What are the variables that a roaster can tweak? Temperature, time, and lots of others I assume? Which variable can produce vegetal flavors? I’d never thought much about roasting but now I’m curious!
5 mins or under and it’s scorched, 10 mins or over and it’s baked. I want in the middle.
Too dark and oily and it’s not great. Too light and I miss the bitterness and it tastes weak. I need to stir it a lot or the roast is uneven. I spray water on it at the end to arrest the roasting.
Temperature is controlled in two ways: direct heat input (e.g. gas flame heating the outside of the rotating drum) and air flow (moving air through the roasting drum to the exhaust). It's not 1 measurement though: there is bean temperature (measured by a probe stuck into the pile of beans) and air temperature.
As far as time goes, when keeping the end temperature equal, spending more time in the roasting process means that the difference between interior and exterior of the bean are closer in temperature. When you plot air and bean temperature against time, you can derive additional information: how much energy is in your roasting drum and the rate at which the bean temperature is changing.
I'm going to preface this by saying that this is an ongoing field of research. We're still learning about what is happening in a coffee bean at various stages of the roasting process. For example, we're not quite certain exactly what is happening at "first crack" (the first time you can start to hear the beans popping), or why some coffee beans simply don't have as audible of a first crack.
We can attribute the first "rules" established for consistent coffee roasting to Scott Rao, who published some of his observations in a book in the early 2010s. Some of those rules were: (1) ensure that the rate of change of the bean temperature ("rate of rise") is constantly decreasing, and (2) prepare to adjust your roast as you begin first crack to prevent the "crash and flick" (a sharp decrease followed by a sharp increase in the rate of rise). The current thinking is that the release of moisture during first crack causes the temperature to crash, and the removal of that moisture causes the temperature to uncontrollably rise back up again. Not handling this properly often results in undesirable hollow and bready flavors; this is frequently referred to as "baked coffee".
As far as vegetal goes, that is often because of roasters cutting their roasts too short (and perhaps roasting too quickly). In this case, the bean does not get hot enough for sufficient flavor development, so it more or less retains a lot of the undesirable flavors of essentially "raw" coffee.
Note that these are "rules" instead of rules because there are a plethora of edge cases out there.
This is why roasting is really really difficult. And why even some of the best roasters out there end up leaning a lot on blends and their milk drink business.
And sorry, I gotta call out everyone who suggests this: most of your home-roasted coffee is gonna taste like ass lol. I tried home roasting a bit with a fancy setup and with a Fresh Roast. I sure saved a lot of money per pound of coffee, but I always got a fraction of the quality and the flavors were never consistent. But what I gained was insanity and the realization that home-roasting isn't for me.