This week I take a quick look at the major hop alpha acids and the role they play in brewing beer. I also discuss some of the recent research on cohumulone and how higher cohumulone hops may not be as harsh as is widely advertised.
Hop Alpha Acids
When you purchase hops, typically the alpha percentage(AA%) is listed somewhere on the package. This is a measure of the total alpha acids by weight in the dried hops you are purchasing. Alpha acids are converted into isomerized alpha acids during the boil phase of the brewing process and this produces the majority of the bitterness we perceive in beer. Brewers typically estimate the post boil bitterness in IBUs which is calculated directly from the hop schedule, boil time, hop quantities and alpha acids. I should note that the IBU level is not typically the level of bitterness in the finished beer due to changes that occur in fermentation as well as dry hopping, but it provides a good guideline for designing recipes.
While we tend to look at alpha acid as a single number, there are actually five hop acid analogs that make up the majority of the alpha acids in a hop code. The five are humulone, cohumulone, adhumulone, prehumulone and posthumulone. The five analogs are similar in their molecular structure, but all are transformed during the boil into iso-alpha acids.
Cohumulone and humulone make up the majority of the alpha acids in a typical hop. Each of these make up between 20 and 50% of the alpha acids. Adhumulone is a fairly consistent 10-15% of alpha acids for almost all hop varieties and the remaining two, prehumulone and posthumulone play a minor role in overall acid content. Each of the compounds isomerizes at a slightly different rate though this effect is not included in IBU estimation equations.
The Cohumulone Controversy
You will typically only see cohumulone listed on a hop specification sheet. You can safely assume that aduhumulone levels are near 10-15% and that humulone makes up the vast majority of the remainder of the alpha acids. So it is fairly easy to estimate adhumulone and humulone levels from just the cohumulone number. The highest cohumulone hops like Topaz, Bullion and Vic Secret have cohumulone levels in the 48-54% range. The lowest (hops like Vanguard) have cohumulone levels in the high teens. In general, aroma hops tend to have less cohumulone.
There is an interesting controversy over cohumulone and its effect on bitterness that remains a bit of an open issue (outlined here). A general rule of thumb with brewers is that hops with a higher cohumulone levels have a harsher bitterness. However, recent research has called this assertion into question.
The original research which you can find here was done by Dr Lloyd Rigby, a Canadian scientist way back in 1972. Dr Rigby brewed two beers using equal amounts of humulone and cohumulone and gave them to testers in a simple beer taste test. The testers found that beers made with higher levels of cohumulone were considered harsher, rougher and less pleasant to drink. This led to the rule of thumb above, which drove many craft breweries to seek lower cohumulone hops, and has even driven hop breeding and growing in a direction favoring low cohumulone, aromatic hops over the last 50+ years.
Subsequent research, however, has determined that Dr Rigby’s original experiment may have been flawed. The problem is that cohumulone and humulone actually isomerize at slightly different rates. In addition pH levels during fermentation affect each differently which also affects the perceived bitterness. So using equal amounts of the isolated alpha acids did not actually produce two different samples with the same bitterness level. So it is possible that the “harsh” beer with higher cohumulone simply had much higher bitterness than the one made with just humulone.
Brulosohpy did an experiment in 2016 where they made two identical beers except one used a high cohumulone hop (Chinook) and the other used a low cohumulone hop (Simcoe). In his experiment the tasters were evenly split over which beer they preferred, though the majority could tell the difference between the Simcoe and Chinook hops in a triangle test.
A similar test was presented earlier by Mike Karnowski from Green Man Brewery at the 2014 AHA conference, where he estimated that the original Rigby experiment could have had as much as a 62% difference in IBUs. He subsequently tried brewing beers with Cascade (35% cohumulone) and Simcoe (19% cohumulone) and then measured the actual IBUs. While the beer was estimated at 40 IBUs the Cascade (high cohumulone) beer had 42 IBUs measured and the Simcoe (low couhumulone) beer had 33 IBUs for a 27% difference!
The fact that measured IBUs differ from estimates is not a surprise – I outlined the major limitations of estimating IBUs here and also why the estimated IBUs will rarely matched the measured IBUs in a finished beer. However Mike went on to do more measurements and showed fairly conclusively that high cohumulone hops tend to produce finished beers with higher measured IBU bitterness levels, partially because cohumulone isomerizes at a higher rate, and partially because the isomerized cohumulone survives the pH drop during fermentation better. So with high cohumulone hops, more isomerized acid makes it into the finished beer.
He also went on to create beer samples with high and low cohumulone that had identical lab-measured IBU levels. Like the Brulosophy test, he found that there was not a correlation between higher cohumulone hops and perceived harshness when the IBU levels were actually the same.
So my overall conclusion from more recent literature is that higher cohumulone hops are not necessarily more harsh, but instead tend to produce higher levels of perceived bitterness in the finished beer. The reason for this is that the cohumulone isomerizes at a faster rate than other humulones, and also tends to survive the pH drop during fermentation better. So you can make good beer with higher cohumulone hops, but you may need to moderate the hops slightly to compensate for this effect.
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