One of the issues roasters ask me about most frequently has to do with the difference between roasting for filter or espresso. It's a frustrating topic, not least because I don't know exactly what others mean by "filter roast" and "espresso roast." Most roasters roast darker for espresso, but one roaster's "filter" may be darker than another roaster's "espresso" roast; there are no universal definitions. Years ago a client implied on his blog that I advocate "omniroasting" (i.e. roasting the same for espresso and filter.) That was an assumption he made because I had asked him why he roasted differently for espresso and filter. To him, my question implied that I believed in omniroasting, but that wasn't my intention.

I don't have a strong opinion about omniroasting, but I'd like to share some thoughts and let you draw your own conclusions. My hunch is that most roasters roast espresso darker because of some combination of these three factors:

Temperature

Underextraction

Milk

Temperature

When brewing filter coffee, the ratio of water: grounds (by weight) is usually something like 17:1. When brewing espresso, the water involved in the process (water that gets into the cup + water absorbed by the grounds) relative to grounds is typically 3:1 or 4:1. Let's do a back-of-the-envelope calculation:

Batch brew:

water: 1700g @ 94° c (201°f)

grounds: 100g @ 25° c (77°f)

If you were to mix the grounds and water together, and the average temperature will be near 90°c. (194°f) (I know this a gross oversimplification of slurry temperature. Please bear with me and focus on the forest, not the trees.)

Espresso:

water: 70g @ 94° c (201°f)

grounds: 20g @ 25° c (77°f)

average temperature of the system: 79° c (174°f)

Not only is the average temperature much higher in a batch brew, but espresso extraction is much more "front loaded," meaning a disproportionate amount (>50%) of coffee solids are removed in the first few seconds of an extraction-- and, that early extraction happens at a very low temperature, perhaps around 70° c. When brewing filter, extraction is less front-loaded and early extraction occurs at much higher average temperatures.

Hey Rao, does this really matter?

Good question. When's the last time you set your kettle temperature to something like 81° c (178°f) for a hand pour? Probably never, but if you did, what was the result? I'm guessing the coffee was rather sour. My point is that the lower temperatures of espresso extraction tend to make coffee sourer. And to combat sourness, roasters tend to roast darker.

Underextraction

Before there were VST baskets and the coffee refractometer, espresso underextraction was the norm in specialty cafes. Poor grind quality (still an issue, though there are finally a few espresso grinders out there able to extract normales above 20%), and the popularity of dosing 18g--20g (into baskets meant to hold 14g) contributed to frequent underextraction. For several years, third-wave cafes and their predecessors were almost always serving sour, sub-19% espresso extractions.

Underextraction, like low extraction temperatures, often makes coffee sourer. I believe chronic underextraction contributed to the popularity of roasting darker for espresso than for filter coffee.

Milk

I'd estimate that more than 80% of espresso shots pulled outside of Italy end up in milk-based drinks. Simply put, most roasters and consumers seem to prefer espresso to be roasted a little darker when it is to be consumed in milk rather than black.

Should you roast darker for espresso?

If you're underextracting and combining your espresso with milk, yes, almost certainly. If you're confident your extractions are high enough (let's say >19.5% on most grinders, >20.5 on an EK) to avoid sourness, the answer is not so clear. Even when roasting for black coffee (ie not combined with milk) to be pulled at a high extraction, there is still the issue of most of the extraction occurring below 80° c, and the potential for sourness. I assume the extra sourness of such low extraction temperatures would still lead most roasters to prefer slightly darker roasts for espresso than for filter.

The More Important Consideration

When roasters ask me about roasting for espresso vs. filter, I usually emphasize that whatever my opinion of the issue, it's more important to focus on whether one should roast differently for black coffee (no milk) vs. white coffee (cappuccino, latte, etc). To me, the difference in roasting for black vs. white coffee is significant; the difference in roasting for filter vs. espresso is modest. That delicate, lightly roasted Yirgacheffe that tastes sublime as a straight espresso may drown in a cappuccino. Likewise, I've had delicious cappuccini made from shots I would have found flat or slightly roasty, had they been served black.

The Bottom Line

Don't worry too much about omniroasting or other philosophical issues. Try not to be dogmatic about how light or dark a roast must be, what's popular, or what you're "supposed to do." Measure often, taste blindly, and try to be logical (ie "if extraction temperature is lower, coffee may taste sourer, so perhaps I should try roasting darker and see if it's better.") In the end, go with what tastes good, even if the internet disagrees or this year's barista competition finalists did the opposite. I promise in a year or two the winner will do the opposite of what this year's winner did!

I've only touched upon the surface of this issue, so please share your thoughts.