Dialing in a light roast on a single-boiler machine
Light roasts on a cheap single-boiler machine are supposed to be a losing battle, and for the first week with this bag, a washed Ethiopian roasted nine days before I opened it, they were. Sour, thin, with a sharp finish that made my jaw tense. I kept a log of every shot and changed one variable at a time, which took eleven shots and most of a 250 g bag to sort out.
Shots one to four: 18 g in, grind at my usual setting for medium roasts, 36 g out in 28 seconds. Sour and watery. Grinding finer, two clicks at a time, got me to 36 g in 38 seconds by shot four, which was less sour but now bitter at the end and still thin. That is the classic sign that the problem is not the grind alone.
Shots five to seven: temperature. The machine has no display, so I use the trick of flushing a measured amount of water through the group after the heating light goes out and timing from there. Waiting 25 seconds after the light instead of pulling immediately gave what I estimate is 3 or 4 degrees hotter. The shot at 18 g in, 36 g out, 34 seconds, with the hotter water, lost most of the sourness. The fruit came through, something like dried apricot, and the finish was cleaner.
Shots eight to eleven: ratio. Lighter roasts are denser and less soluble, so pulling longer helps. 18 g in, 45 g out, a 1:2.5 ratio, same grind, same temperature trick, 40 seconds. That was the shot. Sweet, bright, a little tea-like, no jaw tension. Pushing to 1:3 made it thin again.
So the recipe for this bag on this machine: 18 g, grind five clicks finer than medium roast, wait 25 seconds after the heating light goes out, 45 g out in about 40 seconds. It is a long pull and the machine is near its limit. Two things I noticed along the way: the first shot of the morning was always worse than the second, which I put down to the group head being cooler than the boiler, and preinfusion by hand, pumping for three seconds and pausing for five, reduced channeling in the finer grinds enough to see in the cup. Would I buy a light roast for this machine again? Yes, but a 250 g bag that takes 200 g to dial in is an expensive way to learn.