Fermentation management

Beer quality, yeast type, fermenter design and operation are all intimately related. In particular, the design of many traditional fermenters arose because they were suited to the quality of beer produced and the properties of the yeast used (see Chapter 5). Modern vessels tend to be used for any beer quality and yeast type although perhaps with some compromises being made. All fermentations proceed through a number of stages. Several variations are possible and the terminology can be somewhat confusing.

(1) Primary fermentation: This is the true fermentation, which commences as soon as yeast, oxygen and wort come together. This stage ends when all fermentable sugars have been utilised, in which case the wort is termed 'fully attenuated'. Alternatively, primary fermentation may be arrested by treatments that encourage the yeast to separate out from the wort. In this instance, the wort would be termed 'partially attenuated'. The present gravity of the wort at the end of primary fermentation is the 'attenuation' or 'racking gravity'.

(2) Secondary fermentation: Secondary fermentation refers to any part of the fermentation process that occurs after primary fermentation and involves reactions requiring the presence of yeast cells. These reactions may take place in a brewery vessel or within a cask or bottle.

(3) Conditioning: Conditioning is the stage in the process that occurs after primary fermentation and prior to packaging, in which the immature green beer is converted into mature beer. An obligatory part of conditioning is adjusting the carbonation level of the beer and thus the presence of C02 in beer, or its development, is referred to as 'condition' or 'coming into condition', respectively. If the C02 is derived from the action of yeast as in a cask ale this is termed 'natural' or 'cask conditioning' and is an example of secondary fermentation. The process of conditioning traditional lager beers by holding for a period of cold storage in the presence of yeast and fermentable sugar is termed 'lagering', or 'cold conditioning', and is another example of secondary fermentation.

In some cases, beer in fermenter is held at a relatively high temperature after the completion of primary fermentation. The object of this treatment is to ensure that all available a-acetolactate is decarboxylated to diacetyl and the yeast has an opportunity to reduce diacetyl to acetoin and butanediol (see Section 3.7.4.1). Therefore, this phase may be referred to as a 'diacetyl' or 'VDK stand/rest'. It may also, however, be termed warm conditioning or in German, Ruh storage. It is another example of a secondary fermentation.

Apart from adjustment of carbonation, the conditioning process also involves clarification by promoting precipitation of materials, which if not removed may cause hazes in packaged beers. This is also accomplished by storage at low temperature and the processes involved are purely physicochemical in nature. In addition, flavour maturation may occur by venting undesirable volatile components of beer to atmosphere and carbonation may be controlled by gas sparging. None of these adjustments requires the presence of yeast cells. This stage may also be referred to as cold conditioning, although, particularly in America, it may be called 'ageing'.

6.5.1 Traditional top-cropping systems

Within this category, especially in the case of UK ale fermentations, a variety of systems are used. Some of these, such as the Burton Union system or the Yorkshire square, are unique and their peculiarities are described in Sections 5.3.4 and 5.3.5. In the case of closed or open square vessels, the fermentation process is comparatively simple. Essentially the process consists of just a single stage, since there may be no requirement for a separate conditioning phase. Thus, conditioning and secondary fermentation may be performed in the cask. Primary fermentation is rapid, typically 72-120 h, because temperatures tend to be relatively high (15 to 22°C). Occasionally the dropping system may be used where the fermenting wort is transferred from one vessel to another, usually 24 h after pitching (see Section 5.3.3).

Management is restricted to application of appropriate attemperation, removal of the yeast crop (see Section 6.7.1) and ensuring that the desired degree of wort attenuation is achieved. Differing qualities of beer require specific degrees of attenuation. Thus, if a secondary fermentation process is used, as in a cask-conditioned ale, it is usually necessary to ensure that some fermentable residue remains at the end of primary fermentation. Conversely, if a secondary fermentation is not required, or where additional sugar is added to the cask, a fully attenuated primary fermentation may be needed. The degree of attenuation at the end of primary fermentation is regulated by the controlled application of cooling, to both arrest activity and promote separation of yeast. Where a reasonably high residual yeast count is required, little or no cooling is applied and the process stage is ultimately terminated by emptying the vessel.

