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Valentine (1996), which apparently overcomes some of the disadvantages of the system described already. The sensor utilises what was described as a selective narrow band in the near infra-red, which allowed good discrimination between yeast cells, other particulates and gas bubbles. The linear operating range extended to 2 x 109 cells ml 1 and therefore would be suitable for measurement of yeast concentrations in pitching slurries. This would allow the setting up of an automatic pitching system in which the concentrated slurry could be added quickly, as in conventional pitching, and overcome the problem of having to dose in yeast throughout wort collection. It would still be necessary to correct for viability by separate analysis.

More recently, the application to the enumeration of suspended yeast particles of another optical system has been described (Holmes & Teass, 1999; Holmes, 2000). This system was applied to the control of yeast pitching rate in an automatic system. In comparison to an older manual system based on laboratory analysis, improvements in fermentation consistency were reported.

6.1.4.5 Use of radiofrequency permittivity. A biomass sensor has been developed which detects cells by virtue of their dielectric properties (Harris & Kell, 1986; Harris et al., 1987). Suspensions of cells possess several recognisable frequency-dependent dielectric dispersions (Schwan, 1957). One of these, is called the (3-dispersion. It is caused by a combination of build up of charge at the surface of the relatively nonconducting plasma membranes and partially restricted lateral motions of the charged lipids and protein components within the membrane. These are generated in response to an applied field (Woodward & Kell, 1990, 1991; Ferris et al., 1990). When the applied signal is of the frequency of radiowaves, the value of the (3-dispersion of a suspension of yeast cells is proportional to the volume fraction of biomass bound by a biological membrane. Since yeast cells of a given strain and in the same physiological

Head amplifier electronics module

Screw fitting retaining nut

Inert resin probe c. 12 x 2.2 cm electronics module

Inert resin probe c. 12 x 2.2 cm

Body of in-line fitting

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