Geisenheim University Measures the Elasticity of Coffee Beans to Determine Roasting Progress

Energy-efficient and resource-saving: inline process control for coffee roasting

The coffee bean is isolated from the coffee cherry using various methods and several steps to complete the process, during which mistakes can be made and things can go wrong. The green bean should be uniform, undamaged and free from particles or microbial and chemical contamination. The roasted bean is, of course, more interesting: it should also be intact and, above all, roast evenly to the desired degree in order to develop the desired aroma profile in the coffee (beverage).

The Institute for Food Safety at the Geisenheim University is working on the project in cooperation with Probat from Emmerich, a leading manufacturer of coffee roasting systems. The researchers mainly record changes in the color and mechanical-elastic properties of coffee beans during inline roasting, i.e. inside the machine and during the roasting process. The color of the beans reflects the superficial roasting progress. The elasticity of the beans, which changes during roasting and is determined via sound measurements using AI (artificial intelligence), provides information about the inside of the beans and thus the roasting process. In order to bridge the gap between the sound signal and the elastic properties, as it were, elasticity measurements were carried out on beans of different treatments and roasting degrees at the testing machine manufacturer ZwickRoell in Ulm, Germany.

The reason for this approach is obvious: until now, the roasting result of the coffee beans has only been determined after roasting. During the roasting process, only a random visual check is carried out on the color of the bean. Despite modern process technology and the high level of experience of the master roasters, roasting errors occur often, “some of which are unusable or have to be blended at a high cost,” explains Severin. In some cases, this leads to “coffee of inferior quality with reduced margins.”

Severin: “Our accurate inline process control, which includes the actual roasting process, allows us to produce coffee of the desired sensory quality without the need to reject product. As a result of this and the precise process control, we are able to achieve a significant reduction in CO2 emissions. This is one of the reasons why the German Federal Environmental Foundation (DBU) is funding the project under the name “Energy-Efficient and Resource-Saving Coffee Roasting,” says Severin.

Elasticity measurement: new findings on coffee bean roasting and acoustics

For the tests at ZwickRoell, coffee beans of the Peru Arabica and India Robusta varieties were measured, both in their green state and in various degrees of roasting and pre-treatments (undried, dried, roasting duration, roasting temperature). The test results showed the following: green, unroasted coffee beans could be virtually crushed without showing noticeable breakage. Differences between the two types of coffee were also clearly evident: India Robusta beans required higher forces than the Peru Arabica beans for the same level of deformation. For further testing, both types of coffee beans were dried, which showed that the broke more easily and were less elastic than the green coffee beans. The fastest to break, however, were the roasted coffee beans, for which additional fracture details could be observed during further deformation.

“The findings of the elasticity measurement therefore offer exciting insights into the interpretation of acoustic phenomena in coffee roasting and the changes during the roasting process,” summarizes Severin. It could also be used to answer questions about the structure and roasting progress within the coffee bean. These correlations will be investigated further in future studies.
What does the researcher himself think about coffee consumption? Severin: “I agree with Honoré de Balzac, who once said: “Alone, coffee is the black oil that always gets this fantastic working machine back up and running.” However, in the evening I turn the machine off and favor a nice glass of Rheingau Riesling.”