Continuous fermentation

The use of very large combined fermentation and conditioning vessels in a uni-tank process is in essence a strategy which seeks to increase productivity by a combination of shortening process times and reducing capital costs. Thus, the uni-tank process is more rapid than the traditional separate fermentation and conditioning tank approach and with very large vessels there is an opportunity to minimise the ratio of surface area to volume. An alternative approach is continuous fermentation in which a single vessel (or group of linked vessels) produces a continuous beer stream. In this case, high productivity is possible because the downtime and costs associated with vessel filling, emptying and cleaning of the batch process are much reduced. Con tinuous fermentation should be very efficient since beer losses are low, conditions are arranged such that rates of fermentation are rapid but yeast growth is low and volume output is high relative to the capital costs of the plant. When fully operational with a stable flow rate, the beer should be of consistent quality. The degree of consistency should be better than that achievable with batch fermentations since the variability due to inconsistency in the physiological condition of individual batches of pitching yeast is eliminated.

Continuous beer fermentation is not a new concept. Kleber (1987) describes a system, that of Van Rijn, which was patented in 1906. Similarly, Ricketts (1971) referred to continuous beer fermentation systems which date from the end of the nineteenth century. However, the period of most intense interest in this approach was during the 1960s, coincident with the introduction of large uni-tanks. At this time several systems were developed, some to the point of commercial exploitation. Subsequent experience showed that few of the anticipated benefits of continuous fermentation were actually realised and most breweries, with some notable exceptions, have continued to use the traditional batch approach albeit using large-capacity vessels.

The reasons for the abandonment of continuous fermentation were given by Portno (1978) and with some caveats, they remain valid today. The major strength of the batch system, using several vessels, is that it is flexible. It is able to cope with both seasonal, or shorter-term fluctuations in demand and can easily be adapted to vary the spectrum of production of several beer qualities. Conversely, the benefits of continuous fermentation are only realised fully when the systems are operated for long periods with minimum downtime for changes in beer quality. In this respect, they are suited only to breweries producing a single or very limited portfolio of beer qualities. The inflexibility extends to rates of beer production since this parameter can only be varied within small limits with most continuous systems.

Most breweries have been designed to work within the confines of the batch system. Brewhouses are sized to provide sufficient wort to fill the largest fermenters within a convenient period of time. With a continuous system the rate at which wort is produced must be sufficient to supply the needs of the fermenter at all times. Inevitably, this requires some type of pre-fermenter wort collection vessel. Downstream of the fermenter the brewery must be capable of handling a continuous supply of green beer. The consequences of failure in a continuous system pose a serious threat to production. Thus, emptying, cleaning, start-up times and establishment of stable running conditions are lengthy procedures, possibly taking as long as two weeks. The prolonged nature of continuous fermentation has inherent risks. Extended running times increase the opportunities for microbial contamination, and, perhaps more seriously, yeast 'variants' (Section 4.3.2.6) may be selected for with the concomitant risk of undesirable changes in beer quality. Continuous systems are more sophisticated than many brewery batch fermenters, and therefore skilled personnel must be on-site, night and day, to provide technical support. The advantages and disadvantages of continuous fermentation compared to the batch process are summarised in Table 5.6.

Notwithstanding the disadvantages outlined previously, there has in recent years been a resurgence of interest in continuous fermentation. Where fermentation capacity is simply to be extended within an existing brewery, already using batch

Table 5.6

BREWING YEAST AND FERMENTATION Advantages and disadvantages of continuous beer fermentation compared to a batch process.

Advantages

Disadvantages

• Rapid rates of conversion of wort to beer.

• Lack of flexibility with respect to number of beer qualities.

• Limited ability to vary output rate.

• High efficiency fermentations

- Low yeast growth

- High ethanol yield.

• Efficient vessel utilisation.

• Complex costly vessel(s) and ancillary plant.

• Requirement for continuous skilled technical

• Only one or few fermenting vessels needed.

• Reduced beer losses.

• Consistent beer.

support.

• Serious consequences of break-down or

• Reduced usage of detergents/sterilants.

microbial contamination.

• Possibility of selection of yeast mutants.

