Coffee roasting: Color and aroma-active sulfur compounds.

摘要

Roasting is the most important unit operation to develop flavor, aroma and color in coffee technology. Roasting is a highly complex process that combines heat transfer to the beans, moisture loss, and many chemical reactions.;The color kinetic parameters of coffee roasted at constant temperatures of 170°C, 180°C, 190°C and 200°C were determined in a lab oven. First and second order kinetics were considered for L* values. Color of ground coffee changed from green (L*=64.08+/-1.36, a*=-1.34+/-0.18, b*=14.12+/-0.43) to dark brown (L*=12.54+/-0.78, a*=7.63+/-0.77, b*=8.84+/-1.23) during the various isothermal roasting processes. The color change reaction rate for L* followed Arrhenius behavior with activation energy of 128.7 KJ˙mol-1˙K-1 and 145.9 KJ˙mol-1˙K-1 for first and second order reactions, respectively. Prediction of L* values in whole-bean roasting under non-isothermal conditions demonstrated that second order kinetics produced better agreement with experimental data.;Color of the Agtron Roast Color Classification System was assessed using various instruments. Machine Vision under polarized lighting obtained the highest sensitivity. Cross-matching between sensory analysis results and the matching obtained from the instruments demonstrated also that Machine Vision under polarized lighting was capable of matching the color of coffee roasts with the classification disks 60.1% of the time.;Analysis of aroma-active sulfur compounds demonstrated that the time-temperature profiles significantly affect their final concentrations. Sulfur-volatiles are known to produce many off-flavors, though pleasant aromas are also caused by many sulfur-containing volatiles when at the right concentrations. Total sulfur-volatiles were significantly higher in moderately dark and dark coffee roasts when a long-time profile was applied, in contrast with a short-time profile for the same final degree of roast. However when individually analyzed, compounds such as 2-furfurylthiol, a major character impact volatile, were produced in significantly higher concentrations in the short-time profile.;Findings from this study can help make color prediction possible during roasting in real time. There is evidence that roasting profile greatly affects sulfur-compound formation, but further studies need to be conducted to determine the best sulfur volatile balance that will lead to coffee with better aromas.