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.
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.
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.
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.
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.