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* Viability determined by methylene blue staining.

^ Values obtained from calibration curves of biomass meter output versus yeast dry weight.

* Viability determined by methylene blue staining.

^ Values obtained from calibration curves of biomass meter output versus yeast dry weight.

bilised yeast bioreactors since readings were unaffected by the presence of inert carrier.

A schematic diagram of an automatic pitching rate control system using the biomass meter is shown in Fig. 6.8. The system operates as follows. The volume of wort to be pitched and the desired pitching rate are selected and the controller computes the total viable yeast mass or cell number required. The sequence commences when the controller selects the calibration for the appropriate yeast strain, opens the valve path from the yeast storage vessel to the wort main and switches on the yeast pitching pump. As the yeast slurry is injected into the flowing wort, output from the biomass meter and a flow cell allow calculation of the total viable yeast mass, or total yeast cell number, pitched. When this figure matches the calculated required value, the controller terminates the sequence by closing the valve path and switching off the yeast pitching pump. It is possible to substitute the in-line flow meter for output from a load cell located on the yeast storage vessel and obtain acceptable results. However, flow cells offer the most precise control.

The automatic control system described here produces demonstrable improvements in the consistency of fermentation performance, compared to conventional methods. This is illustrated in the data presented in Fig. 6.10. Here is shown the standard deviation in vessel residence time for 2000 hi lager fermentations for a period of months before and after commission of an automatic pitching rate control system of the type depicted in Fig. 6.8.

9 11 Month

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