Recap
- Moisture content is the most important factor determining how a coffee roasts. This moisture takes a lot of energy to heat up and to evaporate, causing the buildup of pressure in the bean that leads to first crack.
- At higher pressures, the boiling point of water increases to far above 100°C, so water continues to evaporate throughout the entire roasting process. Additional water is also created by chemical reactions during roasting.
- The water content and water activity at different stages of the roasting process strongly affect the rate of chemical and physical changes in the coffee, such as the Maillard reactions.
- Mass lost during roasting includes water and organic matter. The amount of organic matter lost during roasting is closely linked to roast colour and roasting temperature.
- Beans increase in volume continually throughout the roast, becoming more porous, weaker, and more brittle.
- Polymers in coffee undergo glass transitions at high temperatures, becoming rubbery and more flexible, which allows beans to swell and crack. Water helps this to happen at lower temperatures; as the beans dry out, they may revert to a brittle state.
- The exact causes of first and second crack are unknown. First crack might result from the macrostructure of the bean breaking open, perhaps as the result of a glass transition near the bean surface. Second crack could be related to the fracture of the cell wall structure.
- Chemical reactions during roasting can be exothermic or endothermic, but exothermic reactions probably have a small effect on the overall roast curve. Changes in the roast profile such as the ‘plateau’, ‘crash’, and ‘flick’ are likely the results of changes in water evaporation from the beans.
- Pressure has many effects on roasting reactions, but there’s currently no practical way to manipulate pressure during roasting.
Glossary
Exothermy / Endothermy The presence of exothermic or endothermic reactions, or the overall balance between them. Exothermic reactions release heat energy, while endothermic reactions absorb heat energy
Hydrolysis A chemical reaction in which a water molecule breaks one or more chemical bonds in another molecule,