Commercial Acv Fermenting Vessels Designs

Beer outlet

Fig. 5.34 Cascade continuous fermentation system.

cylindroconical design, and the top-cropping deck was omitted. The fourth vessel was essentially a conditioning tank in which the green beer was cooled and provision was made for adjustment of levels of carbonation.

In 1956, Mr Morton Coutts, Technical Director of the Dominion Breweries Company of New Zealand, patented a method for wort stabilisation and continuous brewing process, the contents of which are described in Coutts (1966). This was the forerunner of the only successful continuous brewing systems now remaining in use. The Dominion Group of New Zealand operates continuous brewing systems at several of its breweries, with two new ones commissioned in 1993 (Stratton et al., 1994). The Coutts process is another multi-vessel approach. A diagrammatic representation of a modern incarnation is shown in Fig. 5.35(a).

High-gravity wort is cooled and stored in a receiving vessel where trub is separated out by gravity sedimentation. During transfer from the receiving vessel to the continuous fermenter, the wort is diluted with sterile water to a desired specific gravity. In the first tank of the continuous fermenter, the hold-up vessel, the wort is diluted 1:1 with process fluid recycled from the second vessel. In addition, a stream of yeast is also added which is recycled from the end of the process. The yeast strain is described as being highly flocculent and having a relatively high oxygen requirement. The holdup tank volume is 6% of the total and its purpose is to improve the microbiological robustness of the system. Thus, blending back of partially fermented wort reduces the pH to less than 4.2, adds ethanol at a concentration of approximately 2% v/v and introduces yeast in the exponential phase of growth. All of these measures mitigate against adventitious contamination. The contents of the hold-up vessel are aerated vigorously. Provision of appropriate quantities of oxygen at this stage has been shown to have a critical influence on the extent of subsequent yeast growth and achieving a desirable balance of beer esters.

The second and third tanks are fermentation vessels occupying respectively 63% and 31 % of the total fermenter volume. Both are stirred and passage from one to the other is via gravity feed. Beer then passes into a conical separator where the flocculent

Gravity Feed Production Brewery
Water

I C02 recovery

A: Hold up vessel

I C02 recovery

A: Hold up vessel

Fig. 5.35 Coutts system of continuous fermentation, (a) Diagrammatic, (b) Schematic of Auckland Brewery continuous plant (from Dunbar et al.. 1988).

yeast is allowed to settle. The excess yeast stream is directed towards a washer where the beer is removed via a countercurrent of deaerated water. The resultant beer and water mixture is used for original-gravity adjustment of the green beer as it is transferred to conventional maturation vessels. Use of the recovered green beer in this way, it is claimed, reduces losses to a minimum.

Several refinements have been incorporated in the most modern installations (Stratton et al., 1994). Notably, advances made with regard to hygienic design of conventional fermenters have been applied to the continuous fermenter. Provision has been made for an integrated yeast propagation facility and collection of carbon dioxide. Control of the fermenter has been improved by automatic monitoring of suspended yeast solids (solids concentration meter) and specific gravity (density meter).

The improvements in hygiene have allowed the system to be designed to operate for periods of up to one year's continuous use, thereby maximising the advantage of short down time compared to batch fermenters. It has been demonstrated that the flow rate can be varied to suit demand to give residence times between 36 and 97 hours using a wort of specific gravity of 13.2°Plato, whilst still producing beer of acceptable quality. A detailed study of the kinetics of the system has revealed that a metabolic gradient exists between the hold-up vessel and the yeast separator. This equates to the changes seen during a batch fermentation, although wort recycling causes some blurring of these events.

Most of the sugar uptake occurs in the second vessel; however, glucose and fructose are assimilated in the hold-up tank, and therefore maltose utilisation is not repressed during active fermentation. Amino acid assimilation was also shown to be ordered but residual amino nitrogen in the beer was higher than a comparable batch fermented beer, although the extent of yeast growth was similar. VDK (a-acetolactate) concentrations in the outflowing beer increased with increase in dilution rate. However, within the range of dilution rates used, the subsequent maturation period was sufficiently long to allow reduction of vicinal diketones to acceptable levels.

