Caroline McFarlane - Academia.edu (original) (raw)
Papers by Caroline McFarlane
Chemical Engineering Science, 1998
Continuous culture fermentations of Escherichia coli W3110 have been carried out at controlled di... more Continuous culture fermentations of Escherichia coli W3110 have been carried out at controlled dissolved oxygen levels of 40% and 10% of saturation. Satisfactory and reproducible results were obtained. Agitation speeds of 400 and 1200 rpm at an aeration rate of 1 vvm have been used as well as an aeration rate of 3 vvm at 400 rpm. The upper levels of these variables represent much higher agitation and aeration intensities than those normally used in practical fermentations. The fermentations were monitored by mass spectrometry and optical density, and cell samples were studied by flow cytometry, SEM, and TEM. Protocols were developed so the state of both cell membranes and cell size could be measured by flow cytometry. Under all the conditions of agitation and aeration, flow cytometric analysis indicated that both cell membranes were intact and that a cytoplasmic membrane potential existed; also the cell size did not change, results confirmed by SEM and TEM. There were no detectable changes in off-gas analysis or optical density during the continuous fermentation nor in the cell structure as revealed by SEM or TEM, except at the highest agitation intensity. Under the latter conditions, after 7 h, the outer polysaccharide layer on the cell was stripped away. It is concluded that any changes in biological performance of this E. coli cell line due to variations in agitation or aeration intensity or scale of operation cannot be attributed to fluid dynamic stresses associated with the turbulence generated by impellers or with bursting bubbles.
Multiparameter flow cytometric techniques developed in our laboratories have been used for the &a... more Multiparameter flow cytometric techniques developed in our laboratories have been used for the "at-line" study of fed-batch bacterial fermentations. These fermentations were done at two scales, production (20 m(3)) and bench (5 x 10(-3) m(3)). In addition, at the bench scale, experiments were undertaken where the difficulty of achieving good mixing (broth homogeneity), similar to that found at the production scale, was simulated by using a two-compartment model. Flow cytometric analysis of cells in broth samples, based on a dual-staining protocol, has revealed, for the first time, that a progressive change in cell physiological state generally occurs throughout the course of such fermentations. The technique has demonstrated that a changing microenvironment with respect to substrate concentration (glucose and dissolved oxygen tension [DOT]) has a profound effect on cell physiology and hence on viable biomass yield. The relatively poorly mixed conditions in the large-scale fermentor were found to lead to a low biomass yield, but, surprisingly, were associated with a high cell viability (with respect to cytoplasmic membrane permeability) throughout the fermentation. The small-scale fermentation that most clearly mimicked the large-scale heterogeneity (i.e., a region of high glucose concentration and low DOT analogous to a feed zone) gave similar results. On the other hand, the small-scale well-mixed fermentation gave the highest biomass yield, but again, surprisingly, the lowest cell viability. The scaled-down simulations with high DOT throughout and locally low or high glucose gave biomass and viabilities between. Reasons for these results are examined in terms of environmental stress associated with an ever-increasing glucose limitation in the well-mixed case. On the other hand, at the large scale, and to differing degrees in scale-down simulations, cells periodically encounter regions of relatively higher glucose concentration.
Chemical Engineering Research & Design, 1997
Activated manganese dioxide can replace the enzyme catalase as catalyst of the hydrogen peroxide ... more Activated manganese dioxide can replace the enzyme catalase as catalyst of the hydrogen peroxide decomposition in the steady state peroxide feeding method for KLa determination in aeration equipment. The equivalence of both techniques was experimentally demonstrated in deionized water. Manganese dioxide is thus a reliable alternative for the application of the method in media like strong electrolyte solutions where catalase is quickly deactivated, but some limitations of its own have also to be considered with oxidizable fluids, e.g. polypropylene glycol.
