Understanding the auditory cues like the “First Crack” and “Second Crack” is pivotal for those who wish to master the art and science of coffee roasting.
What is First Crack?
First Crack represents a significant milestone in the coffee roasting process, where the coffee beans transition from being “endothermic” to “exothermic.”
In simpler terms, until the First Crack, the beans are absorbing heat from the environment which is an endothermic process. However, once the First Crack occurs, the beans start to release energy, which is an exothermic process.
During roasting, the coffee beans initially go through a drying phase, transforming in color from green to yellow to brown. Internally, the beans’ temperature rises, converting the trapped water into steam.
As pressure builds up inside the bean, the steam tries to escape, leading to the audible “First Crack,” which resembles the sound of popcorn popping.
Physically, the beans increase in size, and the crease in the middle may start to expand. This is also the point where the silverskin can come loose, turning into chaff.
While moisture content is a crucial element, the First Crack is also influenced by other factors such as bean density, size, and even the specific varietal of the coffee. However, it’s primarily the conversion of water to steam that drives this phenomenon.
The “energy balance” shifts dramatically at this point. Until now, the roaster had to apply heat to effect changes in the coffee beans. Post-First Crack, the beans themselves begin to undergo rapid changes, releasing energy, which can affect the rate at which they roast.
What is Second Crack?
Following the First Crack, the coffee continues to heat up and undergo transformations. However, the Second Crack is different both in its chemistry and its implications for the final coffee product.
While the First Crack is primarily driven by the conversion of water into steam, the Second Crack is more about the build-up and release of various gases, especially Carbon Dioxide, but also other gases like Carbon Monoxide and Nitrogen.
At this stage, the cell structure of the coffee bean has weakened, becoming brittle primarily due to the breakdown of cellulose.
When the pressure from the trapped gases becomes too great for the fragile cell structure to contain, it results in a Second Crack, which is audibly distinct from the First Crack. It’s more of a snap or a crackle, akin to the sound made by Rice Krispies when milk is added.
This is also the stage where oils can start migrating to the surface, sometimes giving the bean a shiny appearance. Coffee roasted to this level often has flavors that are less acidic, more bitter, and can exhibit smoky or even burnt notes.
The Second Crack represents a narrow window of time. Roasting beyond this point can result in coffee that is over-roasted, or even burnt, making it vital for the roaster to understand both when and why Second Crack occurs to have better control over the roasting process.
The Key Takeaway
Both the First Crack and Second Crack are not merely audible cues but intricate chemical and physical milestones that reveal the complex transformations occurring inside coffee beans during roasting.
Knowing these phases well is key to mastering the art of coffee roasting.