By Emily Jackson, Operations Manager & Production Roaster.
The chemical reaction that turns green coffee brown
Today, we are going to look at two of the chemical and physical reactions that occur in the coffee roasting process. These reactions change raw green coffee from a pale green colour the density of a pebble, to a medium brown colour with a brittle structure. This allows it to be ground, brewed and consumed.
The first part of this process is called exothermic activity, followed by a Malliard reaction.
Exothermic Activity, exo- : "outside" a process or reaction that releases energy to its surroundings, usually in the form of heat.
Malliard Reaction, chemical reaction between amino acids and reducing sugars that gives browned food its distinctive flavour.
We're going to be demonstrating these two reactions with a recipe for caramel popcorn.
You will need butter, sugar, water and some un-popped corn.
Exothermic Reaction: Pop your corn to first crack
For thermal stability, we need mass. In order to have an exothermic reaction, we need to have a buildup of heat energy in the corn (or coffee) that we are heating up. With a greater mass of corn, we are less likely to have burnt bits, scorching or inconsistencies, as the heating process is slower.
We need enough corn here to cover the bottom of the pan. This will guarantee even heating throughout the cooking process, and a more even exothermic reaction.
Get your corn on the heat. We're covering it with foil for heat energy retention, shaking to really agitate the kernels through the heating process.
We adjust the heat as the corn begins to exotherm (‘pop’). We call this first crack. What has happened for this to occur? The heat energy in the pan has built up, allowing the pressure of moisture within the cell walls of the corn to build up to 1.5 bars of pressure.
Exothermic reaction complete, and we have popcorn!
Malliard Reaction: Browning sugars to caramel
Now let's make caramel. This represents the Malliard reaction, or ‘non enzymatic sugar browning’. This is when an outside source makes naturally occurring sugars brown. If you have sliced into an apple and left it, allowing the natural sugars to brown, you have observed enzymatic browning.
Put the sugar in the pan and add water to create thermal stability, and turned on the heat. Once the water is hot enough to boil, it will create thin bubbles that easily burst. As the Malliard reaction progresses, the bubbles will get smaller and the colour will progress from light tan in colour to medium brown. The bubbles getting smaller is indicative of the sugars hardening in the browning process. This shows itself in coffee, as the brittleness of roasted coffee.
Turn off the heat once the sugar turns brown, then add butter to begin the cooling of the sugar. In coffee roasting, we cool the coffee with high powered fans that pull the heat off of the coffee. Adding the butter, allows the sugar to stop cooking and going too far into the sugar browning process.
Combine the two
The texture of the sugar has changed completely from white granulated sugar, into a smooth glossy sauce with the addition of heat and water. In coffee roasting, we change the coffee from its raw state (green, kind of grassy, and dense like a pebble) to it’s roasted state (brown, rich and flavourful, and brittle texture for grinding and brewing), with the addition of heat.
And here you can see how the simple addition of heat can lead to physical and chemical reactions that make something ordinary into something extraordinary.
Come roast with us
If you already roast at home and want to delve deeper in the roasting theory, or would like to try your hand at roasting on an Ikawa Home Roaster, you can JOIN EMILY ON OUR HOME ROASTING WORKSHOP — make real coffee beans go pop, not just popcorn!