When we visualize chemistry, it is quite common to picture a laboratory with test tubes and various pieces of equipment. Mix the contents of two test tubes together and bam!

Something new is created! Rule number one about chemistry: if chemicals aren’t in the same space physically, then they can’t react with each other. Rule number two: sometimes, chemical reactions need a little help getting going and being sustained. This help can come from external energy (heat, typically) or an enzyme (a molecule that facilitates chemical reactions without being used up in the reaction and without requiring much, if any, energy to push the reaction forward).

Roasting coffee satisfies both those rules. The bean itself is the laboratory and the cells that make up the bean are the test tubes. The cell walls and the material within the cells comprise the raw ingredients of all the chemical reactions that take place during roasting.

Roasting provides the energy source that begins and sustains the chemical reactions.

While there are enzymes of all sorts in the cells, their role in the creation of what we know of as coffee is poorly understood. Most likely, enzymatic reactions don’t play a significant role in producing the coffee we know and love.

Actually, a coffee cell is more than just a test tube—it is also a pressure cooker. Plant cell walls are thick and durable. Thus, when the contents strive to get out, they cannot do so easily. When the cell becomes heated up from roasting, some chemicals change from liquids to gases and some new gases are formed. These gases will take up more space than they did as liquids or solids, so they push against the cell walls, creating pressure, just like a pressure cooker. While the cell walls eventually break from the pressure (more on this later), the increased pressure conditions do help shape the roasting process. 

Did you know?

While caffeine content might decrease somewhat as roasts get darker, the difference is so small that a daily coffee drinker’s body would probably never notice the difference.

A great deal of research has been produced on green coffee chemistry and roasted coffee chemistry. Scientists have strived to identify the chemical reactions that occur

during roasting as well as identify the compounds in green coffee and the resultant compounds that end up being created from the roasting process. To recount all that data here would be fairly meaningless and it would bore us all to tears. The truth is, while some groups of chemical reactions are known and lists of compounds exist, no nonscientific, practical use for the consumer or small business yet has come from any of it.

In short, we don’t know much about what compounds in green coffee are important precursors for specific compounds in roasted coffee. Nor do we know what compounds in roasted coffee determine specific flavors for us. Yet, we know there are hundreds of compounds in the green and roasted beans, some of which might make it into our cups.

We also know there are more than 1,000 volatile compounds in roasted coffee, less than 30 of which create the generic “coffee smell” experience, while others are recognizable as specific aromas when smelled on their own. Unfortunately, we don’t know exactly what makes coffee taste acidy or sweet or floral or what tastes like chocolate and blueberry are connected to. When it comes to coffee taste chemistry, we’re still in the dark ages.

“Science may never come up with a bet er of ice communication system than the cof ee break.”


These details aside, we know not only that roasting is important, but it is important how one roasts. At its simplest, coffee roasting is adding heat to coffee. However, how one applies heat (what kind of roaster is used) and when heat is applied throughout the roast have significant impacts on the final taste of the coffee. All modern roasters have at least one temperature probe inside the roasting chamber. This probe measures the air temperature at the location in which it is placed. When the chamber is filled and beans cover the probe, the temperature registered is an approximation of the bean’s actual internal temperature. This temperature can help inform the person roasting of what should be adjusted during the roast and in future roasts. The manipulation of time and temperature during roasting is called roast profiling, which can be illustrated on a graph with a curve. The decision of what constitutes an acceptable roast profile is—or should be—taste, although length of the roast and bean color are valuable metrics as well.

The roast profile curve can be used to help a roaster manipulate the taste of a coffee. It can also be used to help us explore some basic coffee roast chemistry. When green coffee is first put into the roasting chamber, the temperature drops precipitously—the green beans are absorbing heat. Before too long, the temperature begins to stabilize and rise again. During this rise, chemical reactions begin to occur as evidenced by the evolution of novel compounds and a color change. Also, water is evaporated; coffee beans typically have a moisture content of 9 to12 percent and, by the end of the roast, have about 2

percent moisture. In addition, some of that water likely takes part in chemical reactions.

The slope at which the temperature rises is very important to coffee roasters. The slope is a measure of how fast the roast is progressing, with steeper slopes reflecting faster roasting during this roast phase. Controlling the speed of roasting also controls the speed of chemical reactions since it is a reflection of the amount of heat being added into the system. However, what this means chemically is unknown.

Once the bean color is decidedly light brown, the beans begin to crack or pop and undergo a size expansion. The cracking is the same phenomenon that happens to popcorn kernels as they transition from kernels to popcorn and is much like a balloon popping from being overfilled. Corn kernels pop because water, trapped inside, converts to steam. The steam creates pressure that eventually breaks the cells, giving us popped corn. The cracks in coffee are also caused by gases producing excess pressure and breaking out of the cells.

Carbon dioxide is likely the major gas contributing to this jailbreak, and water is presumed to be fairly important.

Shortly after this crack, the roast could end and the coffee drunk. If left to continue, the bean color progresses through darker shades of brown and eventually into black.

Somewhere between medium brown and very dark brown, the beans crack again. This, too, is the result of gases breaking more cells. Carbon dioxide is the main culprit here, but accomplices are certainly present.

Although we seem to know very little about roast chemistry, we actually know quite a lot. We really lack knowledge of coffee flavor chemistry and how the two connect.

Current scientific instrumentation, computer power, and software are helping change this dearth of knowledge. Advances are coming, especially as more people become both coffee fiends and scientists. We just need to be patient!

Did you know?

Used coffee grounds can be used to generate biodiesel that can power cars, as a substrate to grow mushrooms, and even converted into a potable, though not necessarily tasty, alcohol! 

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