Yeast recovery

Recovery of yeast during and at the end of fermentation is a three-part process. First, when the fermenter is emptied the yeast concentration within the green beer must be reduced to a level appropriate for the type of beer being produced. Second, a fraction of the recovered yeast must be retained for re-pitching. Third, beer entrained in separated yeast may be recovered. The separation of yeast from green beer during vessel run-down is described in the next section of this chapter. Methods used for removal of the bulk of the yeast crop, especially that destined for re-pitching, are discussed here. The procedures used are much influenced by the type of fermenter and yeast used; however, the same general principles apply. Although numerous reviews have been published on yeast cropping, the reader is directed to O'Connor-Cox (1997) for a readable discussion of an essential part of the brewing process.

The manner in which yeast is cropped can influence the manner in which it performs in subsequent fermentations and some general guidelines must be followed to avoid deleterious effects. Strain purity must be maintained, and therefore cropping must be conducted in such a way as to prevent contamination. Plant should be designed and operated to good hygienic standards and in this regard, cropping systems associated with closed fermenting vessels have a clear advantage. The plant associated with cropping should not subject the yeast to stresses such as excessive shear forces or sudden pressure changes. In addition, in order to avoid unnecessary metabolic activity in the yeast, the temperature should be maintained close to 2 to 4°C and in closed systems exposure to air should be avoided.

Yeast should be cropped from fermenters as soon as possible. The conditions which yeast is exposed to in fermenter, particularly during the later stages, are stressful. Thus, the combination of starvation due to lack of nutrients, high carbon dioxide and ethanol concentrations and possibly elevated pressures all have the potential to cause deterioration in the yeast crop. In extreme cases, the imposed stresses are sufficient to cause cell death and autolysis with consequent adverse effects on beer quality. In this regard, tall bottom-cropping fermenters impose the greatest stresses on yeast. It should be remembered that by the time a crop has formed, the yeast within it plays no further positive role in the fermentation, and therefore it should be removed as soon as is convenient.

Yeast to be used for re-pitching should be as free as possible from contaminating wort solids in order to minimise errors of pitching rate control in the next fermentation. Some entrapment of solids with yeast is inevitable, particularly with bottom-cropping fermenters. However, it is possible to manage the process such that the best possible separation of yeast and solids is achieved.

6.7.1 Top-cropping systems

Traditional fermentation vessels used with top-cropping yeast strains are provided with specific systems for skimming the yeast head. This includes a manual operation in which the yeast is pushed into a drain. Frequently a so-called 'parachute' arrangement is employed. This consists of a drain through which the yeast crop is discharged. The inlet is extended into a broad conical collector, the height of which is adjustable to ensure efficient and controllable separation of the yeast head from the beer. Alternatively, the yeast head may be removed with a suction pump.

The timing of crop removal is dependent on the type of fermentation and the properties of the yeast. Management of the process is dependent on visual observation and the experience of the brewer. In particular, in ensuring that the yeast to be retained for re-pitching is relatively free from trub and is the fraction that will produce the required performance in the next fermentation. Thus, during the course of the fermentation, several yeast heads may be formed and these are all cropped but not necessarily retained. The experienced brewer will be aware of how the appearance and timing of formation of the head relates to overall fermentation performance and which yeast cuts should be kept or discarded.

Selection of the appropriate yeast head and the design and operation of top-cropping fermentation vessels assists with separation of yeast from trub. Thus, the first head is heavily contaminated with trub and is discarded. In addition, much of the larger particles of trub form a sediment and in this respect these systems are self-cleansing. In the dropping system this is further refined by starting fermentation in one vessel, then, after 24 h or so, transferring the fermenting wort into a second vessel (see Section 5.3.3). This serves the dual purpose of ensuring that the actively growing yeast is uniformly suspended in the transferred wort and allows much of the sedi-mented trub to be left behind in the first vessel. It is, of course, profligate in its use of large numbers of vessels.

After removal from the fermenter the cropped yeast is collected into a separate open vessel, a wheeled trolley or more frequently a closed tank termed a 'yeast back'. Yeast to be used for re-pitching may be stored in this form without further treatment. More often the entrained beer (barm ale) is separated from the yeast by pumping the contents of the yeast back, through a cooled plate and frame filter. The yeast cake is recovered from the filter plates and discarded or stored on metal trays or bins in refrigerated rooms. The filtrate can be pumped back into the fermenting vessel from which it originated, or collected into separate recovered beer tanks for blending back at a later stage and at a rate thought appropriate by the brewer. De Clerck (1954) commented that recovered barm ale has a different composition to the mother beer from which it was obtained. It may have very high bitterness levels, due to the propensity of these substances to bind to yeast cell walls (see Section 4.4.3) and it usually contains high concentrations of yeast metabolites derived from shock excretion (see Section 3.7.6). De Clerck (1954) suggests blending back at a low rate to avoid adverse flavour effects, and then only in certain beer qualities. However, it seems likely that these deleterious effects can be ameliorated by careful handling of the slurry and appropriate attemperation.

