Ah, esters - those magical flavours and aromas that are produced during fermentation. It separates Pale Ales from Bitters and ales from lagers, at least superficially. If you've ever entered a beer into competition, you'll have likely run into a cryptic note about your beer having too many or not enough esters for the style, so what are they and how do you control them?
At it's most basic, an ester is an acid where one of the hydroxyls have been replaced by an alkyl, usually carboxylic acid and an alcohol. Yeast produce all sorts of acids during the conversion of sugars to energy through various pathways in the presence or absence of oxygen and/or glucose. I'll spend some time in a later article going over those pathways and how to influence them, but for now, we'll just assume that yeast is spitting out generic acids and generic alcohols nicely prepackaged for our enjoyment.
The esters, of which 90 different species exist in beer, are highly strain dependent. That means the esters produced by an English ale strain will be very different from those made by a Belgian ale strain and so on, though the way in which they are produced is fairly similar. Of those 90, only a handful are important to flavour and aroma.
- ethyl acetate - acetic acid + ethanol - pear
- isoamyl acetate - acetic acid + higher alcohol - banana
- isobutyl acetate - acetic acid + higher alcohol - raspberry/floral
- ethyl caproate - caproic acid + ethanol - apple/pineapple/blackberry
- phenylethyl acetate - acetic acid + higher alcohol - red apple/rose/honey
You'll notice that most of these use acetic acid, which is what they will degrade to in the presence of oxygen, leading to vinegar, as well as one containing caproic acid. It will degrade to goaty/cheesy over time. You'll also notice that "higher alcohol" is being used generically and exactly which higher alcohol is combined with the acid makes a large difference. This is where species of yeast play a role - an English ale will produce more isobutyl alcohol where as a Belgian strain will produce more isoamyl alcohols.
So how does one influence these esters and more importantly, how does one keep the ester production consistent? After all, most of these are desirable in certain styles and less so in others. To start, choose the appropriate strain. Making an English bitter with a Belgian strain will lead to disappointment, even if you do everything right.
Second, temperature plays a role, but not as large a role as you might think. To double the concentration of ethyl acetate, for example, you'd need to raise fermentation temperature from 10-25 degrees Celsius. I wouldn't recommend mucking about with the temperature as it's going to do other things to your beer aside from altering the ester profile.
Three other levers exist - Oxygen concentration, gravity and pitch rate. Gravity is generally determined by your style, unless you're planning on diluting, which leaves us with oxygen and pitch rate.
You should never underpitch - however, it should be noted that what BeerSmith recommends for a standard ale is around 200 billion cells per 5 gallon batch, while both White Labs and Wyeast recommend half that amount. The confusion comes from the rough formula °P x (Million cells per mL). For a fruitier, more estery british style ale, aim for about 0.75 million cells per mL. For a cleaner, American style ale, aim for 1 million cells per mL. Then multiply by °P - however, since most homebrewers measure gravity and not °P, you'll need to convert that. For the lazy, who don't like formulas, you can do that here. It should be noted that this applies to standard gravity ales and higher gravity ales have their own formulas, as do lagers.
The last lever then, is oxygen. It's also the lever that will produce the most results. For fruiter styles, less oxygen is needed, and so forth. I'll make myself somewhat unpopular here and say that if you want a clean ale, you won't get it without using pure O2. I'm subjecting myself to a litany of comments from people who will regale me with stories of how they only splash and don't have any esters - that's simply not true, you just don't notice them. Even lagers have esters and there are multiple studies to back that up.
Anyway, back to oxygen. If making british ales, which I tend to do, splashing is sufficient. You'll want to do this when it's at pitching temperature and aerate it as much as possible through stiring or simply letting it fall during transfer to the fermenter. This will give you, ideally, 8-12 ppm of oxygen. Oxygen is used by yeast to create sterols that repair the cell wall after budding has finished. Without this, the yeast will leach esters into the beer, along with some other nasty byproducts that it might not be able to clean up if the pitch rate isn't sufficient or the yeast isn't healthy.
For American style ales, I recommend an O2 tank and a sintering stone, also called a ceramic candle in some literature. Again, you'll want to do this at pitching temperature as oxygen and other gasses dissolve better in cold liquid vs hot. I'll go over how to measure dissolved oxygen in a future article, but for now, suffice to say that you're aiming for 12-18 ppm. For lagers, and high gravity ales, you're going to want to be on the upper end of that scale. Beyond that is difficult to achieve and will also damage the beer.