Biotechnology Progress, 1995
This second part of a four-part series of papers reports on the hydrodynamic and power characteri... more This second part of a four-part series of papers reports on the hydrodynamic and power characteristics of two commercial, high solidity ratio, hydrofoil impellers, the Prochem Maxflo T and the Lightnin A315, in an air—water system. Studies were conducted primarily in a mixing vessel of diameter 0.56 m. The influence of operating conditions (impeller speed and gas flow rate) and sparger position and geometry on bulk flow patterns, gas filled cavity formation, and power consumption have been investigated in detail. In the turbulent Reynolds number region, the unaerated impeller power numbers are constant with average values of 1.30 for the Prochem and 0. 76–0.84 for the A315, depending on impeller position relative to the base of the vessel. Gassed power consumption is closely linked to cavity type and flow pattern. In particular, the development of large cavities causes a significant reduction in power drawn, impeller pumping capacity, and gas dispersion capability. As with downward pumping mixed flow impellers, two characteristic gas loading regimes, direct and indirect loading, can be identified. Under certain conditions, severe hydrodynamic instabilities over the transition between the two regimes give rise to very large torque and power fluctuations. Changes in reactor geometry can have a significant effect on impeller performance. Thus, sparger type and position have a strong influence on relative power drawn, gas handling capability, power required to disperse gas and stability of operation. Of the configurations studied, the use of a large ring sparger (DS/D = 0.8D) at a large sparger—impeller separation is suggested, since relative power drawn, gas handling capability, and energy efficiency in dispersing gas are all enhanced.
Biotechnology Letters, 2001
Multi-parameter flow cytometric techniques have been used to study the effects of three ceramic p... more Multi-parameter flow cytometric techniques have been used to study the effects of three ceramic powders CaO, MgO and ZnO on the physiology of individual, exponentially growing E. coli cells. Whilst all three powders inhibited reproductive growth, depending on their concentration, the mechanism of action of CaO and MgO was different to that of ZnO as shown by fluorescent staining techniques developed in our laboratory.
Chemical Engineering Science, 2000
Several studies have been conducted over the last decade to accumulate data on the growth of food... more Several studies have been conducted over the last decade to accumulate data on the growth of food-borne bacteria as a function of different environmental variables, such as temperature or pH. It has been demonstrated that such data can be used to predict bacterial growth in food products, both under conditions of constant and fluctuating temperatures. The purpose of the present study was to combine bacterial growth modelling with a heat transfer model describing the spatial temperature changes within a solid object, and to validate the model experimentally. Firstly, experimental growth data were attained for Escherichia coli W3110 immobilised in agar at fixed temperatures. Growth data were then fitted using predictive microbial models to represent growth in lag, exponential and stationary phases. When compared to growth in liquid cultures, similar values were found for maximum exponential growth rate. Next, experiments were conducted whereby a vessel was filled with agar inoculated with E.coli and conduction cooled in a water bath. A finite difference scheme was used to model heat transfer from the vessel, and bacterial growth was consequently modelled as a function of temperature inside the vessel. Experimental results for bacterial growth showed good agreement with theory. The results show that it is feasible to combine engineering and microbial models.
Chemical Engineering Science, 1998
Journal of Biotechnology, 1999
Multi-parameter flow cytometric techniques have been developed for the 'at-line' study of bacteri... more Multi-parameter flow cytometric techniques have been developed for the 'at-line' study of bacterial cultivations. Using a mixture of specific fluorescent stains it is possible to resolve an individual cells physiological state beyond culturability, based on the presence or absence of an intact polarised cytoplasmic membrane, enabling assessment of population heterogeneity. It has been shown that during the latter stages of small-scale (5 l), well mixed fed-batch cultivations there is a considerable drop in cell viability, about 17%, as characterised by cytoplasmic membrane depolarisation and permeability. These phenomena are thought to be due to the severe and steadily increasing stress associated with glucose limitation at high cell densities, during the fed-batch process. Such effects were not found in either batch or continuous culture cultivations. The possibility of using these findings for improved process control using 'on-line' flow cytometry are discussed.
