Lesson 5 - Yeast Biochemistry SAQ

**Your brewery wishes to increase its fermentation capacity and has decided on a dual approach of installing some new vessels with a capacity of 5000 hL, twice as big as currently used. In addition, it has been decided to increase the gravity of the wort from 15 ºP to 18.5 ºP. In order to reduce pressure on the brewhouse, which is not being upgraded, the increase in wort gravity will be via the addition of glucose syrup added directly to the current all-malt wort on the hot side of the paraflow. The brewhouse regime is not planned to be altered. The larger vessels will require six batches of wort, as opposed to three, and will increase the fill time from eight to fifteen hours. Pitching rates and wort oxygenation will be increased pro rata to allow for the increase in wort concentration. All yeast will be pitched at the start of addition of the first batch of wort.

Using your biochemistry knowledge of the growth of yeast in wort, what are the potential problems with this plan?

Response:

At first sight, the plan is a sensible approach to dramatically increasing fermentation productivity without an excessive capital spend. Since the new regime apparently matches the old one in terms of all the usual parameters, fermentation performance and beer analysis should not be markedly changed. However, a closer look shows that there have been two big changes. These are the longer fill time and the alteration to wort composition.

We shall consider these in turn:

Wort composition has changed a lot. The current wort is all-malt and therefore will likely contain mainly maltose and the usual spectrum of other fermentable and non-fermentable sugars.

The new wort contains a much higher proportion of glucose. We know that glucose is taken up first by yeast and that its presence represses the uptake of maltose. One of the consequences of this is that the start of maltose uptake will be delayed. It would be expected that the time to ferment the wort will be lengthened simply because of the greater concentration of fermentable sugars present; however, it will probably be even longer than anticipated because of the increase in glucose concentration and its repressive effects on the uptake of maltose (and longer chain sugars).

The second effect is that the sugar adjunct will dilute other wort components, most notably wort FAN. The ratio of sugar to FAN will be much higher, and this will affect how much yeast is grown. It will also probably change the beer flavour, prompting a need for other remedial actions.

The fill time is also an important consideration. In a typical fermentation, yeast will begin to bud around four to six hours after pitching. This will often be before all wort is pumped into the fermenter if all the yeast is pitched at the start of fermentation.

In the new scenario, the fill time is roughly doubled from eight to fifteen hours. In the original fermentations, most of the glucose will have disappeared soon after filling has completed. The uptake of maltose, the bulk sugar, will commence soon after fill. Under the new set of conditions, this will not be the case. Fresh glucose and, a lot more of it, is added with each new wort batch over fifteen hours, and so the repression of maltose utilisation will last much longer and result in even longer fermentation time.

The same argument will also apply to the uptake of any other wort nutrients which are used in an ordered manner. In particular, as we already know, this applies to amino acids. The combined changes in FAN concentration with respect to sugars and the longer fill time, will possibly also change the concentrations which are formed of yeast-derived beer flavour compounds.

Finally, let us consider the increased pitching rate and wort oxygenation. Should these have any effect? It is inevitable that there will be more oxygen present, but the Crabtree effect will continue to operate because of the presence of fermentable sugars: metabolism will continue to be fermentative even when conditions are aerobic. The amount of oxygen made available per cell has not changed, and therefore, total yeast growth should also be similar. The dilution of FAN, mentioned already, may be more influential in this regard. Again, the longer fill time may have unexpected effects since oxygen acts as a repressor of many genes, and some of these are involved in the formation of beer flavour compounds, as we will discuss more fully in later lessons.