Lesson 3 - Fermentation Principles SAQ

**You have been asked to manage the introduction of high gravity brewing into your brewery. Current fermentations use malt wort with a gravity of 12 °P. For greater economy, the plan is to increase the gravity of the wort to 17 °P using a sugar adjunct largely made of maltose. At the same time the fermentation temperature is being increased from the current 15 °C up to 18 °C. Assume the efficiency of fermentation is 88%.

a)    How much additional ethanol (abv) would you expect the fermentation to generate? Assume the adjunct is pure maltose and use typical efficiency figures for fermentation. The molecular weights of maltose and ethanol are 342.3 and 46.1, respectively. The density of ethanol is 790 g/L.

b)    How would you expect the changes to affect fermentation cycle times?

c)    What process changes would you need to carry out with regard to fermentation management?

d)    Draw up a checklist of what you would monitor once fermentation trials commence to ensure the process is under control and working satisfactorily.

Response:

a) The wort concentration will be increased by 5 °Plato. For practical purposes the Plato scale is the same as Brix which is the concentration expressed as weight to weight.

Therefore, 5 °P = 5 °Brix = 5 g/100 g solution. Since our solute is water we will assume that the concentration of maltose will be 5 g/100 ml or 50 g/L.

We know that in theory 1 mole of glucose will give rise to 2 moles of ethanol. Since maltose consists of 2 molecules of glucose the yield increases to 4 moles of ethanol per mole of maltose consumed.

The molecular weights of maltose and ethanol are 342.3 and 46.1, respectively so one mole of maltose weighs 342.3 g and one mole of ethanol weighs 46.1 g.

We have 50 g of maltose in each litre of wort so we can work how many moles there are:

50/342.3 = 0.146 moles of maltose

For each mole of maltose we ferment we get 4 moles of ethanol so if we multiply this by 4 we can calculate how many moles of ethanol we will yield:

0.146 x 4 = 0.584 moles of ethanol.

If we multiply this by the molecular weight of ethanol we can find out how many g of ethanol we have yielded:

0.584 x 46.1 = 26.9 g/L

This is at 100% efficiency so we need to adjust this figure to account for the 88% efficiency of fermentation:

(26.9 g/L / 100) x 88 = 23.7 g/L

We know that ethanol has a density of 790 g/L and we have 23.7 g so we can work out the volume of ethanol. If there are 790 g in a 1000 mL then each g is:

1000 mL / 790 g= 1.266 mL/g

So if we multiply our g of ethanol by this we get the volume of ethanol:

23.7 g x 1.266 mL/g = 30 mL of ethanol per litre of beer

So to work this out as a percentage we divide the volume of ethanol by the total volume and multiply by 100:

(30 mL / 1000 mL) x 100 = 3% abv

b) Moving to high gravity brewing has a downside in that there is more extract to ferment and so there will be an extension in the cycle time. It is difficult with any certainty to predict what this will be. In this case, though, the fermentation temperature is also being increased and this will offset some of this loss.

c) When bringing in changes of this type it is essential to consider the likely impact on all processes. It will be necessary to increase the pitching rate and wort oxygenation. With regard to yeast handling, it might be necessary to uprate the capacity of the yeast storage plant to ensure it can cope with the increased pitching rates. Similarly, the oxygenation system must be able to cope with the increase. Potentially, it might be necessary to move from the use of air to pure oxygen.

d) The first step would be to monitor fermentation performance using the new conditions. The points to check would be cycle times, time to gravity, and (if required) time to achieve VDK specification. If necessary, proving trials would identify optimum values for pitching rate and wort oxygenation.

A key consideration will be beer flavour. It is highly likely that the addition of such a high proportion of pure sugar adjunct will have a significant impact on beer esters. If this proves to be the case further changes will need to be made. Again, wort oxygenation and pitching rates will be the levers likely to be used to bring flavour back into line. In particular, there may be opportunities to use the timing of oxygenation and pitching during vessel fill to make adjustments to flavour.

The new fermentation process will increase the stress levels on the yeast strain as a result of its exposure to more concentrated worts and higher alcohol concentrations. It will be vital to monitor the health (viability) of the yeast crops and beer pH / beer foaming potential. If undesirable effects are noted, avenues of investigation would be timing of crop removal and the number of times the yeast can be serially re-pitched. If the latter needs to be reduced, this might also increase the pressure on yeast storage capacity and also propagation capacity.