How to brew decaf

Almost everything written about brewing decaf traces back to one assumption: that decaf under-extracts, so you should grind finer. It gets repeated in roaster blogs, in shop training, and in most of the guides currently ranking for this query.

The only controlled study I can find says the opposite, and the trade press has started saying the opposite too. Here is what actually holds up.

What the one controlled study found

Mark Al-Shemmeri, a coffee process researcher, ran a comparison of decaf against a reference coffee from the same lot across matched roast profiles, working with Caravan Coffee Roasters. The findings that matter:

Green bean density and porosity: no significant difference. This is the first casualty. The widely repeated line that decaf is “lighter and less dense because the caffeine has been removed” — which Perfect Daily Grind itself published in 2017 — is not supported by the measurement. Moisture content was about 1% lower in the decaf; density and porosity were not meaningfully different.

Decaf ground finer at the same grinder setting, with a median particle size 8–38 μm smaller and a greater percentage of fines. More fines and a finer median both push toward faster extraction, not slower.

Al-Shemmeri explicitly flagged that this contradicted what the roastery staff believed — they were convinced decaf needed a finer setting. That is a clean example of controlled data overturning shop-floor lore, and it is why I would not trust the folk rule here.

Roasting: decaf ran hotter, hit first crack at a higher temperature, showed lower mass loss, and developed darker and more homogeneously. That last part is the practically important one.

So which way do you grind

Honestly: unresolved, and anyone telling you otherwise is guessing.

• Perfect Daily Grind’s 2026 position is coarser — “decaf extracts faster than caffeinated coffee”, so coarsen to avoid over-extraction and bitterness.
• Most roaster blogs and most baristas say finer, on the under-extraction premise.
• The controlled data is consistent with the coarser camp: if decaf already grinds finer and throws more fines at a fixed setting, and roasts darker for the same visual target, both effects speed extraction up.

The defensible advice is narrower than either camp wants: decaf needs its own dial-in, and the direction is not predictable from the fact that it is decaf. If your decaf is bitter, dry or hollow in the finish at your normal recipe, try coarser first — that is the move most people never make because the folklore points the other way.

The thing that is actually well-supported

Colour is an unreliable roast cue for decaf. Decaffeinated green coffee is already several shades darker than normal green — the processing discolours it — and it develops darker and more uniformly in the roaster. Roast by measurement and by time-temperature curve, not by eye or by ear.

This matters to you as a drinker even if you never roast, because it explains most bad decaf. A great deal of decaf on the market is over-roasted, not badly decaffeinated, and it is over-roasted because the visual feedback lies. If a decaf tastes ashy and flat, the roaster probably took it too far, and no brew adjustment will fix it.

For anyone roasting: lower charge temperature and gentler heat application, on the basis that the cell structure has already been steamed and swollen. Perfect Daily Grind suggests 190–204 °C charge. That one is trade advice, not measurement.

What is folklore

“Decaf degasses faster.” I could find no published study measuring CO₂ degassing rate in decaffeinated versus regular roasted coffee — not in the SCA’s staling literature review, not in the time-resolved gravimetric degassing work. It circulates constantly in roaster blogs. There is nothing behind it that I can find. If you have been resting your decaf differently on this basis, you are acting on an untested assumption.

“Decaf is less dense so you need less of it.” Contradicted above.

“Decaf stales faster.” Same category as the degassing claim. Unmeasured.

A practical starting point

Rather than a rule, a protocol. Take whatever recipe works for your regular coffee — same brewer, same ratio, same water, same temperature — and change exactly one thing at a time:

1. Brew the decaf on your existing recipe first. Do not pre-adjust. You need a baseline in the direction the coffee actually goes, not the direction you expect.
2. If it is bitter, dry, or thin-and-harsh: go coarser, two clicks at a time. This is over-extraction and it is the more common failure.
3. If it is sour, weak, and watery: go finer, or raise the temperature to 94–96 °C.
4. Change one variable per brew. Two changes tell you nothing.
5. Write it down. Decaf lots vary more between processes than you might expect, and a dial-in you can’t reconstruct is a dial-in you’ll redo.

Al-Shemmeri’s other finding is worth ending on: chemical and sensory differences between decaf and regular persisted even when particle size and extraction were matched. Whatever a decaf tastes like, it is not simply an extraction artefact you can dial away. Some of it is just what the coffee is now.

Sources

• Mark Al-Shemmeri — the effect of decaffeination on roasting and grinding performance
• Perfect Daily Grind — does decaf deserve better (2026)
• SCA — what is the shelf life of roasted coffee: a literature review on staling