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Coffee Roastery Equipment, Commercial Coffee Roaster News
Heat Transfer Failures
This was a not atypical internet forum post this morning:
Looking for feedback from other profile roasters.
How many times during a roast are you reducing your gas pressure/flame height? Do you find if you reduce too frequently, the woody matrix of the bean (aka cellulose) is thermally degraded in a way that reduces the overall complexity/cup structure, especially when roasting for espresso, a tendency toward acetic or quinic acid (vinegar/cranberry) with a somewhat toast/roasty finish? Finishing temps are between 403-410 depending on origin. Previously I was increasing airflow with a baffle to limit energy transfer efficiency (Carl Staub’s term for what people now call RoR). Thoughts?
The short answer is no, but there is a lot going on here that points to a very common lack of understanding of coffee roasting.
The “the wood matrix” …”thermally degraded” “with a “tendency toward acetic or quinic acid (vinegar/cranberry) with a somewhat toast/roasty finish” is exactly what I’d expect from an overly conductive roast where the outer seed develops too fast to allow enough heat to penetrate the inner seed. Comparing the difference between ground and whole bean color values of this flavor profile will nearly always show an excessive spread between inner and outer seed development. The only way “reducing heat too frequently” can degrade cellulose or create early onset roast flavors is if you have excess heat in your system to begin with.
Why does this happen?
The purpose of airflow through a commercial drum roaster is to dissipate heat through the roaster, convey heat to the coffee and remove chaff and smoke that would otherwise impart roast notes earlier in the roast. You can, and occasionally should, exhaust excess latent heat via increased airflow, but mostly as an element of overall machine predictability and control.
This condition points either to machine design issues or machine control problems.
Specifically slow drum speed or a low airflow relative to the batch size.
Slow drum speed inhibits loft time of the coffee inside the drum. This limits convective heat transfer with the result that excess heat is required to drive that batch more conductively.
Slow drum speed also limits the removal of chaff. Moreover, the higher gas setting required to drive a more conductive roast frequently causes retained chaff to burn. This imparts smoke and roast flavors at lower finish temperatures.
A competently designed and operated roaster that shows 330F green/yellow transition verified by a scent change from drying straw to baking bread in the tryer and 385F first crack, most quality washed Arabica coffees cross over into roast flavors around 407F.
Slow drum speed forcing higher conductive heat transfer with a full batch of coffee to the same roast target will often exhibit “roast” flavor as low as 400F.
Low airflow causes problems
Low airflow roasts create similar problems with heat transfer. Low airflow can inhibit the diffusion of heat through the roaster creating control problems and potentially uneven roasts. Low airflow can also reduce convective heating efficiency requiring excess application of fuel for greater conductive heat transfer in nearly exactly the same way as a slow drum speed. Even when absent more severe heat defect in the form of tipping, scorching and facing, excessively conductively overheated coffee often presents with less clarity and complexity.
This is so because conductive heat transfer is very efficient. Highly conductive roasts more rapidly heat the outer seed with the result that the duration of Maillard supporting temperatures is marginally shorter. This can lead to less development of complexity overall. Additionally, because sugars and aromatics liberated and previously created by Maillard activity in the outer seed are more quickly destroyed by excess conductive heat as the outer seed temperature exceeds Maillard boundaries due to an excess of conductive heat transfer.
Low airflow can inhibit the diffusion of heat through the roaster creating control problems and potentially uneven roasts
The source of low airflow roast flavor defects can be operator error, problems with the machine design, or backpressure from poor venting schemes that inhibit airflow.
Precise application of heat over time is the key to successfully roast flavor profiling specialty coffee for complexity, clarity, and sweetness. For best results, the operator must anticipate and very tightly control the velocity of chemical and mechanical change in the seed.
How we fix these issues
Currently, this quality of control is only available to machines and installations and roast styles that deliver an optimal balance of convective and conductive heat transfer.
As long as this is my blog and I actually sell roasters and supporting services, I’ll share my belief that operators shouldn’t need to be scientists or physicists or engineers to get better coffee.
To that end, we do a lot of the heavy lifting:
- Unique the industry and in recognition of the requirement of stable and controllable airflow, we provide clear guidance, engineered venting kits and consult on venting and roastery layouts for nearly all new installations.
- All of our models feature very precisely controllable variable speed 3-phase motors. This is important because many manufactures “fake” unreliable speed control by starving single phase fan motors of voltage with variable resistors.
- Also unique to the industry, all of our models incorporate very sophisticated custom built thermometry and drum pressure sensors that feed and record airflow data as part of the roast profile. The drum pressure gauge forms a feedback loop to ensure that fan settings, venting crud, and weather do not affect airflow and the quality of the roast.
- Every model also features variable drum speed to allow operators to tune the drum speed to maintain consistent levels of conduction, convection, and seed contact and loft times for any given batch size. This allows a previously unknown level of flexibility of each model for optimal results from 20% to 120% of capacity.
- All roaster and exhaust fans are centrifugal blowers more akin to vacuum motors than traditional roaster fans. These fans offer extreme headroom allowing an operator to maintain optimal flow even when the fan and venting requires cleaning and loses efficiency. Unlike conventional underpowered “squirrel cage” blowers, the dedicated and oversized centrifugal cooling fan overcomes the resistance of coffee in the cooling tray with no decrease in CFM flowrate. This allows an operator to stop the roast progression within seconds of exit from the drum eliminating carryover cooking and degradation of flavors in the cooling tray.
- Most importantly, we run the most successful and most comprehensive roastery training program in the world. If you want to really understand what happens in the drum and why, do yourself a favor and register for our class.
To the casual observer, roasting coffee looks simple. It’s a lot more complex when you are actually trying to make coffee taste better. Roasters are exactly the same. They look simple too, until you actually try to build a better thing. I’ve spent 20000 hours figuring out how to build better roasters. If you ‘d like to take advantage of that, give me a call.
Steve
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