Mechanical treatment of microorganisms using ultrasound, shock and shear technology

Yusaf, Talal (2011) Mechanical treatment of microorganisms using ultrasound, shock and shear technology. [Thesis (PhD/Research)]

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Microorganism disruption using ultrasound treatment is the focus of this thesis. There has been aboard spectrum of theoretical and experimental work on microorganisms disruption methods undertaken in the past. However, there is a lack of fundamental understanding on the actual reason of microorganism disruption using ultrasound. The reported literature in the microorganisms and cell disruption research field indicates that shock wave and shear effects occur together in typical ultrasound processing systems and may both contribute to microorganism disruption. However the question of whether the real cause of disruption is shock and/or shear remains unanswered.

To address this issue, two independent mechanical devices – a shock apparatus and a shear apparatus were developed for this study. An ultrasound apparatus operated in a batch configuration was also used for microorganism disruption.

The ultrasound work includes a detailed experimental characterisation of processing conditions associated with the ultrasound treatment. The heat transfer through the ultrasound chamber and the suspension mixing during the ultrasound treatment was evaluated using theoretical and experimental approaches. It was found that one second was sufficient to have complete suspension mixing in the ultrasound chamber and 13.5% of the total ultrasound energy was lost to the surroundings as heat. Saccharomyces cerevisiae was selected as a sample microorganism in this study, and a log reduction of 4 was achieved when ultrasound treatment was used.

To determine how the yeast cell wall disrupts using a shock treatment, a finite element model was developed and the simulation results showed that von Mises stress generated due to dynamic external pressure loading was concentrated at the bottom part of the cell wall of the yeast. A vertical gas gun was commissioned to apply a dynamic load on a water-filled tube. To understand the relationship between the dynamic stress and the microorganism behaviour when subjected to external pressure, a plastic bag full of yeast suspension was placed at the bottom of the tube. The result showed that the yeast disruption rate using the shock wave treatment was relatively modest when an external shock loading pressure of around 115 MPa was used.

In the case of shear stress treatment, analysis of the intense turbulent flow region of the apparatus combined with the experimental results demonstrated that when the energy dissipation rate in the turbulence region is high and the eddies are smaller than the size of the cell, the likehood of yeast disruption is high. The microorganism mechanical properties combined with the calculated energy dissipation rate were used to simulate the yeast disruption efficiency using shear stress. The results showed that a maximum yeast log reduction of 4 was achieved with the shear apparatus in the absence of pressure rise.

The specific energy required for yeast disruption in these three mechanical methods was evaluated and a comparison was made with two relevant conventional methods: homogenizer and Ultra High Temperature (UHT) treatments.

It was found that the specific energy required to achieve a log reduction of 2.5 was 108 MJ/kg in the case of shear and around 0.905 MJ/kg in the case of ultrasound. In the case of shock treatment, the maximum log reduction achieved was 0.57 which required 0.00477 MJ/kg. Therefore, on the assumption that log reduction is proportional to the specific treatment energy, for a 1 log reduction, 0.008 MJ/kg is required for the shock treatment, 0.46 MJ/kg is required for the ultrasound treatment, and around 48 MJ/kg is required for the shear treatment. These results show that shock wave treatment requires less specific energy to achieve the same yeast log reduction as the shear or ultrasound treatment. This implies that the cause of microorganism disruption using ultrasound is shock wave energy.

Additional work in the finite element simulation and shock treatment apparatus is recommended to extend this study to different microorganisms and cells.

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Item Type: Thesis (PhD/Research)
Item Status: Live Archive
Additional Information: Doctor of Philosophy (PhD) thesis.
Faculty/School / Institute/Centre: Historic - Faculty of Engineering and Surveying - Department of Mechanical and Mechatronic Engineering (Up to 30 Jun 2013)
Faculty/School / Institute/Centre: Historic - Faculty of Engineering and Surveying - Department of Mechanical and Mechatronic Engineering (Up to 30 Jun 2013)
Supervisors: Buttsworth, David
Date Deposited: 20 Sep 2011 04:08
Last Modified: 22 Aug 2016 01:31
Uncontrolled Keywords: microorganisms; disruption; microorganism disruption; ultrasound
Fields of Research (2008): 09 Engineering > 0913 Mechanical Engineering > 091399 Mechanical Engineering not elsewhere classified
Fields of Research (2020): 40 ENGINEERING > 4017 Mechanical engineering > 401799 Mechanical engineering not elsewhere classified

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