METHOD FOR DETERMINING GAS HOLD-UP

Abstract: A method and apparatus for determining gas hold-up in a fermentation culture, in which, prior to aeration and during fermentation, a high frequency alternating signal is input to electrodes in electrical contact with the culture, the magnitude of the reactive current signal leading the voltage in the electrode circuit is determined and the difference in said magnitudes prior to aeration and during fermentation is used as a measure of the gas hold-up. The system may be combined with various methods of measuring biomass to give a gas hold-up corrected value of biomass.

Description: METHOD FOR DETERMINING GAS HOLD-UP This invention relates to determination of gas hold-up in a fermentation process and has wider implications for determination of cellular biomass or biovolume and to processes in which such determination is made- One of the most important variables in a fermentation or other process using biological cells is the reactor biomass concentration, that is the concentration of microbial or other biological cells in the reactor, because the productivity of a process under a given set of conditions is proportional to the biomass concentration. For monitoring purposes during a fermentation it is particularly useful to be able to estimate the biomass on a real time basis rather than at some time in the past. This generally requires the utilisation of physical properties that can be measured in situ rather than properties which require sampling and analysis. However, the conditions within fermentation reactors do not lend themselves to the use of all physical techniques. Our European Patent No. 0281602 and published European Patent Application No. 0282532 describe a method and apparatus for the determination of biomass in a culture medium in which a signal dependent on the electrical capacitance or dielectric permittivity is generated, at a suitable frequency or range of frequencies, between electrodes mutually spaced in the medium, and determining from the capacitance or permittivity dependent signal, the volume fraction enclosed by the cytoplasmic membranes of the cells. The basis of this approach is that biological cells, in contrast to macromolecules, ionic solutions and gas bubbles, have molecularly thin lipid membranes which (when measured at suitable frequencies) can be shown to have a large electric capacitance per unit membrane area. When suspended in a conductive medium, the measured capacitance exhibits a frequency dependence known as the beta dispersion. By measuring the capacitance at suitable frequencies it is possible to estimate the magnitude of the beta dispersion and in turn the biomass concentration. In a development of the above approach a conductance signal representative of the culture conductivity at respective high and low frequencies may also be used. These methods work in many processes, but a problem that limits their accuracy is the varying quantity of gas that may be present in the culture. During fermentation the medium in a bioreactor may consist of multiple phases of gases, liquids and solids, and because the permittivity of gases is significantly lower than that of liquids, the presence of gas bubbles decreases the measured capacitance or conductance of the suspension causing errors in the biomass concentration estimate. This problem can be minimized by maintaining a constant level of aeration during the reaction and calibrating the apparatus for that level. However in some circumstances it may not be convenient or possible to keep gas hold-up constant even when aeration rate is constant and it is desirable to have a measuring technique for gas hold-up. Accordingly the invention provides a method of determining gas hold-up in a fermentation culture, the method comprising, prior to aeration and during fermentation, inputting a high frequency alternating signal to electrodes in electrical contact with the culture, determining the magnitude of the current signal leading the voltage in the electrode circuit and using the difference in said magnitudes prior to aeration and during fermentation as a measure of the gas hold-up. The invention also provides apparatus for determining gas hold-up in a fermentation culture, the apparatus comprising means for inputting a gas high frequency alternating signal to an electrode circuit that includes electrodes in electrical contact with the culture, means for inputting the voltage over the inter electrode gap to a phase sensitive detector, means for imposing a 90° phase lag on a current signal from the electrode circuit and for inputting said lagged signal to the phase sensitive detector to produce a signal related to a quadrature component of current in the electrode circuit. The invention is now described by way of example with reference to the accompanying drawings in which: Figure 1 illustrates an apparatus for combined conductance measurement of biomass and gas hold-up correction; Figure 2 schematically illustrates a fermentation reactor provided with the equipment of Figure 1. Figure 3 schematically illustrates an alternative apparatus using a separate frequency for gas hold-up measurement; and Figure 4 schematically illustrates a system in which frequency signals are applied sequentially rather than simultaneously. As mentioned above, the electrical behaviour of a cellular suspension or tissue is strongly dependent on frequency, the frequency dependent change relative to the value for an aqueous solution