One of the trickier things to master as a brewer (and you'll soon figure out why) is the hopping rate. Now you can make a perfectly lovely beer that’s slightly too bitter or slightly under bittered and if you’re going for a classic American IPA, you can straight-up dump the entire bag of hops in your kettle if you really wanted to, though you'll waste a lot of wort and money.
However, to make a perfect beer, at least a coherent understanding of hop utilization is necessary. This becomes crucial when we start talking about styles where subtlety and balance are the key descriptors. Kölsch, Helles, Pilsner, American Lagers, anything Czech. Even Browns and Reds can suffer if over or under bittered. If you’re working with a style that has between 10-30 IBUs, an understanding of hops utilisation can mean the difference between nailing the style every time or banging your head in frustration.
To start, we know a lot about hops. Too much even. The problem is that there are a ridiculous number of variables and unless you’re running high pressure liquid chromatography on each batch of hops, you’re not going to be able to make a precise prediction about the actual IBUs in your beer. Even breweries don’t do this, but we’ll talk about what some of the big boys do to compensate later.
An IBU is 1 mg of isomerised alpha acid in 1 L of liquid. 1g of 5% AA hops has 50mg of alpha acid because 5% of 1g = 50mg. So if you add 1g of hops to 1L of wort with an AA content of 5% and boil it for one hour, (with no evaporation for the sake of simplicity), your litre of wort will have 17.5 IBUs, assuming everything went right and you got the maximum utilization of 35%.
In reality, that never happens, but we do know about what will influence the utilization rate up or down. This won’t allow you to make exact calculations, but it will allow you to troubleshoot if your beer is coming out too bitter, or not bitter enough.
To start, hops with a higher alpha acid percentage will have a better utilization rate than hops with a lower alpha acid percentage. Researchers pin this on the increased vegetal matter in lower AA hops adsorbing some of the isomerised acids, but no serious study has been done to find the reason. What this means is that 1g of 10% AA Magnum will give you more bitterness than 2g of 5% Hallertauer, even though they both contain 100mg of alpha acid. Traditional hop utilization formulas do not account for this.
Second, high gravity wort will result in reduced utilisation. This is well known and is usually accounted for in hop utilization formulas. BeerSmith uses Tinseth’s formula by default (though you can change it) and tends to produce fairly accurate numbers. Gary Tinseth has his own page here: http://www.realbeer.com/hops/ where you can calculate hops utilization yourself.
Moving on, elevation will change your utilization rate, assuming all other things are constant. This is because hops isomerise based on temperature and time. If you increase the pressure (near sea level) you will raise the temperature. If you move up to, say 2000 ft, you decrease atmospheric pressure, so water boils at 97°C. That small change will reduce hops utilization by about 10%. This works the other way as well – some breweries have boil systems that operate under pressure. (Volatile stripping is done in a separate step). Increasing the temperature can isomerise alpha acids extremely quickly, as well as produce the other reactions necessary during wort boiling, including creating protein-polyphenol complexes, reducing the pH, killing microbes, and creating foam-stable proteins. It should be noted that you can temperature stress your wort, but I’ll talk about that at another time.
Other things that are going to alter your utilization that aren’t taken into account in any formula is the addition of oxidized beta acids. Generally, we don’t want oxidized hops because of isovaleric acid (though this is also produced by brettanomyces and other infections) but beta acids in your aroma hops (which are usually an unknown quantity) will solublise without heating and are 66% as bitter as iso-humulone. Because these hulupones (oxidized beta acids) don’t’ need to be isomerised, they will contribute to bitterness just by dry hopping – particularly if they’re added after primary fermentation where the stripping action of CO2 and adsorption by trub won’t affect them. Technically, they don't fit into the IBU formula since it's only a calculation of iso-alpha-acids, but tastebuds don't know that.
Speaking of dry hopping, they can also reduce bitterness. The hop leaf material will adsorb bitterness, though we at least have a rule of thumb for this one; if your IBUs (up to this point) are less than 20, you’ll add bitterness by dry hopping. If they’re over 30, you’ll reduce it.
Stripping and Adsorption
Once we've finished the boil and moved on to fermentation, the yeast have more to say before we can figure out our final bitterness. Yeast cell walls will adsorb iso-alpha-acids once they die and decay, forming part of the trub, which is inevitable. What isn't inevitable is how vigorous the fermentation is. Generally, you want a healthy, vigorous fermentation, in which CO2 produced by yeast will strip out the more volatile iso-alpha-acids. This means that if your fermentation isn't as vigorous as normal, you're going to end up overshooting your bitterness targets. Minerals and excess trub or hop vegetal matter will all adsorb iso-alpha-acids leading to undershooting your target bitterness.
Thrown into all of this is a theoretical maximum dissolution of iso-alpha-acids at 8g/L, or 80 IBUs.
But wait! There’s more!
Not all iso-alpha-acids are the same! Iso-humulone is more bitter than iso-cohumulone and iso-adhumulone. Iso-humulone, however, is more volatile and boils away more rapidly, while iso-cohumulone is more soluble, but then there’s iso-cis-cohumulone, which is more bitter than iso-trans-cohumuone…
At this point, you’re probably pulling your hair out and wondering how anyone can possibly cope with all of this. The short answer is that you can’t. The long answer is that you start with a rough IBU calculation, do some recipe development and since you understand the variables that move IBUs up or down, you work from there. Then you hope that your hop supplier doesn’t crater or your water doesn't change!
So to summarise, things that will increase hopping utilization:
- High alpha acid content
- Low gravity wort
- Dry hopping below 20 IBUs
- Higher boiling temperatures
- Lower elevation
- Soft water
Things that will decrease hopping utilization:
- Low alpha acid content
- High gravity wort
- Dry hopping above 30 IBUs
- Lower boiling temperatures
- Higher elevation
- Excess trub
- Hard water