International Journal of Food Science and Technology, 2001
Bacterial recovery from heat injury is in¯uenced strongly by the nature of the recovery medium us... more Bacterial recovery from heat injury is in¯uenced strongly by the nature of the recovery medium used to resuscitate cells. This study used agar gradient plates, in combination with image analysis, to study the synergistic eect of NaCl concentration (% w/v) and pH on the recovery of Brochothrix thermosphacta after heat treatment. Initially, exponential, early stationary and late stationary phase cultures of B. thermosphacta grown in all-purpose tryptone (APT) broth at 25°C were heat-treated at 50°C to measure thermal resistance. Late stationary phase cultures were found to be 2±3 times more heat-resistant than the other two, with a D D-value of 14.8 min. Exponential and early stationary phase cultures were thermally treated and inoculated onto agar gradient plates (modi®ed APT medium) and incubated at 25°C. In the instance of the late stationary phase culture, there proved to be too low a cell concentration to obtain con¯uent growth. These plates had gradients of pH (4.0±7.4) and NaCl (1.5±8.1% (w/v)) running at right angles across them. After 48 h incubation, bacterial growth on these plates was measured by image analysis. In all bacterial cultures (heat-treated or control), optimal growth was found at pH 6.8 and 1.5% NaCl (w/v) concentration. The range of salt concentrations and pH values over which growth could be observed was shown to be reduced as a consequence of heat treatment. Overall, it is suggested that the gradient plate technique, in combination with image analysis, could be useful in determining combinations of dierent environmental factors which are eective in preventing the recovery of heat injured bacterial cells.
Chemical Engineering Science, 2000
Several studies have been conducted over the last decade to accumulate data on the growth of food... more Several studies have been conducted over the last decade to accumulate data on the growth of food-borne bacteria as a function of di!erent environmental variables, such as temperature or pH. It has been demonstrated that such data can be used to predict bacterial growth in food products, both under conditions of constant and #uctuating temperatures. The purpose of the present study was to combine bacterial growth modelling with a heat transfer model describing the spatial temperature changes within a solid object, and to validate the model experimentally. Firstly, experimental growth data were attained for Escherichia coli W3110 immobilised in agar at "xed temperatures. Growth data were then "tted using predictive microbial models to represent growth in lag, exponential and stationary phases. When compared to growth in liquid cultures, similar values were found for maximum exponential growth rate. Next, experiments were conducted whereby a 9 l vessel was "lled with agar inoculated with E.coli and conduction cooled in a water bath. A "nite di!erence scheme was used to model heat transfer from the vessel, and bacterial growth was consequently modelled as a function of temperature inside the vessel. Experimental results for bacterial growth showed good agreement with theory. The results show that it is feasible to combine engineering and microbial models.
Chemical Engineering Science, 2003
A mechanism has been elucidated for the coalescence-mediated break-up of bubbles in gas-liquid sy... more A mechanism has been elucidated for the coalescence-mediated break-up of bubbles in gas-liquid systems. Images taken from dynamic systems (a coalescence cell and laboratory-scale bubble columns) show that in some instances the coalescence of two bubbles is accompanied by the formation of a much smaller daughter bubble. Following the coalescence process an annular wave is formed due to the very rapid expansion of the hole following the instant of ÿlm rupture. As the wave moves along the length of the bubble, away from the point of rupture it causes a rippling e ect which distorts the newly coalesced bubble and may result in the formation of an unstable extension. Instabilities due to the annular wave pinch o a portion of this extension, resulting in the generation of a small daughter bubble. In coalescence dominated systems the process results in the generation of signiÿcant numbers of bubbles much smaller (100 -200 m diameter) than the Sauter mean diameter (3-4 mm). ?
Chemical Engineering Research and Design, 1997
ABSTRACT
Biotechnology Progress, 1995
This first part of a four-part series of papers presents a review of the literature on high solid... more This first part of a four-part series of papers presents a review of the literature on high solidity ratio axial flow hydrofoil impellers. Interest in the application of highflow, low power number, hydrofoil impellers, such as the Prochem Maxflo T and the Lightnin A315, has developed over recent years and has largely been stimulated by reports of improved fermentation performance relative to the Rushton disc turbine. Initially, the review briefly examines the characteristics of the radial flow Rushton turbine, as traditionally used in fermenters, with particular reference to its weaknesses. It also discusses the use of pitched blade turbines (mixed flow impellers) for air dispersion since these show some characteristics similar to those of the axial flow hydrofoils. The consideration of these two impeller types provides the background to explain the advantages and potential problems associated with high solidity ratio, axial flow, downward pumping hydrofoils. These aspects are discussed in relation to existing literature which is still relatively scarce. Finally, the advantages of retrofitting are introduced. Parts 2-4 report on a recent, detailed, fluid dynamic study of the Prochem Maxflo T and Lightnin A315 hydrofoil impellers which extends our knowledge of these important impeller types.
