Our most recent cupping club focused on roasting coffee, the chemical reactions that take place and what makes a good roast. Emily Jackson — our Roastery Operations Manager and Production Roaster led the discussion with her knowledge of thermodynamics and her incredible caramel popcorn making (to demonstrate the Maillard reaction), backed up by Head Roaster Simon.
Coffee Roasting - art or science?
Roasting coffee transforms the chemical and physical properties of green coffee beans into roasted coffee products. The roasting process is what produces the characteristic flavour of coffee by causing the green coffee beans to change in taste. Coffee roasting is considered a “craft industry”, but it also involves a lot of science. Roasting all depends on the individual roaster’s approach to roasting coffee — so coffee roasting is both an art and a science.
What does a good roasting curve look like?
The blue curve represents the bean temperature and the red curve represents the air temperature in the roaster. The turn on the blue curve after the dip is called the turnaround or T1. This is the point at which the beans are no longer cold and begin to warm. The Maillard reaction begins at the turnaround (this is hotly debated though) and starts the positive rate of rise in temperature. The Maillard reaction is seen physically in the difference in colour of the roasted coffee.
There are a few different reactions happening here. Endothermic reaction — the absorption of heat energy — happens from the start of the roast up until the first crack and briefly after. Exothermic — the release of heat energy — is the first crack, where the energy that has been absorbed through the endothermic reaction is released. These are both catalysts for other chemical reactions in the roasting process such as, the Strecker reaction — the formation of new amino acids and volatile organic compounds that comprise flavour.
What does a bad roasting curve look like?
Often roasting faults come together — in this case the temperature is not smoothly rising and we see a ‘crash and flick’ in the rate of rise. It’s not all chemistry, physics is equally important, in this case the momentum is key in ensuring the chemical reactions start, finish, and continue. A baked roast is a roast that has lost momentum and internal development stops sugars fusing, which kills the complexity, aroma and sweetness resulting in a flat tasting cup. Even long development times (after first crack) can be underdeveloped where momentum is lost.
Maillard Reaction: what is it and why does it matter?
The Maillard reaction is non-enzymatic browning that occurs between 140-165/170 degrees celsius. Heat and moisture are the catalysts for the reaction between different amino acids at these temperatures and the simple sugars are reduced. Beyond 165/170 degrees carmelisation occurs in the remaining sugars, which creates a hardening on the cellulose within the coffee. Pyrolysis happens at approximately 220 degrees, when the heat causes a chemical change inside the bean, leading to the release of carbon dioxide. The colour then changes to a medium brown and the beans lose 13 per cent of its weight.
Endothermic and Exothermic reactions
Endothermic (heat energy absorption), happens at beginning of roast until first crack, and briefly after first crack has finished. Exothermic reaction (heat energy release) is also known as the first crack, when a build up of pressure within the cell walls of the coffee bean causes the cell walls to crack once caramelisation occurs and the coffee makes a popping or cracking sound (think popcorn).
The timing of the first crack varies per roast because individual coffee beans may be of different sizes, moisture content, and density. In coffee you cannot have exothermic reactions without endothermic reactions first because they rely on each other — this not always true in for everything though, think spontaneous combustion.
How to apply this knowledge
The best way for a person to experience both the Maillard reaction and exothermic activity first hand at home, is by making caramel popcorn.
- First of all, in a large heavy bottomed pan with a close fitting lid, pour 150 grams of popping corn kernels, with 20 ml of high heat oil, such as safflower or coconut oil or melted butter.
- Heat on a medium high heat with the lid, tightly on the pot. Move the pot back and forth on the burner to ensure even heat distribution and that the kernels do not burn.
- After a few minutes, you will hear a popping sound. Lower the heat to a medium low heat, and continue to move the pan. Once the pops become 2 seconds apart, turn off the heat. Be careful when removing the lid, as a lot of steam will escape.
Now that you have freshly popped corn, you can move to the next step: caramel!
- In another heavy bottomed pan with a close fitting lid, add 250 grams of fine caster sugar and 50 mL of water. Heat on a medium high heat with the lid on while the sugar melts.
- Once the sugar has melted, carefully removed the lid. The sugar mixture should be just turning yellow. This is the beginning of the Maillard reaction.
- Turn the heat to medium and stir with a wooden spoon until the colour reaches a lovely medium brown colour, gradually reducing the heat of the burner to low. Use a wooden spoon as using a metal spoon will result in a heat transfer, and you will end up with a burn.
- Once the sugar has reached the colour of your liking, add 100 grams of cold butter. Please ensure a lot of care at this point — the sugar mixture will bubble and release a lot of steam once the butter is added, stir until the butter has melted and pour over the popcorn and once combined, pour onto a sheet pan to cool.
- Once cool, break up into pieces and serve.
Enjoy your home chemistry experiment!