6.5.2 Traditional lager fermentations

This type of fermentation is associated particularly with the production of traditional European lager beers. Fermentations are performed in shallow open or closed vessels at low temperatures. Typically, the initial temperature is in the region of 6°C. The combination of low temperature and long processing times produces beers with subtle floral flavour notes, which are not as evident in similar beers produced at higher temperatures in a more rapid process (see Section 6.5.3). Pitching rates are comparatively high (c. 20-25 x 106 cells ml *) to ensure a rapid start and reduce the risk of contamination. Satisfactory fermentation progress is judged by the appearance of the surface of the fermenting wort. Thus, the head goes through a number of characteristic stages during the fermentation. The appearance and disappearance of these types of head is used to gauge the degree of cooling to be applied.

Within 8-16 h of starting, a fine white head appears, which is the first visible manifestation of the onset of gaseous C02 evolution. During this early phase a sediment forms in the base of the vessel, consisting of cold break precipitated from the wort. This sediment has the potential to contaminate the yeast crop and this may be avoided by using a dropping system analogous to that described for top-cropping ale fermentations (see Section 5.3.3). Alternatively, a single vessel system may be used and in this case it is usual to attempt to leave this behind when the overlay of yeast crop is removed (see Section 6.7.2).

Another variation is the system described in German as Darauflassen, in which the fermenting vessel is initially part-filled and then after 24 h a further aliquot of unpitched oxygenated wort is added. The sequential addition of wort promotes good yeast growth though a kind of primitive fed-batch approach. After 48 h a rocky or cauliflower head of foam appears on the surface of the vessel, in German termed krausen, ('frill' or 'ruffle'). During this phase, the temperature is allowed to increase gradually by a further 2 or 3°C. After approximately 72 h the fermentation reaches its most active phase and the head grows and assumes a deeply separated appearance. This phase is termed 'high krausen' and during this period of maximum exothermy the temperature is held at a maximum of 8 to 9°C. As the degree of attenuation increases, the vigour of the fermentation declines and the head collapses to form a thin brown and bitter-tasting pellicle containing tannins and other materials. This may be removed by skimming, or tanks may be specifically designed so that it is left behind when the vessel is emptied. As with top-cropping ale fermentations, it is necessary to control the end-point of traditional lager fermentations by careful application of cooling. This is to ensure that sufficient suspended yeast cells and fermentable extract remain to fuel the prolonged cold secondary fermentation.

The secondary fermentation is performed in closed chilled 'lagering' tanks. This process may last from a few weeks up to several months. Typically, the beer is run into lagering tanks at a temperature of roughly 5°C and over a period allowed to decrease to 0°C. Occasionally a more rapid treatment lasting just a few weeks at — 1°C to 0°C may be practised. If the secondary fermentation is too sluggish, additional fermentable extract and yeast cells may be added in the form of actively fermenting wort, a treatment known as krausening. The conditioning process has three functions: adjustment of carbonation, full development of a filterable chill haze and final flavour development. The flavour changes encompass a multitude of complex reactions. These range from simple C02 gas stripping of volatile components such as acet-aldehyde and H2S, through to the production and removal of desirable and undesirable flavour-active by-products of yeast metabolism. Of vital importance during this stage, remaining acetohydroxy acids spontaneously decarboxylate to form undesirable vicinal diketones, in particular diacetyl. Yeast cells must be present to reduce the vicinal diketones to much less flavour-active diols (see Section 3.7.4).

6.5.3 Modern closed fermentations

Fermentations performed in modern closed vessels such as cylindroconicals may be of either ale or lager types. In the case of ale fermentations, the process is managed as described already for traditional vessels. However, at the end of primary fermentation the bulk of the yeast is induced to settle out in the base of the vessel by the application of rapid chilling. This happens even though the same top-cropping strains of yeast are used. The altered flocculation is probably a consequence of the combination of smaller surface area, more efficient cooling and much better mixing during primary fermentation. Probably these factors prevent the formation of aggregates of yeast cells and entrapped C02 bubbles, which characterise the formation of yeast heads in top-cropping fermentations.

