Lesson 2 - Yeast Morphology and Evaluation

Welcome back. So, what were your thoughts? Are all yeast cells the same?

Yeast cells are very small, and we use a lot of them in commercial fermentations. Imagine a fermentation of say 1,000 hL, pitched at 10 million cells/mL – think how many cells there must have been at the start!

The maths is: 10 x 10⁶ x 1,000 (per litre) x 100 (per hL) x 1,000 (per 1,000 hL)

That equals 1 x 10¹⁵ cells, or 1 with 15 zeros after it – a big number! In order to make things easier for ourselves, we assume that all of these cells are identical. However, this is not correct. The asymmetric method of budding means that individual cells will bud at different rates during fermentation, some possibly more than others. Individual cells can only bud a certain number of times before they die. Like us, yeast cells have a finite lifespan during which they age. Like us, older and very young cells cannot function as well as those in middle age. Like us, older cells are bigger than young ones. This is normal and would perhaps not matter if we did not crop and re-pitch our yeast.

• Yeast crops contain a mixture of cells of varying ages.

• Bigger cells tend to be older. In some bottom cropping operations, these cells will drop out first, followed by middle-aged ones, and finally by younger cells (assuming they are all equally flocculent – which is not necessarily the case). Depending on which fraction of the crop you keep for re-pitching, it is possible that you may select a less than perfect fraction.        

• The more times we re-pitch our yeast, the more these risks grow.

• We can remove this potential risk by using dried yeast, once only.