Novel Solvent Systems for the Extraction of Plant Metabolites

Author: Tristan Kilmartin

Kilmartin, Tristan, 2018 Novel Solvent Systems for the Extraction of Plant Metabolites, Flinders University, College of Science and Engineering

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Plant extracts play an important role in human society with extracts finding uses in food, medicine and art to name a few. The preparation of tea and coffee along with the extraction of opioids from the opium poppy are just a few examples. There are a myriad of different technologies that have been developed over the years to achieve extraction of different chemical compounds from plant material. These utilise an extensive range of solvents, temperatures, contact methods, and pressures in order to selectively extract the components of interest. However, many of the solvents and processes are hazardous; either due to the chemical nature of the solvent or the extraction process. Further hazards can be posed by the environmental fate of the solvents, including mobilisation of toxic and or hazardous solvents and degradation products through air, soil, and water, and through the production of greenhouse gases. Industrially, there is often a trade-off between extraction efficiency and hazards in order to achieve an optimal extraction process for the target compounds, and in many cases hazardous, non-sustainable materials are used. Consequently, there is a need to produce ‘green’ extraction processes that utilise environmentally friendly, nonhazardous solvents, ideally obtained from renewable feedstocks.

This thesis describes the development of novel ‘green’ solvent systems for the extraction of metabolites from C. officinalis (Asteraceae). Systematic studies of glycerol-based composites in maceration and ultrasonic extraction processes led to the development of a novel solvent, which exhibited significant increases in peak areas and phenolic content when utilised in an ultrasonic extraction process.

Initial studies were conducted with aqueous acid and base, supercritical CO2, and natural deep eutectic solvents and extracts were compared with those from a traditional hydroethanolic extraction process. Aqueous acidic and basic solutions, along with supercritical CO2 were eliminated from the study on the basis of preliminary results. Preliminary extracts prepared using Natural Deep Eutectic Solvents (NaDES) made from two or more of sugars, acids, and glycerol exhibited a very similar extract profile to that of the traditional hydroethanolic process, with the glycerol based extracts having the most promise on the basis of their similar extraction performance and lower viscosity compared to the other NaDES tested.

Further investigations into the use of glycerol-based composites as extraction solvents, found that glycerol composites containing either sugar or acid demonstrated increased phytochemical content when compared to those prepared in neat glycerol. Strong positive correlations were observed between the extent of solvent chaotropicity and the extract phytochemical content (HPLC total peak area), phenolic content (Folin-Ciocalteu assay), antioxidant activity (KMnO4 chemiluminesence response), and radical scavenging activity (2,2-diphenylpicryl-1-hydrazyl assay). Results indicate that solvent mixtures with higher chaotropicity would be superior in their extraction efficiency for these types of plant extracts.

In summary, due to the often hazardous nature of current solvents and extraction processes, the development of alternate ‘green’ solvent systems is highly desirable. The research presented here represents a step in the development of novel solvent systems for the extraction of plant metabolites.

Keywords: Solvent, Plant, Green

Subject: Chemistry thesis

Thesis type: Doctor of Philosophy
Completed: 2018
School: College of Science and Engineering
Supervisor: Claire Lenehan