Lesson 6 - Yeast Propagation

We probably all know what propagation means in the context of horticulture. It simply means growing new plants from an existing parent stock. We use the same principle in brewing where a culture of yeast is used to grow more cells until we have enough for brewing.

Definition of propagation

Propagation is the periodic introduction into a brewery of a yeast culture of defined purity, quantity and quality. In this lesson, we will consider this more and what it means in real brewing situations.

Why propagate?

Some brewers use spontaneous fermentations. This is where the wort is inoculated with wild yeast and the bacteria present in the atmosphere. Wort cooling is arranged to maximise this chance contamination using open vessels with large surface areas, sometimes called coolships.

Both the atmosphere and surroundings are controlled to make sure the natural microflora is not disturbed. This microflora adapts over time to become dominated by yeasts and bacteria that can grow in wort. In a sense, this is a process of propagation but not one with much intervention by the brewer.

A few brewers use mixed cultures made up of two or three selected yeast strains, or more rarely, mixtures of brewing yeasts and bacteria. Often the fermentations are re-pitched continuously without a propagation step. In these circumstances, it is difficult to be confident in what is actually present at each pitching. Some craft brewers use commercial starter cultures made up of defined mixtures. These can be used once only, and no propagation step is needed. If fermentations are cropped and re-pitched, the uncertainty mentioned already applies.

In more conventional practice, all breweries make more yeast than they can use so at first sight, we would think there is no need to go through all the processes of growing more.

However, in most brewing operations, yeast deteriorates more with every time it is cropped and re-pitched. Deterioration might be a result of cellular damage or genetic mutation as a result of the stressful conditions that yeast encounters in some very intensive fermentations. It might be that a portion of the crop has been collected which is enriched in cells which are in some way compromised or it might be that the yeast is contaminated with beer spoilage organisms.  One option is to keep re-pitching until there is some obvious evidence that the behaviour of the yeast has declined, and then throw it away. This is not a very sensible way to run a brewery, and most brewers stop using yeast before behaviour has changed – replacing it with a new culture.

Yeast taken from top-cropping ale fermentations, if care is taken to get the best cut, is possibly the best quality that can be achieved. Most non-yeast solids drop out, and so a clean, pure crop can be yielded. However, there is still a risk of picking up contaminants, increases with each cropping.

For the vast majority of brewing fermentations, yeast is bottom cropped. This process can create even more problems:

• Although we can try to avoid most of it, some contamination with cold break/trub always happens. The more times yeast is recycled, the worse this becomes as it causes pitching-rate control errors.

• We also have to decide which part of the crop to keep. Bottom crops are heterogeneous, and the more flocculent cells drop out first, as do bigger (older and less vital) cells. Less flocculent type cells drop out last. We could try to keep just the middle of the crop, but this is an imprecise and inexact method.

• Many of the modern intensive fermentations (using relatively high temperatures, very concentrated worts and big vessels) are particularly stressful for yeast. Apart from causing viability losses, the risk of mutations arising is a lot higher. The more times the yeast is recycled, the bigger the risk of ending up with an inferior quality yeast, or a significantly different variant.

Add all these together and you have a strong case for limiting the total number of pitches. Many of the bigger brewers using these very stressful fermentations are limiting the number of cycles to just three to five. For worts of moderately high gravity a range of five to ten cycles is typical. This puts a lot of pressure on propagation plant and explains why efforts have been made to make them higher yielding and with quick turn-round times, as we will see later.