Secondary fermentations

The changes which occur in the secondary fermentation associated with traditional lagering and warm conditioning processes have been described previously (see Sections 6.5.2 and 6.5.3). The application of immobilised yeast reactors for secondary fermentation is described in Section 5.7.2. Two other secondary fermentation processes are worthy of mention, those used for the production of cask- and bottle-conditioned beers.

6.9.1 Cask conditioning

As with so many aspects of brewing, history has produced many variations to the basic process. Undoubtedly beers were originally fermented in the wooden casks from which they were also dispensed, and therefore no distinction was made between primary and secondary fermentation. In this case it would have been difficult to produce a bright product and losses would have been considerable. However, by restricting the escape of carbon dioxide towards the end of fermentation, it would be possible to effect a crude control of carbonation. This approach is still in use for the production of a few traditional beers, for example, in Belgium (de Clerck, 1954). It may also be used in domestic brewing operations using combined plastic fermenting bins and dispense containers. These are built to withstand a degree of pressurisation to allow dispense of carbonated beer.

The advent of comparatively large-scale top-cropping ale fermentations performed in open brewery tanks and the desire for bright beer with condition brought about a need for a separate secondary fermentation. Thus, because of the relatively low carbonation levels achievable in shallow open fermenters, there was a perceived need to have a second fermentation, in the container from which the beer was to be dispensed, purely to generate additional carbon dioxide. An added bonus was the opportunity to make other flavour and colour adjustments.

In the modern process, the beer is usually transferred from fermenter into a separate racking tank. Here adjustments to colour may be made as well as the addition of priming sugars to achieve a desired level of fermentability. Isinglass finings may also be added to the racking tank. Occasionally, casks are filled directly from fermenter, in which case the additions described must be added to each cask, prior to filling. Further flavour adjustments are possible by dosing various hop products directly into casks. Originally beer was dispensed into coopered wooden casks but most breweries now use aluminium replacements, of varying sizes. These have a longer life span, are more hygienic and require less maintenance. The features of a modern aluminium cask are shown in Fig. 6.42.

Filling casks (or 'cask racking') is performed using a multi-head 'racking back'. This consists of a stainless steel main attached to the racking tank. A number of flexible hoses carry beer from this main to the casks. Prior to filling, casks are cleaned and sterilised by steaming and the keystone sealed with a wooden bung. With the cask on its side, the filling head is placed into the bunghole and the valve opened. The beer in the racking main is held under a carbon dioxide top pressure which forces beer into the casks. The volume dispensed may be controlled automatically. When the cask is full, application of carbon dioxide gas pressure forces beer fob out of the cask and back up a separate return pipe. This passes into a collection main from which it is added back to the bulk beer. The opening in the cask is then sealed by driving in a wooden bung known as a 'shive'.

Fig. 6.42 Cross-section through an aluminium beer cask.

Handling of casks after filling is much dependent on the particular beer type. In addition, physical factors that influence the rate of secondary fermentation are important. These include temperature and the quantity of air introduced into the cask during filling. Generally, casks are held in the brewery for up to seven days, preferably at a temperature within the range 13 to 16°C. During this time sugar is metabolised by the yeast to generate carbon dioxide and there is a concomitant modest increase in ethanol concentration. The process is completed when the cask is taken to the point of dispense. A brass or plastic tap is driven though the wooden bung in the keystone, taking care not to introduce any air and the cask is placed on a framework known as a 'stillage'. Before dispensing the beer, a peg, termed a 'spile', is driven through the shive. This allows a small ingress of air to into the top of the cask so that beer can be dispensed from the tap. Usually about 2 days are allowed to elapse between

Spile

Spile

Bottom chime

Fig. 6.42 Cross-section through an aluminium beer cask.

Stillage

Stillage

'stillaging' and dispense to allow for final development of condition and yeast to form a compact sediment under the influence of the finings. Spiles are made from wood and are available with different degrees of porosity, depending on the cut of the grain (soft or hard), which gives a crude regulation of the quantity of air allowed to enter the cask. Hard pegs may now be made from impermeable plastics.

