where y = concentration of cells in the effluent; x = concentration of cells in the fermenter; (i = specific growth rate; D = dilution rate.
A further division of systems is possible based on the extent of mixing within the fermenter. Where mixing is assumed to be perfect, as in the simple chemostat, the system is described as homogeneous. Conversely, where a gradient of conditions exists between the point of entry and exit, the system is defined as heterogeneous. As with the partially-closed category Portno (1970) also recognised partially-
heterogeneous systems in which linked series of tanks could include elements of both homogeneity within an individual vessel yet maintaining a fermentation gradient throughout the entire system. Portno (1968a) also distinguished two classes of heterogeneous closed systems, namely, single and two-phase. The former describes a plug flow arrangement in which cells and substrates/products move simultaneously through the reactor. In the latter, yeast and substrate/product traverse the fermentation system at different rates. Closed and open, homogeneous and heterogeneous systems are, of course, not mutually exclusive.
The requirements of an ideal brewing continuous fermentation system were discussed by Portno (1978). They remain a useful set of criteria and are reiterated here. The first three points relate to performance of the system with respect to productivity and flexibility. Thus, a continuous system should be capable of maintaining a high yeast concentration within the reactor in order to ensure a constant fermentation rate close to the maximum rate of a batch fermentation. Preferably, the system should function with any yeast strain. Output from the fermenter should be variable within reasonable limits in order to retain at least some of the flexibility of the multi-tank batch approach.
The remaining criteria relate to the need for the sum of the conditions within the continuous system to be sufficiently similar to those that occur within a batch culture so that acceptable beer is produced. Many important beer flavour compounds are produced in proportions that are relative to the extent of yeast growth. It follows that should conditions within a continuous fermenter deviate greatly from those in a batch culture the resultant beer is likely to be equally non-standard. With a homogeneous system, particularly where the retention index is high, it is difficult to control yeast growth such that its extent is similar to that of a batch system. Therefore, the most important of this second group of criteria is the need for a fermentation gradient to be established within the continuous fermenter. In other words, heterogeneous continuous systems offer the greatest likelihood of success.
In a batch fermentation, yeast assimilates wort sugars in an ordered fashion with sucrose and glucose being used first followed by maltose, the major wort sugar (see Section 3.3.1). Utilisation of maltose is inhibited in the presence of glucose.
In a homogeneous continuous system the continual in-feed of fresh glucose-containing wort disrupts maltose utilisation, and, therefore, a requirement of a batch fermentation - that all fermentable sugars are assimilated - is not fulfilled. In partially closed systems, these effects are compounded. Such systems are characterised by low growth rates, indeed the latter is directly proportional to the retention index. Induction of a-glucosidase is a growth-related process, and therefore, in partially closed continuous systems, the presence of glucose inhibits maltose utilisation and the low growth rate restricts the synthesis of the enzyme responsible for maltose hydrolysis. Under these circumstances, the presence of glucose in the in-feed can cause a progressive increase in the concentration of maltose in the fermenter.
In some closed or partially closed continuous fermenters, flocculent yeast is used and it is this characteristic which retains the yeast. Maltose is a powerful defloccu-lating agent, and therefore where its assimilation is impaired its concentration may increase to a point where the resultant de-flocculated cells may be washed out of the fermenter. In both partially closed and open homogeneous continuous brewing fer-
menters the complex interactive effects of the varying spectrum of sugars may produce oscillations in the present gravity and yeast concentration (Portno, 1968a, b).
The lack of a fermentation gradient can have other undesirable effects. In a homogeneous continuous system, operating at a low dilution rate, the yeast has, in effect, to grow in the presence of beer. The resultant high concentrations of products and concomitant absence of wort substrates can affect yeast physiology, beer composition and produce instability in the fermenter. Thus, some yeast strains are pleomorphic under such conditions and may develop a pseudomycelial form (see Section 18.104.22.168). Such cells may not be able to flocculate and as in the case of the presence of maltose, wash-out may occur. Greater than usual degree of esterification of higher alcohols is possible where the concentrations of the latter are high, resulting in beer with abnormally high ester levels. In extreme cases, in the absence of wort sugars and under aerobic conditions, it is possible for the yeast physiology to become derepressed, again with consequent undesirable effects on beer composition. It is vital, therefore, to ensure that oxygen is provided at a point where sugar levels are still high.
