Lesson 3 - Fermentation Principles SAQ

1. **Answer both parts of this question:

a)    A wort contains 180 g/L of extract. Using a simple mass balance, calculate the amount of ethanol per litre that would be produced if the wort was fully attenuated.

b)    What could make the ethanol yield lower and how can you control this in the brewery?

Response:

a) Here’s a mass balance that would be expected for a typical fermentation:

2.0665 g extract → 1 g ethanol + 0.9565 g CO₂  + 0.11 g yeast

If we use the extract figure given in the question, our yield of ethanol  per litre would look like this:

180 g extract → [(1 / 2.0665) x 180] g ethanol + [( 0.9565 / 2.0665) x 180]  g CO₂ + [(0.11 / 2.0665) x 180] g yeast

Or

180 g extract → 87.10 g ethanol + 83.31 g CO₂ + 9.58 g yeast

b) In a mass balance, what goes in must come out so the carbon that would have been in the ethanol must have gone somewhere else. Under fermentative conditions CO₂ is produced in proportion with ethanol so the carbon will not have been lost there. That leaves yeast, or rather yeast cell growth as the culprit.

Anything that shifts carbon flow from pyruvate, away from acetaldehyde and ethanol, into new biomass will reduce the ethanol yield. The key parameters to control are therefore pitching rate and the initial oxygen concentration. Very vigorous fermentations with excessive yeast growth will give lower yields of ethanol.