The beans utter their first sound: A story about the "crack" sound during roasting.
During roasting, the quiet beans suddenly crackle and pop. It is in this instant, known as "first crack," that most of a coffee's flavor is determined.
Roasting begins in earnest with a sound.
For a while after starting the roast, the inside of the machine is silent. As the beans lose moisture, their color changes silently from the greenish hue of raw beans to yellow, and then to a pale brown.
However, at a certain moment, the silence is broken. A crack. Another crack. Soon, a series of popping sounds begin: pop, pop, pop. It's similar to the sound of popcorn popping.
In the world of roasting, this sound is called "haze" (Japanese for "crack" or "pop"). The kanji character used means "to burst." It refers to the phenomenon itself, where the beans burst from within.
In the previous article, I mentioned DTR (Development Time Ratio) as a roasting indicator. It is the number representing "what percentage of the total roasting time is spent from the beginning of the first crack until the end of roasting." This time, I'd like to delve into "haze" itself, which is the starting point. For a roaster, this is the most reliable signal and the core of the roasting process.
What's happening inside the bean
The true nature of the "haze" (crack) becomes apparent when we look inside the bean during roasting.
Inside the heated beans, various chemical reactions, including the Maillard reaction, are taking place. Simultaneously, two gases are produced. One is water vapor, created from the moisture remaining in the beans by heat. The other is carbon dioxide, a byproduct of the chemical reactions.
The cells of the bean harden like glass when heated, and gradually swell due to internal pressure. However, the continuously generated water vapor and gas seek an escape. Eventually, the internal pressure exceeds the strength of the cell walls, and the bean can no longer withstand it and bursts.
This is "first crack." The popping sound is, in fact, the sound of the bean cells bursting.
At this point, the beans visibly swell, their wrinkles smooth out, and their color deepens considerably. Chaff flies, and the aroma instantly becomes vibrant. It is the moment of greatest change in roasting.
And importantly, it is around this first crack that the coffee's flavor and acidity develop most richly. This is why roasters listen to this sound as a signal that "the real work begins now."
From this first crack until the end of roasting, how much time is taken—in other words, how the DTR is designed—is where the roaster's skill shines. If roasting is stopped immediately after the first crack, the coffee will be light. If the time is extended from there, sweetness and richness will develop deeply. Even with the same first crack as a starting point, just a few tens of seconds from this point can dramatically change the impression of the cup. The most critical point in roasting lies in the time after this sound occurs.
First Crack and Second Crack — Two Distinct Sounds at Different Times
The "haze" or "crack" occurs twice during roasting.
After the first crack has finished, the roasting machine temporarily becomes quiet. However, the chemical reactions and expansion have not stopped; changes continue within the beans. As roasting progresses further, a second sound emerges in the dark roast range.
This new sound is different from the first crack. If the first crack is a distinct "pop, pop" as each bean bursts loudly, the second crack is a smaller, faster, continuous "patter, patter, patter." It's a finer sound, like the beans are crumbling. This is the "second crack."
Just before the second crack, the color of the smoke from the roaster changes, and the emission of combustion gases increases rapidly. Inside the beans, deeper structural damage occurs, and trapped gases lose their escape routes, repeatedly causing small bursts. Oil squeezed from within seeps onto the surface of the beans, making them glisten. Dark roasted beans appear black and shiny as a sign that they have been roasted to this stage.
If the first crack is "a loud, opening sound dominated by water vapor," the second crack can be described as "a finely crumbling sound dominated by gas and oil." From a single "pop" to a rapid succession of "pats." Even though the word "haze" is the same, the sound, and what's happening internally, are completely different.
Haze serves as a map for roast degree.
Roasters listen intently to the "haze" because it is the most reliable guide for understanding the roast degree. This sound holds insights that thermometer readings alone cannot reveal.
The classifications of roast degrees, such as "light roast" and "dark roast," are ultimately determined by these two cracks. Here's a general correspondence:
- Stop roasting around the start of the first crack — Light roast. This range is characterized by bright acidity and pronounced floral or fruity notes.
- Stop roasting around the end of the first crack — Medium roast. This is a classic roast degree with a balanced acidity and sweetness, found palatable by many.
- Stop roasting around the start of the second crack — Full City roast. Acidity mellows, and bitterness, body, and chocolate-like richness emerge.
- Stop roasting around the end of the second crack — French roast. A deep, strong, and bitter range.
- Continue roasting beyond the second crack — Italian roast. The deepest roast degree.