Biotechnology and Bioengineering, 1999
High cell density fed-batch fermentations of Escherichia coli W3110 have been carried out at spec... more High cell density fed-batch fermentations of Escherichia coli W3110 have been carried out at specific growth rates of less than 0.3 h −1 , to investigate the effect of glucose limitation on the physiological state of individual cells. After an initial exponential batch phase, the feed rate was held constant and a final dry cell weight of approximately 50 g per litre was achieved. The fermentations were monitored by mass spectrometry whilst measurements of pH, DOC, CFU/mL, TCN, OD 500nm and residual glucose concentrations were made. Satisfactory and reproducible results were obtained. Flow cytometric analysis of cells in broth samples, based on either of two multi-staining protocols, revealed a progressive change in cell physiological state throughout the course of the fermentations. From these measurements it was concluded that the loss in reproductive viability towards the end of the fed-batch process is due to cell death and not due to the formation of a "viable but nonculturable state" as had previously been reported. Since the presence of a high proportion of dead or dying cells at any time during a fermentation has a detrimental effect on the synthesis of any desired product it is proposed that an on-line flow cytometric analysis and control strategy could be used as a means of increasing overall process efficiency.
Brochothrix thermosphacta is a common meat spoilage bacterium. The morphology of this bacterium c... more Brochothrix thermosphacta is a common meat spoilage bacterium. The morphology of this bacterium changes from coccobacilli and short rods to chains during growth, which may give a false estimation in numbers using some enumeration techniques. Methods for the quantification of this bacterium have been compared. Turbidimetric readings showed good agreement with cell dry weight indicating that the former provides a good measure of the change in cell mass during growth. The turbidimetric method also correlated well with bacterial numbers determined by plate counts, flow cytometry and 7 9 manual counts (by microscope) over a limited range of 10 2 10 cells / ml. Flow cytometry and manual counts gave a linear 5 9
Chemical Engineering Science, 1998
Continuous culture fermentations of Escherichia coli W3110 have been carried out at controlled di... more Continuous culture fermentations of Escherichia coli W3110 have been carried out at controlled dissolved oxygen levels of 40% and 10% of saturation. Satisfactory and reproducible results were obtained. Agitation speeds of 400 and 1200 rpm at an aeration rate of 1 vvm have been used as well as an aeration rate of 3 vvm at 400 rpm. The upper levels of these variables represent much higher agitation and aeration intensities than those normally used in practical fermentations. The fermentations were monitored by mass spectrometry and optical density, and cell samples were studied by flow cytometry, SEM, and TEM. Protocols were developed so the state of both cell membranes and cell size could be measured by flow cytometry. Under all the conditions of agitation and aeration, flow cytometric analysis indicated that both cell membranes were intact and that a cytoplasmic membrane potential existed; also the cell size did not change, results confirmed by SEM and TEM. There were no detectable changes in off-gas analysis or optical density during the continuous fermentation nor in the cell structure as revealed by SEM or TEM, except at the highest agitation intensity. Under the latter conditions, after 7 h, the outer polysaccharide layer on the cell was stripped away. It is concluded that any changes in biological performance of this E. coli cell line due to variations in agitation or aeration intensity or scale of operation cannot be attributed to fluid dynamic stresses associated with the turbulence generated by impellers or with bursting bubbles.
