Engineering researchers turn up the heat for cheaper coffee

David Riddell

A research project by a Waikato University engineering student may help hold back the escalating price of coffee. Cameron Kelly is now into the second year of his PhD and hopes to find a way to automate the coffee-roasting process.

“It’s certainly an opportunity whose time has come,” said Mr Kelly’s supervisor, Professor Jonathan Scott.

“If you ask people how much they spend on gasoline and how much they spend on coffee, coffee is the second-largest dollar volume commodity traded on the surface of the Earth, right after fossil fuel. Currently, roasting machines require skilled operators and the roasting process adds substantially to the cost of the beans. So you need only a little leverage to make money here.”

Automated coffee roasting was difficult to achieve because the beans varied considerably from batch to batch, Professor Scott said. “The timing of the different stages depends on the source of the beans, how big they are, and how thoroughly they have been dried.

“If you were to discover that having a certain amount of heat for a certain period of time gets you a great cup of coffee this week, you may not get the same result next week, because something’s changed.”

Professor Scott said he had videotaped several expert roasters at work.

“It seems they have to experiment a bit … if you ask them what they’re doing, it’s almost a subliminal skill.”

The skill of the roaster was vital because a 15-second error in a 15-minute process could ruin a batch, he said. “When people roast beans there’s a bunch of markers they look for.

“The domestic aficionado likes the smell but the commercial guys don’t get that because health and safety requires them to vent their systems outdoors.

“There’s a lot of divided opinion and hearsay but it all makes you feel that if there are so many different ways and they’re all generating drinkable coffee, it ought to be possible to automate it.”

While most coffee roasters tumble the beans in a horizontally rotating drum, the unit Cameron has built is based on a second principle, in which heated air blows up through the beans with sufficient force to lift and tumble them. This allows him to control the temperature to within a tenth of a degree, and to adjust it at different stages of the process.

Temperature measurement, Professor Scott suspects, will provide the most practical key to automated roasting.

“The beans contain a certain amount of water, so as you start to roast them they tend to sit a little above 100 degrees Celsius while the water evaporates, then the temperature takes off and the beans begin to change colour, then a bit further beyond that they crack.”

Cracking is much like popping corn.

Coffee beans go through two cracking points as they roast, almost doubling in size at the first crack, which happens about 190 Celsius. At this point the beans have a relatively light colour and delicate flavour, but if they are roasted further they continue to darken and crack a second time. While all this is happening, the beans go through phases where they’re responding differently to heating.
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In the early stages, as the water is being driven off, they are purely absorbing heat but then they get to a point where they are actually giving off heat of their own, because some of their components are burning. By measuring the temperature of the air arriving at the bottom of the roasting chamber and exiting the top, and knowing the flow rate, Mr Kelly can calculate how much energy the beans are absorbing.

“So you should be able to see the exothermic phase, when the beans are giving off their own heat, and perhaps that will give some useful signature.”

For now, Mr Kelly is measuring temperatures with thermocouples.

“But I’ve borrowed an infrared camera that should allow me to measure temperature very precisely,” he said. “To do that, I first need to see if I can get the control I want between the camera and the computer, and I’ll also need to put an artificial sapphire window into the side of the roasting chamber because the camera doesn’t work through glass.

“And that’s very expensive.”

Professor Scott said this area of research had a lot of speculation and not a lot of science. “There are lots of people who claim to have produced automated systems but they’re like a cruise control.

“You’ve still got to steer and you’ve got to decide what speed to set it at. So … the time has come for a coffee machine that will give us a decent drink without having a person attached to it.”

– Waikato