I'm definitely going to get parts (or potentially all) of the following blog post wrong. Warning that we'll be discussing two branches of physics: circular motion and heat transfer.
What has prompted this particular blog post was an article on Medium about how coffee beans move in a coffee roaster's drum. There's a reason why you can't just buy green beans and roast them in your oven like you roast a chicken and potatoes. The reason is that if you don't keep moving the beans they end up scorching on the surface where they receive conductive heat and your coffee tastes burnt and bitter. That's why you come to people like me, who use a specialised piece of equipment that balances conductive heat and convective heat transfer to ensure that your beans are well roasted and taste good. You'll likely remember that there are three ways by which heat is transferred from one object to another: convective, conductive and radiative. Convective heat transfer occurs when a hot fluid (in this case air) transmits heat to an object - think about standing in front of an air conditioning unit set to heat. Conductive heat transfer occurs when there is direct contact between two objects - think of a fry pan searing a steak. Radiative heat transfer occurs when an object receives heat via electromagnetic waves - think of the sun providing heat to the Earth.
Coffee roasters tend to employ convective and conductive heat transfer to roast beans. There are different roaster architectures which emphasise different heat transfer forms. A classic drum roaster (what I have) uses both convective and conductive. Fluid bed roasters (they work like a massive hair dryer) employ 100% convective heat transfer. Some drum roasters use perforated drums where some radiative heat transfer can occur if the roaster uses infrared burners.
The reason I've gone into some detail about the different forms of heat transfer is that there is a way to alter the proportion of heat transferred to the coffee bean by convection and conduction - one can change the speed at which the drum is rotating.
Changing gear (impending pun warning), imagine you're sitting in a car which is going around a corner. You'll experience a sensation that arises because the atoms in your body have momentum in the line of your initial direction of travel and your seat and seat belt are forcing you to go around the corner with the car. The force you're experiencing is centrifugal force. That experience increases in relation to the speed at which the car travels around the corner.
If we increase the speed of the drum rotation the coffee beans will experience greater levels of centrifugal force and remain in contact with the carbon steel drum wall for longer thereby increasing the proportion of conductive heat transfer responsible for the roasting of said coffee beans.
I'm unsure what the taste differences arising from conductive versus convective roasting are. I've read claims that fluid drum roasters (100% convective) produce clean coffees with higher acidity and lower body and texture than drum roasters. I haven't seen much real evidence to support that assertion.
When I get some time I'll try some roasting at different drum speeds and report back!