In a later report (Dunbar et al., 1990) the possibility of continuous warm maturation for the removal of vicinal diketones was tested. Outflow of green beer from the yeast separator was directed towards an unstirred maturation vessel maintained at 15°C. Beer issuing from the maturation vessel was chilled in-line prior to conventional cold conditioning treatment. Residence time in the maturation vessel was varied by altering the flow rate through the continuous fermenter. Using this system, it was shown that there was a critical residence time, 32 hours with the plant under test, above which the diacetyl precursor, a-acetolactate was removed at an adequate rate to forego the need for further warm maturation. With shorter residence times, a-acetolactate concentrations remained high. It was concluded that at fast flow rates the yeast was still generating a-acetolactate in the maturation vessel, potentially at a greater velocity than the rate of decomposition to diacetyl.

The Coutts approach is also used at New Zealand Breweries Limited (Davies, 1988). The first plant was installed in 1958 at the Palmerston North Brewery and this became the first in the world to be totally dependent on continuous fermentation. Similar installations were made at four other breweries, the most modern at the Christchurch brewery in 1976. This plant is similar to that shown in Fig. 5.35(b) and uses two rectangular stainless steel fermenting vessels with capacities of 1500 and 1100 hi. The throughput is 72 hi per hour.

In 1982, there was a requirement at the New Zealand Breweries' Christchurch plant to increase capacity up to 2 million hi per annum. Coincident with this, the method of payment of excise liability was changed from a levy on wort extract to an end-product duty. After a consideration of relative costs, it was concluded that the cheapest option was to generate the increased capacity by installing conventional cyclindroconical fermenters. Complete costing of the project indicated that obtaining similar volumes through the continuous approach would be 20^12% more expensive. This, coupled with a requirement to produce a much wider product portfolio would seem to have heralded the end of further investment in continuous fermentation by this company.

Another system contemporary with the Coutts process was the Canadian Labatt ABM fermentation process described by Geiger (1961). This patented approach (Geiger & Compton, 1961), used to produce lager beer, was a heterogeneous open system with yeast recycling (Fig. 5.36). The key characteristic of the system was the physical separation of yeast propagation and fermentation. Wort was collected twice each week and held in hygienic storage tanks for up to four days at 8°C. From the storage tanks, the wort was pumped into the first vessel in which growth of yeast was encouraged by the maintenance of highly aerobic conditions. Yeast plus wort was then pumped into the anaerobic fermentation vessel. The second tank was described as being a fluidised bed reactor in which very high yeast concentrations were maintained, thereby allowing rapid rates of fermentation to proceed. For some trials, a second fermentation vessel was used. The out-flowing beer passed through a yeast separator and thence was cooled in-line prior to transfer to a conditioning tank. Yeast from the separating vessels could be either recycled into the fermentation vessel(s) or directed to waste.

Aerobic propagation vessel Anaerobic fermentation vessels

Chilled green beer

Aerobic propagation vessel Anaerobic fermentation vessels

Chilled green beer

Yeast recycle

Fig. 5.36 Labatt ABM continuous fermentation system.

Yeast recycle

Fig. 5.36 Labatt ABM continuous fermentation system.

In the pilot scale plant system described, the two main vessels each had capacities of 120 hi and the yeast separation vessel was of approximately 20 hi. When operated at 15°C, the overall residence time was 30-40 hours and beer was produced at the rate of 6hlh 1. It was claimed that yeast growth was some 25% less than that seen in a comparable batch fermentation. Beers were considered a close organoleptic match for those produced by conventional means. Analytically, matching was also close although esters and phenylethanol were significantly elevated in the continuous product. However, in some trials, a single homogeneous tank system was used and in this case the beers were considered unacceptable on the basis of both taste and analysis. The system was apparently quite resistant to contamination and on one occasion was run for four months without breakdown. It was used for commercial brewing albeit only briefly.