Multiparameter flow cytometric techniques developed in our laboratories have been used for the &a... more Multiparameter flow cytometric techniques developed in our laboratories have been used for the "at-line" study of fed-batch bacterial fermentations. These fermentations were done at two scales, production (20 m(3)) and bench (5 x 10(-3) m(3)). In addition, at the bench scale, experiments were undertaken where the difficulty of achieving good mixing (broth homogeneity), similar to that found at the production scale, was simulated by using a two-compartment model. Flow cytometric analysis of cells in broth samples, based on a dual-staining protocol, has revealed, for the first time, that a progressive change in cell physiological state generally occurs throughout the course of such fermentations. The technique has demonstrated that a changing microenvironment with respect to substrate concentration (glucose and dissolved oxygen tension [DOT]) has a profound effect on cell physiology and hence on viable biomass yield. The relatively poorly mixed conditions in the large-scale fermentor were found to lead to a low biomass yield, but, surprisingly, were associated with a high cell viability (with respect to cytoplasmic membrane permeability) throughout the fermentation. The small-scale fermentation that most clearly mimicked the large-scale heterogeneity (i.e., a region of high glucose concentration and low DOT analogous to a feed zone) gave similar results. On the other hand, the small-scale well-mixed fermentation gave the highest biomass yield, but again, surprisingly, the lowest cell viability. The scaled-down simulations with high DOT throughout and locally low or high glucose gave biomass and viabilities between. Reasons for these results are examined in terms of environmental stress associated with an ever-increasing glucose limitation in the well-mixed case. On the other hand, at the large scale, and to differing degrees in scale-down simulations, cells periodically encounter regions of relatively higher glucose concentration.
Chemical Engineering Research & Design, 1997
Activated manganese dioxide can replace the enzyme catalase as catalyst of the hydrogen peroxide ... more Activated manganese dioxide can replace the enzyme catalase as catalyst of the hydrogen peroxide decomposition in the steady state peroxide feeding method for KLa determination in aeration equipment. The equivalence of both techniques was experimentally demonstrated in deionized water. Manganese dioxide is thus a reliable alternative for the application of the method in media like strong electrolyte solutions where catalase is quickly deactivated, but some limitations of its own have also to be considered with oxidizable fluids, e.g. polypropylene glycol.
Biotechnology Progress, 1995
This second part of a four-part series of papers reports on the hydrodynamic and power characteri... more This second part of a four-part series of papers reports on the hydrodynamic and power characteristics of two commercial, high solidity ratio, hydrofoil impellers, the Prochem Maxflo T and the Lightnin A315, in an air—water system. Studies were conducted primarily in a mixing vessel of diameter 0.56 m. The influence of operating conditions (impeller speed and gas flow rate) and sparger position and geometry on bulk flow patterns, gas filled cavity formation, and power consumption have been investigated in detail. In the turbulent Reynolds number region, the unaerated impeller power numbers are constant with average values of 1.30 for the Prochem and 0. 76–0.84 for the A315, depending on impeller position relative to the base of the vessel. Gassed power consumption is closely linked to cavity type and flow pattern. In particular, the development of large cavities causes a significant reduction in power drawn, impeller pumping capacity, and gas dispersion capability. As with downward pumping mixed flow impellers, two characteristic gas loading regimes, direct and indirect loading, can be identified. Under certain conditions, severe hydrodynamic instabilities over the transition between the two regimes give rise to very large torque and power fluctuations. Changes in reactor geometry can have a significant effect on impeller performance. Thus, sparger type and position have a strong influence on relative power drawn, gas handling capability, power required to disperse gas and stability of operation. Of the configurations studied, the use of a large ring sparger (DS/D = 0.8D) at a large sparger—impeller separation is suggested, since relative power drawn, gas handling capability, and energy efficiency in dispersing gas are all enhanced.
Biotechnology Letters, 2001
Multi-parameter flow cytometric techniques have been used to study the effects of three ceramic p... more Multi-parameter flow cytometric techniques have been used to study the effects of three ceramic powders CaO, MgO and ZnO on the physiology of individual, exponentially growing E. coli cells. Whilst all three powders inhibited reproductive growth, depending on their concentration, the mechanism of action of CaO and MgO was different to that of ZnO as shown by fluorescent staining techniques developed in our laboratory.