being termed a 'dispersion' . Three major sources of dispersion are recognised and are termed alpha, beta and gamma dispersion. Beta dispersion, as explained in our prior European Patent No. 0281602, is a property of intact cells and is therefore of use in analysing cell content via the electrical behaviour of media incorporating intact biological cells. Alpha and gamma dispersions are not predictably dependent upon intact cell concentration, and occur, respectively, at lower and higher frequencies than beta dispersion being related to mobile ions at cell surfaces and dipolar rotation. Beta dispersion is essentially due to intact cells having a poorly conducting, relatively ion-impermeable, cellular membrane, whereas the interior of the cell is relatively conducting. The position of the beta dispersion curve on the frequency axis is a function of cell radius, membrane capacitance and internal and external electrical conductivities, whilst the magnitude of the dispersion depends upon the concentration, in terms of percentage volume, of the cells in the medium as well as other variables such as cell size. Our previous patent utilises a calibrated measurement of capacitance in the frequency range of the beta dispersion as a measure of biovolu e. However as measurement of capacitance in higher conductance media becomes more difficult, an alternative technique utilising conductance measurements has been developed. This alternative technique utilises the fact that at lower frequencies cell membranes block the flow of current through the interior of the cell, but at higher frequencies current flows through the cell via the membrane capacitance. It can be shewn that the difference in conductivity through the suspension at different frequencies equals 9/4 (volume fraction x internal cell conductivity), i.e. it depends upon, inter alia, the radius and volume fraction occupied by the cells. During fermentation the liquid media in reactors are aerated and although the rate of aeration can be maintained constant to maintain, as far as possible, constant gas hold-up there are occasions when changes in the rate of aeration are desired. Also, as fermentation progresses changes in viscosity may also result, giving rise to changes in gas hold-up. The presence of gas bubbles, reducing the unit volume concentration of water, reduces the capacitance over the whole range of frequencies used in biomass measurement because of the lower permittivity of gas compared with water. The change in capacitance caused by gas hold-up has an effect both on background capacitance and also upon the measured magnitude of the β dispersion itself because the gas bubbles also displace cells. Hence in the capacitance measurement technique the measured β dispersion changes with gas hold-up. In the above described conductance method water displacement also changes the measured values. In this technique the difference between two measurements is taken, so the background element of the change may be eliminated, but this still leaves the inaccurancy in measured dispersion arising from displacement of the cells. The present invention is principally concerned with enabling measurement of gas hold-up in order to establish corrected values for biomass as measured for example by either of the techniques referred to above. At high frequencies, ideally above the β dispersion range, capacitance is principally determined by water content. Thus in the invention it is proposed to utilise a high frequency capacitance measurement before onset of aeration, and during reaction. Subsequent changes in the capacitance at the higher frequency after onset of aeration and reaction are then attributed directly to gas hold-up, for example a 20% reduction in capacitance indicating approximately 20% gas content. This estimate of gas hold-up can be used to correct the biomass concentration estimate provided by means such as the capacitance method in EP 0281602 or, as described hereinafter, the conductance method. In either instance the following relationship applies: Corrected biomass - 100 x direct biomass concentration estimate 100 - G.F.(%) concentration signal G.F. = Gas Fraction % Conventional sampling followed by laboratory analysis methods, i.e. after the gas bubbles have cleared from the sample, measure corrected biomass and thus the present invention enables in-situ determination of the conventional analysis amounts, which may be subsequently verified from time to time. Preferably the frequency used for the capacitance measurement is above the β dispersion range. In some instances measurements may need to be taken within the top end of the β dispersion range, for example if the dispersion range is very extended. This would cause some inaccurancy of the gas fraction estimate; if this is unacceptably large, a curve fitting technique may be used to determine the true high-frequency (i.e. above β dispersion) capacitance value. A measure of gas fraction, as opposed to just aeration rate, is also useful in its own right as a means of monitoring and assisting in the optimisation of some fermentations. A circuit for a preferred embodiment of the invention is shown in Figure 1, in which a gas hold-up measurement is combined with conductance measurement to determine biomass. The conductance measurement utilises measurement at two frequencies generated by oscillators 1 and 2, with oscillator 2 at high frequency. The outputs of the oscillators are combined, amplified, input to an automatic level