How-to: Roasting Coffee in the Oven

November 24, 2021

This is not a definitive guide, this is meant to provide beginners and people that are interested in coffee roasting with a framework and a starting point from which they can begin to experiment and figure out how to improve their roasts.

With a little understanding of what’s happening to the beans and what professional roasters do and what they look for, you can have delicious roasted coffee at a fraction of the cost.

Roasting coffee only needs to be as complicated as you want it to be. 

A Quick Primer on Roasting

Once you hit 'run' on the roaster, your coffee will go through 5 distinct stages:

• Drying stage: the beginning stage that dries out the bean in preparation for roasting, getting rid of any excess moisture. The beans will start green and then slowly turn a yellow colour. They will change from smelling grassy and floral to smelling like hay or dried grass.
• Browning stage: Browning is the actual start of the coffee roast. This stage is where the beans go from smelling hay to baked bread. The beans also start to change colour from pale yellow to light brown. 
• First Crack: The first crack occurs when the beans expand and swell from gas build-up to the point that the outer part of the seed cracks and releases the pressure (mainly caused by water vapour). It's similar to what happens to popcorn, and it sounds a little bit like it, too, just a little sharper. This typically occurs between 385-405 degrees. It depends on the bean and your roaster.
• Development Stage: Starts after the first crack. Variable in length depending on personal preference and partially determined by the coffee itself. The roaster can choose to extend the development stage by lowering the temperature or, with fluid-bed roasters, by adjusting the fan speed. The aim of this phase is to caramelize the sugars, roast off some of the acids present in the green beans, and develop the volatile flavour compounds that give coffee those unique and refined notes that we always look for under the familiar roasted and caramelized coffee flavour we all know.
• Second Crack: The second crack happens sometime after the first crack, depending on the length of the development time. The development time can technically overlap a little bit with the beginning of the second crack. Still, once the beans start popping rapidly, that's no longer considered 'development.' The Second crack, like the first crack, also occurs because of the build-up of gas inside the bean and typically starts as the beans approach 435-440 degrees. It sounds similar to green cedar leaves crackling on a fire.

During the roast, the coffee beans will dry, and the moisture content of the beans is reduced from 8-12% to less than 0.1%. The drying turns the seeds brittle and allows them to be ground. The reduction in moisture also means the beans lose mass. Roasting coffee typically results in an 18-20% loss in weight. Meaning, that if you roast 130 grams of green coffee, you will get 104-107 grams of roasted coffee. While the beans lose mass, they can almost double in volume. Each coffee bean expands as it roasts due to the creation of gasses on the inside, mainly from the evaporation of water and the generation of carbon dioxide.

Roasting caramelizes the natural sugars found in the coffee and burns off the acids present in the green beans, creating a balanced flavour. It also helps develop volatile flavour compounds and releases coffee oils from the cell structure, which greatly enhance the flavour and the mouth-feel of the eventual beverage when the coffee is ground and brewed.

Roasting is just like cooking; you're aiming for the sweet spot between under-done and over-done. When roasting, you can under-develop your coffee and end up with sour, grainy and vegetal notes, a weak body, and a short-lived, unsatisfying flavour. If you over-develop, the coffee will taste flat and unbalanced because over-exposure to high temperatures has caused the decomposition of the volatile flavour compounds. Development time is critical, but it's hard to give recommendations about how long the coffee should stay in this phase because it depends on the coffee you're working with and what you want to do. Like with a V60 brewer or an espresso machine, you will need to experiment and dial in a roast over time. Keep a timer handy!

 Roasting Coffee in the Oven

You will need: oven-safe pan or baking sheet to roast your coffee (cast iron pan preferable), spatula or stirring spoon, oven mitts, timer, bowl.

1. Pre-heat your oven with the pan in it to 450 degrees F. It is preferable to use a convection setting.
2. Measure out about 90-100 grams of green coffee beans. 90-100 grams is an estimate. You may be able to roast more or less, it just depends on the size of the base of your pan. Please make sure that the amount of coffee you measured you fits comfortably into your pan without crowding BEFORE you get the pan hot.
3. Once the oven has reached 450 degrees, open the door and pull out the rack with the pan. Spread the beans onto the hot pan into one even layer. It is very important that none of the beans are overlapping or sitting on top of each other as this will lead to an uneven roast. Leaving a bit of room between the individual grains is also advisable as the beans will swell during the roasting process.
4. Make sure to set your pan near the back of the rack, away from the oven door. Slide the rack back into the oven and close the door. It is very important that you do not open the door of the oven during the roast unless absolutely necessary as this will vent out the heat and slow down your roast.
5. Turn on your exhaust fan and/or open a window. 
6. Set your timer for between 13-15 minutes.
7. At around 7-8 minutes you may need to rotate the pan or stir your beans if they are roasting unevenly. This should be the only time you open the oven door.
8. Between 6-8 minutes into your roast, you should start hearing some light popping and cracking. This is first crack. Once you hear first crack, it is important that you stay by the oven and watch your beans because the roast will proceed much faster after this point
9. Use a colour reference! Either print off a picture or use another roasted coffee that you like as a reference to figure out when you need to pull off your beans. How to tell when the beans are done without a colour reference?
1. For a medium roast, the beans will be taken off as they approach second crack or just at the beginning of second crack. The beans should be a rick chocolate brown color, have no wrinkles on the surface, have a cream/tan center line and should have a satiny surface that reflects the slightest bit of light. This is caused by the coffee oils rising to the surface of the bean
2. A dark roast should be pulled out of the oven during second crack. The beans should be dark brown (not black) and have a glossy, oil-covered surface with a dark center line. Please be careful if roasting dark in the oven as it is really easy to burn your coffee (and make your house smell not so great).
10. Once your beans have reached the desired colour, pull the beans out of the oven and pour them into a heat-resistant bowl. This is really important because if the beans stay in the hot pan, they will continue roasting.  

Bonus tip: the chaff (the outermost papery layer on the bean) will stay on the coffee when roasting in the oven and will end up in your bowl while the beans are cooling. To remove the chaff, go outside with the bowl and toss the beans lightly in the bowl while blowing. The chaff will fly out, leaving only the beans. Don't do this in your kitchen or you will have chaff all over your floor and your counters.

Some more advice: track your development time. Underdeveloped coffee can taste grassy and grainy, overdeveloped coffee can taste bitter and unpleasant. Once you hit first crack and all of the beans are popping, start a timer on your phone and track how long it takes for the beans to reach the desired roast degree. You can slow the roast down by lowering the oven's temperature or by opening the door for a couple of seconds to vent out some of the hot air. Experiment! see what works for you.

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Also in Home Roasting: Demystified