Accelerated batch fermentation

Several approaches have been suggested for increasing vessel productivity by manipulation of the physical and biochemical parameters that regulate the progress of fermentation. These may be loosely grouped, as others have done, under the heading of accelerated batch fermentation. The simplest methods require no special modification to vessels but merely changes to the way in which they are managed. In this respect, they are not distinct fermentation systems; however, mention is made here for the sake of completeness. A detailed discussion of the effects on performance of fundamental control parameters is given in Chapter 6. These simple methods are attractive economically since they offer the promise of increase in productivity from existing plant.

Batch fermentations, depending on the type, are made up of some combination of the stages given below:

(1)

vessel filling;

(2)

primary fermentation (lag phase, exponential phase and phase of decline);

(3)

warm conditioning;

(4)

warm yeast cropping;

(5)

chilling;

(6)

cold bottom yeast cropping;

(7)

vessel emptying;

(8)

CiP;

(9)

conditioning tank filling;

(10)

cold conditioning;

(11)

vessel emptying;

(12)

CiP.

Clearly, some of these operations may be simultaneous, or overlapping, for example, primary fermentation and yeast skimming from a top-cropping ale fermentation. However, since they all contribute to the total vessel residence time it is useful to consider, albeit briefly, how the time taken to accomplish each phase may be shortened.

Vessel filling, chilling and emptying are largely pre-determined by the design of the fermenter and associated plant and little improvement is possible to existing installations. Manual yeast cropping procedures may be replaced by automated methods that may be more rapid and efficient, as described in Chapter 6, although again the actual time savings are likely to be modest. Where a period of cold conditioning is required, the time-savings which accrue from the use of a uni-tank approach have been discussed previously (Section 5.4.3). By default, therefore, the most profitable approach to reducing vessel residence time is to shorten the time taken for primary fermentation (and warm conditioning, if required).

The parameters that influence the duration of primary fermentation are wort composition, temperature, initial wort dissolved oxygen concentration, yeast pitching rate, strain-specific yeast properties, pitching yeast physiological condition and physical factors pertaining to the vessel and its management. Yeast strain-specific properties include flocculence character, cell surface hydrophobicity and oxygen requirement, amongst others. Physical factors would include parameters such as pressure, dissolved carbon dioxide concentration and the extent of mixing during fermentation. All of these interact in a complex manner and together determine the outcome of fermentation.

It may be readily appreciated that most of these factors could not be classed as process variables. Manipulation of wort composition to effect improvements in vessel productivity, as in high-gravity brewing, is discussed in Section 2.5. Deliberate alteration of yeast strain-specific characteristics by, for example, genetic manipulation, to produce variants with more desirable brewing properties could be a route to accelerating batch fermentation (see Section 4.3.4). This leaves dissolved oxygen concentration, yeast pitching rate, pitching yeast physiology, temperature and other physical parameters as being amenable to manipulation.

Increasing pitching rate, temperature and dissolved oxygen concentration all result in a more rapid primary fermentation, mainly by increasing the rate of wort attenuation in the exponential phase, although also by shortening the duration of the initial lag phase. The reduction in primary fermentation time can be very dramatic providing the parameters are increased to a sufficient degree. However, the apparent benefits are somewhat illusory since significant rate improvements can usually be obtained only at the expense of unacceptable changes in beer flavour and other quality parameters. Thus, changes in pitching rate and dissolved oxygen concentration, in particular, influence the extent of yeast growth during fermentation. Concomitantly, this will also change the spectrum of flavour-active metabolites whose concentration in beer is related to yeast growth.

With some fermentations, it is possible to gain significant advantage by modest increases in temperature alone. This is because temperature exerts most effect on yeast growth rate and not growth extent. However, this strategy must also be treated with caution since some lager beers, traditionally fermented at low temperature, can lose much of the subtle flavour character if too high temperatures are used. It has been suggested that the use of increased pressure can ameliorate the negative flavour effects due to elevated temperature whilst retaining the gains in time saving (Kumada et al., 1975).

Primary fermentation can be shortened by the use of improving vessel agitation, either mechanically or by gassing with carbon dioxide. As with the other approaches some caution is required to avoid changes in beer flavour. Masschelein (Haboucha et al., 1967; Masschelein, 1986a) reported that forced agitation resulted in increased yeast growth and concomitant reduction in esters and elevated levels of higher alcohols. The same author (Masschelein et al., 1993) suggested that the problem could be overcome by using a fed-batch approach. In this pilot scale study, stirred batch growth was allowed to proceed until the wort was 40% attenuated, at which point the wort feed was started. Wort was added to the culture at an exponential rate to keep the residual fermentable extract at a constant 40% attenuation. This approach allows yeast growth rate to be regulated by the medium feed rate, as in a continuous system (see Section 5.6). In this case, yeast growth extent was reduced to that measured in a comparable unstirred batch fermentation. By inference the production of flavour-active metabolites dependent on yeast growth could also be manipulated by this method whilst retaining advantages in productivity. Application at commercial scale remains to be demonstrated.

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