What Do Roasting Plants Manufacturers Say About First and Second Crack?

Coffee roasting passes through homogenization, first crack, a resting phase, and second crack, and each stage changes the flavor in the cup. Green beans hold roughly 10% to 14% moisture after drying. When heated, that water turns to steam and expands about 1,700 times in volume, raising internal pressure and pushing the boiling point into a superheated state, which drives aroma diffusion.

Equipment built by roasting plants manufacturers controls how heat reaches the bean during those phases, so the same green coffee can taste quite different from one machine and profile to another. Components such as organic acids in coffee and sugars stay inside the shrunken dried bean and react once heat arrives, which is why moisture content, batch size, and airflow settings matter as much as the final roast color.

Coffee roasting is not merely a matter of applying heat. From the moisture changes inside the green bean to the expansion and rupturing of cellular tissue, every stage quietly determines the final flavor in the cup. This article takes the perspective of commercial roasting machines to systematically sort through key stages such as homogenization, first crack, the resting phase, and second crack. It explains the principles behind the diffusion of coffee bean aroma and clarifies the taste differences corresponding to different roast levels, helping coffee enthusiasts gain a deeper understanding of how roasting influences flavor direction.

Pressure first, then bursting — this is precisely the core mechanism by which coffee bean aroma is diffused

Key knowledge you need to understand

Coffee roasting involves multiple stages. This article uses commercial roasting equipment as an entry point to sort through each roasting phase. Because intense chemical reactions occur inside the bean during roasting, these reactions directly determine the coffee's final flavor expression.

Homogenization

─Heat is transferred into the green bean, inside and out, bringing all beans into a state of readiness for uniform reaction.

Every green bean is composed of a large number of cells, and each cell contains a cell wall, cell membrane, cell sap, and nucleus. After harvesting, green beans undergo drying to facilitate storage and transport. During this process, a portion of the moisture in the cell sap and cell walls is evaporated, leaving only about 10%–14% moisture. As moisture decreases, cell volume shrinks, and the green bean as a whole becomes smaller. However, components such as organic acids and sugars remain within the bean.

When dried green beans are heated, heat is conducted into the bean and triggers changes. First, the remaining moisture inside the bean is heated and turns into steam, and when water becomes steam its volume expands dramatically (by roughly 1,700 times), so the pressure inside the heated bean also rises.

The rise in pressure also raises the boiling point of water, putting the steam in a superheated state. It may be more intuitive to understand the principle of boiling point elevation by thinking of a pressure cooker: at high altitudes, air pressure is low, so the boiling point drops and rice is prone to being undercooked. If pressure is applied to the pot at this point, the internal pressure increases and the boiling point is forcibly raised — the pressure cooker is designed precisely on this principle. In a pressure cooker at 1 bar, steam temperature can reach about 125°C, and the same phenomenon occurs inside the bean's cells.

Because pressure rises, the cell volume that had shrunk due to drying expands again. Coffee beans come from coffee cherries, and their cellular structure is the same as that of trees and plants. Therefore, once the volume expands to a certain extent, at a certain moment the tissue can no longer withstand the pressure and ruptures. At this point, part of the high-pressure steam formed inside the cells is released, while another part transfers into adjacent cells. Through this process, steam at the same temperature and pressure becomes uniformly distributed throughout the entire cell population, and this phenomenon occurs almost simultaneously in virtually all green beans in the batch.

Generally speaking, green beans include a mix of large and small beans, and each bean also has areas of varying thickness, along with a relatively dry surface and interior tissue that contains more moisture. If these complex factors are ignored and heat is simply applied continuously, the small beans will scorch first, while the large beans may be underdeveloped, or only the surface may be scorched while the interior remains half-raw. Therefore, the beans must be brought as a whole into a state of readiness for uniform reaction.

Inside the bean, moisture migration prepares the entire tissue for uniform reaction. Outside the bean, heat conduction between beans and the convective heat generated by their shared rotation bring all beans into a state of readiness for uniform reaction. This stage, which prepares the beans as a whole for uniform reaction, is called "homogenization."

