Understanding Yeast Metabolism
Understanding yeast metabolism, how it works and what can influence it will go a very long way to help you, as a brewer, avoid myths and other pitfalls that arise on the internet based on someone's experience that never seem to work out for anyone else. Below, I'm posting a simplified version of the metabolic pathways that brewers yeast will use. In most instances, I have only included the end products and the hubs where multiple outcomes may occur, depending on what the brewer does or does not do. Use this chart to help you decide the flavours you want from your yeast and the best ways to get there.
I have made the hubs yellow, for easy identification and the inputs of NAD+ green and the outputs of NAD+ red. If you don't know what those are, keep reading because it's not quite as scary as your high school biochem textbook might have made it seem.
Lag, Log, Stationary, Dormancy.
Yeast, like many organisms, go though phases in which certain metabolic pathways are open and others are closed. These open and close in response to environmental and cellular conditions and it helps us to group them into phases.
In the Lag Phase, yeast do not take up nutrients. This is because their cellular membrane is made impermeable by trehelose, a sugar which will be dismantled. Glycogen is also present, which is the main storage carb for yeast. It is broken down into glucose to prime the machinery of the cell. Since no nutrients are available, the pathways branching off from pyruvate toward diacetyl is open. Oxygen can pass through the cell membrane, so the path toward Acetyl CoA is also open. To simplify our understanding, it is helpful to think of newly formed yeast as starting in Lag Phase, even if the rest of the population has moved on. In an underpitched situation, you essentially have an elongated Lag Phase.
In the Log Phase, yeast cells multiply logarithmically. Ideally, we want them to reproduce no more than 3-4 times. More than that leads to an extended Lag Phase and more than that leads to a more uniformly older population, ill prepared for dormancy. If you do not crop and repitch your yeast, it's not much of a concern, however.
In the Stationary Phase, yeast stop growing, usually because they run out of some nutrient. They switch to preparing for Dormancy and reduce the diacetyl and acetaldehyde they created in order to replenish coenzymes that will be needed to restart when conditions are better. Some brewers refer to this as "cleaning up", though it's not like they're doing it because they made a mess.
During Dormancy, yeast bind to each other and flocculate as a survival strategy. Oxygen is the chemical that will revive yeast and encourage them to come out of dormancy.
Once the Lag Phase has finished, the yeast cell will enter the Log Phase. in this phase, the yeast cell can absorb nutrients from outside the cell. It now has access to valine, so that branch of of the pyruvate hub will close. If the yeast cell is immature or too old, it may take a while for it to shut down completely, which is why pitch rate is important.
At the top of the chart you will find glucose. Yeast will use glucose, fructose, sucrose, maltose and maltotriose in that order. (In the case of lager yeast, it will also use melibiose.) If glucose levels are above 1%, the cell will not take up any other sugars (which require ATP to take up). Other species and mutations will cause the yeast to use even more sugars, but for all intents and purposes, the yeast cell will convert everything to either glucose or fructose before it enters the pathway. In the case of sucrose, invertase needs to be secreted to turn it into fructose and glucose before it can cross into the cell.
Pyruvate is the product of fermentation. Although yeast use oxygen, they do not (under brewery conditions) go through respiratory behaviour. The net result of one glucose molecule is 2 pyruvates and 4 ATPs, which if you remember from high school, is used as an energy source for all living things. Because there are 2 pyruvate molecules, everything branching off from that hub is doubled.
You will notice that two green coloured NAD+ molecules are required to get from glucose to pyruvate. These are coenzymes and they pick up a hydrogen ion when they enter the system. The yeast cell has a limited amount of these coenzymes and so they need to ditch that hydrogen ion somewhere else in the process. Once it gets to pyruvate, it can either do that by converting it to ethanol (remember there are 2 pyruvates, so you get 2 ethanols and also 2 NAD+) or by converting diacetyl which, for simplicity's sake, is a byproduct of making valine.
This path only opens up when oxygen is present, usually at the beginning of fermentation. The branches from that path lead to sterols, lipids and unsaturated fatty acids that they yeast need to prepare for a period of starvation. If no oxygen is present, the cell will still make Acetyl CoA, but it will make it from acetaldehyde instead of pyruvate. Oxygen will also divert some products from the Krebs "Cycle" to sterols and unsaturated fatty acids.
"Cycle" is put in quotations because in a brewery situation, it never actually completes. Oxygen is required, however, to convert many of the acyls resulting from it into lipids and sterols. If the yeast is using the acetaldehyde pathway to get to the Acetyl CoA rather than pyruvate, those acyls will join up with alcohols and produce esters. Organic acids will be produced either way.
Obviously, this is the end product that we want, but this is not always the case. Depending on the situation, you will end up with higher level alcohols (fusels) instead. This arises more frequently when the yeast is stressed. Yeast stressors are as follows: high gravity, high temperatures, high hopping, old, immature or low viability yeast. If you look at the names of esters, you'll see that some are from ethanol (ethyl) and some are from higher alcohols (isoamyl, isobutyl). The biggest factor in which higher alcohols are produced is based on strain. How much of it is produced is based on brewer control.
Putting it all together
Oxygen is the most important factor controlling ester formation, with gravity coming in second. High gravity means more nutrients, more stress, more growth, which all feed into more esters. While you can compensate for low oxygen with lower gravity, you cannot do the same in reverse. This is because there is a limit to how much oxygen will remain dissolved. (While it might seem like it would be possible to continually aerate, there are effects not shown here that render that impossible)
Temperature speeds up reactions, but some reactions are naturally quicker to execute than others. This leads to asymmetries, such as more higher alcohols then acyls or acetates able to turn them into esters. Temperature raised near the end of fermentation takes care of this problem.
As mentioned previously, the point of the chart is to plot what will happen in various brewery situations. Lets see if we can figure out what will happen if we follow some common advice.
underpitching your hefeweizen yeast to get more banana
Underpitching will result in a longer Lag Phase; it has no effect on ester production. Things that open up those pathways are lower oxygen rates and ramping up temperature near the end of fermentation. In the case of hefeweizens, (though not shown in this chart) increasing the proportion of wheat from 50% to 70% will also increase the amount of acetates to join up with isoamyls.
adding sugar near the end of fermentation will increase esters and reduce acetaldehyde in Belgian ales
I'm really not sure how this one gained any traction at all. At the end of fermentation, nutrients are low. Adding anything to the fermenter, especially at a home brew level, will add (some) oxygen. It won't be enough to decrease esters, butl uptake of sugars will cease until the glucose falls below 1% in a situation where all the nutrients will be gone. With no nutrients present, the yeast will make valine and likely crash into dormancy unprepared and leave you with a bunch of diacetyl.
Keep this page handy to see if you can predict what might happen whenever you run across advice, particularly if the person giving it to you can't explain why it works.