Why airflow matters

Many years ago, the latest “fad” was the pimped up roaster that had a variable speed drum controller. That is, being able to change the rpm of the drum was…..at the time…..seen as the duck’s nuts. Some of the yanks were raving about unbelievable cup quality improvements via either decrease or increase in the rpm using their shop roasters.

Of course, industrial roasters have had this feature for a while and it’s quite common for agitation speed to change throughout the roast cycle on large roasters, but the point was made very clear that agitation was just that – moving the bean around to prevent excessive or concentrated thermal transfer. If the roaster was designed to make use of low speed in the early stage and faster towards the end because of the changes in thermal transfer between conduction and convection, then there was a case (or argument) to be heard about agitation speed control, but on the whole, most of the shop roasters that were being talked about at the time really could not have produced such exaggerated improvements from simply changing the agitation speed. The Placebo effect at it’s best me thinks.

Heat source is another area that many roasters obsess about. Some shop roasters are designed with barely enough thermal energy to run consistent full batches back-to-back, or if they were designed properly, someone has made a right royal fist of the installation via either substandard fuel calibration or poor venting.

Transfer type also has a role to play in understanding the thermal dynamics of your roasting device. Conduction (contact with, e.g. drum surface heated) and convection (surrounding air, e.g. fluidized bed) are the two most common types of transfer methods during roasting. Unfortunately, the 1-eyed convection zealots tend to stretch the truth a little in espousing the virtues of their roaster superiority over old-school drum types when in fact many drum roasters employ both conduction and convection transfer of heat. Most air-roasting pundits would be more than a little upset to discover that some drum roasters can employ very high convection rates that match their so-called fluidized “clean” superiority.

Then we arrive at airflow. It’s not a topic that gets a lot of air-time (pardon the pun) as most people generally refer to airflow as being the critical function of exhausting fumes, smoke and chaff. However, airflow is more important than providing a pathway for by-product to escape, it is a vitally important and highly undervalued variable that requires careful management during the roasting process.

Some roasters use airflow in tandem with heat source to provide either dramatic change (advance or retard) the roasting progress or to deliver finer control throughout the duration of a roast given the stages at which beans transition from absorbing heat (endothermic) to emitting heat (exothermic). In other words, heat sources can have lag or latency, whereas airflow change can be more instant.

Whilst we all tend to be focused on time and temperature parameters, a.k.a. the profile, a very interesting dynamic relating to airflow could be causing more issues that you think – flat, dull, baked, dry flavours – all common causes from time/temp + airflow issues.


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