Lesson 6 - Yeast Propagation

Table: Markers for a successful propagation

Propagation theory

There are two thought processes used to decide how a propagation plant should be designed and used. Generally, these underpin both traditional and more modern plant.

In both cases, the aim is to produce a new culture that has the same physiological condition as pitching yeast. If this is achieved, the new culture should behave in the same way as conventional cropped pitching yeast.

A traditional propagation plant is often multistage. The first steps are aimed at generating growth. The later stages gradually acclimatise the new culture so that the yeast physiology is similar to cropped pitching yeast. What this means in practice is that the earlier stages use a relatively high temperature, to give rapid growth rates, and some oxygen/aeration to promote growth. In the later stages, both oxygen levels and temperature are reduced to values closer to the conditions used for a standard fermentation. In general, oxygenation is treated with caution and often use discontinuous aeration regimes. The plant is not designed with good oxygen transfer characteristics in mind.

This traditional method has a lot of underlying logic but brings problems as well. Cycle times for the whole of the brewery propagation cycle can be several weeks and since yields are generally quite low step-up ratios are also small, typically 1:5, so many stages are needed. This means several uncertain transfers and multiple tanks to run. Long cycle times are difficult to live with if several strains are in use, and the number of permitted cycles is small.

The modern approach is focused on speed and yield. Since all brewing yeast strains have optimum growth temperatures greater than 30 °C, propagation temperatures are quite high by brewing standards, typically 20-25 °C. This greatly shortens cycle times. High yields are made possible by the use of efficient continuous oxygenation systems which guarantee high rates of gas transfer. The relatively high yields make it possible to use fewer stages and bigger step-up ratios, typically 1:20, or greater.

As previously mentioned, the theoretical basis of modern plant designs makes use of the Crabtree effect. Providing glucose is present, metabolism is always fermentative, and mainly alcohol is produced. This also happens under aerobic conditions, but more of the carbon from pyruvate is pushed towards acetyl-CoA and from there to new biomass. In other words, the yeast should be similar in condition to conventional pitching yeast, but aerobic conditions ensure that you get more of it.

Yields from traditional plant are not very high, typically 60-100 million cells/ml; a continuously oxygenated plant should double this.

The best method for propagating yeast

There are many aspects within our industry where yeast and brewing collide; therefore, the real answers often lie somewhere in the middle of traditional and modern practices.

Many traditional propagation plants do not really deliver what is required. Often, the first generation fermentation is slower than the standard; the beer is also not typical and has to be blended. A partial explanation for this is that the relatively low yields of this type of propagation plant mean that the first fermentation is underpitched.

This is a significant problem where a brewery has to handle several yeast strains each which can only be re-pitched a limited number of times.

Modern high-yielding propagation plant solves some of these problems. Cycle times for the brewery phase take a few days, and the yeast is apparently of high quality based on usual methods for assessing viability and vitality.

Although the standard pitching rate is matched, first generation fermentations are often slower than standard, and beers are still blended. This is perhaps a bit surprising; the new yeast culture has been grown under aerobic conditions, and although the metabolism is repressed it should contain enough sterol and unsaturated fatty acid reserves not to need wort oxygenation when the yeast is pitched into the first fermentation. Since first generation fermentations are oxygenated as normal, we might have expected a more vigorous fermentation than is usual.

In an article published in 2010 (Boulton, C.A., Miller, K. & Smart, K.A. (2010). Yeast physiology, propagation and fermentation performance. Proceedings 23rd Annual Meeting of the ASBC, Rhode Island, USA) a production-scale fully aerobic propagation plant was studied. The main findings were that although the wort was fully attenuated, and the yeast had stopped budding at the end of the process, the cells had apparently arrested in mid-cycle. A significant proportion of the cells still had small buds on them, unlike normal pitching yeast in the resting G0 phase.

 After pitching into aerated wort, they had to complete this aborted cycle before moving into normal growth, which accounted for the longer than usual lag phase seen in first generation fermentations. Based on these observations, it seems likely that the newly propagated yeast needs a final treatment to allow it to complete growth and move into the resting phase. Perhaps similar to the acclimatisation stages used in some traditional propagation procedures.