Several continuous brewing fermentation systems have been described in the brewing literature although the majority were restricted to the scale of the laboratory or occasionally pilot plant. A description of some of these, which may be termed experimental continuous systems, is given here since they appeared mainly during the late 1950s and '60s, the period of burgeoning interest in continuous fermentation. They are worthy of inclusion since they provide a record of the development stages of continuous brewing and they represent working models of some of the different classes of system as outlined in the previous section. The results of trials using these various models provided data on which the design of some of the later commercial systems were based.
Comparatively few systems were developed to production scale. For reasons of historical completeness, details of these are provided although most are now defunct. Only breweries in New Zealand still use a continuous process, albeit over a period of some 30 years and very successfully.
22.214.171.124 Experimental continuous fermentation systems. Hough and Rudin (1958) described a simple system, shown in Fig. 5.24, which consisted of a series of linked stirred round-bottomed flasks. The first flask was connected to a wort reservoir via a reciprocating pump and the outflow from the final flask was directed towards a beer receiver. The contents of each flask were attemperated by immersion in constant temperature water baths. This efficacy of this system was tested using either one flask or linked series of two or three flasks. Thus, it could be used either as a simple open homogeneous system, or as a partially-heterogeneous open system.
Results were presented of the effects of varying process parameters such as temperature, flow rate and wort gravity using a top-fermenting ale yeast strain. It was claimed that after a single inoculation with yeast, steady state conditions were achieved in each flask and these could be maintained over a period of several weeks.
The resultant beer was similar in analysis and flavour to that derived from batch fermentations using similar wort and yeast. Two or three flask systems were found to be more efficient than the single flask arrangement. In a subsequent paper the effect of varying yeast strain was tested (Rudin & Hough, 1959). This study concluded that both top and bottom fermenting yeast strains could be used in the laboratory system to produce palatable beer. In some cases, where mixtures of strains were used, one strain became dominant over a period of time.
A simple partially-closed system was described by Harris and Merritt (1962). In this single vessel system, escape of yeast was restricted by placing the outlet tube within a secondary containing tube (Fig. 5.25). The contents of the flask were stirred magnetically, such that the bulk of the culture was homogeneous. However, in the region adjacent to the exit tube the lack of turbulence allowed yeast in this region to settle, and thereby be returned to the stirred culture. Provision was made for direct aeration of the culture.
A more sophisticated heterogeneous closed system was devised by Hough and Ricketts (1960). This apparatus (Fig. 5.26) may be viewed as the progenitor of the commercial tower continuous fermenter (Section 126.96.36.199). The fermenter consisted of two linked tubular sections, one vertical and the other inclined at an angle of approximately 25°. Wort is introduced at the top of the vertical tube and is mixed via a top-mounted motor and drive shaft arrangement. Fermenting wort and yeast move from the base of the vertical section into the adjacent inclined unstirred tube. The apparatus was designed specifically for use with a flocculent yeast strain. In the inclined tube section, the lack of mixing allows such yeast to settle out so that beer issuing from the exit line at the top of the inclined tube was shown essentially free of yeast.
Fig. 5.25 Partially-closed homogeneous continuous fermenter (re-drawn from Harris & Merritt, 1962).
Fig. 5.26 Heterogeneous closed continuous fermentation system (re-drawn from Hough & Ricketts, 1960).
Fig. 5.26 Heterogeneous closed continuous fermentation system (re-drawn from Hough & Ricketts, 1960).
The system was tested with a number of yeast strains of varying flocculence. The time taken for steady state conditions to become established was directly proportional to the degree of flocculence of the yeast strain used. Similarly, increased flocculence was associated with the establishment of progressively higher yeast concentrations within the fermenter, less yeast in the effluent and higher rates of fermenter productivity. Productivity using a flocculent strain was significantly greater than that which could be obtained using a simple open homogeneous continuous system. Although the beers arising from the system were claimed to be palatable and a good match for a batch-made product, no detailed analytical data was provided. It was reported that both the concentrations of ethanol and nitrogen were elevated in the continuously made beer, perhaps suggesting that an exact match for the batch product had not been achieved.