Typically, ale fermentations performed in cylindroconicals are fully attenuated and after chilling and cropping are subjected to a short low-temperature conditioning process in a separate tank. The conditioning phase is purely to adjust carbonation, precipitate chill haze, and allow some loss of undesirable flavour volatiles through gas purging. On completion of conditioning the beers are filtered and packaged.

Lager fermentations performed in cylindroconical vessels tend not to be of the traditional variety. Instead, there is an increasingly common trend throughout the world to produce lager beers using a rapid and high-temperature fermentation. Primary fermentation may be performed at temperatures anywhere between 10 and 20°C, although usually within the lower quartile of this range. When primary fermentation is completed, the comparatively high temperature is maintained for a period of warm conditioning. This is to allow the reduction of diacetyl to acetoin and 2,3-butanediol. For this reason, this phase is also referred to as the diacetyl rest or stand. When the concentration of diacetyl and its precursor, a-acetolactate, have fallen to a pre-determined concentration, the diacetyl rest is terminated by the application of chilling. This causes the bulk of the remaining suspended yeast to settle into the cone of the vessel from which it is cropped.

The green beer may then be transferred into a separate conditioning tank via a continuous centrifuge, which reduces further the suspended yeast count. Condition ing is carried out at a low temperature for no longer than a few days. As with chilled and filtered ales this part of the process serves simply to adjust carbonation, develop chill haze protection and clarify the beer. Yeast cells play no part in the process, hence the need for prior elimination of diacetyl precursor. In some cases, the period of cold conditioning may be performed in the same vessel as used for primary fermentation (see Section 5.4.3).

6.5.4 Troubleshooting abnormal fermentations

Inevitably, all breweries experience occasional abnormal fermentation performance. This should be a rare problem; however, when it occurs it is necessary to have procedures for identifying the causes and putting into action an appropriate plan of countermeasures. The procedures should encompass an analysis of the observed symptoms, investigative tests to obtain further information, identifying causes and taking remedial actions. The action plan needs to consider how best to deal with the individual problem fermentation, and thereby rescue that batch of beer. It should also establish if the individual abnormality is an isolated incident or part of a bigger brewery-wide problem.

Early detection of abnormal fermentation behaviour is dependent on the methods of monitoring performance. Therefore, atypical behaviour is usually identified as deviations from the normal patterns of profiles of physical parameters such as specific gravity and temperature. Nevertheless, the personal experience of the brewer is also particularly useful. The appearance of the fermenting wort can be an invaluable aid for early identification of non-standard behaviour at a time when there may have been little or no change in measurable parameters. In this regard, the use of open fermenting vessels is an obvious advantage. Where closed vessels are employed, the practitioner is more reliant on physical measurement. The discussion here concentrates mainly on large-scale closed fermentations performed in cylindroconical vessels.

The symptoms of abnormal performance are typically fermentation rates that are too slow (or too rapid) and those that indicate altered patterns of yeast growth or physicochemical behaviour (Table 6.4). Clearly, the symptoms are not mutually exclusive. Inevitably, slow and/or sticking fermentations will be viewed as being the most troublesome as they impact directly on the efficiency of the whole brewery, and therefore most of the following discussion relates to dealing with this situation. Should these circumstances arise, it will be necessary to take steps to try to correct the problem and thereby bring the fermentation back on track and minimise effects on the beer. Where abnormalities relate to features such as over-attenuation or the effects of contamination (either microbiological or other), the problem may not be manifest until after the fermentation is complete. In this case, the investigation can only be of historical value.

The causes of fermentation abnormalities are obviously related to abnormalities in those factors that influence performance, i.e., wort composition, control of the major fermentation variables, yeast phenotype/genotype and microbiological integrity (Table 6.5). To establish the precise cause of a problem an ordered hierarchy of investigations should be followed. The majority of slow fermentations probably arise

Table 6.4 Symptoms of abnormal fermentation performance.

Stage

Primary symptom

Secondary symptom

Primary fermentation

Too rapid.

High coolant demand. Excessive fobbing.

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