In the same period as the Labatt process, and also from Canada, O'Malley (1961) presented plans at both pilot and production scale for a system of continuous wort production, fermentation and maturation. Unfortunately this very innovative system was another casualty of the 1970s volte-face over continuous fermentation and it was never implemented at commercial scale.

Hopped wort was produced in a moving belt continuous combined mash lauter tun and a cylindrical multi-stage horizontal copper. The fermenter consisted of four linked rectangular tanks, the first with a sloping bottom and the other three with bottoms of triangular cross-section (Figs 5.37(a) and (b)). The combined tanks were covered by a dome, which contained manways, sight glasses and carbon dioxide outlets above the second and third compartments. Temperature control was via wall jackets and internal baffles in the fourth tank.

The fermenter was an example of an open heterogeneous single-phase type. Thus, as in a plug flow reactor, the stages of batch fermentation were intended to take place in the individual vessel compartments. To better mirror the batch process no provision was made for mechanical agitation. Pitched wort was fed into the first vessel where it remained for 8 hours, equating to the batch lag phase, and was then pumped with a minimum of turbulent flow into the second chamber. Here active fermentation commenced. After a further 18 hours the wort passed into the third chamber where an increase in temperature promoted even more vigorous fermentation. When used with top-fermenting yeast, collection of the crop was accomplished by laterally mounted chutes attached to the second and third tanks. Wort attenuation was nearly complete after 24 hours' residence in the third vessel, and, after this time, the beer proceeded into the fourth tank where it was cooled. Sedimented yeast and other solids could be removed via drain cocks at the base of vessels two, three and four. The same route could be used to collect bottom-fermenting lager yeast crops. The purpose of the baffle at the entrance to the final tank was to direct the flow of nearly bright beer upwards once the lower route was blocked with sediment. Total residence time in the fermenter was 3.5 days at a flow rate of 6 hi per hour.

Chilled beer exiting from the fermenter entered a series of linked maturation vessels. These wall-cooled tanks were of similar construction to the fermenting tanks, having angled bases for collection of precipitated material. Flow through the maturation vessels was slow with very little turbulence and a decreasing temperature gradient was maintained between the entrance and exit. Total residence time in the maturation vessels was three weeks at a flow rate of approximately 6 hi per hour, equivalent to a fluid flow rate of 17 cm per hour.

Williamson and Brady (1965) described another continuous system developed by the Carling Brewing Company (USA) and Canadian Breweries Ltd. A pilot scale facility was constructed at the Cleveland plant and a production scale system at Fort Worth. This was also a progressive multi-vessel system similar to the Coutts process.

The production scale facility (Fig. 5.38) consisted of an initial vertical rectangular pitching tank in which aerated wort was introduced after mixing with recycled yeast. This vessel was fitted with internal baffles designed to provide a long path length to ensure a sufficiently long residence time to complete the lag phase of fermentation.

Vessel 1

Vessel 2

Vessel 3

Vessel 4

Vessel 1

Vessel 2

Vessel 3

Vessel 4

(a)
Fig. 5.37 Continuous fermenter, (a) Side view, (b) Front view. (After O'Malley, 1969.)

There were two fermentation vessels, the first roughly double the capacity of the second. Both vessels were agitated and attemperated at the maximum temperature used for the batch process. High fermentation rates were achieved by maintaining high yeast concentrations. Beer exiting from the second fermenter entered a vertical conical-bottomed yeast separating vessel in which the specially chosen flocculent strain was allowed to settle. The relatively yeast-free beer was taken from the top of the separating vessel and chilled and carbonated in-line before transfer to a maturation tank. Yeast and beer taken from the bottom of the separation tank was pumped to a yeast concentrator. In this small conical tank yeast was separated from the beer by a pressure treatment and sent to waste. A mixture of beer and yeast was

Yeast recycle

Yeast recycle

In-line chiller

Fig. 5.38 Forth Worth continuous fermenter: A, pitching vessel; B, first fermenting vessel; C, second fermenting vessel; D, yeast separating vessel; E, yeast concentrator, F, spent yeast vessel; G, maturation vessel (Williamson & Brady, 1965).