Design of a pilot scale continuous fermenter was also provided. In essence this was a scaled-up version of that shown in Fig. 5.25. More detailed description of a pilot scale continuous facility was provided in a later paper (Hough et al., 1962). It consisted of two stirred, attemperated cylindroconical vessels each fed with wort supplied from either of two stainless steel reservoir tanks at a rate controlled by two variable speed feed pumps (Fig. 5.27). Each wort feed pipe was fitted with a trap arrangement, which prevented back-growth from fermenter to wort reservoir. Each fermenter had a total working capacity of approximately 0.7 hi, but smaller volumes could be used by lowering the position of the exit main. If required the outflow from one fermenter could be coupled to the inflow of the second. Each fermenter was stirred by two electrically driven paddles. A third blade swept the liquid surface acting as a foam breaker.
If required, retention of yeast in the fermenters could be encouraged by the use of an adjustable sleeve, which fitted over the exit main and allowed the yeast to settle. The outflow from each fermenter was fed via a yeast separation vessel. This comprised a chilled stainless steel cylindroconical vessel fitted with an internal vertical partition. Beer and yeast was fed into the top of one compartment. Yeast settles in the
conical base of the vessel and the beer passes under the base of the partition and out via an exit pipe located in the upper wall of the adjacent compartment.
Several laboratory continuous fermentation systems have been described in a series of papers by Portno (1967, 1968a, b). These were used to investigate fully the kinetics of continuous wort fermentation and form the conclusion that heterogeneous systems, which generate a fermentation gradient, offered the greatest likelihood of success, as described in Section 5.6.1.
The gradient-tube continuous fermenter was a plug-flow reactor and is illustrated in Fig. 5.28. It consisted of a tube, 70 metres in length, immersed in a constant temperature water bath. The tube had a diameter of c. 0.5 cm, which was claimed to be sufficiently narrow to virtually eliminate back-mixing. Ideal plug flow behaviour was further assisted by the formation of carbon dioxide bubbles in the tube, which tended to partition the flow into a series of segments. Pasteurised wort, mixed with a desired proportion of yeast slurry, was fed into the tube and fermentation proceeded as the mixture passed through. Fermentation performance was gauged by analysis of process fluid removed from a number of sample points arranged at intervals throughout the length of the tube. Beer exiting from the tube passed into a device that separated yeast and beer into two streams. The yeast could be recovered from the base of the separating device and mixed with fresh wort and re-circulated back through the tube.
Performance of the gradient tube fermenter was governed, predictably, by temperature, flow rate and the proportion of yeast introduced with the in-flowing wort. Results, which were obtained from analysis of samples taken at points throughout the
length of the tube, confirmed that a gradient had been set up which mirrored the changes associated with a batch fermentation.
The centrifugal continuous fermenter was a partially closed type, which did not rely solely on flocculence for retention of yeast cells. In this case, yeast sedimentation could be controlled using an internal centrifugal rotor (Fig. 5.29). The fermenter consisted of a cylinder fitted with a variable speed stirrer and a bottom-located wort in-feed tube. The drive shaft of the stirrer was hollow and this provided a route for exit of beer. A centrifugal rotor was attached to the drive shaft, which contained a number of chambers, each contiguous with the hollow drive shaft.
Beer exiting upwards through the drive shaft is subjected to a downward centrifugal force of a magnitude which is a function of the stirring speed. This force separates yeast from the beer and returns it to the fermenting vessel. It was observed that for any given yeast strain there was an optimal rotation speed at which yeast retention was maximal. At sub-optimal rotation speeds it was concluded that the centrifugal force was insufficient to retain yeast. At supra-optimal rotation speeds the yeast concentration in the outflow increased, due, it was considered, to yeast deflocculation caused by turbulence in the fermenter.