The beer may be dispensed directly from cask to glass, although more usually a separate cellar is used. Beer is pushed from here to a separate tap under carbon dioxide top pressure or via a pump. Cellarage of cask beers is preferable since a constant cool temperature is more easily achieved. As the beer level falls, the casks are raised using wedges known as 'thralls'. The fining behaviour of each batch of beer is assessed in the brewery by retaining samples, which are dispensed into glass ended casks. These are held at an optimum temperature in the brewery sample cellar and the clarity of the beer judged by direct observation when the cask in placed in front of a bright light source.

Figure 6.43 shows a charming historical photograph of cask racking and tapping post stillage. Although the engineering has changed, the principles remain true today.

Fig. 6.43 'Tapping cask beer' (courtesy of the Bass Museum. Burton-upon-Trent, England). 6.9.2 Bottle-conditioned beers

Rarely, beers may be subjected to a secondary fermentation after packaging into bottles. Essentially the process is similar to cask conditioning although careful control is required. With many traditional beers, a cloudy product is entirely acceptable and in these cases, the only requirement of secondary conditioning is to ensure that sufficient carbon dioxide is generated to suit the style of beer. The presence of suspended yeast cells is of no particular significance; indeed they will enhance the nutritional value of the product.

With some bottle-conditioned beers there is a requirement for both carbonation

Fig. 6.43 'Tapping cask beer' (courtesy of the Bass Museum. Burton-upon-Trent, England). 6.9.2 Bottle-conditioned beers

Rarely, beers may be subjected to a secondary fermentation after packaging into bottles. Essentially the process is similar to cask conditioning although careful control is required. With many traditional beers, a cloudy product is entirely acceptable and in these cases, the only requirement of secondary conditioning is to ensure that sufficient carbon dioxide is generated to suit the style of beer. The presence of suspended yeast cells is of no particular significance; indeed they will enhance the nutritional value of the product.

With some bottle-conditioned beers there is a requirement for both carbonation and clarity and this presents a more demanding technical challenge. In this case a base beer is used which has been brewery cold conditioned and filtered to remove sources of haze. Base beers are as nearly as possible completely attenuated. This allows accurate control of addition of priming sugars. This is important since too much sugar would generate excessive carbon dioxide volumes with potentially disastrous consequences for the bottles.

In a typical process, beer is packaged containing sufficient primings to generate 2-3 volumes of carbon dioxide and yeast at a concentration of 1-2 x 103 cells ml 1. During these operations, great care must be taken to reduce as much as possible the risks of microbiological contamination. After bottling, beers are held in the brewery for a period of about two weeks at 15 to 20°C for condition to develop and allow microbiological checks to be made, prior to release for sale. During this time, the yeast proliferates and the cells gradually settle out. Any yeast strain that is capable of generating condition may be used. However, during the secondary fermentation alterations to flavour also occur as a result of products of yeast metabolism being excreted into the beer. In this respect, a strain providing the desired flavour is important. Frequently yeast strains are used which have a chain-forming morphology. This is useful for formation of a mat of yeast growth, which forms a stable sediment in the bottle. This may more easily be left behind allowing bright beer to be dispensed. Use of bottles with reasonably deep punts is also useful in this regard.

A modern manifestation of bottle-conditioned beers is the practice of sterile packaging beers that contain a very low yeast count into bottle or can. In this instance, the yeast is claimed to give protection against the development of stale flavours associated with oxidation and ageing in small-pack products. Yeast cells are freely suspended throughout the beer but at a concentration low enough to render them invisible to the naked eye. Apart from the need for strict control of hygiene during handling and packaging, it is essential to ensure that there is no opportunity for the yeast cells to proliferate. Therefore, precise control of residual fermentable extract and in-pack oxygen is obligatory.

Brewing Yeast and Fermentation Chris Boulton, David Quain Copyright © 2001 by Blackwell Publishing Ltd

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