What Happens When You Roast Coffee?

Maybe you wanted to make your own coffee, but growing is difficult and you don’t live on a mountain in the middle of Ethiopia, so you would only be able to grow poor little coffee plants. Or maybe you did grow some coffee but when you processed it you just wound up with a fermenting pile of mush. Either way, somewhere along the line you just gave up and decided to buy some green beans online. Roasting can’t be that hard, right? It’s just, what, baking?

Well, you’re in luck. Anyone with an oven or a stove, or (for the desperate folks out there, a popcorn popper, can roast coffee). Unfortunately, roasting good coffee can be tough. It’s not as simple as just tossing the beans in a heated drum and walking away.

No matter what you’re using, when you toss a bean in the heat, the beans will dry out before anything else happens. Even though some of that moisture was dealt with in the processing, there’s still a little hanging on. One surprising side effect of this is that drier coffee beans will actually roast quicker. So sometimes in the depths of our Maine winters the coffee will roast quicker, despite Jack Frost’s best attempts to cool the process down.

Once the beans are dry they will start to caramelize. That’s right, the same process that makes a decadent sweet is behind coffee’s awesomeness. Okay, maybe not so surprising. There are a lot of sugars in those green beans, but we don’t want them to just be sweet, we want them to be complex.

Coffee is not Kool-Aid
Picture by Arun Joseph is licensed under CC BY NC SA 2.0

Once the beans reach 394 degrees Fahrenheit (give or take a few) they will crack. Think popcorn. It’s an audible sound that is called first crack and is considered the point when coffee becomes drinkable, by most.

Coffee might be popcorn, kinda. Don't tell coffee I said that.
Picture by Janet Lackey is licensed under CC BY NC 2.0

If you keep the heat going until the beans reaches 437 degrees Fahrenheit (again, give or take a few—coffee is complicated), they will actually pop again. At this point, all of the sugars have been caramelized, most of the delicate flavors have been burned off, and there will be a lot of ‘darker’ flavors—think chocolate and smoke. This is also the end of a stage called pyrolysis, which began with first crack, for all of the science-y types out there.

Coffee: a more reasonable and fashionable outlet for the seasoned nerd

Picture by Andrea Allen is licensed under CC BY 2.0

A good roaster wants to balance a nice sweetness with a rich complexity. If you roast a bean too long you risk losing some of the bean’s more delicate flavors, and if you get crazy and decide to keep on going, all you will taste will be the roast. Some appreciate the darker roasts for their chocolate flavor and rich mouthfeel, but some consider dark roasts to be burnt, tasting like carbon and even plastic if the roast goes too far.

Coffee is also not plastic.
Picture by Brenda Anderson is licensed under CC BY NC SA 2.0

There’s not just the level of the roast to worry about, but also how it got there. If you don’t treat the coffee well, heat it gently and keep it constantly moving, you risk beans that are unevenly roasted. This could mean one side is burnt while the other is a nice rich brown, or that maybe the outside is just where you want it to be, but the inside is still green. The resulting coffee can taste, at the same time, burnt and vegetal, a kind of unfortunate conundrum.

Now you know the process: next time we will talk a little bit more about what roasters use on different levels, and how you might go about roasting coffee at home.

Processing, Pt. 2

So we've looked at how to get the bean out of the fruit, but once you have it, what do you do with it? The answer is not to grind and brew, or even roast. Those beans have a ways to go, and before they can even be shipped, they have to be dried to a low moisture content, because moisture, rot and bacteria are like a group of young hooligans that are always hanging out together. The beans are usually dried in beds outside, where they are shifted frequently to speed the process and prevent mold. 

(Photo by skinnydiver is licensed under CC BY-SA 2.0)

The process is also done partially or entirely by machine on some farms. It is important that the process is done properly, neither too quick nor too slow. If it's down too quickly, it risks damage to the beans, while if it's done too slowly unpleasantness may set in via rot or fermentation. Either way, you get some pretty nasty coffee.

I’m not pointing any fingers or anything 
(Photo by JeepersMedia is licensed under CC BY 2.0)

After the coffee has been dried, there's still the parchment around the bean, which is removed by dry milling. Some producers delay milling and let the beans rest with the parchment intact for a few months, which increases the shelf-life of the green beans. Once the beans are milled they must be sorted by density and color to insure consistency within batches. For a roaster, this means he doesn’t have to curse because half of the batch burnt before the other half could finish. For you this means if you buy a pound of coffee it’s going to taste basically the same throughout the whole pound. This can be done by hand, using density tables, and judging color by eye, but infrared is being used more frequently throughout the world.

(Photo by sarahemcc is licensed under CC BY 2.0)

Mostly, the beans are sorted to remove any defect beans, but they can also be graded into different batches labeled by number or letter, or separated into normal coffee beans and peaberries, which are smaller, round coffee beans that are the result of a genetic defect (which isn't to say there is anything bad about them, just to say that they are different; some even hold the peaberry in higher respect than the plain ol’ bean).

Roasted peaberries up close 
(Photo by Tschi is licensed under CC BY-SA 2.0)

After the beans are sorted, they are shipped to importers who distribute them, or sometimes even directly to roasters who have relationships with individual farms. The final step between these green beans and the coffee beans that most people think of is roasting, which we'll cover next post.