What's interesting is that between the two cracks, the coffee's flavor slowly shifts its focus from "the world of acidity" to "the world of bitterness." Even with the same beans, what's in the cup becomes entirely different depending on which crack, and at what moment, the heat is stopped. If you consider that the taste of your usual cup is the result of the roaster deciding at which crack to stop, your perspective might change slightly.
The "haze" doesn't always show up the same way.
Annoyingly, this "haze" signal doesn't always arrive at the exact same temperature or with the same intensity. Its arrival varies depending on the characteristics of the beans.
Beans processed by the natural method, which are high in moisture, or Mandheling beans, with their soft outer skin that releases expansion force, may experience first crack a bit later and at a higher temperature. Large-bean varieties like Pacamara require a lot of energy to burst, so their first crack also tends to be delayed. The temperature of the "haze" also fluctuates depending on the batch size, heat input, and the design of the roasting machine itself.
That's why experienced roasters don't just blindly trust the thermometer readings. They try to assess the beans' condition comprehensively through their ears, eyes, and the rising aroma. Rather than saying, "It's 1st crack because it reached XX degrees," they listen to "how these beans are sounding right now." This is where the difficulty and profound depth of roasting lie. Since beans arrive with slightly different characteristics each time, simply following the same recipe won't lead to the same cup.
Transforming "sound" into "numerical data" — between prediction and reality
How do we accurately transfer that split second, "the haze has arrived," heard by ear, to the next roast? This has been a long-standing challenge in mastering roasting. Sound disappears immediately. Memories of "it felt good" are surprisingly unreliable.
What's interesting about Kaffelogic is that it allows you to enjoy "creating changes" starting precisely from this first crack. How much DTR (Development Time Ratio) you raise after the first crack — that slight difference manifests as a change in flavor. With the same beans, even a slight extension of the DTR changes how sweetness and richness develop. Experiencing this difference firsthand is a unique pleasure of home roasting.
And this is where the Nano 7's home roasting size really comes into play. Because the amount of beans is manageable, the sound of the "haze" — that popping sound — reaches your ears clearly. This allows you to observe when the haze occurs with your own ears and record that moment. The Nano 7 precisely controls temperature and records the entire roasting process as a profile, so how the bean temperature changes, where the first crack occurs, and how many seconds later roasting was stopped — the entire trajectory, including DTR, remains as data in your hands.
On the record screen, the default profile's expected timing for "color change" and "first crack" is displayed as a vertical yellow prediction line. This provides a rough estimate of "around when the haze will occur." However, as we've already seen, the actual timing of the haze varies depending on the beans. That's why it's crucial to record the actual haze observed with your own eyes and ears, and to check the deviation from the prediction line.
For example, when actually roasting, the observed and recorded first crack may occur slightly before the prediction line—that is, earlier. Seeing this difference itself provides a clue for the roaster. The prediction is merely a guideline; ultimately, you rely on the roaster's own intuition. The Nano 7 allows you to interpret this "gap between prediction and reality" and apply it to the next roast, transforming what was once a task of a craftsman's experience and intuition into accessible data.
Furthermore, because the Nano 7 can perform serious roasting with as little as 50g of beans, you can repeatedly conduct small experiments like "what will it taste like if I stop at this moment of the haze?" or "what happens if I extend the DTR a little more?" without wasting precious green beans. You can seamlessly switch between roasting by ear and roasting refined by data, all within one machine.
Listening intently, the gateway to roasting
There were sounds I couldn't hear when I was just brewing coffee. When I started roasting, those sounds were added. The first distinct crack echoing in the quiet. The continuous popping sounds. The silence that falls, then the second sound of finer crumbling. Each of these is a signal of the bean's condition.
Once you can distinguish the "haze," roasting transforms from "a task of managing time and temperature" into "a dialogue of interpreting the bean's condition." Today's beans started cracking earlier than usual, today they are popping well — these individual differences become the very satisfaction of roasting.
The cup you drank today also had a "haze" that someone heard somewhere. Based on that sound, acidity, bitterness, and aroma were shaped. If you're going to try roasting yourself, what you should first pay attention to might be this sound, rather than temperature numbers. Roasting begins with waiting for that first pop.
May you have a good cup today, too.
The temperature and timing of the "haze" (crack) vary depending on the type of beans, moisture content, roaster design, and batch size. Please read the temperature ranges and roast degree correspondences in this article as general guidelines only.
Tomoki Sakurai
You might also want to read — The World in a Cup of Coffee — A Story of the Coffee Origin Map