Chemical Engineering Science, 2000
Several studies have been conducted over the last decade to accumulate data on the growth of food... more Several studies have been conducted over the last decade to accumulate data on the growth of food-borne bacteria as a function of different environmental variables, such as temperature or pH. It has been demonstrated that such data can be used to predict bacterial growth in food products, both under conditions of constant and fluctuating temperatures. The purpose of the present study was to combine bacterial growth modelling with a heat transfer model describing the spatial temperature changes within a solid object, and to validate the model experimentally. Firstly, experimental growth data were attained for Escherichia coli W3110 immobilised in agar at fixed temperatures. Growth data were then fitted using predictive microbial models to represent growth in lag, exponential and stationary phases. When compared to growth in liquid cultures, similar values were found for maximum exponential growth rate. Next, experiments were conducted whereby a vessel was filled with agar inoculated with E.coli and conduction cooled in a water bath. A finite difference scheme was used to model heat transfer from the vessel, and bacterial growth was consequently modelled as a function of temperature inside the vessel. Experimental results for bacterial growth showed good agreement with theory. The results show that it is feasible to combine engineering and microbial models.
Chemical Engineering Science, 1998
Journal of Biotechnology, 1999
Multi-parameter flow cytometric techniques have been developed for the 'at-line' study of bacteri... more Multi-parameter flow cytometric techniques have been developed for the 'at-line' study of bacterial cultivations. Using a mixture of specific fluorescent stains it is possible to resolve an individual cells physiological state beyond culturability, based on the presence or absence of an intact polarised cytoplasmic membrane, enabling assessment of population heterogeneity. It has been shown that during the latter stages of small-scale (5 l), well mixed fed-batch cultivations there is a considerable drop in cell viability, about 17%, as characterised by cytoplasmic membrane depolarisation and permeability. These phenomena are thought to be due to the severe and steadily increasing stress associated with glucose limitation at high cell densities, during the fed-batch process. Such effects were not found in either batch or continuous culture cultivations. The possibility of using these findings for improved process control using 'on-line' flow cytometry are discussed.
International Journal of Food Science and Technology, 2001
Bacterial recovery from heat injury is in¯uenced strongly by the nature of the recovery medium us... more Bacterial recovery from heat injury is in¯uenced strongly by the nature of the recovery medium used to resuscitate cells. This study used agar gradient plates, in combination with image analysis, to study the synergistic eect of NaCl concentration (% w/v) and pH on the recovery of Brochothrix thermosphacta after heat treatment. Initially, exponential, early stationary and late stationary phase cultures of B. thermosphacta grown in all-purpose tryptone (APT) broth at 25°C were heat-treated at 50°C to measure thermal resistance. Late stationary phase cultures were found to be 2±3 times more heat-resistant than the other two, with a D D-value of 14.8 min. Exponential and early stationary phase cultures were thermally treated and inoculated onto agar gradient plates (modi®ed APT medium) and incubated at 25°C. In the instance of the late stationary phase culture, there proved to be too low a cell concentration to obtain con¯uent growth. These plates had gradients of pH (4.0±7.4) and NaCl (1.5±8.1% (w/v)) running at right angles across them. After 48 h incubation, bacterial growth on these plates was measured by image analysis. In all bacterial cultures (heat-treated or control), optimal growth was found at pH 6.8 and 1.5% NaCl (w/v) concentration. The range of salt concentrations and pH values over which growth could be observed was shown to be reduced as a consequence of heat treatment. Overall, it is suggested that the gradient plate technique, in combination with image analysis, could be useful in determining combinations of dierent environmental factors which are eective in preventing the recovery of heat injured bacterial cells.
Chemical Engineering Science, 2000
Several studies have been conducted over the last decade to accumulate data on the growth of food... more Several studies have been conducted over the last decade to accumulate data on the growth of food-borne bacteria as a function of di!erent environmental variables, such as temperature or pH. It has been demonstrated that such data can be used to predict bacterial growth in food products, both under conditions of constant and #uctuating temperatures. The purpose of the present study was to combine bacterial growth modelling with a heat transfer model describing the spatial temperature changes within a solid object, and to validate the model experimentally. Firstly, experimental growth data were attained for Escherichia coli W3110 immobilised in agar at "xed temperatures. Growth data were then "tted using predictive microbial models to represent growth in lag, exponential and stationary phases. When compared to growth in liquid cultures, similar values were found for maximum exponential growth rate. Next, experiments were conducted whereby a 9 l vessel was "lled with agar inoculated with E.coli and conduction cooled in a water bath. A "nite di!erence scheme was used to model heat transfer from the vessel, and bacterial growth was consequently modelled as a function of temperature inside the vessel. Experimental results for bacterial growth showed good agreement with theory. The results show that it is feasible to combine engineering and microbial models.