6.7.2 Bottom-cropping systems

In traditional open fermenters used with bottom-cropping yeast, the beer is removed first, at the end of primary fermentation, leaving a yeast sediment in the base of the vessel. This is then removed manually. Attempts are made to ensure that the fraction retained for re-pitching is taken from the middle of the sediment. Thus, the lowest layer is enriched in trub, whereas that which settles last is typically in poor condition.

In closed fermenting vessels, using bottom-cropping yeast strains, it is not possible to see the crop develop, as it is with a top-cropping system. Therefore, it is necessary to gauge the optimum time for yeast removal from other measures of fermentation progress. Usually the fermentation is judged complete when the wort is fully attenuated and the period of warm conditioning, if practised, is over. At this time, the vessel contents are chilled to between 2 and 4°C to encourage the bulk of the remaining suspended yeast to settle into the base of the vessel. In cylindroconical fermenters, this is facilitated by provision of cone cooling jackets. The latter are also useful for maintaining high yeast viability where vessel residence times are long since they prevent localised warming due to the exothermy of the packed yeast. Although chilling encourages yeast sedimentation, much of the crop is formed during the warm phase of active fermentation - especially if the yeast is flocculent. It is a misconception that this yeast contributes to diacetyl reduction during warm conditioning, and for quality reasons, both of beer and yeast, it is better to remove a crop as soon as possible after the achievement of attenuation gravity.

Cahill el al. (1999a) studied the efficacy of cooling in sedimented yeast using a specially designed apparatus. This allowed the effects of attemperation of solid yeast plugs to be assessed using a wall jacket arrangement, as is the case in the cones of cylindroconical fermenters. These authors demonstrated that thermal gradients developed rapidly such that a differential of 11°C was measured between the cooling surface and a point 1.2m into the packed yeast mass.

Loveridge el al. (1999) suggested that much of this problem could be avoided by removing an initial 'warm crop' of yeast 24 h after the attainment of attenuation gravity. This was followed by a second conventional crop taken when cooling had been applied after the attainment of VDK specification. The first crop was retained for re-pitching purposes and the second cold crop was disposed of (Figs 6.37(a), (b)). The results were based on comparison of a substantial number of trials (n = 180) and control (n = c. 420) lager fermentations of 15°Plato wort performed in both 2000 and 4000 hi cylindroconicals. The viability of the early-cropped yeast was always greater than that of the conventional cold crop. Furthermore, the suspended solids content of the warm crop was greater than that of the cold crop. A similar differential in mean

6 8 9 Days

Fig. 6.37 Comparison of conventional 'cold' yeast cropping (a), and (b) 'warm' yeast cropping fermentation regimes (from Loveridge et al.. 1999).

Fig. 6.37 Comparison of conventional 'cold' yeast cropping (a), and (b) 'warm' yeast cropping fermentation regimes (from Loveridge et al.. 1999).

cell size was also noted (Table 6.8). Significant process improvements were noted using this approach (Fig. 6.38). Total residence time for more than 70% of trial fermentations pitched with the warm cropped yeast was less than 10 days, compared with fewer than 30% pitched with the conventional late crop.

Table 6.8 Analysis of early 'warm' and conventional 'cold' yeast crops (from Loveridge et al„ 1999).

First crop Second crop

Mean Range Mean Range

Cell size (nm) 7.23 -0.15/ + 0.19 6.54 —0.68/ + 0.58

Slurry wet weight 53.0 -10.0/ + 9.0 25.0 —16.0/+ 13.0 (% weight to volume)

Exit FV Exit FV Viability Viability VDK <9 <10 days <12 days >80% >90% days

Fig. 6.38 Comparison of fermentation and yeast performance measures for yeast cropping via conventional (□) and early warm (■) regimes (from Loveridge et al„ 1999).

Exit FV Exit FV Viability Viability VDK <9 <10 days <12 days >80% >90% days

Fig. 6.38 Comparison of fermentation and yeast performance measures for yeast cropping via conventional (□) and early warm (■) regimes (from Loveridge et al„ 1999).

In another recent study, Hodgson et al. (1999) investigated yeast cell age distribution in cone crops. Thus, as the crop was removed from the cone of the fermenter, successive samples were removed that could be equated to the distribution from top to bottom of the sedimented yeast. The average age of the cells within each sample was determined by bud scar staining using the fluorochrome, calcofluor. In addition, the predicted future performance of yeast from each sample fraction was assessed in EBC tall tube fermentations. The results indicated that the mean age of cells decreased from bottom to top of the crop. Yeast taken from the middle to top of the crop, judged as being middle to young in age, produced the best fermentation performance. It was recommended that this fraction should be retained for re-pitching purposes.

