Lesson 7 - Yeast Propagation SAQ

**Conventional brewery propagation and the preparation of active dried yeast use very different processes. Discuss how the physiological state of yeast arising from each process differs and speculate as to what effects these differences might have on fermentation performance when each yeast type is used.

Response:

The only common feature of the two processes is that both are aerobic. In both cases, the provision of oxygen is designed to maximise yields. The key differences are the influence of sugar.

Brewery propagations use wort as the growth medium. Conditions are made aerobic by the addition of air or pure oxygen. The aerobic conditions maximise the yields of yeast, but the presence of high sugar concentration in the wort ensures that the physiological condition of the yeast is similar to conventional pitching yeast cropped from fermentation. Controlling the process at a relatively high temperature ensures that the growth rates are high, giving short cycle times.

Active dried yeast is grown in fed-batch conditions. This is a highly aerobic process and uses a different growth medium, usually molasses. The key difference, apart from oxygenation rates, is that the sugar is added to the growth vessel over the whole propagation, which is why it is called fed-batch. Feeding the sugar in this way makes sure that all the carbon is used for new biomass formation, and no alcohol is produced.

A comparison of the physiology of yeast arising from both processes indicates that they are very different. These differences are a consequence of the Crabtree effect. In the case of brewery propagations, the yeast is always repressed since although cultures are aerobic, the sugar is present until the very end. Usually, the propagation will be ended at the same time that sugar runs out. If oxygenation was continued after exhaustion of the sugar, the cells could become derepressed. In this case, oxidative growth on alcohol produced in the propagation could be used as a substrate, and a further cycle of growth could occur. This second growth stage is called diauxy (literally two growth phases). In a properly managed propagation, this should not be allowed to happen. When managed properly, the newly propagated yeast should be in the same physiological condition as yeast cropped from a fermentation. However, this is not quite the case.

Brewery propagations are aerobic, as we have discussed, and so the yeast should be capable of synthesising sterols and fatty acids (the yeast components that are essential for proper membrane function and which are required for growth under anaerobic conditions). Conventional pitching yeast is depleted in these lipids because they are diluted during anaerobic growth in fermentation. This is why, when this yeast is re-pitched, a supply of oxygen is required. So, newly propagated yeast, in theory, should require less oxygen when pitched into wort than cropped pitching yeast.

Active dried yeast is always derepressed since this is a consequence of the fed-batch process used to make it. This is because of the propagation phase, where conditions are always aerobic, but sugar concentrations are very low – less than what is needed to trigger the Crabtree effect. As a consequence, yields are very high. There are other effects too; yeast grown under derepressing conditions synthesise much more sterol and unsaturated fatty acids compared to yeast propagated aerobically in wort. This is why propagations using dried yeast do not require oxygen.

We may ask then, what difference does it make if derepressed yeast is pitched into wort? In theory, perhaps not too much at all because as soon as the cells are exposed to the sugar in wort, they will adapt to a repressed state, the same as conventional pitching yeast. This will take some time, of course, and perhaps this is why there is still a lag phase when dried yeast is used in brewery fermentations.