“Green beans smell green-earthy, so they ust be heat treated in a process called roasting to bring about their truly delightful aroma. Roasting in the temperature range of 200-250C causes profound change The beans increase in volume (50-80%) and change their structure and color. The green is replaced by a brown color, a 13-20% loss in weight occurse, and there is build-up of the typical roasted flavor of the beans. Simultaneously, the specific gravity falls from 1.126-1.272 to 0.570-0.694, hence the roasted coffee floats on water and the green beans sink. The horny, tough and difficult-to-crak beans become brittle and mellow after roasting.
Four major Phasses are distinguished during the roasting process; drying, development, decomposition and full roasting. The initial changes occur at or above 50C when the protein in the tissue cells denatures and water evaporates. Browning ocurs above 100C due to pyrolisis of organic compounds, accompanied by swelling and an inital dry distillation: at about 150C there is a release of volatile prodicuts (water, CO2, CO) which results in an increase in bean volume. The decompositon phase, which begins at 180-200*C, is recognizable by the beans being forced to pop and burst (bursting by bracking along the grooe or furrow); formation of bluish smoke; and the release of coffee aroma. Lastly, under optimum caramelizaiton, the full roating phase is achieved, during which the moisture content of the bean drops to its final level of 1.5-3.5%. The roasting process is characterized b a decrease in old and formations of new ompunds…The running of a roasting process requires skill and experience to achieve uniform color and optimum aroma development and to minimize the damage through over-roasting, scourching or burning.”
(Food Chemisty by: Professor Dr.-Ing. H.-D. Belitz, Professor Dr.-Ing. W. Grosch)