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* Pitching rate controlled on the basis of addition of known weight of slurry of pre-determined centrifuged wet solids content and corrected for viability as measured by the methylene blue procedure.

* Pitching rate controlled on the basis of addition of known weight of slurry of pre-determined centrifuged wet solids content and corrected for viability as measured by the methylene blue procedure.

a uniform size. This is not the case with flocculent yeast and significant errors may arise.

The data in Table 6.1 also shows that though greater consistency of pitching rate was achieved, on average the turbidometric system tended to pitch fewer yeast cells than the conventional technique. This potential for underpitching can arise in two ways. First, the automatic system has no correction for viability, and, second, although the double sensor arrangement allows for non-yeast solids in worts it does not correct for such material, which may be entrained in the injected yeast slurry. In the case of the fermentations discussed here, the effects of underpitching would seem to be significant because on average vessel residence times were longer where the turbidometric system was employed (Table 6.2). Of course, it is possible that other factors were involved in these discrepancies, for example, the effects of continuous yeast dosing versus one-shot pitching, as discussed earlier. Nevertheless, the turbidometric system has utility and any tendency to underpitch could be corrected for by simply increasing the set-point.

Another yeast sensor using an NIR detection system was described by Byrnes and

Table 6.2 Vessel residence times, measured as time to application of crash cooling, for fermentations using conventional and turbidometric pitching rate control. Each datum point is the mean for 6 individual fermentations (Boulton & Besford, unpublished data).

Time to crash cool (h)

Table 6.2 Vessel residence times, measured as time to application of crash cooling, for fermentations using conventional and turbidometric pitching rate control. Each datum point is the mean for 6 individual fermentations (Boulton & Besford, unpublished data).

Time to crash cool (h)

Beer quality

Conventional pitching rate control

NIR turbidometric pitching rate control

Lager 1

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