Optimisation Challenges in Pressurised Hot Water Extraction of Polyphenols: Extraction and Degradation Kinetics
Abstract: Popular Abstract in English Extraction of valuable substances from plants using a solvent has been practised in many fields. One of the examples is making coffee or tea using hot water. In agricultural and food industry, there are tons of wastes containing valuable compounds that can be extracted before they go to incineration. Extractions are usually performed in organic solvents such as methanol, acetonitrile, and heptane. As concerns about global warming and primary energy consumption are on the rise, it is of importance to find sustainable solvents to substitute the fossil-fuel-derived ones. Water is not the most commonly used solvent because many compounds have limited solubility in water. However, as extraction techniques quickly advance, using pressurised hot water above its atmospheric boiling point has shown its potential in extracting valuable compounds from plant material. One of the many applications of using pressurised hot water extraction (PHWE) is to extract polyphenols from plants. Polyphenols are a group of widely distributed antioxidants in plants. The extracted polyphenols can be used as additives in food or nutraceuticals as antioxidants and/or colorants. Although PHWE has been successfully applied to extract polyphenols from plant materials, there are still challenges to tackle to make PHWE a more effective technique. For example, it is a common strategy in PHWE to increase the temperature in order to improve the total yield of extraction, because higher temperature usually leads to higher solubility of target compounds in water and faster transportation (diffusion) of the compounds from the plant to the extraction solvent. However, many polyphenols are susceptible to heat. Increasing the temperature will also increase the risk of degradation of polyphenols. In order to achieve optimised extraction yield degradation needs to be calculated and minimised accordingly. This thesis introduces mathematical methods to calculate the simultaneous degradation during an extraction process. Theoretically maximised extraction yield could be calculated, by adding back together the amount of degraded polyphenols. PHWE of polyphenols was also carried out in continuous flow mode, which means that extraction was performed by pumping fresh solvent through the plant material. Compared to PHWE in static batch mode and conventional extraction using methanol at low temperature, PHWE in continuous flow mode exhibited significantly higher extraction yield. Another challenge is to determine the rate-limiting step during an extraction process. An extraction process includes desorption of analytes from the plant cell, diffusion through the cell and finally solubilisation in the extraction solvent. Mathematical models can be used to describe these steps. In the plant cell the same analytes may have different adsorption properties on the cell wall. This can be generalised as strongly adsorbed analytes and weakly adsorbed ones. The strongly adsorbed analytes need to have stronger "initial desorption" in order to be effectively extracted. To mimic the strong and weak adsorption to the plant cell, specifically designed polymers were synthesised to bind similar analytes with different strength. In this way the initial desorption can be modelled and distinguished from the other steps during extraction process. A well-established extraction model based on experimental data can be used to see if extraction techniques, such as ultrasound-assisted extraction have any effect on the initial desorption. It can also be used to predict extraction yield under similar experimental conditions. Another approach is to select model compounds of similar solubility but different chemical structure, thus different adsorption properties on the cell wall. Mathematical tools such as the Hansen Solubility Parameters for calculating the solubility and elution by characteristic point method for determining the type of adsorption could also be used to select model compounds.
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