Citrus peels, which otherwise contain biologically active substances, essential oils and similar compounds that are otherwise desirable on the market, pose a major problem in the fruit processing industry, because the peels themselves pose a disposal problem due to their high content of organic matter, the low pH of extracts the peels is also problematic.
Waste citrus peels are an excellent source for obtaining essential oils from the flavedo - the outer, colored part of the peel. Essential oils are complex mixtures of volatile organic compounds that have the characteristic smell of plants or their fruits. In oranges, the major part of the essential oil is the monocyclic monoterpene limonene, which has a lemony fragrance.
Limonene can be extracted from peels through solid-liquid extraction, with various techniques falling into traditional and modern ones. Traditional techniques include batch extraction, Soxhlet extraction, cold pressing, and water or steam distillation. Modern Techniques have been developed to improve the drawbacks of individual traditional techniques or to emphasize their strengths. Additionally, it is essential to reduce the use of organic solvents or replace them entirely with greener solvents, including natural deep eutectic solvents (NADES). The latter are mixtures of two or more pure compounds in an optimal ratio, resulting in a melting point lower than the melting point of an ideal mixture. The compounds in NADESs are primary metabolites and/or bio-renewable substances that act as hydrogen bond donors or hydrogen bond acceptors.
The aim of master's thesis was preparation of NADESs and compare with organic solvents how much limonene they extract. I wanted to use the common synergistic effect of NADES with limonene for application in creams, that's why I used various amino acids as hydrogen bond donor and choline chloride or glycerol as hydrogen bond acceptor. Extraction with organic solvents (methanol, ethanol, ethyl acetate and acetone) was performed with batch extraction and using Soxhlet method, while extraction with NADESs was performed only with batch extraction since they are non-volatile. For analysis of limonene in extracts GC-MS was used, and for the analysis of extracted proteins UV-Vis spectrometry was used. Since proteins are contaminant I wanted to extract as few of them as possible. Results showed that yields of limonene obtained with the Soxhlet technique were higher than those obtained with the batch extraction. Results also showed that none of the prepared deep eutectic solvents extracted limonene.
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