Catty notes in hoppy beer and when ribes aroma becomes a fault

Ribes, catty and blackcurrant leaf

Brewers borrowed the word ribes from the botanical name for blackcurrant, Ribes nigrum. Crush a blackcurrant leaf and you get the smell: green, fruity, a bit sharp. Push it further and it becomes what tasters call catty, the smell of a tomcat's spray or a neglected litter tray. Sauvignon Blanc wine drinkers know the same spectrum, from gooseberry and passion fruit to, at the extreme, something best described politely.

Catty sits on the standard beer flavour wheel and it can mean two different things in two different beers. That is the first thing to understand.

The compound behind it

The main culprit in hoppy beer is a sulphur compound called 4-mercapto-4-methylpentan-2-one, usually shortened to 4MMP. It belongs to a family of aroma compounds called thiols, which carry much of the tropical and blackcurrant character of modern hops. Thiols are potent. Our noses detect 4MMP at levels measured in nanograms per litre, so a tiny change in concentration moves it from pleasant fruit to cat.

Hop varieties differ a lot in how much 4MMP they bring. Brewers commonly associate strong blackcurrant and catty notes with varieties such as Simcoe, Summit, Citra and Nelson Sauvin, though crop year and growing region change the picture. The same variety can smell of passion fruit one year and of cat the next.

When catty is a hop note

In a fresh, heavily dry-hopped IPA, a light blackcurrant or catty edge is often just the hops talking. It is common in beers built on the varieties above and many drinkers enjoy it at low levels. It becomes a fault when it dominates, when it covers the fruit and citrus the brewer intended or when the brewer did not want it at all.

Judging it is about balance. Smell the beer from a little distance, then put your nose in the glass. If the catty note is the first and loudest thing you notice, it has crossed the line. If it hides behind citrus and stone fruit and adds a green edge, it is probably working.

When catty means the beer is old

Catty has a second meaning. On the beer flavour wheel it is listed with papery and leathery as a stale flavour, the kind that develops as beer ages and oxidises. A pale lager with no expressive hops that smells of blackcurrant leaf or tomato leaf has usually spent too long warm. In that context it is never a hop choice. It is a freshness problem.

Indian conditions make this more common than it should be. A lager that spends weeks in a warm warehouse or a hot truck ages much faster than one kept cold. Our guide to tomato and ketchup notes in old hoppy beer covers the related staling notes in IPAs.

Serving temperature changes the verdict

The same IPA can seem fruity at one temperature and catty at another. Straight from a fridge at 3 or 4 °C, many aroma compounds stay in the beer and the glass smells muted. As it warms towards 10 to 12 °C, more of them reach your nose, thiols included. A beer that seemed clean in the first sip can turn catty halfway down the glass.

That matters when you judge a beer. Taste it cold, then again after ten minutes. If the blackcurrant grows into something unpleasant as it warms, that is a beer close to the edge. Brewers running a tasting panel serve at a set temperature for exactly this reason, so that results from different days can be compared fairly.

What brewers do about it

Brewers who want the fruit without the cat tend to work on three things:

• Hop selection. Rub and smell each lot before buying and run a hop tea sensory test to see what a variety gives in liquid rather than in the hand.
• Blending varieties. Pairing a catty hop with citrus or stone fruit varieties keeps the blackcurrant in support.
• Dose and timing. Very high dry-hop rates concentrate everything, catty included. Brewers dialling in a recipe often find a lower rate tastes fruitier.

Copper is worth a mention. Copper ions bind thiols and reduce them, which is one reason old copper brewhouses were associated with cleaner sulphur profiles. Most modern kit is stainless steel, so this effect is largely gone.

A practical way to calibrate your nose

Thresholds for thiols vary widely. Some drinkers barely notice catty notes. Others find them in nearly every modern IPA. Knowing where you sit helps you judge other people's tasting notes as well as your own.

Your first step: buy a punnet of blackcurrants or a blackcurrant cordial, smell it next to a Simcoe or Citra hopped IPA and decide where pleasant fruit ends and cat begins for you.

Common questions

What makes a beer smell catty?

Usually 4MMP, a thiol from certain hop varieties. In an old lager without strong hops, catty can also be a sign of staling.

Is catty aroma always a fault?

No. A light blackcurrant edge can suit a hoppy IPA. It is a fault when it dominates or when it shows up as a staling note in an old beer.

Which hops give catty notes?

Brewers often link Simcoe, Summit, Citra and Nelson Sauvin with blackcurrant and catty character, though crop year and region change it.

What does ribes mean in beer tasting?

Ribes comes from the botanical name for blackcurrant. It describes the blackcurrant leaf aroma that can become catty at higher levels.