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Yeast concentration Remote strain output (4-20 mA) yeast selection

Yeast concentration Remote strain output (4-20 mA) yeast selection

Fig. 6.7 Schematic layout of in-line yeast concentration measurement using the biomass meter.

From yeast storage vessel

From yeast storage vessel

Fig. 6.8 Automatic pitching rate control system using the permittivity biomass meter.

measurements are made, the tanks must be stirred to homogeneity. In the case of inline measurements (Fig. 6.7), periodic fluctuations in yeast concentration as slurry passes the probe will be taken into account. However, it is assumed that there is not a gradient in yeast concentration across the section of the pipe. Provided that homogeneous conditions are maintained, readings are unaffected by the morphology of the yeast. Thus, there is no effect due to the use of chain forming or flocculent strains.

Fig. 6.9 Biomass meter (courtesy of Aber Instruments).

A major advantage of the biomass probe is that it is responsive only to viable yeast cells - at least those cells that would be classed as viable using the methylene blue staining test (see Section 7.4.1). The data in Table 6.3 compares biomass meter predicted yeast concentrations, measured as dry weight, to actual measured dry weight for a number of pitching yeast slurries removed from brewery storage vessels. In addition, the viability of the yeast in each slurry was determined by methylene blue staining, allowing the viable yeast dry weights to be computed. As may be seen, there was a good correlation between measured viable dry weights and those predicted by the biomass meter, although the former were on average slightly higher. This may be explained in that the measured dry weights contained varying proportions of trub and this was not taken into account with the biomass meter derived values. This illustrates a further benefit of the meter. The correlation between biomass meter output and viable yeast mass has been confirmed by others, for example, Kronloff (1991). This author also reported that the device had utility for biomass measurement in immo-

Table 6.3 Comparison of biomass concentration for pitching yeast slurries measured directly and using a permittivity biomass meter (from Boulton et a!., 1989).

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