The brewing industry in the United Kingdom has had a relatively brief 'courtship' with continuous cul ture. Two types of continuous brewing have been used
(i) The cascade or multistage system.
(ii) The tower system.
Hough et al. (1976) described the cascade system utilized at Watneys' Mortlake brewery in London. The process utilized three vessels, the first two for fermentation and the third for separation of the yeast biomass. The specific gravity of the wort was reduced from 1040 to 1019 in the first vessel and from 1019 to 1011 in the second vessel. The residence time for the system was 15 to 20 hours, using worts in the specific gravity range of 1035 to 1040, and it could be run continuously for 3 months. However, it is believed that the system was abandoned due to problems of excessive biomass production. This process appears to have been used widely in New Zealand with greater success (Kirsop, 1982), but apparently newer installations are of the batch type.
A typical tower fermenter for brewing is illustrated in Fig. 2.12. The system is partially closed in that
Table 2.3. Comparison of the performance of different operational modes of an animal cell fermentation (After Griffiths, 1992)
Operational Cell No. Product yield Length of run mode (x 10~6 cm"3) (mgday-») (mg month-») (days)
Batch 3 100 200 7
Semi-continuous 3 200 600 21
Fed-batch 6 200 500 14
Perfusion 30 + 3000 12000 > 30
Continuous 2 300 1200 > 100 culture
relatively little yeast leaves the fermenter due to the highly flocculent nature of the strains employed. Thus, the system is a type of internal feed-back. Wort is introduced into the base of the tower and passes through a porous plug of yeast. As the wort rises through the vessel it is progressively fermented and leaves the fermenter via a yeast-separation zone, which is twice the diameter of the rest of the tower. Hough et al. (1976) described the protocol employed in the establishment of a yeast plug in the tower and the subsequent operating conditions. Prior to the fermentation, the tower is thoroughly cleaned and steam sterilized, aseptic operation being more important for the continuous process than the batch one. The vessel is filled partially with sterile wort and inoculated with a laboratory culture. The initial stages are designed to encourage high biomass production by the periodical addition of wort over about a 9-day period. A porous plug of yeast develops at the base of the tower. The flow rate of the wort is then gradually increased over a further 9 to 12 days, by which time an approximate steady state may be achieved. Following the establishment of a high biomass in the fermenter the system is operated such that the wort is converted to beer with the formation of approximately the same amount of yeast as would be produced in the batch process. The beer produced during the 3-week start-up period is usually below specification and would have to be blended with high-quality beer. Thus, more than 3 months' continuous operation is necessary to compensate for the initial losses of the process.
The major advantage of the continuous tower process was that the wort residence time could be reduced from about 1 week to 4 to 8 hours as compared with the batch system. However, the development of the cylindro-conical vessel (initially described by Nathan in the 1930s, but not introduced until the 1970s, see also Chapter 7) led to the shortening of the batch fermentation time to approximately 48 hours. Although this is still considerably longer than the residence time in a tower fermenter, it should be remembered that beer conditioning and packaging takes considerably longer than the fermentation stage, so that the difference in the overall processing time between the tower and the cylindro-conical batch process is not sufficient to justify the disadvantages of the tower. The major disadvantages of the tower system are the long start-up time, the technical complexity of the plant, the requirement for more highly skilled personnel than for a batch plant, the inflexibility of the system in that a long time delay would ensue between changing from one beer fermentation to another and, finally, the difficulty in matching the flavour of the continuously produced product with that of the traditional batch product. Thus, the continuous-tower system has fallen from use, with virtual universal adoption of the cylindro-conical batch process.
Was this article helpful?