The relationship between higher alcohols and esters
Moving right along.
So far we’ve talked briefly about esters, but only mentioned controlling them with oxygen and trub. Now let’s go a little deeper and discuss where they come from.
Starting from glucose, the cell moves it through the fermentation pathway (even in the presence of oxygen) to an acid called pyruvate. From here, it can go to either diacetyl through the ilv pathway (talked about in Yeast Flavors 3) or through the PDH bypass to become Acetyl CoA outside the mitochondria or to acetaldehyde to alcohol or Acetyl CoA inside the mitochondria. There’s a few branches here, but the important part to remember is that there is a reservoir of Acetyl CoA which becomes Acyls through the metabolism of fatty acid and a bunch of alcohols at the end.
Putting those two together is going to make esters. That requires two more things – energy and oxygen. Yeast spend energy making esters, which means they actually serve a purpose, which is similar to diacetyl reduction – to replenish a coenzyme that the yeast need, in this case, free CoA. Acetyl CoA is the starting point for making a bunch of stuff, including lipids and sterols and without oxygen to help replenish a free CoA part, the yeast make esters to do so. At any rate, while oxygen is present, the yeast cell won’t make esters as it’s an energy intensive process. That means that all the alcohols remain alcohols.
Higher Level Alcohols
The other part of the equation is alcohols – it’s not just all the happy fun drinkable kind, ethanol. Yeast growth, and anything that promotes it, creates more higher level alcohols. Those are the ones people refer to as fusels. This happens because of the need to maintain the redox balance. The creation of higher alcohols converts more NADH to NAD+ which is needed to convert glucose to pyruvate – no NAD+ and the yeast simply stop functioning. (This is also why yeast reduce diacetyl.) Temperature, gravity, yeast nutrients and yes, even oxygen, promote the growth of yeast which in turn promotes higher alcohols, and in turn, the fuselly, nail polish remover flavours you want to avoid.
This, right here, is why I write this blog. You’ve likely all heard about how adding nutrients or ramping the temperature up or down or adding oxygen will help your fermentation. It all needs to be taken in as a package to get the outcome required. Altering one thing without understanding the rest will create other issues that seemingly provide contradictory advice to control it.
Adding oxygen alone will repress (or down-regulate for you biochem nerds) alcohol-acyl transferase which puts the alcohols and acyls together. This means all the fusel alcohols that were produced from that same growth-promoting oxygen will make it into the wort unhindered. How do we fix this?
Limiting nutrients will definitely slow down growth, but is a poor choice because it will trigger the ilv pathway and promote diacetyl formation, as well as not leaving the yeast healthy enough to retake it up.
That leaves temperature, pitch rate and gravity. Lowering the temperature will slow down yeast growth, meaning fewer higher alcohols. Moving your pitch rate higher, between 1-2 million cells per mL will also lower the growth rate as there are fewer yeast generation cycles that can happen before the sugar runs out. Lowering the gravity will also limit the amount of yeast generation cycles, as well as keep yeast healthier and more able to take up diacetyl before they flocc out. The last one is more of a brewery concern as some breweries will concentrate the wort and then dilute it to get more out of less space, but I mention it so that when you brew higher gravity beers, you know that it will be a factor to consider.
To conclude, the brewer’s levers of power over yeast are as follows and need to be fully understood before using:
Yeast growth – promotes ester formation, fusel alcohols, diacetyl production
- Moving up: promotes yeast growth and rapid production of diacetyl, followed by a sharp decline.
- Moving down: restricts yeast growth and diacetyl production. Esters formed will be contained in the wort
- Ideal use: Keep lower to start and ramp up near the end of fermentation to promote the rapid decline in diacetyl.
- Moving up: promotes yeast growth and rapid production of diacetyl followed by a sharp decline. It also limits ester production by suppressing the enzymes that join acyls to alcohols. This promotes more fusel alcohols being produced
- Moving down: slows yeast growth, but restricts fusel alcohol production in doing so. Ester production is high as the genes that manufacture the enzymes are up regulated.
- Ideal use: 4-8ppm/L for more esters, 10-15 ppm for fewer.
- Lower pitch rate: promotes yeast growth as there’s simply more room for them to grow. Expect higher fusels, esters and diacetyl production. Be extremely careful underpitching as everything else needs to be right or the rampant growth will result in a large proportion of immature cells at the end with no diacetyl uptake capabilities and lots of fusels and esters. Basically a recipe for a hot mess.
- Higher pitch rate: promotes lower yeast growth as there are fewer sugars to take up. This is the ideal lever to chose if the goal is low esters/low fusels. Nutrients and oxygen need to be higher to avoid a large portion of elderly cells, but this is more of a concern if you do yeast cropping.
- Ideal use: 0.75 million cells/mL for a higher ester production in conjunction with lower oxygen, 1.5-2 million cells/mL for the cleanest profile, along with higher oxygen and nutrients, especially with high gravities.
- Higher gravity: means more room for yeast to grow and all the accompanying consequences
- Lower gravity: means less room to grow.
- Ideal use: Brew your beer, but be aware that this will usually require a higher pitch/O2/nutrient content than you would use otherwise.
- Higher nutrients: promote yeast growth. All malt brews generally don’t have a problem with nutrients, but if you use adjuncts, you might want some. It will also reduce the need for valine, which will promote a high amount of diacetyl produced, but a rapid uptake at the end. Ester formation will be unchanged, but fusel alcohols will be produced at a higher rate unless something is done to restrict yeast growth, such as lowered temperature. They can also be used with lower pitch rates and low oxygen rates if esters are desired, which results in higher alcohols being converted to esters. Use with caution and only if you understand how the other levers will affect your beer.
- Lower nutrient amounts: occur with a high portion of adjuncts, or if you’re growing a starter culture on a lower nutrient medium.
- Ideal use: I’ll use it when growing large starters or if I’m really trying to pump the esters, but that’s about it. It’s a good way to have things go sideways real quick