How to ride the roasting curve

Zest Specialty Coffee’s Aryan Aqajani dissects roasting variables and explains why the time spent between roasts is as crucial as monitoring temperature throughout the roasting process.
roasting curve

For a barista, finding out all the things that can massively influence the end flavour of coffee, such as origin, processing, and brewing, is game changing. As a coffee roaster, I’ve had a similar moment. 

Although we know coffee flavour is developed further during the roasting process, we’re not seeing enough transparent discussions about roasting in the coffee community, not the way we do about brewing and equipment. 

I’ve been lucky to roast on many different machines throughout my roasting career. While the small but intricate differences (machine to machine, roast to roast, bean to bean) can be a frustrating learning curve when you have production deadlines to meet, consistency and mastering the variables becomes the holy grail that drives you, and curiosity takes over.

Aryan Aqajani
Aryan Aqajani is a roaster and Q-Grader at Zest Specialty Coffee.

The variables are almost greater when roasting than they are in brewing.

Similar to espresso machines, roasting machines have their differences. We have drum, fluid-bed, centrifugal, tangential, and hot air roasters, and interestingly, not all drum roasters are equal. They are built with different materials, with various heat application. Beyond the machine, there are so many other variables affecting flavour outcome. 

The density and water activity in the bean, as well as the processing of the green coffee, has great effect on roasting. We closely monitor the duration of the drying phase (when coffee turns from green to yellow), the duration of Maillard phase (when coffee turns from yellow to brown until the first crack), and the development time (from first crack to when the roast is finished). When aiming for complete consistency we also analyse the rate of rise, which is how fast the bean temperature changes over a specific time.

Temperature is key. By considering batch size, total roast time, and bean density, we choose different charging temperatures. And depending on the roast degree, a certain end temperature.

One thing I’ve always kept a close eye on is what we call temperature probes, which monitor the bean temperature inside the drum, the inlet temperature (the air coming to the drum), and the exhaust temperature (the air going out of the roaster).

Nowadays, specialty roasters are using logging software to keep track of these temperatures, which allow us to “profile” coffee beans, and taste and tweak to get the best out of them, time after time. 

Keeping track of the temperatures, how they change, and the time it takes is integral. I am able to go back to these profiles to try and replicate the flavour profile as closely as possible each time, which means I learn even more about the variables and go even deeper into flavour appreciation. 

During my production roasting shifts, I even discovered that the time between roast batches, when the coffee is cooling, can massively influence the thermal energy of the roaster. Depending on the batch size and how efficient the fan is, the cooling process can take few minutes, and this consequently affects the flavour of the coffee with each roast. This discovery has now changed the way I roast. 

It became clearer to me, after several times analysing the roast profiles, that perhaps this time spent between roasts is as crucial to monitor as any other variables throughout the roasting process. 

This became more obvious as I started to scrutinise my roasts. I found that usually the roasted batch with longer waiting time tasted more flat and dull, and lacked vibrancy, while the batch with a shorter gap between roasts was more lively. 

Since then, I’ve attached a small timer next to the cooling tray to monitor the time spent for cooling the coffee, and also the gap between roasting batches. Comparing these times next to the roasting qualities, over a period of time, has helped me get a step closer to more consistent roasts.  

With every new discovery we’re better able to control that consistency, and in turn, impress more coffee drinkers with the complex flavour from around the globe.

This article appears in the February edition of BeanScene Magazine. Subscribe here.