Info

Larger than normal.

Crop forms sooner than expected.

Low yeast count in green beer.

Low or normal yeast count in green beer.

Table 6.5 Factors influencing abnormal fermentation performance.

Parameter Factor

Wort composition Starting gravity

Sugar spectrum FAN/TSN

Metal composition (zinc addition) Presence of inhibitors

Fermentation variables Yeast pitching rate

Dissolved oxygen tension Temperature

Yeast Correct strain

Viability

Physiological condition/vitality Genotype

Microbiology Contamination with:

other production yeast wild yeast bacteria because of a failure to adequately control pitching rate and/or wort oxygenation, or using pitching yeast that was in a stressed condition. Records of the collection of individual fermentations should be sufficiently detailed and accurate to allow confirmation, or otherwise, that correct conditions were established at the outset and that the condition of the pitching yeast was satisfactory. Of course, in-line control and logging systems are particularly suitable for this type of checking procedure (or ensuring that fewer problems arise in the first place).

If control of the initial parameters is in doubt, a sample of the fermenting wort should be removed from the vessel and examined. Direct microscopic examination of fermenting wort is an invaluable diagnostic aid. If the yeast count is lower than predicted and viability is high, an error in pitching rate is likely. If the yeast count is as expected and viability is acceptable but there is little evidence of budding, an error in dissolved oxygen control is probably indicated. In both of these cases, the corrective action should be to add more oxygen, via direct injection of gas, and, if available, mechanical agitation. The window of time for such remedial action is narrow. In addition, if the cell count is very low, more yeast may be added prior to the application of oxygen. If the diagnosis is correct this should restart the fermentation, or increase the rate of attenuation. Additions and/or agitation result in the release of dissolved carbon dioxide which during active fermentation can be sufficiently 'explosive' to result in major losses of fermenting wort from the vessel (see Fig. 6.35).

Fig. 6.35 Rousing an active fermentation - flooding of Station Street. Burton-upon-Trent (with permission from the Burton Daily Mail).

It would not be expected that the normal attenuation rate would be fully reestablished since sticking fermentations are notoriously difficult to recover. In extreme cases, it may be necessary to transfer the fermenting wort to another vessel, which may re-start fermentation simply by ensuring that the yeast is re-suspended. Another approach is to divide the problem fermentation between two vessels and top each up with fresh oxygenated wort. The latter measure should be treated with caution since it can result in doubling the problem! These extreme remedial actions produce a strong possibility that the resultant beer may be out of specification and blending at a suitable rate is advised.

Microscopic examination of fermenting wort provides an opportunity to identify heavy contamination with bacteria, although probably not with another yeast.

Microbial contamination should be a very rare occurrence and the consequences would depend on the nature of the foreign organisms. In the case of a gross bacterial contamination, the situation may be irretrievable and the beer would have to be destroyed. The effects of contamination with another yeast may not be evident at all during active fermentation although alterations in cropping behaviour and subsequent beer processing may occur. In addition, over-attenuation of worts may be observed. Of course, changes in beer flavour and aroma may result from contamination with other yeast and a decision regarding appropriate action would need to be based on the severity of the beer abnormality. In any case, it would be advisable to destroy the yeast crop.

Some fermentation abnormalities relate directly to the yeast. Pitching yeast in a stressed physiological condition will probably produce sub-standard fermentation performance, as described already. In this regard, the utility of the so-called vitality tests is discussed in Section 7.4.2. Occasionally, changes in yeast genotype (see Section 4.3.2.6) may produce atypical fermentation performance. Such changes may produce effects such as sudden shifts in yeast flocculence. Accordingly, effects on fermentation may be higher than normal yeast counts in green beer or increased solids content of sedimented yeast crops. The latter situation may also be associated with failure to reach expected attenuation gravity and/or prolonged diacetyl stand-times. If changes in genotype are suspected the yeast should be discarded and a fresh culture introduced.

Sticking fermentations which contain low cell counts and non-budding yeast and which do not respond to oxygenation and/or repitching may indicate a wort nutrient deficiency or the presence of an inhibitor. Further evidence of the latter could be abnormally low yeast viability. A suitable test for these eventualities is to remove a sample of the fermenting wort and incubate it overnight, preferably using an orbital incubator to ensure aerobiosis. Good yeast growth should be observed and the expected attenuation gravity achieved. If not, a wort problem is indicated. Providing there is no reason to suspect that the batch of wort is atypical severe nutrient deficiencies should be very rare. However, inadvertent failure to add a zinc supplement is possible. Proprietary nutritional supplements, so-called 'yeast foods', are available and the effect of dosing these into the wort, at the recommended rate, may be efficacious.

The presence of growth-inhibitory substances in wort is more difficult to detect and deal with. The effects of contamination may become evident in two ways. First, there is the situation where there has been gross contamination of a single batch of wort, which leads to obvious atypical fermentation behaviour. An example of this would be accidental contamination of wort with a cleaning agent such as sodium hydroxide. Here a simple and obvious test, if this type of accident is suspected, is to measure pH. The remedial action would depend on the severity of the contamination. In extreme cases, it might be necessary to destroy the wort; less serious contamination might be solved by judicious blending.

The second type of contamination is that where a low level of a yeast toxin is introduced at some stage of wort production. Contaminants may be introduced with any raw material. For example, in two recent communications (Boeira et al., 1999a, b) inhibition of the growth of brewing yeast by Fusarium mycotoxins was described.

These toxins are possible contaminants of barley, wheat, rice and maize and they may persist into wort and beer. Another obvious source of contamination is via the water supply. Procedures such as carbon treatment of brewing liquor and appropriate checking procedures should preclude this type of problem. Nevertheless, the possibility of introducing toxins in brewing liquor via pollution of groundwater must be guarded against. Since pollution may be a growing threat in many parts of the world great vigilance is required on the part of the brewer. Contamination problems of this type may have multiple effects on yeast metabolism, depending on the nature of the toxin and the concentration. Typically, the initial effects may be slight but possibly cumulative. It is therefore essential to monitor fermentation performance to check for long-term drift.

The ability to distinguish between isolated and more general problems is much dependent on the quality of the records maintained within the brewery. It is usual to retain records, either as hard copy or in electronic form, of profiles of present gravity, temperature and VDK (if appropriate) and analysis of the yeast at pitch and crop. It should be possible to compare these results for any given set of fermentations grouped on the basis of common parameters of the sort detailed in Table 6.6. In addition, similar data should be available to allow comparison of the analysis and organoleptic quality of the beer devolving from each fermentation. Variations in these data as a function of time (or another variable) can be monitored using statistical methods such as the cumulative sum (CUSUM) procedure.

Table 6.6 Identification of common factors possibly relating to atypical fermentation performance.

Parameter

Potential common variable

Fermenter

Individual fermenter

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