Lesson 5 - Fermentation and Beer Flavour SAQ

***Your brewery is developing a new beer. Your role in the team is to develop a suitable fermentation regime. It is intended that the beer will be low carbohydrate and made by an intensive process. For this reason, it will use a combination of very high gravity brewing, high refined maize starch adjunct levels, and supplementation with an amyloglucosidase enzyme.

Initial trials indicate that fermentation cycles are much longer than planned for. The major problem is a very extended VDK stand time.

Fermentation vessels have a capacity of 6,000 hL, have a fill time of 18 hours, and require six batches of wort.

Discuss the likely causes of the problem and identify some measures you might consider taking to shorten fermentation cycle times.

Response:

This is a very testing fermentation which will be stressful to the yeast. The combination of very high gravity and the use of amyloglucosidase will generate very high concentrations of ethanol.

In the first instance, checks will need to be carried out to ensure the yeast is suitable for use in this high stress situation. Assuming the yeast is sufficiently ethanol-tolerant, fermentation management will need to be set up to minimise yeast stress. Measures such as early warm cropping will probably be essential. Of particular relevance to VDK performance, it would be sensible to check if the yeast is prone to form petite mutants. These are known to remove diacetyl very poorly and these mutations are formed more commonly in some brewing strains. If this proves to be the case, the strain should be replaced by a more robust one. If this is not possible, it would be necessary to restrict it to very few re-pitchings, or even limit it to single use.

An obvious and major problem is wort composition. It is likely that FAN levels will be low. This is probably the major factor producing the extended VDK stand time. In this situation, depletion of wort valine would occur quite early in fermentation, causing the ilv pathway to be active for a long time. The result is a very extended and low VDK peak, since intracellular valine concentrations would also be low meaning that the ilv pathway would not be inhibited by valine.

What actions can be taken to improve matters? Assuming it is not possible to modify wort composition, the list of possible actions is quite limited. The wort fill time is also long and if the yeast is all pitched with the first batch of wort, as is common, the situation may be made worse. In this case, the problem is the disruption to the ordered uptake of wort amino acids caused by adding fresh wort to already growing yeast. Since valine is group B, not taken up until mid-fermentation, the potential for inhibiting the ilv pathway is also limited. In other words, continued addition of group A amino acids ensures that the ilv pathway remains active for longer as does the formation of α-acetolactate, the immediate precursor of diacetyl. If possible, it would be best to delay pitching for as long as possible to reduce the severity of this effect.

What else might be done? The sharpness of the VDK peak is influenced by the vigour of fermentation. It would make sense to control fermentation to provide a good yeast growth extent and rate. The first form of control would be to use high wort oxygenation levels and reasonably high pitching rates. The second would be to use as warm a temperature as can be used without compromising yeast health and beer flavour.

Lastly, if these measures do not improve matters more radical steps might be needed. The easiest of these would be to consider the process aid that contains the enzyme α-acetolactate decarboxylase. This enzyme converts the α-acetolactate precursor directly into acetoin without diacetyl being formed at all.