Heat and mass transfer model for a coffee roasting process

Abstract

The roasting of coffee is a complex process and it takes years of experience to be able to produce a quality cup of coffee (as well as consistently reproducing the same quality coffee). Although there are various factors that can influence the final taste of coffee, from the green bean processing method to the roasting equipment used, the most crucial part in coffee flavour development is the roasting process. Even the highest quality green coffee beans can be spoiled with improper roasting procedures. No set rules exist to produce a specific roast of coffee and roasting techniques differ from roaster to roaster. It is the objective of this study to model the roasting process for the purpose of system optimisation and control. The usefulness of the model to be implemented to predict the quality of the final roasted coffee (in other words the degree of roast) was also considered. In order to model the roasting process, the heat and mass transfer that take place during roasting were investigated and further quantified by means of heat and mass transfer models. Three heat and mass transfer models were identified from literature to be able to adequately model the moisture content and temperature of the beans during roasting. From these models, the roasting process was modelled and the predicted roast profiles were obtained. For model validation, several experimental roasting procedures are conducted. A comparison between the experimental and modelled results (for the 9.09 wt% green beans) showed that all three proposed models could predict the roast profiles fairly well, with some deviations occurring with prolonged roasting times. However, all three moisture loss models consistently overestimated the moisture loss that occurs during roasting, which improved somewhat for the longer roasting times. Two of the proposed models were found to be very sensitive for the higher initial moisture contents, where the predicted roast profiles showed higher overestimations than with the normal green coffee beans. The third model performed fairly similar with all initial moisture contents and no adverse reactions (such as the significant levels of overestimation seen with the other two models) to the increased moisture content could be observed. All three moisture loss models still showed a degree of overestimation of the moisture content during a roast. The degree of roast of the roasted coffee beans was determined from the final moisture content, the roast loss percentage (which includes moisture loss, volatile release and dry matter) and the progression of the roast. It was found that some defining roasting characteristics of the coffee beans (referred to as the first and second crack) consistently occurred at the same temperatures, with the first crack occurring at about 175 to 180 °C, and the second crack occurring at temperatures above 200 °C. From this, it was concluded that a rudimentary roast degree prediction can be made based on the progression of the roast and the final roast temperatures obtained. It was finally concluded that all three models can be used in the optimisation and control of the coffee roasting process, although further investigation is needed into the optimisation of the moisture loss models. In conjunction with the end of roast temperatures, the predicted roast profiles could be used to give a simple prediction of the degree of roast. This could help to control the roasting process more effectively and assist in reproducing high-quality products