Lesson 5 - Yeast Biochemistry SAQ
This lesson enables you to test yourself on the material covered in the Yeast Biochemistry lesson, and determine the level to which you have met the learning outcomes.
Suggested Response - Q4
*Identify the two principal compounds used by yeast cells to store carbohydrates and indicate their primary functions.
Response:
The two carbohydrate storage materials are glycogen and trehalose.
Glycogen is a true storage carbohydrate; it is formed during fermentation and can be as much as 40% of the yeast’s cell dry weight. It is used to help cells survive through the period of starvation that occurs between cropping and re-pitching. This is why it is important to slow down (by maintaining a low temperature) yeast metabolism as much as possible during storage. Glycogen is also used to supply carbon and energy for the sterol and unsaturated fatty acid synthesis that happens in the aerobic phase of fermentation.
Trehalose is used by cells as a stress protectant. It can bind to the cell membrane; this helps to stabilise this vital structure when cells are stressed. It is formed in lower quantities than glycogen, usually around 5% of the dry weight, but can be as much as 20%. High levels show that the yeast is being subject to stress; for example, yeast cropped from very high gravity fermentations. This might sound a bit like the question of which came first, the chicken or the egg? To explain more simply, if we subject yeast to a fairly mild sub-lethal stress like a short heat treatment, it will make more trehalose. If the cell is then subjected to a much bigger stress like a higher temperature treatment, or indeed, any other stress, it is already in a better position to deal with it.