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+640 hi wort possible to arrange to pitch the culture at a lower than maximum count but during the exponential phase when the yeast is still very active. This ensures that the onset of fermentation is rapid.

High-yield propagators allow larger step-up ratios. Assuming a terminal count from propagator of 250 x 106 cells ml 1 and a target pitching rate in the first fermentation of 15 x 106 cells ml \ this would allow a scale-up ratio of 1:16. Thus, a typical cylindroconical fermenter of 1500 hi capacity could be serviced by a 100 hi propagator. Furthermore, a single seed vessel of 8 hi working capacity would be sufficient to provide an inoculum for the final propagation stage. It is perfectly feasible to operate a two-stage propagation system of this type within a total seven day cycle time. Although the barm ale arising from propagation will have a non-standard volatile spectrum, the relatively high dilution factor at pitching makes this less of a problem.

A system of this type is shown in Figs 7.7 and 7.8 (Boulton & Quain, 1999). It consists of two tanks each of similar design. Both are serviced by a dedicated CiP system and can be steam sterilised, together with all associated pipework. Sterile wort is delivered to each vessel via an in-line heat exchanger, which serves to both sterilise wort and cool it to a desired starting temperature. Sterile oxygen is introduced to the vessels via a bottom mounted stainless steel diffuser. High rates of oxygen transfer are promoted by constant mechanical agitation and the presence of internal baffles to increase turbulence. Attemperation is achieved by circulating coolant through external jackets. During operation, vessels are top-pressured using sterile inert gas.

Fig. 7.7 Schematic of a two-tank aerobic yeast propagation system (from Boulton & Quain. 1999).

In-tank dissolved oxygen probes can be used to monitor and control oxygenation rates.

Propagation of yeast on wort limits the yield of yeast because metabolism is always catabolite repressed (see Section 3.4.1). Much greater yields could be achieved if the yeast was grown under derepressed conditions such as is practised in bakers' yeast production (Barford, 1987). This approach is capable of generating very high yeast concentrations under appropriate conditions, typically five times greater than those achievable in brewery propagators. In addition, derepressed cells accumulate roughly five times greater concentrations of sterols compared to derepressed cells. Theoretically this yeast should have no requirement for wort oxygenation and produce satisfactory fermentation performance at much lower than usual pitching rates (Quain & Boulton, 1987b).

Derepressed growth of yeast may be achieved by growth on an oxidative carbon source, such as glycerol and ethanol. Alternatively, as is the case in the bakers' yeast industry, a fed-batch approach is used. Here, the principal carbon source is separated from the remainder of the growth medium and added at an exponentially increasing rate during the entire time course of cultivation. This ensures that the carbon source

Fig. 7.8 Two-tank aerobic propagation system at Bass Brewers Limited. Alton Brewery (kindly supplied by Jim Appelbee. Bass Brewers).

remains at low concentrations at all times and repression is not triggered. These approaches have yet to be applied in production scale brewing. However, Quain and Boulton (1987a) reported that brewing yeast strains grown oxidatively on mannitol exhibited excellent storage properties and produced more rapid fermentations than similar yeast grown under repressed conditions. A potential use for these very high yielding systems is that yeast might be used in a 'single trip' or 'pitch and ditch' fermentation regime. There would be an on-cost for the increased requirement for propagation; however, the advantage would be improved fermentation consistency due to use of pitching yeast with consistent physiological condition.

Masschelein el al. (1994) described laboratory scale studies in which yeast was propagated in a fed-batch system in which the wort feed was controlled by a computer such that steady state conditions were established. In this instance, the conditions were controlled so as to maintain constant sugar concentration and oxygen tension. Thus, the yeast physiology was not derepressed; however, evidence was provided that it was consistent and highly active.

7.2.2.3 Use of dried yeast. Bakers' yeast propagated under oxidative conditions is frequently dried to render it into a stable form in which it can be more easily stored and transported. Provision of small-scale dried pure cultures for propagation in the brewery laboratory has been discussed (see Section 7.1.1.3). Provision of dried cultures sufficient in quantity for direct use at production scale has been applied to wine yeast and it has been suggested that brewing yeast might be treated similarly. For example, it was proposed as a method of utility where a central facility could supply yeast to a number of satellite breweries, with no propagation facilities of their own (Lawrence, 1986b).

The potential advantages of such approaches are manifold. There would be no requirement for brewery or laboratory propagation plant. If dried yeast were used for all fermentations, there would be no need for, or at least a reduction in, intermediate storage vessels. It would be predicted that the condition of individual batches of the yeast inoculum would be relatively constant, therefore offering the possibility of more consistent fermentation performance. Dried yeast is resistant to storage over periods of several months provided it is kept at cool temperatures and is vacuum packed. This facilitates transport and is particularly suitable for small-scale craft brewers, franchise-brewing operations, or in situations where particular beer qualities may be produced infrequently.

The application to brewing of dried yeast has been described (Muller et al., 1997; Fels et al., 1999). These communications reported the results of fermentation trials at a scale of 300 hi. The dried yeast had a viability of just 65%, compared with a more typical value of 95% for the usual production brewing strain. However, it was claimed that fermenter residence times could be matched, provided that pitching rates were corrected for viability. It was necessary to réhydraté the yeast, prior to pitching, by suspending in wort for 30 minutes at 20°C, before use. The organoleptic properties of beers from fermentations using dried yeast were within the range of conventional controls.

Undoubtedly, this method will be of utility, but at present probably in niche applications. It is unlikely to see widespread adoption in the mainstream large-scale commercial brewing if only because of inertia and cost.

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