Robert W. Todd

Dielectric permittivity of microbial suspensions at radio frequencies: a novel method for the real-time estimation of microbial biomass

The radiofrequency dielectric properties of microbial suspensions are a direct and monotonic function of the radius and volume fraction of the particles constituting the suspended phase. Measurement of these properties therefore permits the direct estimation of microbial biomass during fermentations, in situ and in real time. The present approach to biomass estimation does not suffer significant interference from non-cellular particulate matter and retains its linearity at volume fractions two orders of magnitude greater than those at which the Beer-Lambert law fails.

Real-time monitoring of cellular biomass : methods and applications

Abstract We review physical approaches to the problem of devising a real-time biomass probe. Direct measurement of the dielectric permittivity of cell suspensions at radio frequencies provides one possible solution to this problem.

Introduction to the dielectric estimation of cellular biomass in real time, with special emphasis on measurements at high volume fractions

The equations that describe the magnitude of the β-dielectric dispersion of biological cell suspensions are introduced. It is then demonstrated how this magnitude can be used to monitor cellular biomass concentrations in real time. These equations are then shown accurately to describe experimental data obtained over a wide range of cell sizes and volume fractions.

Correction of the influence of baseline artefacts and electrode polarisation on dielectric spectra.

The deconvolution of biological dielectric spectra can be difficult enough with artefact-free spectra but is more problematic when machine baseline artefacts and electrode polarisation are present as well. In addition, these two sources of anomalies can be responsible for significant interference with dielectric biomass measurements made using one- or two-spot frequencies. The aim of this paper is to develop mathematical models of baseline artefacts and electrode polarisation which can be used to remove these anomalies from dielectric spectra in a way that can be easily implemented on-line and in real-time on the Biomass Monitor (BM). We show that both artefacts can be successfully removed in solutions of organic and inorganic ions; in animal cell and microbial culture media; and in yeast suspensions of varying biomass. The high quality of the compensations achieved were independent of whether gold and platinum electrodes were used; the electrode geometry; electrode fouling; current density; the type of BM; and of whether electrolytic cleaning pulses had been applied. In addition, the calibration experiments required could be done off-line using a simple aqueous KCl dilution series with the calibration constants being automatically calculated by a computer without the need for user intervention. The calibration values remained valid for a minimum of 3 months for the baseline model and indefinitely for the electrode polarisation one. Importantly, application of baseline correction prior to polarisation correction allowed the latters application to the whole conductance range of the BM. These techniques are therefore exceptionally convenient to use under practical conditions.

Modelling real-time simultaneous saccharification and fermentation of lignocellulosic biomass and organic acid accumulation using dielectric spectroscopy.

Dielectric spectroscopy (DS) is routinely used in yeast and mammalian fermentations to quantitatively monitor viable biomass through the inherent capacitance of live cells; however, the use of DS to monitor the enzymatic break down of lignocellulosic biomass has not been reported. The aim of the current study was to examine the application of DS in monitoring the enzymatic saccharification of high sugar perennial ryegrass (HS-PRG) fibre and to relate the data to changes in chemical composition. DS was capable of both monitoring the on-line decrease in PRG fibre capacitance (C=580 kHz) during enzymatic hydrolysis, together with the subsequent increase in conductivity (G=580 kHz) resulting from the production of organic acids during microbial growth. Analysis of the fibre fractions revealed >50% of HS-PRG lignocellulose had undergone enzymatic hydrolysis. These data demonstrated the utility of DS biomass probes for on-line monitoring of simultaneous saccharification and fermentation (SSF).

Nonlinear dielectric spectroscopy of biological systems: principles and applications.

Biological cells can be seen, electrically, as consisting of conducting internal and external media separated by a more-or-less non-conducting cell membrane. The classical, linear, β-dielectric dispersion results from the charging up of this nominally ‘static’ membrane capacitance according to a Maxwell-Wagner type of mechanism, and typically occurs in the radio frequency range. However, because practically all of the external macroscopic field is dropped across the 5 nm thick cell membrane, there is an effective and substantial amplification of the field across this membrane. This is predicted, and is found, to produce substantial nonlinearities when attempts are made to measure harmonics of the single-frequency exciting field. The nature (odd vs even) and magnitude of these harmonics changes substantially with cell status and environment, providing opportunities for using the cells themselves as sensing elements to describe their surroundings. Electrode polarisation effects producing nonlinear dielectricity can confound these measurements and must be bypassed or taken into account. Nonlinear dielectric spectroscopy (NLDS) provides a wholly non-invasive approach to cellular characterisation and diagnosis.