At very high yeast retention rates, it was not possible to achieve a satisfactory steady state. Thus, a trial was described in which the rotor speed allowed 96.5% of the yeast to be retained. After 2 days' batch growth on an all-malt wort of specific gravity, 10°Plato, continuous flow was initiated. The yeast concentration during this period increased from 10 to 20 gl 1 dry weight. During the first 4 days of continuous operation, the yeast concentration remained constant, after this time it progressively decreased, falling to approximately 8gl 1 at day 11. The decrease in yeast concentration was accompanied by a progressive reduction in yeast viability, from close to 100% at the end of batch growth to less than 40% at day 11. As the yeast concentration decreased, there was a concomitant increase in levels of wort a-amino nitrogen. The specific gravity was constant and low throughout the first 4 days of continuous operation; after this time it increased to a value close to that of the inflowing wort.
This instability was ascribed to the decrease in yeast growth rate and fermentative ability brought about by the high retention rate. As the growth rate decreased, the extent of assimilation of wort sugars also fell until the concentration of maltose was sufficiently high to cause deflocculation of the yeast and consequent wash out. At lower retention rates, a steady state could be established which produced conditions closer to those seen at the end of a batch fermentation. No explanation was offered for the observed dramatic reduction in yeast viability. Equally, no details were provided with respect to addition of oxygen. It must be assumed that this was at a constant rate, that which was dissolved in the in-flowing wort. It follows that as the yeast concentration within the fermenter increased, oxygen must have become progressively limiting. Perhaps depletion of sterol, due to oxygen limitation, was the reason for the progressively increasing proportion of dead cells in the effluent stream.
Another system was devised by Portno (1968a) to study further the apparent instability of closed systems. This device, which was almost an immobilised fermenting system, is depicted in Fig. 5.30. It was designed to operate under fully closed conditions, or at any chosen retention index. It consisted of a glass tube through
which wort was circulated. Enclosed within the tube was a central resin-impregnated fibreglass tube, the end of which was attached to a glass bulb. The resin tube contained the yeast; however, pores, roughly 2 (im in diameter, allowed transfer of nutrients and metabolites between the inner and outer tubes. Yeast was circulated from the base of the inner tube and back into the upper bulb. The latter also served as a foam reservoir. In operation it was noted that a yeast deposit formed on the inner surface of the fibreglass tube. To prevent this, an automatic siphon back-flushing arrangement ensured that yeast was removed by the induced liquid washing action.
Using this apparatus, it was confirmed that it was not possible to achieve a stable steady state in a fully closed continuous system. Thus, with time, the yeast viability, as judged by methylene blue staining, gradually decreased. This was mirrored by a gradual increase in the specific gravity of the wort. Within these trends, both of these parameters also showed regular oscillations. Sugar analyses revealed that the oscillations in specific gravity were due principally to fluctuations in the concentration of maltose.
Since it appeared that open or closed homogeneous continuous systems were inherently unstable, Portno (1968b) devised a laboratory heterogeneous model fermenter to confirm that such an approach would be capable of producing satisfactory beer under stable steady state conditions. The apparatus, shown in Fig. 5.31, consisted of two linked stirred and attemperated flasks, the first of which was fed with a constant supply of pasteurised wort. The overflow from the second flask led to a chilled vessel in which the yeast was allowed to form a sediment. The overflow from the sedimentation vessel was attached to a further reservoir in which surplus yeast collected and from the top of which, beer was taken for analysis. The sedimentation vessel was also attached to another vessel in which yeast could be aerated and then recycled into the first fermenting vessel. The sedimentation, aeration and surplus
yeast vessels were each fitted with top-mounted spherical flasks to contain foam. Foaming was suppressed by applying heat to these flasks. Control of the dilution rate and the rate of yeast recycling provided a means of regulating the conditions within each fermentation flask.
It was demonstrated that steady states could be achieved, in each flask, over a wide range of dilution rates. The oscillatory behaviour seen in homogeneous systems was entirely lacking and high yeast viability was retained over several days. Furthermore, the resultant beers were similar in analysis to those obtained from batch fermentations using similar wort and yeast.
