The Coffee Roasting Process

When coffee is roasted, the bean undergoes a series of chemical reactions that transform it to its ultimate consumable state. Heat is applied to the bean through the roasting machine (there are many different roasting methods out there). One of the most common roasting techniques is using a drum roaster where artisan roasters use sight, sound and smell to determine when the coffee beans are done roasting. The truth is, anyone can quickly learn to be a great roaster, even at home. If this is something that interests you, I highly recommend you pick up Home Coffee Roasting by Kenneth Davids. Roasting coffee is an art and, although it takes skill to know not only what roast level will bring out maximum flavor and the precise moment during roasting when the beans have reached that level to stop the roast, it’s learnable.

Green coffee beans

As I mentioned, coffee beans aren’t really beans at all, but are actually the seed of a coffee plant which has been picked and dried by one of a few drying processes. Finding quality green coffee beans from around the world is the first step to coffee roasting. Buying green coffee is also relatively cheap. It would be nice if home roasters could purchase green coffee directly from farmers around the world, and although there are a few selling online, it’s not always practical to do this. The main reason is that coffee farmers sell in bulk to importers via huge burlap bags that can hold over 100 pounds of green coffee. Trust me, that’s a lot of coffee. Instead, consumers usually purchase green coffee from importers who break up the large burlap bags that they purchase into 1 or 5 pound bags of green coffee to sell online to home roasters. There are many online green coffee sellers. When I bought in bulk for my coffee business I went through That’s probably too much coffee to buy as a consumer. I always recommend where you can purchase small amounts from 1 pound to 20 pound bags. If you need more, Sweet Marias has a wholesale division at where you can get 50 to 100 pound bags.

Roasting background

Some coffee roasting companies roast ahead of time in anticipation of sales. These same companies also tend to roast every coffee they have exactly the same (no matter what origin and what roast level best fits the coffee). In fact, many of them roast their beans extra dark to hide the stale taste of inferior beans. Beware of any companies who only roast dark. The burnt taste they’re trying to sell you is not really how coffee is supposed to taste with the exception of a few coffees that should be roasted dark on purpose (called a Vienna roast) for making espresso or because it’s the best roast profile for that particular coffee.

The roasting process

After finding the highest quality green coffee beans available, the roasting process can begin. Coffee can be roasted to many different roast levels. Coffee “cracks” twice when it’s roasted. This is part of the natural reaction of the coffee beans to heat. Coffees vary on how long they take to roast but on average can be roasted in about 15 minutes. The first crack occurs around the 12 minute mark and about 3 minutes before the second crack can be heard. The first crack is loud and sounds like those little white poppers that are thrown on the ground by kids on the 4th of July. The second crack is more subtle and sounds like Rice Krispies after adding milk to the cereal. The longer a coffee is roasted, the lighter it weighs and the darker it looks. The different roast levels from lightest to darkest are:

1. City/Light (first crack, a volatile sound of the beans cracking begins)

2. City+/Medium (shortly after first crack is complete)

3. Full City/Medium Dark (second crack is about to happen at any moment)

4. Full City+/Dark (first few moments into second crack, a quieter snapping sound begins)

5. Vienna/Light French Roast/Very Dark (second crack is well underway)

6. French Roast/Extremely Dark (second crack is over and the beans are getting oily)

Here are two pictures I took of coffee beans before and after roasting.

Before: this is green, unroasted coffee from Java.


After: the same beans after roasting to a Full City+ profile. Notice they nearly doubled in size.


After roasting

Coffee doesn’t taste very good immediately after the beans are done roasting. Roasted beans emit carbon dioxide for a period of 48-72 hours (called degassing). If you buy fresh roasted coffee online from a company that roasts the coffee after you order, it should be done degassing and just about ready for use by the time it arrives at your home. If you buy from a local roaster or you roast coffee yourself, make sure to wait a few days after roasting to allow the beans to rest and degas before using them. Coffee consumed within the first 72 hours, while the beans are still degassing, can taste flat and stale. After about 72 hours, the beans reach their peak and taste the best. As long as the coffee beans are stored properly, they will be fresh for 2-3 weeks with a rapid decline in flavor and quality starting at around day 21. In most cases, the whole bean coffee available in grocery stores and coffeehouses is old and stale. Typically, this coffee is roasted months before you buy it. Coffee companies try to preserve the life of their beans by using 1-way valves in their packaging, allowing air to leave the bag while preventing any from going back in.

The hack

Try this experiment. The next time you buy coffee at the store (which I hope you won’t for too much longer) squeeze the bag to let the air and coffee aroma come out. When you get home, put the coffee on the counter and wait a day or two. Then see if more air fills up the bag, allowing you to squeeze more out. If this happens to you, it’s either really good or really bad. It’s possible you have really fresh coffee beans that are still degassing and releasing carbon dioxide, explaining why the bag has filled back up again. That’s the best case scenario and if you do the expiration date math and the beans were recently roasted, this is the likely case. More often than not, though, coffee bags fill back up with air because of faulty 1-way valves that are actually allowing oxygen to go back into the bag, degrading the coffee beans. Buying coffee that is freshly roasted is the single best thing you can do to make a great cup of coffee.

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