control 3 and then to the primary winding of an isolating transformer 4. Induced oscillations occur in an electrode circuit 5 connected to the transformer secondary winding. To alleviate electrode polarisation effects there are two sets of electrodes, outer current electrodes 7, 7' through which the current passes to and from the culture and at which any polarisation effects will occur, and an inner set of voltage electrodes 8, 8' which sample the voltage over a gap intermediate the current electrodes, immune from polarisation. A separate circuit 6 for applying pulses to the electrodes for cleaning purposes from time to time may also be provided. Other electrode configurations may also be used. The voltages sensed by the electrodes 8, 8' are amplified and input to a detector 9 and to band-pass filters 10 and 11. Detector 9 completes a feedback loop for automatically controlling the gain of the applied voltage by means of the automatic level control 3. It will be appreciated that the voltage over the electrodes 8, 8' is a combination of voltages having the two different oscillator frequencies and the band-pass filters 10 and 11 are tuned, respectively, to pass the lower and higher frequency voltage signals which constitute phase reference signals for the phase detectors 12 and 13. From the band-pass filters the voltage signals are input to respective phase sensitive detectors 13 which also receive a current signal input from the electrode circuit, which for example may be taken over a suitable load. The output from the phase sensitive detectors 12 and 13 are signals corresponding to the magnitude of the component of the current signal that is in phase with the respective reference voltage signal, i.e. proportional to the conductance. Thus conductance value signals for the lower and higher frequencies are output, respectively, from phase sensitive detectors 12, 13 into a comparator within a difference and correction circuit 14. The circuit 14 also receives a temperature dependent signal from a temperature sensor 15 located close to the electrodes in the medium which provides a temperature compensation adjustment from a precalibrated reference. The gas hold-up correction is also input to circuit 14. This utilises the higher frequency voltage signal, taken after band-pass filter 11 and input to a third phase sensitive detector 16, along with the current signal. In order to establish the extent of gas hold-up a measure of the capacitance at the higher frequency is required, and thus prior to the phase sensitive device the current signal is input to a phase shifter 11, which imposes a τt/2 lag, so that the output of phase sensitive detector 16 is indicative of the magnitude of the current signal leading by π/2 with respect to the voltage in the electrode circuit, i.e. representative of the capacitance value. This capacitance value is used by the correction circuit, using pre-calibrated values, to apply a gas hold-up correction to the final biomass concentration dependent signal output from circuit 14. It will be realised that if the different frequency signals are applied in rapid succession, rather than simultaneously, signal processing of the different frequencies may be handled within a single processor; for example the signals may all go in sequence to a single phase sensitive device. This approach reduces the requirement for frequency-selective filtering and may be preferred for this reason. A suitable circuit is shown in Figure 4, using only a single oscillator 1, a sequencing circuit 40 and with a sample-hold facility in the difference and correction circuits 14. Figure 2 schematically illustrates a fermentation reactor 20 provided with conductance biovolume measuring equipment as previously described. In order to reduce equipment introduced errors the electrode voltage and current sensing circuits including an amplification stage are preferably incorporated in a probe module 21 close to the electrode circuit. The main electronic processing unit 22 may then be located remote from the apparatus. For simplicity the apparatus and method for combined biomass measurement and gas hold-up correction have been described in terms of utilising a high and low frequency. More than one high and low frequency may be utilised and the results used to compute biomass concentration and other parameters or used for other correction purposes. In some cases, the gas hold-up capacitance is preferably measured at a third, higher frequency, than the conductance measurement frequencies with suitable additional circuitry as shown in Figure 3 with oscillator 30. Alternatively, the high frequency capacitance measurement may be incorporated into a capacitance measuring biomass arrangement to correct the biomass concentration signal for changes in gas hold up. In some instances measurements may need to be taken within the top end of the β dispersion range rather than clear of it, for example if the dispersion range is very extended. In such instances, as with the conductance method above, it is possible to utilise either separate or common high frequency signals for the biomass concentration and gas hold-up measurement. Also, different electrodes may be used for the gas hold up measurement and biomass measurements. The block diagrams shown herein have been simplified for illustrative purposes. Production versions of the systems shown would require further feedback systems for automatic trimming of zero and phase response of the electronics and cables.

