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Assume that, if poorly controlled, the fermentation may take up to a further 3 days to complete. In the worst case the annual productivity would fall to:

Assuming the same worst-case vessel turn-round time of 16 days, the annual productivity for each vessel would be 34219 hi. Therefore, to make up the potential annual shortfall of 157 933 hi, there would be a requirement for an additional five fermenters. Assuming a capital cost of £350000 per fermenter, the potential savings due to an efficient control regime would equate to a saving in new plant costs of £1.75 million. This figure does not include the revenue costs of operating the additional vessels. Alternatively, the same figures could be expressed in terms achieving the same volume output using the same fermenter capacity. In this case, the control system would provide a potential increase in fermenter productivity of more than 20%.

Set against these potential savings is the cost of installation and operation of the control system. Obviously these may be considerable, particularly the initial capital costs of fitting sensors to every vessel. In this regard, control systems that use existing measuring devices, such as thermometers, are particularly attractive. Where new sensors have to be fitted, those that are non-invasive and preferably able to be shared by a number of fermenters attract a significant cost advantage.

6.4.2.2 Fermentation control by yeast oxygenation. Variable pitching yeast physiology can arise in several ways. Inconsistencies in performance of the fermentation from which the yeast was cropped and less than ideal conditions of handling and storage in the intervening period between cropping and re-pitching are probably common causes. An intermediate stage between a passive control system and an automatic interactive regime is to take positive steps to eliminate sources of inconsistency. With respect to yeast, this can be partially achieved by stringent control of storage conditions. Another option is to subject pitching yeast to a pre-treatment that ensures that all the cells are in a desired physiological condition.

A prime cause of inconsistency of pitching yeast condition is variable exposure to oxygen during handling and storage. In this case, yeast may couple dissimilation of stored glycogen to limited sterol synthesis. Failure to correct for this pre-formed sterol by reduction of the concentration of oxygen supplied with the wort will result in a rapid but inefficient fermentation due to excessive yeast growth (Boulton & Quain, 1987). Since the amount of pre-formed sterol is not quantified and is variable, inconsistent fermentation performance and beer analysis will result. A logical extension of this effect is to take positive steps to expose pitching yeast to oxygen in a controlled treatment and encourage sterol synthesis. Historically such a treatment was common practice, particularly in German breweries as a means of improving the vigour of pitching yeast. Proper regulation of the process ensures that all yeast cells achieve a consistent level of sterol synthesis and this source of variability in physiological condition is eliminated. After the treatment, in theory, the yeast will have no requirement for wort oxygenation and good control of fermentation can be achieved by precise regulation of pitching rate.

During the oxygenation process the synthesis of sterol and concomitant utilisation of the carbon and energy released by degradation of glycogen is accompanied by an increase in the rate at which yeast assimilates oxygen. The maximum observed yeast oxygen uptake rate (Adot) coincides with the achievement of steady state maximum and minimum values of sterol and glycogen, respectively, and therefore this parameter can be used as a monitor of the oxygenation process (Fig. 6.23).

The data shown in Figs 6.24(a) and (b) depict the results of a laboratory trial in which a sample of pitching yeast slurry was oxygenated as described in the legend to Fig. 6.23. Samples were removed from the slurry at the times indicated by arrows (Fig. 6.24(a)) and pitched into aliquots of anaerobic wort. The resultant attenuation profiles were as shown in Fig. 6.24(b). As may be seen, samples of yeast removed before the point of maximum Adot produced fermentations with slow but progressively increasing rates. Yeast removed at, or after, the point at which the maximum Adot was observed produced essentially identical fermentation profiles.

The total quantity of sterol synthesised during the oxygenation process was of the same magnitude as that seen in a conventional wort-oxygenated fermentation. Thus, the initial sterol content of approximately 0.2% cell dry weight increased five-fold to around 1% cell dry weight. Sterol synthesis was accompanied by dissimilation of squalene. The decrease in the size of the squalene pool was sufficient to account for the quantity of sterol synthesised (Fig. 6.25(c)). This suggests that carbon released by glycogen dissimilation was not utilised directly for sterol synthesis de novo.

