Cohumulone and the harsh bitterness debate

Alpha acids come in three main forms

The bitterness in beer comes mostly from iso-alpha acids, formed when hop alpha acids isomerise during the boil. Alpha acids are not one compound but a family. The three main members are humulone, cohumulone and adhumulone. They differ only in a small side chain on the molecule. Our guide to alpha acids covers how they form bitterness.

Hop analyses report cohumulone as a percentage of total alpha acids. Across common varieties it ranges from under 20% to over 40%. Many traditional European aroma hops sit around 20 to 28%. Some older American bittering varieties, such as Cluster and Galena, sit near 40%.

The harshness idea dates back to the 1970s

Research published in the early 1970s suggested that beers bittered with high-cohumulone hops tasted harsher than those made with low-cohumulone hops. The finding spread quickly. Breeders began selecting for low cohumulone. Brewers started reading the cohumulone figure on hop certificates as a quality mark.

For years, low cohumulone was simply assumed to mean smooth bitterness. Many brewers still believe it. Some modern varieties are marketed partly on their low cohumulone share.

Later tasting work has been mixed

Since then, sensory studies have given mixed results. Some found a difference in bitterness quality between high and low cohumulone hops. Others found little or none once the bitterness level was matched. Part of the problem is that hops differ in many ways at once. Two varieties with different cohumulone levels also differ in oils, polyphenols and beta acids, all of which affect how bitterness is perceived.

The honest summary is that cohumulone alone does not reliably predict harsh bitterness. It may play a part, but it is not the only factor and probably not the main one.

Cohumulone changes utilisation more clearly than taste

One effect is better established. Cohumulone is more soluble in wort than humulone and isomerises into iso-cohumulone more efficiently. So a high-cohumulone hop can give somewhat more measured bitterness per gram of alpha acid than a low-cohumulone hop at the same alpha percentage.

That matters if you swap one bittering hop for another. Replace a 22% cohumulone hop with a 38% one at the same alpha dose and your IBU reading may come out higher. Some of the perceived harshness in old comparisons may simply have been more bitterness than expected.

Harsh bitterness usually has other causes

If a beer tastes harsh or lingering, look at these before blaming cohumulone:

• Water. High sulphate sharpens bitterness. High alkalinity makes it coarse. Our guide to bittering hops for clean lager bitterness covers the water side.
• Hop material. Large amounts of leafy or vegetal matter, especially from late additions, add polyphenols and astringency.
• Old hops. Oxidised hops bring their own rough bitterness, as our guide to resin and bitterness from old hops explains.
• pH. High kettle or beer pH extracts more harsh compounds.
• Yeast in suspension. A hazy young beer can taste more bitter than the same beer once clear.

The cohumulone figure moves from lot to lot

Cohumulone is measured by HPLC, the same analysis that gives the alpha acid figure. Like alpha acid, it varies with the growing season, the farm and the harvest date. A variety listed at 20 to 25% cohumulone may come in at either end of that range in a given year.

Variety descriptions quote typical ranges, not guarantees. If cohumulone matters to you, read it from the certificate for the lot you actually bought. Treat a published variety range as a guide only. Most hops used in India arrive with a certificate from the supplier, so the number is usually there for the asking.

How to choose a bittering hop today

Cohumulone is still worth reading. Just weigh it with the rest of the certificate. For a clean lager or a delicate pale ale, many brewers still prefer a high-alpha hop with moderate cohumulone, because it adds little vegetable matter for the bitterness it gives. Hop extract, such as CO2 extract, is another option with very little plant material at all.

Whatever you use, measure it. A lab IBU or iso-alpha acid test on a few batches tells you more than any rule of thumb. Here is the floor detail: when you swap bittering varieties, brew the next batch with the same target IBU and taste it next to the old one before you change anything else. Sorting folklore from mechanism is what brewing science is for.

Run a bittering swap trial

Pick your house pale beer. Brew one batch with your usual bittering hop and one with a variety of very different cohumulone at the same calculated IBU. Taste them blind side by side. Your own result, on your own water and malt, will settle the question for your brewery.

Common questions

What is cohumulone?

One of the three main alpha acids in hops, alongside humulone and adhumulone. It is reported as a percentage of total alpha acids.

Does cohumulone cause harsh bitterness?

The evidence is mixed. Early research suggested it did, but later sensory work found little difference once bitterness levels were matched.

Does cohumulone affect IBU?

Yes. Cohumulone isomerises more efficiently, so high-cohumulone hops can give somewhat more bitterness per gram of alpha acid.

What else causes harsh bitterness?

High sulphate or alkalinity, a lot of vegetal hop matter, oxidised hops, high pH and yeast still in suspension.