Claims: CLAIMS 1. A method of determining gas hold-up in a fermentation culture, the method comprising, prior to aeration and during fermentation, inputting a high frequency alternating signal to electrodes in electrical contact with the culture, determining the magnitude of the reactive current signal leading the voltage in the electrode circuit and using the difference in said magnitudes prior to aeration and during fermentation as a measure of the gas hold-up. 2. A method according to claim 1 in which the high frequency is towards the upper range of frequencies at which β dispersion occurs. 3. A method according to claim 1 in which the high frequency is above the range of frequencies at which β dispersion occurs. 4. A method according to any preceding claim further comprising determining biovolume in a culture by inputting respective high and low frequency alternating signals to an electrode circuit including electrodes in electrical contact with the culture, deriving a conductance signal representative of the culture conductivity at each frequency and processing said conductance signals to provide an output indicative of biovolume and utilising said gas hold-up determination to provide a corrected biovolume estimate. 5. A method according to claim 4 in which the conductance signals are generated by determining for each frequency a signal related to the in phase component of current within the electrode circuit. 6. A method according to claim 4 or claim 5 in which the output is subjected to a temperature dependent adjustment. 7. A method according to any of claims 4 to 6 in which the gas hold-up high frequency signal is one of the signals used for deriving biovolume. 8. A method according to any of claims 4 to 6 in which the gas hold-up high frequency signal is a separate high frequency signal not used for deriving biovolume. 9. A method according to any preceding claim in which at least some of the differing frequency signals are input to the electrode circuit simultaneously. 10. A method according to any of claims 4 to 9 in which at least some of the differing frequency signals are input to the electrode circuit sequentially. 11. Apparatus for determining gas hold-up in a fermentation culture, the apparatus comprising means for inputting a gas high frequency alternating signal to an electrode circuit that includes electrodes in electrical contact with the culture, means for inputting the voltage over the inter electrode gap to a phase sensitive detector, means for imposing a 90° phase lag on a current signal from the electrode circuit and for inputting said lagged signal to the phase sensitive detector to produce a signal related to a quadrature component of current in the electrode circuit. 12. Apparatus according to claim 11 further comprising means for comparing the quadrature component during fermentation with a quadrature component determined before onset of aeration of the culture. 13. Apparatus according to claim 11 or claim 12 further comprising means for determining biovolume comprising means for inputting respective high and low frequency alternating signals to the electrode circuit and generating from the relative magnitude of the in phase components of current and/or voltage at the respective high and low frequencies an output signal indicative of biovolume. 14. Apparatus according to any of claims 11 to 13 in which the alternating signals are input to the electrode circuit by induction. 15. Apparatus according to claim 13 or claim 14 in which the means for generating comprises band-pass filters tuned to a respective one of the frequencies and connected to means for obtaining an in-phase component of current signal relative to the respective voltage signal, the outputs of which are input to a difference circuit. 16. Apparatus according to claim 15 including means for sampling voltage signals over an inter-electrode gap and means for determining current signals in the electrode circuit, the voltage signals being input to the band-pass filters and the current signals input, along with the output from the band-pass filters to the means for obtaining the in-phase component. 17. Apparatus according to any of claims 13 to 16 further comprising means for adjusting the output signal indicative of biovolume in dependence upon temperature sensed in the culture. 18. Apparatus according to any of claims 13 to 17 further comprising means for adjusting the output signal indicative of biovolume in dependence upon the magnitude of said quadrature component.

Inventors: TODD ROBERT WILLIAM, TODD, ROBERT, WILLIAM
Applicants: ABER INSTR LTD, ABER INSTRUMENTS LIMITED
Application year: 1993
Main classification: G01N33
IPC codes: C12M1, G01N27, G01N33