• Wort production and beer processing capability

• Reduced need for pitching yeast storage.

must match needs of fermenter. • May not be suitable for all beer qualities.

vessels, there is still little chance that a continuous fermenter would be chosen. However, the approach may now be given serious consideration if a new brewery is to be constructed. The renaissance of interest is a result of advances in hygienic design and facilities for control. These allow a continuous system to be used with a fair degree of confidence that downtime due to plant failure or contamination will be virtually non-existent. More importantly, between the 1960s and the present day, there have been technical advances that have broadened the spectrum of fermentation-associated processes to which continuous systems may be applied. These advances allow improvements to process efficiency that are not possible to achieve with a batch system.

The crucial step forward in continuous technology has been the development of commercial immobilised yeast reactors. This approach has generated sufficient interest to form the subject of an entire European Brewing Convention Symposium -'Immobilised Yeast Applications in the Brewing Industry' held in Finland in 1985. This technology is discussed in detail in Section 5.7; however, the principal advantages of immobilised reactors are that the very high yeast concentrations which are achievable allow very rapid process throughputs. This is of particular benefit when applied to rapid beer maturation. Thus, a single immobilised yeast reactor can eliminate the often very time-consuming warm conditioning step associated with lager fermentation. In many breweries, this can result in reduction in fermentation process times of a few to several days, with consequent improvements in the overall productivity of large capacity vessels. The application of immobilised yeast to beer maturation need not be restricted to new breweries; it may be retro-fitted within any site as an added process at the end of conventional batch primary fermentation.

Immobilised yeast reactors have also found use in new fermentation processes, for example, in the production of low- or zero-alcohol beers. Thus, immobilised reactors are particularly suited to the approach where primary fermentation is restricted to a brief contact between wort and yeast, which serves primarily to remove wort flavour character with the formation of little or no ethanol.

5.6.1 Theoretical aspects

Microbial cultures may be characterised as being of either 'closed' or 'open' type. In the former, as typified by a batch culture, some component that has a positive or negative influence on growth, is contained within the system. An example of such a critical component would be an essential nutrient, or a growth-inhibiting metabolite. In consequence, the conditions within a closed system are in a continual state of transition. The growth rate will always tend towards zero as the essential nutrient becomes exhausted or the concentration of a metabolite increases to a toxic level. In contrast, continuous fermentations are open systems in which by definition components may freely enter or leave. In consequence, it is possible, by the continuous replenishment of nutrients, or removal of potentially toxic metabolites, to produce conditions where the microbial growth rate and by inference the rate of production of metabolites, is constant.

Pirt (1975) describes two basic types of continuous culture, the plug flow system and the chemostat. In a plug flow system (Fig. 5.23(a)) the reactor takes the form of a coil or similar elongated form. Inoculum and growth medium are mixed at the point of entry and fed simultaneously and continuously into the reactor. Within the reactor, the conditions are arranged such that there is a minimum of backward and forward mixing, and thus batch growth proceeds within each discrete 'plug' as it travels through the reactor. The reactor may therefore be viewed as a continuum of batch cultures in which spatial location is related to culture age. The factors that regulate growth rate in a conventional batch culture such as temperature, inoculation rate and substrate concentration, are also influential in a plug flow continuous culture. In addition, the composition of the culture issuing from the reactor will also be a function of the flow rate. By careful regulation of all of these parameters, it is possible to establish a steady state where the product is of a constant and desired composition.

A further refinement that may be introduced to a plug flow reactor is biomass recycling. In this case, a means is provided for separating and concentrating some of the microbial cells from the product stream issuing from the reactor. This biomass is returned to the entry point of the reactor where it is used as inoculum. Used in this way the reactor requires only to be supplied with fresh medium.

A true plug flow continuous reactor is a theoretical entity only as there will always be some degree of backward and forward mixing. In order to turn the concept into practical reality some compromise is inevitable. This can be achieved by using multiple tanks, which provide physical separation of individual stages of the batch culture. The extent to which this arrangement approaches the ideal is a function of the number and size of each individual tank.

The second open continuous culture system is the chemostat (Fig. 5.23(b)). This consists of a stirred reactor to which medium is introduced by an entry pipe. The rate of medium addition is controlled by a variable speed pump. Culture is removed from the reactor via a second pipe, which is arranged in the form of a siphon or weir such

Product stream

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