How many of you have heard this story: a new brewer pulls off a few great batches, dives head first into the hobby, does a bunch of research, buys a bunch of equipment and then all of a sudden, their beer is not so great anymore? How many of you are that person?
One of the things that happens when brewers move from new to intermediate is that they learn just enough to mess things up. As they say: a little knowledge is a dangerous thing. One of the things that inevitably happens is a previous recipe turns out super alcoholic. In order to understand why this happens, we need to understand yeast and a little bit about their metabolic pathways.
Yeast are wonderful organisms, perfectly adapted for fermenting wort into beer. They can function with or without oxygen and generally do what we ask them to. However, they can only use what we give them, and often giving them too much of a good thing will cause unexpected results.
Intermediate brewers know that you should oxygenate your wort. Some of them have even purchased oxygen tanks and stones. They know that oxygen is used by the yeast to make lipids and sterols, but beyond that, we start getting into advanced brewing knowledge.
So here's some advanced knowledge:
Oxygen is indeed used by yeast to make lipids and sterols. These, in turn, are used to help yeast grow. However - yeast can grow without oxygen, they just end up depleting their own reserves every time they bud, which can lead to premature cell death and then they floc out early.
Have you ever asked yourself, why, if yeast can grow to whatever quantity I want, do I need to pitch a certain amount? The answer is higher alcohols. Higher alcohols are caused by yeast growing and oxygen helps them do that while remaining healthy until a different nutrient runs out.
In fact, the presence of oxygen in the wort switches the metabolic pathway from producing ethanol to producing lipids and sterols. Once the oxygen is used up, yeast go back to producing ethanol. These fully charged cells will bud and reproduce until they can't anymore.
There's another part of this equation that becomes important, which is a coenzyme. This coenzyme, called NADH is responsible for carrying and accepting oxygen molecules in order for the metabolic pathway to work. If it's all of it is oxidised, energy production stops and fermentation is over. To prevent that, the yeast cell have ways of balancing NADH by reducing it (the opposite of oxidation). One of these ways is producing ethanol.
If you'll remember, we divert away from making ethanol when we make lipids and sterols. That means the yeast cell needs to balance NADH somewhere else, and that comes by making higher alcohols.
So what you as the brewer want to have happen is for the yeast to grow, but not too much. If there is no oxygen, the yeast will run out of reserves before fermentation is finished. If there is too much oxygen, the yeast will over grow and you'll get beer that tastes like nail polish remover.
This is where pitch rate comes in. Ideally, you want your yeast to reproduce only 3-4 times. More than that will be too much growth and too many higher alcohols. Less than that and there will be other problems I'll cover in a different article. Your pitch rate should be as follows:
- 1 million cells/mL/°P for clean standard ales with 8 ppm oxygen
- 0.75 million cells/mL/°P for estery standard ales with 8 ppm oxygen
- 1.5 million cells/mL/°P for higher gravity ales or standard gravity lagers with 10-12 ppm oxygen
- 2 million cells/mL/°P for higher gravity lagers with 15 ppm oxygen
As a rule of thumb, a standard gravity starter will get you around 100 million cells/mL on a stir plate after 2-3 days at room temperature.
If you don't have a dissolved oxygen meter (which, lets be honest, are expensive and difficult to calibrate) you can use the following rule of thumb:
- Splashing vigorously gets you 8 ppm
- Using an oxygen stone for one minute gets you 10-12 ppm
- Using an oxygen stone for 2 minutes gets you to 15 ppm, if the wort is quite cool.
When using an oxygen stone, you'll want to see tiny bubbles on the surface of the wort and move it around. More than this and you're just pumping oxygen into the atmosphere. For 15 ppm, it's usually easier to make yourself an inline system so that it oxygenates the wort as you transfer it from your whirlpool to your fermenter.
Lastly, you'll notice that a lower pitch rate and the same amount of oxygen results in esters. This is because of another enzyme, Coenzyme A, or CoA for short, acts in a similar manner to NADH, only it carries something different. During the lipids and sterols manufacturing process, if there isn't any oxygen at the end of making a higher alcohol, the yeast will need to regenerate CoA. To do this, it will take one of the higher alcohols, merge it with an acetyl and an ester will be formed. This is often the place where intermediate brewers find their beers that were once great, stop being so great.
If you take a beer, lets say a standard estery IPA that did very well with a lower pitch rate and just splashing for oxygen and then suddenly add a bunch of oxygen, but not change the pitch rate, you'll end up with a huge amount of yeast growth and a corresponding increase in higher alcohols.
So remember - if you're going to add oxygen, increase the pitch rate as well!