How is Whole Bean Coffee Roasted?

When coffee beans are first removed from the fruit of the coffee plant, they don’t look anything like the rich, dark brown beans with which we’re so familiar. Although we commonly refer to all unroasted coffee beans as green, only the immature raw beans are green like peas. Mature raw coffee beans range in color from pale yellow to reddish brown. Regardless of color or maturity, however, raw coffee beans are not suitable for brewing. It’s the roasting process that creates the bean from which we can extract our morning brew. But exactly how is whole bean coffee roasted?

Properties of Green (Raw) Coffee Beans

To understand what the coffee roasting process accomplishes, it helps to understand the properties of green (raw) coffee beans.

If you were to grind up green coffee beans and brew a cup of coffee from them, the resulting beverage would share many of the same properties as your traditional roasted brew. Green coffee beans have the same, or, in some cases, even a slightly higher, make-up of proteins, sugars, acids, and caffeine.

More caffeine sounds like a good thing, right? It does until you realize what it is your brewed green bean coffee is missing.

Raw coffee beans lack the two most important components that make our coffee experience so indulgent and worth having: flavor and aroma!

A cup of brewed green coffee may have plenty of caffeine, but it is still an odorless, bitter beverage.

And no one wants to start their day with that!

The Chemical Changes of Roasting

So now we know that it’s the roasting process itself that literally creates our sensory coffee experience. Until the green coffee beans are roasted, there is no enticing coffee smell. And there is no rich, hearty flavor.

So what exactly happens when a green coffee bean is roasted? It’s called the Maillard reaction.

The Maillard reaction is chemical process named after Louis-Camille Maillard, the French chemist who first described it in 1912. Without getting overly scientific about it, the Maillard reaction is the heat-generated breakdown of starches into simple sugars. In the process, these simple sugars turn brown and change flavor.

The Maillard reaction is what causes bread, cookies, and even steaks to brown. It’s the basic chemical reaction that created the artificial flavor industry. And it’s the reason we enjoy coffee that’s brewed from roasted coffee beans.light and dark roast coffee beans

How Whole Bean Coffee is Roasted

In the roasting process, whole coffee beans are exposed to hot, dry air. The goal of the process is to render green coffee beans suitable for consumption. As noted, it is through the Maillard reaction that the coffee beans are chemically altered to become both flavorful and aromatic. The roasting process also dehydrates the beans, creating the brittle, porous texture that makes them suitable for grinding.

Although there are a number of methods available, most coffee roasting is done using either a drum machine or a hot air roaster.

Drum Machine

As its name implies, a drum machine roaster consists of a perforated cylinder (drum). The raw green coffee beans are placed inside the drum. An external heat source utilizing electricity, natural gas, or even burning wood, is used to roast the beans while the drum continually revolves.

With drum machine roasting, purity of taste can be a concern since oils and smoke are trapped in the rotating drum with the beans. The drum itself also becomes coated with oil and char from the beans over time. Such residue can also taint the flavor of the final roast. It is important that drum roasters be kept as clean as possible to ensure consistency in roasting.

Despite these concerns, drum machine roasting is the predominant form of coffee roasting used by both commercial and home roasters.

Hot Air Roaster

In hot air roasting — also known as fluid bed roasting — green coffee beans are placed in a roasting chamber in which the floor is made of screening or perforated metal. Hot air is then blown into the chamber with enough force to lift the beans. Instead of manually being rotated in a turning drums, the beans “float” and tumble in the hot air until roasted as desired.

Although many consider this a cleaner method of roasting than that of using the drum machine, it is a more difficult process to control temperature-wise. The importance of this will be discussed in the next section. Roasting times tend to be shorter with the hot air/fluid bed roasting method, though, and this does make the process desirable for large-scale roasting.

Roasting Transformation

Regardless of the specific method used, the green, raw coffee bean goes through various stages during the roasting process. The heating process itself has two phases, and adjusting the heat accordingly is important to the outcome of the final roast.

