Light roasts are great! But there are just a lot of straight-up bad renditions of them, as a result of lack of adequate training on either roasting or brewing, to the consternation of many of those coffee snobs. Unfortunately, this just happens when shops follow trends.

Roasting well in general is already quite challenging and is a lot more than just arriving at a certain bean color or temperature. Vegetal flavors are very much a roasting mistake that's being passed off as an inherent characteristic of a light roast. Combine that with techniques better suited to brewing (or pulling shots of) darker roasted, and you have a recipe for a dull, astringent, sour cup.

That being said, a sour espresso shot is always possible regardless of dark the coffee is, so I'd argue it has a lot to do with a cafe owner's willingness to train themselves and their staff to work with lighter roasted coffee.

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!

I roast my own with a heat gun.

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.

This is something that I still consider to be black magic to me, so this is my best attempt at describing a number of the variables.

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.

Thank you for the detailed answer! It makes sense that the vegetal flavors are the raw flavors from the beans themselves. And that temperature is much more complicated than it appears. I also appreciate the Scott Rao pointer, perhaps that will be interesting future reading.

This is mainly what got me into roasting my own coffee. It was becoming a pain to find high quality dark roasts as all of the boutique roasters turned their efforts to light roasts.

I hadn't thought of that as part of why finding good dark roasts has been hard for me. I've been annoyed at light roasts for a long time because I tend to find them acidic to the point that they're not enjoyable. I appreciate the bitterness and toastiness of a good dark roast, but finding good ones has been few and far between in my experience.

How did you go about roasting your own if you don’t mind me asking.

It was a journey! My wife got me a roaster for Christmas several years ago, and I had absolutely no idea how to use it. After tons of reading, YouTube, and trial-and-error I eventually got the hang of it. I still use the same roaster she bought me, but I upgraded to a double-walled chamber to make winter roasting more consistent and temperature probes [1] to record the roast process with Artisan [2]. Since collecting data is fun and makes it easier to get consistent results.

I don't roast beans for cold brew anymore since I drink way too much and it was becoming a chore, but I still roast ~8oz every two weeks for pour overs.

If you're interested there are a lot of great resources online. Sweet Maria's [3] has been a constant go-to for knowledge, equipment, and green coffee beans. And of course, YouTube.

1: https://imgur.com/T0WW90R 2: https://artisan-scope.org/ 3: https://www.sweetmarias.com/

Not OP, but if you just want to experiment and you have a cast iron frying pan, there are instructions out there on how to roast beans with a frying pan. I'll leave the instruction search as an exercise for the reader ('cuz I don't know which ones I used), but basically just keep those beans stirred until they start to pop like popcorn, and you're done. CAUTION: this will make a ton of smoke, as in, if you have a way to do this outside then do it. It's what keeps me from making a habit of it.

That said, much like home-brew beer: best beans I've ever had (granted, I'm no snob). Just writing this makes me want to order a bag of unroasted beans off Amazon and give it another whirl.

Or go the easy route and just order what sibling comment recommends. :-)

I’d recommend getting a stove top popcorn popper and a range hood vent :)

Or if you’re like me and live in an apartment, get a window fan blowing out, and be prepared for your apartment to smell amazing/terrible (depending on your perspective) for a few days

i live in Sydney - not far from UNSW :P Campos coffee are phenomenal roaster imo. There's no hype around these light roasts - i would never cold brew them though - i've tried - and it's an incredibly inefficient form of brewing. But light roast itself is a phenomenal thing :)

Agreed, many supposedly good coffee shops in the Seattle area (e.g. Trabant) produce espresso that is way too sour for my taste. (If you want a point of reference, IMO Espresso Vivace does it perfectly.)