Roasting Science

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Heat Transfer

RS 4.02 Conduction and Convection in Roasting

In Chapter 1 we learned about some effects of the different types of heat transfer on the way coffee roasts. Early drum roasters relied on conduction to transfer heat to the beans, which made roasting a very slow process. Modern drum roasters transfer the majority of the heat via convection. This has greatly decreased roasting times, transforming the quality of the roasted coffee. Hot-air roasters took the principle a step further, allowing roast times of just a few minutes in some industrial-scale machines.

Many roasters believe that conduction and convection can impart different flavours to the beans. Specifically, many advocates of traditional drum roasters report that their method creates roasted coffee with more body, and some claim it produces more-complex flavours.

Apart from these anecdotal reports, scant evidence supports the idea that the method of heat transfer affects the coffee flavour. The few published experiments that compare conductive roasters with convective roasters make no attempt to control the overall roast profile. The only sure difference is that convective heat allows for faster roasting, which enables certain roast profiles that are impossible to obtain in drum roasters.

The biggest limitation of conduction is that it works only through direct contact. When a bean touches the drum wall, only a tiny portion of it makes contact. The heat spreads into the rest of the bean by conduction — and conduction is slower in the bean than it is in the metal drum, because the metal is more conductive. This means that the portion of the coffee bean in contact with the drum heats up rapidly, but that heat takes time to disperse.

With convection, on the other hand, the hot air makes contact with the entire bean surface at one time, so all sides of the bean receive an equal amount of heat. The surface receiving heat energy by conduction is far smaller than the whole mass of the bean, so you have to create a much higher temperature at the bean’s surface to transfer the same amount of heat into the bean. If the resulting drum surface temperature is too high,