Time-Resolved Roasting-Induced Microstructural Evolution of Arabica Coffee Beans using X-Ray Computed Micro-Tomography

The aroma of coffee is deeply connected to the roasting process, which leads to notable changes in the bean's structure and chemistry. While many research studies have explored the chemical alterations occurring in the coffee bean during roasting, the structural changes have typically only been observed broadly, quantified by changes in size, density, fracture strength, or basic sectional views of the coffee bean. Traditional microscopy techniques are insufficient in providing an in-depth understanding of the coffee beans' microstructure, primarily due to their resolution, two-dimensional (2D) limitations, and the destructive nature of the analyses. These limitations are further exacerbated by the fact that the coffee bean undergoes significant three-dimensional (3D) structural changes during the roasting process. These changes are impacted by the coffee bean variety's properties, the bean's initial microstructure (which is affected by the local environment, including altitude, humidity, etc.), and the roasting process. As a result of the limited information about the initial 3D microstructure of the bean, its impact on the evolution of the emergent microstructure during roasting has been largely unexplored. Furthermore, while global measurements in porosity or pore volume at different roast levels have been reported, the heterogeneity in the evolution of porosity in different regions of the bean, which impacts the uniformity of the ground beans, has not been described. In this study, we conducted a time-resolved study of the roasting-induced microstructural evolution of Arabica coffee beans using X-ray computed micro-tomography (XCT). We combined high-resolution XCT data with computational image analysis techniques to examine the evolution of the microstructure of Coffea arabica beans during roasting. Beans sourced from Brazil, Colombia, and Ethiopia were tracked individually throughout the roasting process going from the green bean to the dark roast stage. The individual beans were tracked and scanned at multiple stages of roasting to capture the 3D tomography data of the entire bean. The XCT data, captured at different roasting levels, revealed detailed, time-resolved information about porosity, pore size distribution, and cracking within different regions of the beans. Results showed a significant increase in porosity and pore size from green-bean to dark-roast state, but the rate of increase wasn't linearly dependent on roasting time. Furthermore, beans from different regions showed markedly difference spatial heterogeneity in porosity and cracking. The effects of the initial green bean microstructure, as influenced by the growth environment, on the subsequent structural evolution will be discussed. The data and analyses presented in this study can assist in advancing research in the realm of food engineering by enabling a deeper understanding of the structural characteristics of coffee beans from various regions of the world and by clarifying the link between the processing and microstructure of the bean at different roasting levels. Furthermore, the 3D characterization and analysis presented here can be applied as a framework to study a diverse class of food materials.