I have a 1kg perforated drum roaster and I try and aim for a 20% dev time but I find it's difficult to get a light roast doing that and what I end up with usually is closer to a full city roast. if I want to drop the roast earlier and keep 20% dev time everything will have to run hotter. is that generally the concept of light roast or are there different rules?

Development percentage is really a meaningless parameter. Even Scott mentioned in a blog post that he really hopes roasters aren't dropping their roasts based on development percentage.

I drop light roasts around 385-390*F (370* 1C), which correlates to around 16%.

FWIW, my "dark" roasts are dropped around 415*F with a development of 28-30%. I don't think I roast any coffee these days within the 20-25% range.

Don't tell Scott.

I've been playing with this a lot lately, but I haven't come to any conclusions. If I drop my rate of rise below 10*F/minute at FCs I can get 20% development and still keep the drop temp in the "light roast" range. The thing is, though, it still often tastes undeveloped, and it makes me wonder if there's not enough energy in the development phase. Here's an example. It ended up tasting fine, but I've had better results with shorter and hotter development phases.

Almico wrote:Development percentage is really a meaningless parameter. Even Scott mentioned in a blog post that he really hopes roasters aren't dropping their roasts based on development time.

Did you mean time or percentage? The video suggests you might use the same time after 1C for coffees from the same region or with the same density, moisture content, etc.

Almico wrote: I drop light roasts around 385-390*F (370* 1C), which correlates to around 16%.

That's about where I drop light roasts, too, which usually (but not always) correlates to 14%-16% depending on how light I want to go. Often that's 1:20-1:30 after 1Cs.

I think the time in each phase depends a lot on what flavors you want to bring out and the target brew method. I just watched this Ikawa video from a 2018 SCA Expo panel discussion on roasting for espresso, and found the discussion on time in each phase -- Dry, Maillard and Development -- quite interesting, especially the concept that development takes place in Maillard, too, and lengthening or shorting it can bring out different flavors and characteristics. None of the panelists mentioned development percentage.

Almico wrote:faster roasts build internal bean pressures to a higher degree and therefore force heat transfer from the outside to the inside the seed faster and thus "develop" coffee sooner. So a 1:15 post 1C drop for an 8:00 roast is not the same as a 1:15 drop for a 12:00 roast. A 1:15 drop for an 8:00 roast is more "developed" than a 1:15 drop for a 12:00 roast, given the same drop temperature.

Very well stated. I think this statement is true.

I've spoken with Scott about this at length. He seems to wish he hadn't emphasized DTR as much as he did in Coffee Roaster's Companion and now advises that roasters drop based on color rather than percentage. For what it's worth, my light roast DTRs are much the same as others: usually in the 16% range. But recently I roasted an Ethiopia Anaerobic Dry that I dropped at 13%, and it's stellar (total roast time of around 10 minutes). Still, despite not using it as an indication of when to drop, I find DTR useful because Scott's prescription of gas drops at 12/14/16 does wonders to avoid flicks.

My idea of a 20% development time is something used for say espresso roasting or shop roasting. I think it comes from Scott Rao from his experience with consulting artisan roasteries; who were "under roasting/underdeveloped" as Hoos might say.

I think for me and other home roasters, "light roasting" is where the origin notes of the coffee are the first thing one tastes, florals, fruity, etc. Not sure I've ever had an Indonesian light roasted coffee I liked, Ethiopians are a sure bet for this method. At least on the smaller machines hobbyists use one can drop the roast before 1C ends. What final temperature that ends up being is in the heat control process.

With the OP using a perforated drum, one would guess scorching might be an issue to watch out for; along with airflow being tricky. I've had light roasts from a perforated drum (not mine) that were good, no reason that can't be done. I don't usually run light roast Ethiopians past 1:30 roast development time and try to keep the drop temperature on the low/lower side. If 1C was at 196c, then 203-206c would be in the ballpark for my solid drum 500g roaster.

If you are sticking to and able to repeat one roast profile, then shortening the development percentage will result in dropping your coffee at a lower temperature, resulting in a lighter roast. But if your profiles are not identical then you could very well end up with one roast having less DEV % but darker cause it was dropped at a higher temperature. It all depends on the details of your profile and the bean itself.

I would Drop based on colour or final temperature if you are in uncharted territory with your profile and/or green bean. If it's a profile and bean your familiar with then playing with DTR will get you lighter or darker or whatever.

My 2cents