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Wall of main

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Gold or platinum electrodes

Fig. 6.6 Biomass meter, head amplifier and probe assembly for in-line measurement of yeast concentration (from Boulton & Clutterbuck, 1993).

condition are approximately the same size, the build-up of charge, measured as capacitance, is proportional to the number of cells in the operating field.

Yeast cells, which would be classed as non-viable by vital staining methods such as methylene blue, have disrupted plasma membranes. Cells with disrupted membranes have no dielectric response and do not contribute to the measured capacitance. Similarly, non-yeast particulates are incapable of building a charge. Therefore, the biomass meter is responsive only to the viable fraction of a yeast suspension and the reading is unaffected by trub.

The device available commercially is designed specifically for in-line measurement of yeast concentration in brewing applications such as the control of pitching rate. The sensor consists of a probe made from an inert resin in which are embedded four electrodes made from gold or platinum. The probe is resistant to brewery cleaning and sterilisation regimes. Attached to the probe is a small electronics module, which contains amplification circuitry and generates the radiofrequency field. This module is connected by cabling to an electronics module. This is provided with controls for calibration and a visual display of yeast concentration. A signal of 4-20 mA, proportional to the yeast concentration reading, is output from the principal electronics module for connection to an external controller. An RS232 interface can be used for the same purpose. A multiplexer is available which allows connection of banks of four separate probe and amplifier assemblies.

The meter must be calibrated for each individual yeast strain. This entails identifying and entering strain-specific calibration parameters into the memory of the meter. This places a limit on the number of strains that can be handled, usually a maximum often. When calibrated, the meter reading corresponds to viable yeast concentration. This is usually expressed as spun solids, or suspended cell count; however, any desired unit may be used. It does highlight one drawback of the meter. In order to set up the calibration it is necessary to use slurries whose yeast concentration has been measured by conventional means. As discussed already, these conventional methods do lack precision and inevitably there will be a one-off error in the initial calibration. Obviously, the meter cannot have a precision greater than the precision of the method used to analyse the slurries employed in the calibration procedure.

In practice, the measure of concentration used is somewhat irrelevant since in an automatic pitching rate control system it would never be necessary to refer to it. The pertinent parameter in this case is that of repeatability. Thus, the control system uses a derived measure of yeast concentration and is only of significance in the initial setting up to achieve a desired pitching rate in fermenter. Once this is done, all that is required is that the yeast concentration is measured in a repeatable manner. The calibration for any particular strain may be selected manually, although in automatic systems this would be accomplished using the external controller. The principal features of the biomass meter are shown in Figs 6.7 to 6.9.

In operation, the biomass meter provides an instantaneous reading of yeast concentration. Output is linear within the range 1 x 10 to 1.5 x 109 cells ml 1 (approximately 5-60% wet weight to volume). Therefore, unlike the turbidometric sensor, it is not suitable for measurement of yeast concentration in pitched wort. Readings may be taken in-line or in-tank; however, the operating field of the probe extends to only a few centimetres in front of the electrodes. Hence, if in-tank

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