Fig. 6.10 Consistency of performance, shown as standard deviation in vessel residence time, for 2000 hi lager fermentations over a period of 20 months. Values for each month were calculated from data taken from at least 30 individual fermentations. Pitching rate control was changed at the end of month 11 from a system based on metered addition of yeast slurry of pre-determined viable spun solids to an automatic system using a capacitance biomass meter (Boulton & Box. unpublished data).
Cahill et al. (1999b) observed a decrease in mean cell volume during storage of pitching yeast. They suggested that this could lead to over-pitching errors when using total cell weight or volume as the controlling parameter. They concluded that an image analyser could be used to measure cell volume and correct for this deviation. Control of pitching rate using the biomass meter would produce this error since it measures the total enclosed biovolume within the operating field. Therefore, it corrects automatically for any variation in yeast cell volume.
Proper management of wort collection requires the establishment at the start of fermentation of controlled values for temperature, yeast pitching rate, dissolved oxygen concentration and wort specific gravity. Precise regulation of these parameters is a vital prerequisite to good fermentation control. However, the timing and order in which these events occur also has an important influence on fermentation performance and beer composition. In the case of small volume fermentations, collection times are short, and therefore all yeast cells are exposed to oxygenated wort at roughly the same time. With large capacity fermentations the time taken to complete wort collection may be considerable and a decision has to be taken as to the timing and sequence of addition of the various components which contribute to establishing initial conditions.
To ensure a defined starting point to fermentation it may be considered preferable to fill the fermenter with oxygenated wort at a desired temperature and then initiate the process by pitching in the yeast. This is an undesirable course of action in two respects. First, the risk of microbiological contamination of the unpitched wort would be unacceptable, and, second, delaying pitching to the end of wort collection postpones the start of fermentation and is therefore wasteful of process time.
An alternative option is to pitch yeast continuously throughout wort collection. This has the advantage that the yeast is well dispersed when collection is complete. A variation to this approach is that where several batches of wort are required to fill a fermenter, discrete portions of yeast slurry may be dosed into some or all batches of wort. Continuous dosing and discrete multiple pitchings are not to be recommended since it is very difficult to predict the effects on fermentation performance.
The consequences of discontinuous pitching throughout collection are illustrated in the following extreme but nonetheless real example. In this instance, there was a mismatch between the capabilities of the brewhouse and the capacity of fermenters. Cylindroconical ale fermentations of 1950 hi and original gravity of 15°Plato required five individual batches of wort to fill each vessel. All batches of wort were saturated with air and collection took some 16 hours to complete. The wort was pitched with yeast at a rate of approximately 1 g per litre wet weight. In total, 195 kg of pressed yeast was added in the form of a slurry containing 50% wet weight to volume. Of this, 40% of the total yeast was pitched with the first batch of wort and the remaining 60% with the fourth batch. During fermentation, the temperature was maintained at 22°C. The attenuation gravity of 3.5°Plato was achieved in 50-60 hours.
The rationale behind the pitching regime was that the comparatively small proportion of yeast added with the first worts provided microbiological protection during the prolonged collection period. The larger portion of yeast added near the end of collection was presumed to be responsible for the bulk of the fermentation. In fact, samples removed during the collection period and analysed for yeast concentration revealed that this was not the case (Fig. 6.11). Here is shown the actual measured yeast counts together with predicted yeast counts, assuming no growth had occurred
1st pitch 2nd pitch
Wort volume (hi) CeN count (x10e6 ml_1)
Wort volume (hi) CeN count (x10e6 ml_1)
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