Separation, isolation and enrichment of phenolic compounds from agricultural byproducts with physicochemical methods

Abstract

Phenols are naturally occurring compounds, with great antioxidant potential. They are present in a wide variety of plants, and can act as pigments or even play a part in defense mechanisms. Due to the health benefits they offer, their purification from plant materials and agro-industrial byproducts is of great interest. For this purpose physicochemical separation techniques like solid-liquid extraction, membrane filtration and resin adsorption/desorption were implemented.The products and byproducts of olive tree cultivation are examples of plant materials rich in phenolic compounds. Olive tree cultivation has a long history in the Mediterranean countries, and even today consists an important cultural, economic and environmental aspect of the area. The production of olive oil through 3-phase extraction systems, leads to the co-production of large quantities of olive mill wastewater, rich in phenolic compounds that inhibit its biodegradation. Membrane filtration was used for the exploitation of this byproduct, through fractionation with an inline filtration through Ultrafiltration, Nanofiltration and Reverse Osmosis membranes. The Reverse Osmosis concentrate, containing the low-molecular-weight compounds, was further treated with resin adsorption/desorption. The non ionic XAD4, XAD16N and XAD7HP resins were implemented for the recovery of phenols and their separation from carbohydrates. The recovered phenolic compounds were concentrated through vacuum evaporation reaching a final concentration of 378 g/L in gallic acid equivalents containing 84.8 g/L hydroxytyrosol.Another phenol-rich byproduct of olive fruit harvesting is olive leaves. In order to purify olive leaf phenols, a solid-liquid extraction method was optimized, and the phenol rich extract was treated with membrane filtration. As olive leaves contain phenolic compounds of higher molecular weight than the ones contained in olive mill wastewater, they were separated in the Nanofiltration step of the inline membrane filtration process. The Nanofiltration concentrate was then treated with resin adsorption/desorption for the separation of phenols from the contained carbohydrates. The final concentrate, after the removal of carbohydrates and vacuum evaporation of the sample, contained 98 g/L phenols in gallic acid equivalents.Another example of plant materials rich in phenolic compounds are the products occurring from the cacao tree. In this scope, isolation of phenolic compounds from cocoa powder was also examined. Even though cocoa powder is not a byproduct, it was chosen as a source of phenolic compounds due to its high content in phenols and small particle size that facilitates extraction. The presence of fats in large amounts hindered the entire process, and the phenolic compounds were not concentrated in the membrane fraction expected. Fresh extracts were used in the resin experiments, with the presence of fats and suspended solids being detrimental for the separation of phenols and the conduction of experiments in resin packed beds, as they were quickly clogged. The main result of the experiments concerning cocoa phenols was that only through the synergy of the proposed methods could adequate purification be achieved.Grape cultivation and winemaking consist important agricultural activities in the Mediterranean region. During winemaking, large quantities of solid byproducts are generated, mainly consisting of grape skin and seeds, known as grape marc, rich in phenolic compounds, the same compounds responsible for the beneficial effects of wine in human health. The optimization of a solvent extraction process of grape marc phenols was carried out, followed by fractionation with inline membrane filtration. The fraction of the extract, rich in phenolic compounds was then treated with a resin procedure for the separation of the polar phenolic compounds, from the less polar carbohydrates. The ethanolic effluent of the resin procedure was finally concentrated through vacuum evaporation, reaching a final concentration of 190 g/L in gallic acid equivalents, containing 4.7 g/L catechin. The novelty of the process that was developed and is presented in this thesis, lies in the different stages of phenol separation, first according to their molecular weight in the stage of membrane filtration where lower-molecular-weight phenolic compounds with high-added value are separated from polyphenols. Then the low-molecular-weight phenolic compounds are further purified according to their polarity through an adsorption/desorption process on selective resins consisting of three stages: adsorption, carbohydrates desorption with water and finally phenols desorption with ethanol