Optimisation Challenges in Pressurised Hot Water Extraction of Polyphenols: Extraction and Degradation Kinetics

Sammanfattning: Pressurised hot water is an environmentally friendly alternative to hazardous organic solvents such as methanol and heptane. One of the many applications of using pressurised hot water extraction (PHWE) is to extract polyphenols from plants. The challenges in PHWE lies with the optimisation of extraction yield. One factor that is often overlooked is the risk of degradation of thermally labile compounds. In this thesis, extraction and degradation kinetics of polyphenols during PHWE were studied. Mathematical models were constructed to calculate the maximum extraction yield (theoretical yield), accounting for simultaneous degradation during extraction. Results show that significant increase in extraction yield could be obtained, if degradation effects can be minimised. PHWE in a home-built continuous flow extraction system proved that PHWE in continuous flow mode has the highest extraction yield and efficiency than PHWE in batch mode and conventional solid-liquid extraction at low temperature. Extraction kinetics is another challenge to conquer in order to quantitatively optimise the extraction. Extraction kinetics in continuous flow system can be modelled by thermodynamic and mass transfer equations. Mass transfer includes desorption from the sample matrix and diffusion through the matrix and extraction phase. It is difficult to study the effect of the "initial desorption" of naturally abundant compounds on extraction kinetics. Therefore an artificial sample matrix was designed to bind analytes of similar chemical structure with different strength. By modelling the extraction processes of strongly bound and easily extractable analytes, initial desorption can be distinguished in the extraction process. The results show that strong adsorption has a great impact on the rate of the extraction kinetics. In order to increase the extraction rate, extraction methods should aim at increasing the initial desorption rate, for example by increasing the temperature or changing the composition of the extraction solvent. Other mathematical tools such as Hansen Solubility Parameters for solubility study and elution by characteristic point method for adsorption isotherm determination were also utilised to model extraction kinetics. A pair of compounds of similar solubility but different adsorption property on a cellulose matrix was selected to be the model compounds. By studying the elution profiles of the model compounds in a cellulose-packed column, extraction kinetics will be modelled in the near future. A good model of extraction kinetics could be used to predict the extraction time when changing flow rate, solvent concentration, and temperature etc., therefore reducing the amount of laboratory work.