Oxygenation was accompanied by the formation of ethanol, possibly indicating the likely fate of the carbon derived from glycogen breakdown (Fig. 6.25(b)). In the data shown, the high initial value of ethanol reflected that present in the barm ale entrained in the yeast slurry. The combination of simultaneous exposure to oxygen and ethanol had no effect on yeast viability. This remained high and essentially unchanged throughout the process. The yeast dry weight decreased during the treatment, presumably also reflecting glycogen breakdown (Fig. 6.25(a)). In addition, the pH fell from an initial value of 4.2 to approximately 3.75. Probably this effect is analogous to

Time (hi

Fig. 6.23 Changes in the intracellular concentrations of sterol and glycogen compared with the rate of uptake of oxygen of a slurry of pitching yeast during exposure to oxygen. Yeast (35% wet weight to volume) was removed from a brewery storage vessel and oxygenated without dilution at a temperature of 20°C (redrawn from Boulton & Quain, 1987).

Fig. 6.23 Changes in the intracellular concentrations of sterol and glycogen compared with the rate of uptake of oxygen of a slurry of pitching yeast during exposure to oxygen. Yeast (35% wet weight to volume) was removed from a brewery storage vessel and oxygenated without dilution at a temperature of 20°C (redrawn from Boulton & Quain, 1987).

that seen in the spontaneous first part of the acidification power test, which has also been related to glycogen breakdown (see Section 7.4.2).

The spectrum of individual sterols formed during oxygenation is shown in Fig. 6.26(a). Predictably, ergosterol showed the greatest increase during oxygenation. However, comparatively large amounts of zymosterol were also synthesised. Interestingly, in relative terms, zymosterol was the only sterol to increase during the oxygenation process, whereas, as a proportion of the whole, ergosterol decreased (Fig. 6.26(b)). This was perhaps surprising since zymosterol is reportedly not inserted into membranes but is esterified and stored in lipid particles (Leber el al., 1992).

The changes observed during prolonged oxygenation are shown in Fig. 6.27. Following the initial peak of oxygen uptake rate, which occurred after about 4 hours, this parameter increased again, to an even higher value. The second peak of oxygen uptake rate occurred after some 16 hours' continuous oxygenation. This second burst of high oxygen consumption was accompanied by a concomitant fall in the concentration of exogenous ethanol. Therefore, this probably indicated derepression and the attainment of respiratory competence. Sterol increased in concert with the first peak of oxygen uptake rate; however, there was no further increase thereafter. Thus, the relatively high sterol concentration associated with derepressed yeast, typically 5% of the cell dry weight (Quain & Haslam, 1979), was not seen here.

The storage properties of oxygenated and non-oxygenated yeast are illustrated in

Time (h) (a)

Fig. 6.24 Profileof changein oxygen uptakerate of yeast slurry exposed to oxygen. Samples were removed at the times indicated by the arrows and (a) oxygen uptake rates were measured off-line using a polargraphic method and (b) evaluated by assessment of their fermentation performance in anaerobic wort in EBC tall tubes (re-drawn from Boulton et al., 1991).

Fig. 6.24 Profileof changein oxygen uptakerate of yeast slurry exposed to oxygen. Samples were removed at the times indicated by the arrows and (a) oxygen uptake rates were measured off-line using a polargraphic method and (b) evaluated by assessment of their fermentation performance in anaerobic wort in EBC tall tubes (re-drawn from Boulton et al., 1991).

Fig. 6.28. Within the time-scale of production-scale brewing and at normal storage temperatures (2°C) there were no significant differences. However, at elevated temperatures (18°C) oxygenated yeast lost viability more rapidly than the non-oxygenated control. It must be assumed that this reflected the low glycogen content, and, hence, reduced ability to withstand prolonged starvation, of the oxygenated yeast. Synthesis of trehalose which has been reported to occur during yeast oxygenation (Callaerts et al., 1993) and which has been also observed to occur in the studies described here, did not apparently confer an increased ability to withstand prolonged storage.

The yeast oxygenation method of fermentation control has been applied at

Oxygenation time (h)

Oxygenation time (h)

Oxygenation time (h)

Oxygenation time (h)

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