# Extraction of Antioxidants from Grape and Apple Pomace: Solvent Selection and Process Kinetics
###### Abstract
In the case of grape residues, they are created during the production of wine. The winemaking industry is the principal beverage industry worldwide according to FAO, reaching a production of 77.8 Mt [4], especially in Italy, Spain, and France [8]. Nearly half of grape production is destined for the winemaking process, predicted to reach 258 million hectoliters in 2020, according to the Organization of Vine and Wine (OIV) [8]. The vinification process is composed of different stages, so a wide variety of residues are produced like grape pomace (GP), grape stalks (GK), grape seeds (GS), wastewater and wine lees (WL), among others. Traditionally, grape-derived byproducts have been employed as dyes or fertilizers, or in the food industry. However, those residues have a higher potential, as all of those residues are rich in added value products such as oils, phenolic acids, and polymers such as cellulose, hemicellulose, or lignin. GP is a residue very rich in antioxidants as it comes from grapes, a natural source of these compounds. This residue is produced after vinification, a fermentation process where a partial extraction process takes place (around 40% of phenolics are extracted from grapes during vinification [9]). Thus, there is still a notable amount of remaining antioxidants in GP. Anthocyanins, flavonoids, flavonoids, phenolic acids, and stilbenes are the most important phenolic compounds found in this residue [3]. Polyphenols are secondary metabolites produced by plants as a mechanism of defense with potential applications in the pharmaceutical, cosmetic, and food industries. The particularity of those molecules is their antioxidant activity that provides beneficial actions in human health, as they are able to reduce oxidative stress by donating hydrogen atoms or electrons, preventing atherosclerosis, cardiovascular diseases, acute hypertension, and diverse types of cancer. Those polyphenols are classified in carotenoids and phenolic compounds. The latter are classified in phenolic acids, flavonoids (that includes anthocyanins, flavanols, flavanones, and isoflav), stilbenes, lignans, and tannins [10]. Polyphenols are obtained via extraction of different plant materials and plant-derived materials and phases, both solid and liquid. Most common methods are solid-liquid extraction, Soxhet extraction, liquid-liquid extraction or maceration. However, those methodshave some drawbacks such as requiring a large volume of extraction, low efficiency, and environmental problems due to the use of organic solvents. To avoid those disadvantages, unconventional methods have been developed: for instance, microwave-assisted extraction, ultrasound assisted extraction, and enzyme-assisted extraction, among others. In this case, the advantages are lower solvent requirements, higher yields, less extraction time, and better reproducibility. Still, unconventional methods have generally a higher cost and more complex operation conditions [6,11]. The aim of this work was to develop fast and efficient extraction processes for both wastes (AP and GP), leading to rich antioxidant liquors, by screening different solvents and conditions and to understand process efficiency by a comparative kinetic analysis of the total phenolic content (TPC) and antioxidant capacity evolution with extraction time. Furthermore, a compositional study was performed with the liquors obtained in the best conditions.