This recommendation is somewhat contrary to that of the 'warm cropping' procedure described previously. Thus, it would be predicted that the latter approach would select the older and, therefore, lower quality yeast. The possible disadvantages of this need to be weighed against the benefits of removing yeast, as soon as possible, from the hostile environment of the base of a large fermenter. Of course, the diffi culties of reconciling these recommendations demonstrate an inherent shortcoming of cylindroconical fermenters as compared with selective top-cropping vessels. A combination of the two approaches would be possible by simply retaining a late cut from an early crop.

Removal of the yeast crop may be a manual process. A sight-glass is fitted to the exit main to allow visual identification of the interface between yeast slurry and green beer. This is used as the signal to open and close relevant valve paths to divert the process flow towards an appropriate destination. Solids which settle into the base of fermenting vessels are not of homogeneous composition. The lowest layer is enriched with trub since this forms the first sediment in early fermentation before appreciable yeast growth has occurred. In order to separate this from the bulk yeast crop the first cut may be diverted to waste tanks. This is judged on a volume basis gained by experience of working with the particular vessels and fermentation in question. Care must be exercised when removing the crop. There is a tendency, particularly in cylindroconicals, for the centre of the plug of yeast in the cone to exit before that close to the walls. If the flow rate is too high, mixing effects make it difficult to engineer a clean separation of yeast from beer.

Yeast crops may be used directly to pitch another fermentation, without intermediate storage (see Section 6.1.4.3) but more usually the cropped yeast slurry retained for re-pitching is pumped into attemperated storage vessels (see Section 7.3.2). Yeast slurries typically contain 30-55% wet weight to volume solids, although this may rise to over 60% solids where very flocculent yeast strains are used. Barm ale collected with the yeast slurry is not separated; however, it is not lost since it is returned to the next fermentation when the yeast is re-pitched.

Yeast slurry that is not required for re-pitching, or that is considered to be of insufficient quality, is pumped to waste yeast storage vessels. In this case, the barm ale is recovered by filtration. The efficiency of beer recovery from filtered yeast may be enhanced by incorporating a washing step, using chilled liquor. Thus, the slurry contains two pools of beer. There is an extracellular fraction located in the interstices between the yeast cells. There is also an intracellular fraction within the yeast cells. Some of the latter can be recovered by resuspension of yeast in cold liquor after filtration and re-treating, or by flushing yeast cake with liquor whilst still in the press. This encourages excretion of ethanol from yeast cells because of the altered equilibrium. The pooled recovered beer is stored in chilled vessels and blended back into beer at a rate that does not impair product quality. In a typical fermentation of 1500 hi, some 5-7 tonnes wet weight of yeast may be recovered. This equates to 12.517.5 tonnes of slurry at 40% solids.

The cropping process may be automated using output from a sensor that detects the yeast/beer interface. Optical sensors can be used for this task. A more sophisticated system uses the yeast biomass meter described in Section 6.1.4.5 and shown in schematic representation in Fig. 6.39. In operation, the process is initiated by the controller selecting the calibration on the biomass meter for the yeast strain to be cropped. The valve path is opened and the cropping pump activated. During cropping, the biomass meter measures the viable yeast concentration in the process stream issuing from the fermenter. The first portion of the crop, which is rich in trub, is directed towards a waste tank. When the viable yeast concentration reaches a pre-

To waste tank

To yeast storage vessels

To green beer centrifuge

Pump

Flow meter

Fig. 6.39 Automatic control of yeast cropping from a bottom-cropping fermenter (from Boulton & Clutterbuck, 1993).

To waste tank

To yeast storage vessels

To green beer centrifuge

Pump

Flow meter

Fig. 6.39 Automatic control of yeast cropping from a bottom-cropping fermenter (from Boulton & Clutterbuck, 1993).

determined threshold value the valve path is automatically changed and flow is diverted to yeast storage vessels. This continues until the interface between the yeast slurry and green beer is detected by a sudden drop in measured viable yeast concentration. When a second pre-determined threshold value is reached the process flow is either stopped or diverted to the green beer centrifuge. A flow meter located in the fermenter exit main, together with the viable yeast cell concentration output from the biomass meter, allows calculation and loggng of the total mass of viable yeast diverted to storage vessels. This can be fed into the supervisory information management system, which, together with a log of the quantity of yeast pitched, provides a continuously updated record of yeast stocks, their history and fate.

A profile of yeast cropping from an 8 hi pilot scale cylindroconical fermentation is shown in Figs 6.40(a) and (b). A biomass meter probe was located in the main a short distance from the exit valve on the bottom of the cone of the fermenter. At the end of fermentation, the green beer was chilled to 2°C then emptied in the normal manner. During run-down, readings were taken from the biomass meter at the times indicated. At the same time, analyses were performed for total suspended solids and viable yeast count on samples removed from the process stream at a point adjacent to the biomass meter probe. The results confirmed that the first portion of the yeast crop contained high levels of entrained trub. This was not detected by the biomass meter. Instead there was a close correlation between output from the meter and measured viable yeast concentration (Figs 6.40(a) and (b)).

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