmetic Applications: NADES Extraction of Bioactive Compounds from Hazelnut By-Products

Hazelnut processing generates a variety of by-products, including skins, shells, and defatted (DFT) flour, which contain valuable bioactive compounds. These by-products are rich in polyphenols, fibers, and other molecules that are suitable for incorporation into nutraceutical and cosmetic products. The efficiency of three natural deep eutectic solvents (NADES), such as betaine/sorbitol/water (BS), fructose/lactic acid/water (FL), and fructose/glycerol/water (FG) was compared with a control (C) extractant (ethanol/water). These NADES were combined with two extraction techniques: a conventional method involving heat and magnetic stirring, and ultra-sound-assisted extraction (US). The free radical scavenging capacity (FRC), total phenolic content (TPC), and the polyphenolic profile (HPLC) were evaluated. BS NADES exhibited superior efficiency for the extraction from the skin and shell, while FL was optimal for defatted flour. Although the skin is the least abundant hazelnut processing by-product, it exhibited the highest polyphenol content and antiradical activity, indicating potential for cosmetic applications. The suitability of DFT flour, skin, and the residual panel of extracts for thermochemical and biochemical conversion processes was investigated. Some of the materials were found to be conducive to thermochemical conversion, while others were suitable for anaerobic digestion.