Chemical and Biomolecular Engineering - Theses
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ItemMass transfer enhancement of supercritical fluid extraction from a model herb( 2006)The use of supercritical fluids to separate natural materials from foods has attracted much attention recently. However, the commercial success of supercritical carbon dioxide for this application has been limited because it can only separate non-polar substances and the use of high pressure requires a high capital investment. The current study has explored two different methods to enhance the extraction yield and thereby widen the applicability of supercritical fluid extraction. These methods are 1. The effect of the natural moisture content of the substrate 2. The concurrent use of ultrasonic vibration Ginger (Zingiber Officinale Roscoe), which belongs to the Zingerberceae family is used as a model herb in this work. It is found that the use of a new season high moisture content fresh ginger (96 wt % substrate moisture content) for supercritical extraction provides a greater yield than the use of a dried ginger feed (. 1 wt% moisture content) as is currently employed commercially. However, when more fibrous low moisture content old season ginger is employed (. 85 wt % moisture content), the fresh ginger provides lower yield. This variation is considered to be due to differences in the structure and moisture content of the ginger matrix. Mathematical modeling of the new season high moisture content fresh ginger data shows that the effective diffusivity is higher than that of freeze-dried ginger. However, in the case of old season low moisture content ginger batches, the effective diffusivity is higher in the freeze-dried sample. This shows that when a ginger sample of high moisture content (96% moisture) is used, the high water content acts as a swelling agent to enhance the extraction rate and yield. The solubility parameter is comparable across different ginger batches, which indicates that the entraining effect of moisture is negligible. For further work, it may be possible to consider pre-soaking the old season ginger to increase the moisture content to > 95% (wt %) in order to improve the yield. High Performance Liquid Chromatography (HPLC) and Gas Chromatography — Mass Spectrometry (GC-MS) analyses are performed on the fresh, freeze-dried and oven-dried ginger extracts. This analysis shows that the fresh and freeze-dried ginger extracts contain more natural characteristic flavor and pungency than the oven-dried feed which is commonly used in industry. Under the influence of ultrasound, the extraction yield is increased by up to 30%. This higher extraction yield is attributed to disruption of the cell structure and an increase in the accessibility of the solvent to the internal particle structure, which enhances the intra-particle diffusivity. FESEM images of the surface of different ginger particles shows clear evidence of cellular disruption. While such effects are usually attributed to acoustic cavitation in ambient systems, the absence of phase boundaries in a supercritical system should preclude such a phenomenon. In spite of this, the possibility of cavitational collapse as one of the mechanisms for mass transfer enhancement in supercritical extraction due to ultrasonication is considered. Although, experimental results to prove this phenomenon are inconclusive, the formation of cavitation remains a possible cause of the cellular damage. Rapid changes in density (associated with the pressure fluctuations induced by the ultrasonic wave), sonoporation and acoustic streaming are other possible mechanisms that could be responsible for this mass transfer enhancement. It is recommended that further work is conducted using a high pressuer vessel with a direct viewing port to confirm the presence of cavitation. Chromatographic analysis of the extract from experiments conducted both with and without the influence of ultrasound is also performed. This analysis shows that the proportion of all extracted compounds incleased and that no new degradation products were formed as a result of ultrasonication.