When the roasting process first begins, the beans are absorbing the heat being supplied to the roaster by the external source. Once the beans reach about 347 degrees, however, the process reverses, and the beans begin giving off heat themselves. This is the point where the roaster needs to adjust the external heat source to keep the beans from over-roasting. Which explains why the drum machine roasting method, with its simpler-to-control external heat source, has the edge among coffee roasters over the less-controllable hot air process.

As the roasting continues, the beans will become about 15% lighter in weight but will nearly double in volume. The reduced weight is the result of the loss of water and other volatile compounds that “burn off” in the roasting process. The increased volume is caused by the cellulose structure of the bean that expands as the bean gives off that water and those volatile compounds.

The final result is a dark, plump but airy bean that smells amazing and grinds easily.

Good Listeners Make Good Roasters

Since most of us are familiar with the look of roasted beans, it’s understandable to think that determining when coffee beans have reached their desired roast level would be a visual process. But bean color is actually a poor indicator of the level of roast a bean has achieved. Turns out that, in coffee roasting, seeing isn’t believing, listening is.

That’s because there are two temperature markers in the roasting process that cause the beans to make an audible cracking sound. The first — called, obviously enough, the “first crack” — occurs when the internal temperature of the beans reach 385 degrees. This is considered the start of the “light roast” phase in which much of the bean’s water has evaporated, decreasing the weight, and the size or volume of the bean has begun to increase.

The bean will continue to darken in color and transform in flavor until the “second crack” at about 435 degrees. This is the phase of the dark roasts. But the second crack is caused by the structure of the beans starting to collapse. If roasting continues much beyond the 475 degree range, the beans carbonize or “burn up” and become unusable.

From Bean to Cup

We now understand that the flavor we enjoy in our coffee cup is created entirely through the roasting process. What’s more, the extent to which raw coffee beans are roasted provides us a wide range of flavors. There are “first crack” light roasts, “second crack” dark roasts, and all the “medium” roasts in between.

So how do you find your favorite roast? Try them all! That’s the fun of being a coffee lover.

coffee beans heart


If you want a little guidance for your roasted-coffee experimentation, however, our article The Difference Between Coffee Roasts will get you moving in the right direction to find your favorite.

Are you a Light Roast or Dark Roast coffee person? I’d love for you to tell me about your favorite coffee roast in the comments below.


  1. Chrissie Spurgeon

    Wow, I have learnt so much that I did not know about how coffee beans are roasted!

    Before reading your definitions of light or dark roasting, I thought that I preferred a light roast. However, it turns out that I prefer a dark roast, as in Italian coffee! Who knew?!?

    Thank you so much for your fascinating post.

    Chrissie 🙂

    • Cheri

      I’m so glad you found the article informative, Chrissie. I’ve had some revelations about my own coffee preferences as I’ve researched, too, so I totally get where you’re coming from. It’s great to learn new things so we can understand and, in some cases, improve our choices. Thanks for visiting and commenting. I appreciate it!

  2. Jack

    I really like your website it is perfectly organised with a great easy-to-navigate layout. You clearly know what your talking about, I had never realised the scale of how much attention to detail actually goes into making the beans that ultimately brighten up my mornings! Thanks for a great, enjoyable read I’ll definitely visit again, good day!

    • Cheri

      Hey Jack…I so appreciate your kind comment and feedback! I’ve worked hard to try to provide the most-organized experience I can for my visitors, so I’m especially glad you find the site easy to navigate. And you’re right…there is a lot that goes into that morning-brightening cup of coffee…a lot more than I ever realized until my curiosity got me reading and researching the topic. I’m enjoying sharing what I’m learning with coffee-lovers around the world. Thanks again for reading and commenting. Happy sipping!

