Study on electronic-nose-based quality monitoring system for coffee under roasting

Roasting process needs to be monitored and carefully controlled because it plays as the most important stage for determining flavor quality on the final product in the secondary coffee processing. Common quality monitoring method by applying parameters namely roasting time, roasting temperature and grain color may have disadvantages especially for nonuniform quality of green beans and stirring mechanism of regular roasters; therefore, an alternative quality monitoring model is necessary. Because emitted vapor during roasting may represent the occurred reaction stage, it is possible to indicate the roast degree of the coffee grain. This study evaluated the application of an electronic nose based on semiconductor sensor array for quality monitoring of coffee roasting. The electronic nose designed with gas sensor array was integrated to a mini batch coffee roaster. Data including sensor array response, vapor humidity and temperature were recorded in line to the roasting process and compared to the coffee grain color measured with a universal colorimeter. The experiment showed that the gas sensors respond to the emitted coffee flavors, from which a logarithmic profile as a function of grain color was obtained with the highest slope of-1.6V/color difference of roasted coffee. Aroma patterns obtained from sensor responses were then analyzed with principle component analysis (PCA), by which a distinctive profile between initial and final phases of roasting is obtained. Although the corresponding analysis is still unable to distinguish the levels of light, medium and dark (LMD); high sensor responses indicate a further benefit of this system for developing an analogous quality monitoring system.