The benefits of a multi-vessel heterogeneous continuous fermentation system in terms of stability could be clearly demonstrated. However, such systems are complex and require considerable process pipework and valving. With an eye to future commercial exploitation, Portno (1969) also devised a single tank heterogeneous system, which retained the advantages of the multiple tank, without some of the complexities. This was termed a 'progressive continuous system' and is illustrated in Fig. 5.32. It consisted of a glass tube fitted at each end with stainless steel base plates to give a capacity of 1400 ml. Wort was fed in through the bottom and the beer outlet through a port in the top. The fermenting vessel was separated into five connected chambers by metal discs attached to a central rotating drive shaft. The gap between the discs and the vessel wall was 25 mm. The contents of each chamber were stirred by angled blades attached to the drive shaft. Silicone rubber membranes were attached to each rotating metal disc. These served as one-way valves so that flow was restricted to an upward movement from wort inlet to beer outlet. Good mixing within each chamber was further encouraged by wall mounted baffles, the angle of which could be adjusted by external magnets.
The apparatus was used in conjunction with the ancillary vessels used in the two-stage heterogeneous system shown in Fig. 5.31 and in a similar fashion yeast could be recycled. Using this apparatus it was demonstrated that steady states could be achieved at dilution rates varying between 10 and 100% of the fermenter volume per hour. Analysis of samples taken from each chamber revealed that, as predicted, the composition of the wort and the physiological condition of the yeast changed in the same ordered sequence as would be seen throughout the time course of a conventional batch fermentation. Beers obtained at all dilution rates were comparable with the batch product; however, it was observed that where a high yeast recycle rate was used, levels of esters were enhanced. This was attributed to the relatively high levels of fully attenuated beer introduced with the yeast into the first chamber. This mirrored the abnormally high ester levels that have been observed with homogeneous continuous systems.
The Wellhoener system (Wellhoener, 1954) was a pilot scale continuous fermentation and maturation system consisting of a series of six vessels. The first three vessels were maintained at 10 to 12°C and were used for fermentation. The process commenced by filling the first fermentation vessel with filtered wort and pitching with yeast slurry. When fermentation had started, continuous operation was established by initiating flow of aerated and filtered wort to the first vessel. A pressure differential, using carbon dioxide, was maintained between the three fermentation vessels. This was used to move wort between vessels and reportedly to control the rate of fermentation. Green beer exiting from the third vessel was filtered in-line and directed towards the first maturation vessel, all vessels being held in a cooled room at a temperature of 0°C. Maturation proper and adjustment of carbonation occurred in the second maturation vessel, the third vessel was used as a buffer tank.
The capacity of the fermentation vessels were 40, 30 and 20 hi respectively, and the conditioning vessels, 15 hi each. It was claimed that beer could be produced at a rate of 5 hi per day and the overall residence time was 27 days.
This lengthy residence time seems to have been the major drawback of the system, and, as with many others, it never progressed beyond the pilot scale. However, Wellhoener did introduce a successful discontinuous accelerated fermentation and maturation system as describd by Kleber (1987). The system was suitable for both top- and bottom-fermented beers and used pressure as a method of controlling yeast growth. The method claimed that increased carbon dioxide pressure inhibits yeast replication but has a smaller effect on fermentative metabolism and therefore, by inference, application of pressure can be used to increase fermentation efficiency. In the method described, bright aerated wort was introduced into a first vessel and pitched with yeast. After 6 hours, pre-fermentation pressure was allowed to build up to 0.3 bar by restricting the release of evolved carbon dioxide. At 50% wort attenuation, the pressure was allowed to increase to 0.7 bar, at 70% attenuation to 1.2 bar and 78% to 1.5 bar. After 2.75 days, the wort achieved 79% attenuation, at which point it was transferred to a maturation vessel. During fermentation, the pressure was partially released periodically to encourage mixing of the vessel contents and purge undesirable flavour volatiles and therefore further accelerate the process. During transfer to the maturation vessel the beer was cooled in-line. This efficacy of this system was confirmed by Kumada et al. (1975).
