Lesson 3 - Fermentation Principles SAQ
This lesson enables you to test yourself on the material covered in the Fermentation Principles lesson and determine to what level you have attained the outcomes.
Suggested Response - Q3
**To what extent do each of the following variables influence how much yeast grows during fermentation?
a) Wort concentration
b) Wort oxygen concentration
c) Temperature
d) Pitching rate
Response:
a) Wort concentration can influence how much yeast grows since it affects how much nutrient is made available to the yeast. It is not quite as simple as that because it depends on how the wort concentration is varied. If wort concentration is increased simply by the addition of more fermentable sugar, yeast growth will not be greatly changed. If the concentration is increased by increasing the proportion of pure malt in the grist, this will increase the concentrations of all yeast nutrients and growth will increase in proportion. However, the solubility of oxygen decreases with an increase in wort concentration and this could reduce growth.
b) Wort oxygen concentration has a direct effect on yeast growth because of the amount of lipids, sterols, and unsaturated fatty acids the yeast synthesises in the aerobic phase of fermentation. More oxygen leads to more lipids which leads to more growth. If the wort is under-oxygenated, the fermentation will be slow and yeast growth extent will be small. In extreme cases, the fermentation may stick and yeast crops may have a low viability.
c) Temperature has no direct effect on yeast growth; it only influences the yeast growth rate. Again, other factors have to be borne in mind. An increase in temperature reduces the solubility of oxygen.
d) Pitching rate can influence yeast growth but with some provisos. The important parameter is the ratio of yeast pitching rate and wort oxygenation. This controls how much oxygen is supplied to each yeast cell. At high oxygen levels and relatively low pitching rates an increase in pitching rate will increase yeast growth. The excess of oxygen allows every cell to make abundant quantities of the essential lipids and, therefore, growth increases with pitching rate. If the pitching rate is high but there is limited oxygen, yeast growth drops. This is because less lipid is formed per cell and this limits growth.