Lesson 8 - Fermentation Unit Final SAQ

***After reading the introductory information, answer BOTH parts of this question.

You are part of the NPD team for your brewery working on a project to introduce a new beer. Your role is to oversee planning of the fermentation trials, with a view to developing a suitable regime that meets the brief. You will have to use the existing tank farm of 5,000 hL cylindroconical vessels.

The new beer will be a pale pilsner type lager of 12 °Plato. Your marketing colleagues have decided it should have a marked fruity and floral character, derived from yeast during fermentation, but with the majority of its other flavour characteristics derived from malt and hops.

You will have to use an existing yeast strain that is quite highly flocculent and is currently used for high gravity brewing, which it tolerates but not well, based on the evidence that average crop viabilities are trending downwards towards 85-90%.

For reasons of economy the new beer will be brewed at high gravity (20 °Plato), using a wort made with 85% malted barley and 15% sugar adjunct. Fermentation fill times are 20 hours and require six 800 hL batches of wort.

a)    How would you propose that the fermentations be managed to encourage the development of yeast-derived flavours that meet the marketing brief?

b)    What yeast-derived flavour problems do you think you might encounter and how would you propose to minimise these effects?

Response:

a) From the information given, the aim seems to be that the beer should have a pronounced ester content but with little else. This means that fermentations need to be managed in a way that favours ester formation but not other groups of key flavour compounds such as higher alcohols. It is essential to ensure that undesirable yeast-derived off-flavours such as acetaldehyde (and other aldehydes) and diacetyl are controlled at sub-flavour threshold levels.

How might this be done? Wort composition should not be a particular problem. The proportion of malt used is adequate and FAN levels should be sufficient to ensure good yeast growth. Providing the fermentation is managed adequately, high levels of diacetyl and acetaldehyde should not be present in green beer at the end of fermentation.

Managing the balance of esters and other flavour metabolites, especially higher alcohols, is more problematic. The key will be to control yeast growth and how this relates to the formation of flavour-active metabolites. From the perspective of wort composition, yeast growth extent should be moderately high and this would favour the formation of commensurately high levels of higher alcohols. This means that pitching rate and wort oxygenation will have to be used as the principal control levers. The long fill time provides another method of control. The ratio of oxygen to yeast would be used to regulate total yeast growth, to ensure that the balance of carbon flux between the formation of alcohol and flavour metabolites produces acceptable levels of higher alcohols and other flavour metabolites. In other words, the fermentation conditions would be used to regulate how the yeast cells balance cellular redox.

Ester formation is favoured by the restriction of oxygen-dependent lipid synthesis. When oxygen is present, the terminal step in the ester formation pathway, catalysed by the product of the ATF genes, is repressed. This can be used here by ensuring that oxygen is provided early in the vessel fill but not in the later stages. Under these conditions, ester synthesis is de-repressed. The timing of oxygen addition, the oxygen concentration, and the pitching rate can be used to control both total yeast growth and ester development. Of course, the more that esters are formed, the more that higher alcohols are reduced since some of these will be used to make esters.

b) The use of large capacity fermentation vessels and very concentrated worts are stressful to yeasts. Some strains are more tolerant than others but the information given in the question suggests this yeast stain is reasonably susceptible. Fermentation conditions need to be managed to minimise this stress. Failure to do this will result in loss of viability, undesirable flavour effects, poor beer foam performance, and higher than normal beer pH, which increases susceptibility to spoilage. Higher wort pH also reduces the rates of decomposition of precursor α-acetolactate into free diacetyl and can therefore prolong the time to achieve VDK specification.

The use of a reasonably cool fermentation temperature reduces the stress that yeast is subjected to and would be beneficial for this beer since more of the volatile esters would be retained. A stepped fermentation profile would probably be sensible; cooler in the early stages of active yeast growth to help retain esters, and warmer in the later parts to encourage high rates of diacetyl reduction.

In these stressful types of fermentation, it is essential to remove yeast crops as early as possible. It is when yeast has formed a sediment in the cone of the vessel that it is subject to maximum stress since alcohol concentrations are high, and efficient dissipation of heat by wall cooling jackets is not possible. The warming of the yeast crop can quickly cause significant reductions in yeast viability. This yeast strain is highly flocculent which means it is likely to drop out quite early and therefore be at risk of damage. Potential side-effects are the formation of undesirable sulphur compounds and other products of cell death, together usually referred to as ‘yeast-bitten’.