  3. arnaiemhasan

    Hi Cheri,

    I asked a specialty roaster and they will tell me that coffee is within 2 weeks of roasting .If the beans you purchase have been roasted before you place your order , you have no idea what condition they will be in by the time you brew them . The oxidation could be in full effect and the oils could already be turning and i advice ,get coffee from a seller that roasts after you place to order .

    • Cheri

      Thankfully most commercially-packaged coffee is sealed so that oxidization is not an issue until the package is opened, so those without access to local roasters can still enjoy a good cup of coffee. But I’m totally with you: whenever there is an option to get freshly-roasted coffee from a local roaster, it is definitely worth it! Thanks for visiting and commenting. Happy sipping!

  4. Kenechi

    So this is how a whole coffee green beans is roasted and that’s what makes it have the sweet coffee flavour which I so much cherish. If these things are not industrialized, will the local kind of roasting still be okay and sweet like the industrialized ones? My friend has tried to roast one and I was thinking he was doing the wrong thing though it got burnt.

    • Cheri

      People can definitely roast their own coffee, and I’ll be doing some articles on that subject in the near future…as well as trying it myself. Hopefully I won’t burn it like your friend, but I guess I will until I learn! Commercial roasting is actually relatively new compared to how long people have been drinking coffee, so “home roasting” was once very common. So common that one of the first workers in a commercial roasting facility thought it was a ridiculous waste of time, that no one would purchase pre-roasted coffee! Funny how things change, isn’t it? Thanks so much for reading and commenting, Kenechi.  Happy sipping!

  5. Md moinul Islam

    Wow Cheri,This is an excellent topic which you are telling us.I like coffe  so much.It is very special for me.Most of the time I drink it.A coffee bean is a seed of the coffe plant and the search for coffe.It just like ordinary Cherris,the coffee  fruit is also a so called stone frui. It is the pit inside the red or purple fruit often refered to as a cherry.One of only two states in the United States of America able to grow coffee plants commercially is Hawai, the other being California.your post is very good.This topic is wonderful for those whose are very fond of coffe like me.Thanks for sharing this informative article.               

    • Cheri

      I really appreciate your kind comments, and I’m glad you found the article informative. You are right, there are few places in the U.S. that coffee can be grown which is why the United States is the top importer…because we sure do love our coffee! Thanks so much for visiting!

  6. raju


    this is the first ever website i have been commin which explains very clear about coffee beans transformation from bean to cup good for coffee lovers and makers freshly opened packagei really like iced coffeeei will definately try yr shown method  thanks for sharing your knowledge i like your website and blog

    • Cheri

      Thanks very much for your kind comments, Raju. I’m glad you like the site and found the information on how coffee beans are roasted to be helpful. Like you, I love iced coffee in the hot summer months, and that’s why the post about how to brew it was one of my very first on the site. I hope you’ll find that article helpful, too. Happy sipping!

  7. Lok Which

    Thanks for sharing this informative and educative article on coffee. I do not know much about roasting whole bean coffee but this article has really triggered my interest to try it and I will surely try this out. I will ensure I follow this article in trying that out. Thanks for bringing this to my notice.

    • Cheri

      I’m glad you found the article intriguing. Freshly-roasted coffee is quite a treat, and I hope you’ll soon find that out for yourself. Thanks so much for reading and commenting.

  8. charles39

    Although I come in coffee growing country the process of roasting the beans is New to for we don’t bdothat in our country we only process and export the raw let say I love  coffee it’s aroma is heavenly and it’s has alot benefits also in our bodies. Having said that it seems you tought me very important lesson that I should not dare to forget but learn more into the process of roasting coffee beans I’do hope your subsequent Post will she’d more light into this .

    • Cheri

      I’m glad you found the article informative, Charles. You have me very curious about where you live since the focus is only on processing and exporting raw coffee beans. I think I could learn plenty about that from you! Roasted coffee does have an enticing aroma, that’s for sure, and I will definitely be writing more about the fine art of roasting raw coffee beans that creates that wonderful scent. Thank you for commenting.

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