A complete experimental pilot scale continuous brewery was described by Hudson and Button (1968). This allowed continuous or batch production of hopped wort, which could be directed towards conventional batch or continuous fermenter. The latter was of a partially closed design as shown in Fig. 5.33. It consisted of an attemperated vessel of 10 litre capacity, the contents of which were mixed by an electrically driven stirrer. Aerated wort was fed into this vessel and this caused the fermenting liquid to rise through a vertical column to an upper vessel, the expansion chamber. Here the green beer exited through a shrouded outlet and the retained yeast was allowed to fall back into the main fermentation vessel. The upper surface of the expansion chamber was heated electrically to encourage foam collapse.
188.8.131.52 Commercial continuous fermentation systems. Some of the more applied and largely pilot scale continuous systems were introduced in around 1900, and thus pre-date the laboratory models already described. In some respects the traditional brewing process has always incorporated some continuous elements, for example, the semi-conservative nature of yeast collection and re-pitching. Furthermore, some long established procedures which were introduced with a view to accelerating batch fermentation, such as mixing partially fermented and fresh wort, may be viewed as semi-continuous processes. It is perhaps unsurprising, therefore, that the concept of a fully continuous fermentation process should have come to the mind of these late Victorian brewers even though at that time the practice was not supported by a proper theoretical understanding.
Early semi- and fully continuous systems were reviewed by Green (1962) and Kleber (1987). For completeness, a brief mention is made here. Although all of these systems were intended for commercial use, many did not progress beyond the pilot scale.
A two-tank semi-continuous process was patented by Schneible in 1902. This involved pitching wort in the first vessel and then encouraging vigorous yeast growth by continuous aeration and rousing. The fermenting wort was then transferred into the second vessel. When the wort was attenuated the green beer was removed leaving the bulk of the yeast behind. Fresh wort was then added to this yeast and the process repeated.
A more complex process was patented by Schalk in 1906, which involved six connected vessels. The first vessel was filled with aerated wort and pitched at double the normal rate. After 24-48 hours, when the fermentation was actively proceeding, half the contents of the first vessel were transferred to the second vessel and then both were topped up with fresh wort. As soon as the contents of each vessel were again actively fermenting, half the volume of second tank was transferred to the third vessel and again both of these were topped up with fresh wort. This procedure was continued until all six vessels were filled with fermenting wort. By the time the final vessel was filled, attenuation of the wort in the first was complete. After the green beer was removed from the first vessel for further processing, the procedure was continued by transferring half the contents of the sixth vessel into the first. The sequence could be repeated ad infinitum, provided infection was kept at bay. The advantage of the Schalk process was, therefore, that the requirement for pitching yeast and its handling were much simplified. Of course, the consequences of microbial contamination would be severe in that there would be the potential for losing six batches of beer, as opposed to one in the conventional process. According to Kleber (1987) the process was not exploited commercially.
A contemporary of the Schalk process was that of van Rijn (1906). This was also a multi-tank approach but was truly continuous in that constant wort feed was required. The tanks were arranged in the form of a cascade, thereby allowing gravity feeding from overflow pipes fitted at the top of each vessel. The process was controlled by the rate at which wort and yeast were fed into the base of the uppermost tank. It was claimed that control of the rate of addition of wort allowed a constant yeast population to be maintained in each vessel. Therefore, this is an early example of an open single-phase heterogeneous continuous system.
A similar approach using a cascade of four linked fermenting vessels was patented by Williams and Ramsden (1963). This consisted of four linked jacketed tanks, each with a bottom located in-feed and top located overflow so arranged as to allow gravity flow (Fig. 5.34). Wort and yeast were added to the first vessel and a batch fermentation was allowed to proceed. At a suitable point, continuous fermentation was then initiated by addition of wort to the base of the first vessel. No further yeast was added to the system. The contents of the first and second vessels could be gas roused and provision was made for collection of carbon dioxide from the second. The arrangement of vessels as shown in Fig. 5.34 was specifically designed for use with top-fermenting yeast. In this case, the third tank, in which primary fermentation was essentially completed, was fitted with a top-mounted baffle, which contained the yeast head and prevented it from being carried over into the last vessel. The crop was pumped from the third vessel to a plate and frame filter from which yeast was recovered and the barm ale returned to the in-feed of the fourth vessel. The system could also be used with bottom-fermenting yeast; in this case the third tank was of
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