In-line chiller

Fig. 5.38 Forth Worth continuous fermenter: A, pitching vessel; B, first fermenting vessel; C, second fermenting vessel; D, yeast separating vessel; E, yeast concentrator, F, spent yeast vessel; G, maturation vessel (Williamson & Brady, 1965).

taken from the top of the separator and recycled back to the pitching vessel and first fermenter.

This fermenting system was used in conjunction with plant for continuous wort production for commercial brewing. The beer was reported to be indistinguishable from the product produced by conventional batch fermentation (Williamson & Brady, 1965). Apparently it was susceptible to bacterial infection; however, in spite of this, relatively long periods of uninterrupted use were achieved. Despite this success the plant was decommissioned and the breweries returned to batch fermentation.

Bishop (1970) was responsible for introducing another multi-vessel continuous system into four UK breweries of Watney during the late 1960s. Wort was produced by a conventional batch process and stored in chilled tanks for up to 14 days. For a commercial process it was recommended that three wort tanks were available since with a conventional brewhouse this provided a continuous supply of wort whilst allowing necessary down-time for tank cleaning and re-filling.

The continuous fermenter comprised an in-line wort steriliser, a unique oxygenation column, two fermentation vessels and a yeast separation tank (Fig. 5.39). The process was initiated by filling the first fermenter with wort and inoculating with a pure yeast culture. Wort was pumped into the system at an accelerating rate to match increased yeast growth. After some 2-3 days a steady state was established and a wort flow rate was set which was considered appropriate for the temperature regime used and provided beer of suitable quality. Supply of oxygen at an optimum concentration was considered to be critical to the success of the process. If too low, then fermentation rate would be diminished because of inadequate yeast growth. If too high, it was observed that yeast cells became elongated, although any effect of this pleo-morphism on beer quality was not reported. Accurate dosing of oxygen into the in-

Stirred attemperated fermenting vessels

Stirred attemperated fermenting vessels

Fermentation Vessel Brewery Diagram

flowing wort was achieved using the column oxygenator. This consisted of an inverted U-tube. Wort and oxygen were introduced into the first ascending arm. When the wort passed into the second downward arm any undissolved oxygen bubbles would tend to rise and meet more incoming wort, therefore providing adequate time for complete solution.

Within the first fermenting vessel, at a desired steady state, it was observed that the wort was approximately 50% attenuated and the yeast achieved a concentration of 51b per barrel (c. 2-2.5 gl 1 dry weight). In the second vessel the final gravity was achieved with a yeast concentration of 61b per barrel (2.5-3.0gl 1 dry weight). The yeast settling tank was cylindrical with a conical bottom and contained a centrally mounted helical spiral, cooled by circulating brine. Chilling the beer stopped fermentation and caused flocculated yeast to settle into the cone from whence it was recovered at intervals, pressed and retained. Recovered beer was returned to the outflowing product stream. The yeast was used as a source of pure culture but unlike the other continuous systems already described there was no recycling. Green beer flowing from the top of the separator contained less than 0.5 pounds per barrel yeast (0.1-0.15 gl 1 dry wt). Carbon dioxide gas was recovered from the top of the separating vessel after passing through a mechanical fob breaker.

This system was shown suitable for all beer qualities, ales and lagers; indeed a strong beer (20-22.5°Plato original gravity) was produced by continuous fermentation in two days compared to nine months by a traditional procedure! All beers produced with the system were considered the equal of their batch-produced counterparts. When installed at all four breweries the maximum output was 1.6 million hi per annum. Each fermenter was rated at approximately 6500 hi per week. On one occasion, a fermenter was operated continuously for 13 months, which would seem to belie the oft-made criticism of continuous systems that they are prone to infection. As with the majority of the other systems described already, and despite the apparent success, the plant is no longer used and the breweries have reverted to using batch fermentations.

