# Extraction of Antioxidants from Grape and Apple Pomace: Solvent Selection and Process Kinetics
###### Abstract  
Polyphenols have become a research target due to their antioxidant, anti-inflammatory and antimicrobial activity. Obtention via extraction from natural sources includes the revalorization of food wastes such as grape pomace (GP) or apple pomace (AP). In this work, GP and AP were submitted to a liquid-solid extraction using different solvents of industrial interest. Process kinetics were studied measuring the total phenolic content (TPC) and antioxidant capacity (AC), while the extraction liquor composition was analyzed employing chromatographic methods. Extraction processes using water-solvent mixtures stood out as the better options, with a particular preference for water 30%-ethanol 70% (\(v/v\)) at 90 \({}^{\circ}\)C, a mixture that quickly extracts up to 68.46 mg GAE/gds (Gallic Acid Equivalent per gram dry solid) and 122.67 TEAC/gds (TROLOX equivalent antioxidant capacity per gram dry solid) in case of GP, while ethylene water 10%-ethylene glycol 90% (\(v/v\)) at 70 \({}^{\circ}\)C allows to reach 27.19 mg GAE/gds and 27.45 TEAC/gds, in the case of AP. These extraction processes can be well-described by a second-order kinetic model that includes a solubility-related parameter for the first and fast-washing and two parameters for the slow mass transfer controlled second extraction phase. AP liquys were found to be rich in quercetin with different sugar moieties and GP extracts highlighted flavonoids, cinnamic acids, and anthocyanins. Therefore, using identical extraction conditions for AP and GP and a comparative kinetic analysis of TPC and AC results for the first time, we concluded that ethanol/water mixtures are adequate solvents for polyphenols extraction due to their high efficiency and environmentally benign nature. p 2022  
essential oils, etc. [3]. In the present work, we paid attention to two types of residues of importance in the agro-food sector: grape (GP) and apple pomaces (AP). Cider production produces a waste called apple pomace. A mixture of pulp, peels, pips, and stems of the whole apple, which is produced in the peeling, slicing, and pressing operations in the production of cider. AP is rich in sugars and in certain types of phenolics such as quercetin. So, although the residue is not very rich in phenolics, the phenolics from AP have high industrial interest. Furthermore, in 2020, worldwide apple production accounted for 86.4 million of tons of waste. However, compared to other fruit wastes such as citrus peels, around 30% of the initial weight is lost in their processing in the case of apples. However, from the total apple production only 30-40% of them are processed for different purposes, principally for juice obtention prior to cider production. In Spain, the annual production in 2020 was of 522,000 metric tons according to FAO statistics [4]. If the aforementioned estimations are considered, apple waste production amounted to 156,000 tons in Spain in that year, which were used for animal feeding and compost production. Additionally, AP has a valuable composition of other polymeric compounds that are the main constituents: cellulose, hemicellulose, pectin, and lignin [5]. These relatively poor added value applications are a waste in the sense of the potential of this biomass as a source of sugars, polysaccharides, and antioxidant molecules. In any case, the recovery of the aforementioned chemicals requires the development of processes that respond to the needs of economical optimisation [6]. AP is, in particular, a natural source of interesting antioxidant molecules and, more specifically, phenolics. In these fruit residues, some examples of the different families of natural phenolics are found: hydroxybenzoic acids, cinnamic acids, and the group of flavonoids. Despite the variety of this kinds of molecules, apple-derived wastes are enriched in particular classes. Flavanols, for instance, catechin and epicatechin and their polymeric derivatives, as well as other abundant flavonoids belong to the flavonoids group, which consists mainly of quercetin and its glucosides [7].