During homogenization, the beans turn coffee-colored and appear as though they have been dried. We are usually accustomed to calling this the "drying phase," but in fact the moisture evaporated in this process is negligible compared with the first crack, when the tissue itself undergoes a qualitative change. Therefore, the term "drying phase" is not accurate.

First crack

─The tissue expands and ruptures, and the aroma produced by roasting begins to increase.

The end of the homogenization stage means the beans now meet the conditions for all of them to undergo the same reaction synchronously. Compared with the green bean state, each cell's volume has already increased substantially, and the tissue as a whole has thickened. If heating continues, the volume will continue to increase, and then, as shown in the next image, the bean's center line will split open. In the green bean state, the center line is tightly interlocked; but as volume increases, the ends on both sides of the bean split open, producing a sound like a tree branch snapping. At first crack, the bean's tissue splits open in this way, and at the same time it begins to release a large amount of moisture from inside the bean.

The process up to this point is called the cinnamon–high–city roasting stage. The stage where first crack begins is called Cinnamon, the stage where first crack is about to end is called High, and the period from the end of first crack until second crack occurs is called City.

Resting phase

─Intense chemical reactions occur inside the bean, determining the coffee's taste.

The period from the end of first crack until the beginning of second crack is called the "resting phase."

As moisture is released, the pressure on the bean's internal tissue decreases, and the tissue temporarily stops expanding. Because the bean has already released a large amount of moisture at this point, if it is heated further, the heat will not be stolen away when steam is generated, and the internal temperature will rise faster than before.

At this time, the high temperature can drive more active chemical reactions, producing gas and causing the tissue to continue expanding. The internal tissue that split during first crack becomes more expanded and filled out. Visually, the wrinkles on the bean's surface gradually disappear, and the bean becomes larger.

At this point, the high temperature has reached every bean, and the chemical reactions inside the bean reach their peak. The aroma produced by roasting increases, making this an important period that determines the bean's taste. Beans before the resting phase are in the "endothermic phase," while beans enter the "exothermic phase" once the resting phase begins. In reality, however, the beans do not release heat; it is simply that compared with the earlier stages, the temperature rises faster during heating, so this name is used for convenience. If the beans react too quickly, they may scorch or develop unwanted off-flavors, so the heat is usually reduced at this stage.

Second crack

─The tissue splits again, producing intense chemical reactions, and the coffee's aroma and acidity increase.

After the resting phase, once the internal tissue has changed and fully expanded, the split internal tissue becomes filled out and the bean continues to grow larger. At this point, a sound of the tissue changing again can be heard, and this is called second crack. It produces a sound like snapping a matchstick, and compared with first crack, the sound is crisper and higher-pitched.

At this time, the pressure within the tissue reaches its highest point, and the gas generated inside the bean begins to be released to the outside. The capillaries formed during the release of this gas in this period are even more closely related to later coffee extraction. In other words, coffee beans that have only undergone first crack have relatively fewer capillaries formed, so extraction tends to be less smooth.

From the occurrence of second crack onward, the bean's temperature rises even more dramatically, so stronger chemical reactions are produced inside, making the coffee's aroma even more intense. At the same time, the aromatic compounds produced by roasting also exhibit strong bitterness.

Using second crack as the reference, a roast level before second crack is called City, and after second crack is called Full City. For coffee that continues to be roasted beyond the Full City stage, we customarily use the following names:

(Vienna roast): the dense second-crack period (also called espresso roast).

(French): the period after second crack ends when oils fully seep onto the surface.

(Italian): the period after second crack ends when the color becomes darker and the oils begin to slowly char.

Roasting refers to the process of heating green beans, and green beans have components similar to wood. Therefore, if the temperature exceeds wood's ignition point of 250°C, the beans will burn into charcoal. So dark roasting refers to roasting only until the bean temperature reaches 250°C. If the final roasted result is not reddish-brown but completely black, then the bean has turned into charcoal.

The above content is compiled by CoffeeHunters, a coffee news website.