Coffee Roasting: What matters the most?

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There is a lot of well meaning “advice” about coffee roasting online, but it’s seldom weighted in terms of the relative impact it makes on your coffee and your craft. We talk to owners about their coffee quality every day. This is a list of the 10 things I find myself talking about the most.

#1: All coffee is good

Simply not liking things is boring. Amateurs criticize; professionals discern. If you want to get a better thing out of the cooling tray, the first step is to check your confirmation bias at the door and instead cultivate a professional curiosity by actively searching for the good in every coffee experience.

If you want your craft to evolve, you need to keep learning. That means trying unfamiliar greens, roasts, and coffees. Occasionally sourced, roasted, and brewed by someone other than yourself. Only inspiration will push you forward. Aversion will bog you down.

#2: Coffee success is tied to the quality of the green

In terms of inputs, greens quality is the single biggest predictor of roast success. Better greens are a big, fat bullseye. A little drunk driving on the roast curve will still land you firmly in the black. There is a lot to know about coffee. If you want to maximize your probability of success, you’ll need professional instruction in greens processing, sample roasting, and cupping scores.

#3: Cupping is king

You can’t hit a target you can’t taste. A little training in professional extraction and sensory analysis goes a very long way. What you’re really doing in these classes is learning common descriptors and associating them with specific flavors. One recent study about the way your brain works shows that the act of “naming” a flavor dramatically increases your ability to perceive that flavor. Your class time should be spent identifying and agreeing on specific coffee flavor attributes in a way that helps you to independently analyze them later.

#4: Venting layout

Poor venting layouts inhibit airflow and airflow control in a way that robs the operator of precise machine control. Few people understand the importance of airflow. Nearly everyone knows airflow through your roaster exhausts combustion CO2 and water vapor, chaff and smoke. What people miss is the fact that 70% of your heat transfer is convective and proper airflow setup and settings diffuse heat throughout your roaster in a way that provides enormously more linear response and by association, much more intuitive machine control. Airflow helps you manage and put to productive use the excess latent heat in your roasting system.

#5: Machine control

Machine control has two parts. The first is the machine itself. Obviously, we build the best machine. Like, duh. That means we build a system with instrumentation and mechanical control characteristics that allow an operator to most successfully tailor their roasts of any given green coffee. The second half is how the operator actually uses all this to make coffee magic. Pinpoint roast control happens when the operator is closest to the thermal equilibrium ROR of any given batch of coffee. It’s a bit like surfing because this equilibrium is constantly changing relative to the roast plan time and temperature targets. All things being equal, machine control is a physical skill. It’s a process of planning, execution, evaluation, and modification over and over again. Practice won’t always make “perfect”, but it will take your craft to a place you can be proud of.

#6: Thermocouples & the feedback loop

Most of the talk about thermocouple deficiencies (noise, calibration, response time) is more correctly traced to problems with thermocouple placement. Get the placement right and nearly all of your data logging problems go away.

#7: Batch Size

For those of you not yet roasting on a Mill City roaster: your roaster has weak (and nearly impossible to clean) squirrel cage fans and weak burner sets, relatively small thin walled drums, and you’re probably stuck with crappy damper control or cheap single phase fan motors starved of voltage for fan speed control. The way you get closer to the thermal equilibrium point of YOUR roaster is to decrease the batch size. Decreasing the batch size effectively increases your airflow and maximizes your effective BTU’s to provide greater per pound process heat and more predictable heat transfer.

#8: Airflow

Airflow is a lot like an old preacher describing the Holy Ghost. You can’t see it, but you can see what it does. The better you control your airflow, the better you diffuse and exchange heat to your coffee. On most roasters, airflow is so frustrating that many people have adopted the practice of a single static setting throughout the entire roast. This sacrifices a significant amount of heat transfer performance. You’ll be forced to increase gas to get greater conductive heat because you’ve sacrificed a percentage of your convective heat. That may prove to be necessary on crude machines, but ALL of our roasters have precisely controllable three-phase variable speed fans and data logging of both fan speed and drum pressure as feedback loops for enormously greater precision, repeatability and roast control.

#9: Have a (roast) plan

Relative to just winging it on every batch and hoping something good happens and for the purposes of selling more and better coffee, control and intentionality taste better. More importantly, control and intentionality creates an archive of experience you can build on. You’ll need to analytically plan your work and do your best to precisely execute your roast plans. That means executing your roast profiles to something like less than total 2F variation and a less than total 15 second deviation to your finish temp. In case you haven’t yet noticed, our RoastPATH roast data system is both plug and play to all of our roasters and is stupendously useful for precision roast exploration and precisely controlled production roasting.

#10: Charge Temps & latent heat

When production roasting, you’ll quickly discover that not all 425F charge temps are the same. For the purpose of repeatability and predictability, you have to control the latent heat in your system from batch-to-batch. All of our digitally controlled roasters automate this by retaining the charge settings from the preceding batch and resetting the roaster to the charge temp, drum speed, gas pressure, and fan speed of the previous roast. Pretty slick, eh?

RoastPATH further refines this function by calculating the relative latent heat deviation from batch to batch and automatically correcting it prior to charging the drum. What you didn’t just hear is the sound of thousands of long suffering accomplished roasters hearing this for the first time wetting their pants with excitement. This is a big deal and something we’ve been quietly chipping away at for quite a while.
If you don’t have a digital machine or RoastPATH yet, you’ll just have to time your preheat very carefully and do the best you can.

#11 Bonus: Finish Temp

All things being equal, the first thing people taste about coffee is the finish temp. Strangers universally begin their order with a roast preference of light, medium or dark. The simple reason for this is chemistry is most reactive at heat. Your coffee’s rate of chemical change is constantly accelerating as your bean temperature increases. So the greatest impact on coffee flavor is automatically your finish temp. A good green with a well executed roast plan that gets you to full development at any given finish temp without heat or roast defect is probably going to produce a pretty good cup of coffee.

In conclusion…

You should note that stuff like declining ROR, crashs and flicks, signal noise, didn’t make my list. The first reason for this is you kind of need to get the big stuff (like all the stuff we just talked about) professionally right FIRST and develop some roasting chops before you start worrying about minutia.

My second concern is that a lot of amateur thought about professional coffee is simply overly critical confirmation bias. People claim to have some double-top-secret-super-Ninja palate training that decisively proves that their coffee is good and your coffee is bad. Who knows? They may be right. But keep in mind that the same people that routinely dismiss Starbucks also routinely fail to tumble to the fact that Starbucks is wildly successful selling “wrongly” roasted coffee. Warren Buffet once opined that “Your customers can be wrong a lot longer than you can be solvent.” There’s something to learn there.

For most of you, this thing is about making money. (Ie. getting a better thing out of the cooling tray and SELLING more of it). My suggestion is to keep your eyes on your own paper. Get the best machine you can afford, experience enough coffee to find something that knocks your socks off, and work towards replicating and/or bettering it.

Free advice and potentially worth what you paid for it from the posh lavish digs of Mill City Roasters, I’m Steve Green.

Thank you and goodnight.

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