The 'tower continuous fermenter', manufactured by the APV Company of Crawley (UK) differed from those commercial systems described previously in that a fermentation gradient was generated within a single tank. Most of the yeast was retained within the vessel, thereby providing conditions for the maintenance of rapid fermentation rates but retaining a certain simplicity of design. The system was used for the commercial production of both ales and lagers, in the former case notably by Bass Brewers Limited in the United Kingdom (Seddon, 1975).

Several variations were developed which used slightly different ancillary equipment, as described in the brewing literature (Klopper et al., 1965; Ault et al., 1969). The system described here was that used by Seddon (1975). Wort was collected hot from a conventional brewhouse into storage tanks to ensure sterility. During transfer from the copper it was centrifuged, in-line, to reduce the loading of solids. To further reduce the possibility of contamination the wort was given a further in-line heat treatment following transfer from storage tank. After cooling, the wort was sparged, also in-line, with sterile air and carbon dioxide at a point immediately before entry into the base of the fermenter. The purpose of the carbon dioxide was to improve agitation at the bottom of the tower and prevent yeast compaction.

The tower consisted of a wall attemperated stainless steel vertical cylinder with a larger diameter head structure (Figs 5.40 and 5.41). A specially selected highly floc-culent, low ester and diacetyl producing and slow growing strain was used in the fermenter. Such yeast tended to settle and form a plug in the base. Yeast concentrations, at the base of vessel, were in excess of 350 g wet weight per litre. Upward flowing wort passed through the yeast plug and was therefore fermented at a very rapid rate. Concentration and retention of the bulk of the yeast at the base of the tower in transient contact with wort allowed the formation of a gradient throughout the vertical axis of the vessel. The tendency for back mixing was reduced naturally by the density gradient due to progressive fermentation and it was further discouraged by the provision of a number of horizontally mounted perforated metal plates. This fermenter, therefore, may be classified as a heterogeneous, closed, two-phase type.

Yeast carried to the top of the fermenter completed the attenuation of the accompanying wort. In the top region of the tower an arrangement of baffles separated evolved carbon dioxide and allowed some of the residual yeast to be diverted into a reservoir. From here, the yeast could be recycled back into the base with the inflowing wort. Different types of separator could be used depending on whether the yeast was an ale or lager type (Klopper et al., 1965). Claimed advantages for the tower fermenter were very rapid rates of attenuation due to the high yeast concentration. Seddon (1975) described use of a battery of four towers each of nearly 1 m diameter and c. 8.5 m tall. Together these were capable of producing ale at the rate of 5000 hi per week. This represented rates of wort attenuation of the order of 3-4 hours. In addition to rapidity, the low rates of yeast growth led to improvements in the efficiency of ethanol yield. Use of several fermenters provided some degree of flexibility in terms of being able to vary production rates. It was also shown that if necessary it was possible to chill the contents of the tower and stop the flow of wort for periods of

Cooling surfaces

Perforated metal baffle

Yeast plug

Cooling surfaces

Perforated metal baffle

Yeast plug

Flow controller

Yeast re-cycle

Beer outflow Yeast sediment

Flow controller

Yeast re-cycle

Oxygen

Wort flow controller

Fig. 5.40 Tower continuous fermenter.

up to 14 days with little loss of yeast viability. Subsequent re-initiation of flow produced a small volume of non-standard beer, which could be blended away.

In practice, these advantages were out-weighed by many failings. The general lack of flexibility of continuous systems compared to batch cultures also applied to the towers. More specifically the towers required constant monitoring to ensure that the flow rate was correctly adjusted to provide beer of consistent quality. Great care had to be exercised to avoid the yeast bed lifting en masse, thereby causing total washout of the culture. The stringent requirements regarding the properties of the yeast strain were too restrictive and limited the number of beers that could be produced.

For these reasons the love affair with tower fermenters proved to be ephemeral, and, by the 1970s, their application in brewing ended. They have continued to find occasional use in fermentations to produce fuel ethanol. Here, this approach may be viewed as a simple method of achieving cell immobilisation without the need for an additional support medium (Christensen et al., 1990; Chen et al., 1994).

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