Chemical Engineering Science, 2003
A mechanism has been elucidated for the coalescence-mediated break-up of bubbles in gas-liquid sy... more A mechanism has been elucidated for the coalescence-mediated break-up of bubbles in gas-liquid systems. Images taken from dynamic systems (a coalescence cell and laboratory-scale bubble columns) show that in some instances the coalescence of two bubbles is accompanied by the formation of a much smaller daughter bubble. Following the coalescence process an annular wave is formed due to the very rapid expansion of the hole following the instant of ÿlm rupture. As the wave moves along the length of the bubble, away from the point of rupture it causes a rippling e ect which distorts the newly coalesced bubble and may result in the formation of an unstable extension. Instabilities due to the annular wave pinch o a portion of this extension, resulting in the generation of a small daughter bubble. In coalescence dominated systems the process results in the generation of signiÿcant numbers of bubbles much smaller (100 -200 m diameter) than the Sauter mean diameter (3-4 mm). ?
Chemical Engineering Research and Design, 1997
ABSTRACT
Biotechnology Progress, 1995
This first part of a four-part series of papers presents a review of the literature on high solid... more This first part of a four-part series of papers presents a review of the literature on high solidity ratio axial flow hydrofoil impellers. Interest in the application of highflow, low power number, hydrofoil impellers, such as the Prochem Maxflo T and the Lightnin A315, has developed over recent years and has largely been stimulated by reports of improved fermentation performance relative to the Rushton disc turbine. Initially, the review briefly examines the characteristics of the radial flow Rushton turbine, as traditionally used in fermenters, with particular reference to its weaknesses. It also discusses the use of pitched blade turbines (mixed flow impellers) for air dispersion since these show some characteristics similar to those of the axial flow hydrofoils. The consideration of these two impeller types provides the background to explain the advantages and potential problems associated with high solidity ratio, axial flow, downward pumping hydrofoils. These aspects are discussed in relation to existing literature which is still relatively scarce. Finally, the advantages of retrofitting are introduced. Parts 2-4 report on a recent, detailed, fluid dynamic study of the Prochem Maxflo T and Lightnin A315 hydrofoil impellers which extends our knowledge of these important impeller types.
Biotechnology and Bioengineering, 1999
High cell density fed-batch fermentations of Escherichia coli W3110 have been carried out at spec... more High cell density fed-batch fermentations of Escherichia coli W3110 have been carried out at specific growth rates of less than 0.3 h −1 , to investigate the effect of glucose limitation on the physiological state of individual cells. After an initial exponential batch phase, the feed rate was held constant and a final dry cell weight of approximately 50 g per litre was achieved. The fermentations were monitored by mass spectrometry whilst measurements of pH, DOC, CFU/mL, TCN, OD 500nm and residual glucose concentrations were made. Satisfactory and reproducible results were obtained. Flow cytometric analysis of cells in broth samples, based on either of two multi-staining protocols, revealed a progressive change in cell physiological state throughout the course of the fermentations. From these measurements it was concluded that the loss in reproductive viability towards the end of the fed-batch process is due to cell death and not due to the formation of a "viable but nonculturable state" as had previously been reported. Since the presence of a high proportion of dead or dying cells at any time during a fermentation has a detrimental effect on the synthesis of any desired product it is proposed that an on-line flow cytometric analysis and control strategy could be used as a means of increasing overall process efficiency.
Brochothrix thermosphacta is a common meat spoilage bacterium. The morphology of this bacterium c... more Brochothrix thermosphacta is a common meat spoilage bacterium. The morphology of this bacterium changes from coccobacilli and short rods to chains during growth, which may give a false estimation in numbers using some enumeration techniques. Methods for the quantification of this bacterium have been compared. Turbidimetric readings showed good agreement with cell dry weight indicating that the former provides a good measure of the change in cell mass during growth. The turbidimetric method also correlated well with bacterial numbers determined by plate counts, flow cytometry and 7 9 manual counts (by microscope) over a limited range of 10 2 10 cells / ml. Flow cytometry and manual counts gave a linear 5 9