This is a learning note of the Roasting Science course on Barista Hustle.

The two roasters I’ve ever used are Giesen and Probat – Giesen when I attended the SCA course and Probat at the roastery. It’s a surprise to me to find the similarities between the roasting machines used in the 1900s and the mordern ones being used today.

| DRUMS AND BURNERS

I confess that I didn’t pay much attention to the drum structure when I was using the machine; but during the course, I learned about hot it was designed to solve certain issues at production, I hold great respects to roasters and inventers back in the days.

The first drum roasters were using the perforated metal drums, then gradually evolved into fully-walled drum and even double-walled drums to protect the beans from the open flame.

And the burners – I never thought that burners would matter this much! – atmospheric burners and forced-air burners are commonly used for roasting machines. Both machines I used are equipped with forced-air burners, and I won’t forget how loud they are!

| DIRECTLY AND INDIRECTLY HEATING

Drum roasters like Giesen and Probat I used convey heat directly to the beans via the drum walls. It is mainly the conduction between the beans and the drum walls that does the magic.

When roasting samples and premium beans, and when doing some roasting experiments, I used hot-air machines like Link and Ikawa, both are fluid-bed, electric roasters. The hot air blows up from the bottom of the roasting chamber, therefore it is the convection that brings heat to the beans.

I learned that there are actually different types of fluid-bed designs. The Link I used is a fully fluidised bed design, and there are spout-bed with a tube blowing air into the chamber and the beans move like a fountain flowing upwards. To avoid beans being blow out of the chamber, some machines come with an asymmetrical chamber so that the beans will fall back down to the bottom.

| RECIRCULATING AND AFTERBURNERS

Recirculating the hot air flows out of the roasting chamber is an efficient way to reduce energy usage. As to afterburners – cannot dress how important they are – are essential to minimise volatile organic compounds (VOCs) in the air goes out of the cyclone. Can’t forget that time when I saw white-grayish smoke bursting out from the ventilation tube and rushed immediately to the chaff bin!

Warm reminder: wear a mask before you open the chaff bin!

| DIRECT VS. INDIRECT: WHO’S GOT BETTER FLAVOURS?

Experienced roasters tell me that the differences in flavours between beans roasted with an open flame direct drum roaster like Probat and beans roasted with a hot-air machine like Link are easily distinguishable. Theoretically it makes a lot of sense. A roaster once told me it’s like making a wok-fried dish on gas vs. heating up something with a microwave.

I don’t think I have enough experience on this, but I did notice that when roasting espresso profiles on hot-air machines like Link, the roundness and the texture are far from a full-bodied Probat produce. It could be the profile configuration but when I think of the metaphor of the wok-fry and the microwave, I started to understand the difference a bit.

| SUMMARY

This chapter introduced the history of cofee roasting gear and its development over the centuries, followed by a deep dive into differences of directly and indirectly heated machines from their structures, features and flavour profiles. A good reminder of my working experience at the roastery!