In recent years, there has been an increasing concern about the disposal of high amounts of citrus residues, mainly orange-peel waste, and potential environmental and economic issues arising from this. However, orange waste biomass contains high value-added compounds, such as essential oils or natural polyphenolic antioxidants, highly demanded for the food, fine chemical and, medical applications; hence, extraction of these compounds is of great interest to valorise orange residues through bio-refinery schemes. Traditionally, petrochemical-based volatile organic compounds (VOCs) have been used as solvents of choice to extract value-added components from citrus waste; however, increasing international regulations are limiting the use of these VOCs for environmental, health and safety reasons. Consequently, the search for green solvents with more benign profiles and the development of alternative extraction technologies for bioactive compounds for the sustainable valorisation of biomass feedstock are considered topics of chief importance. Therefore, this thesis has focused on the development of novel extraction technologies for deterpenation of orange essential oils, as well as the recovery of limonene and polyphenolic antioxidants from orange peel waste, using bio-renewable solvents for sustainable citrus waste biomass valorisation. Firstly, the feasibility of using deep eutectic solvents (DES) for orange essential oil deterpenation by liquid-liquid extraction was investigated. Additionally, the suitability of the quantum chemical COnductor like Screening Model for Real Solvents (COSMO-RS) method to model the thermodynamic behavior of the systems was evaluated. The solvent performance was assessed by calculation of distribution coefficient and selectivity values (both experimental and predicted), with results showing that alkanediol-based DES were effective for selective separation of terpenoids from terpenes in citrus essential oils downstream processing. Secondly, the recovery of polyphenolic antioxidants from orange peel waste through solid-liquid extraction using polyol-based DES was studied. The total phenolic content (TPC) and the antioxidant capacity of extracts confirmed the capability of the proposed DES to extract target natural antioxidants, such as ferulic acid and p-coumaric acid, from citrus peel waste; moreover, DES seemed to promote biomass dissolution, which could be beneficial as pre-treatment for further bioprocesses. Thirdly, recovery of limonene from orange peel using bio-based solvents was investigated through experimental and COSMO-RS studies, with results showing that 2- methyl ethyl tetrahydrofuran (2-MeTHF) and cyclopentyl methyl ether (CPME) are promising solvents for direct extraction of limonene from orange residues. Overall, the studies reported throughout the present PhD project demonstrate the potential of green solvents to replace conventional petrochemical solvents in extraction technologies to recover valuable compounds from orange residues. This will ultimately promote the development of novel biorefinery strategies for the sustainable valorisation of citrus biomass in the wider context of bioeconomy.
|Date of Award
|1 Aug 2020
- The University of Manchester
|James Winterburn (Supervisor) & Maria Gonzalez Miquel (Supervisor)