What is first crack and second crack in coffee roasting?

Coffee roasting is an intricate process that transforms green coffee beans into the aromatic, flavourful beans we use to brew our favourite beverage. Central to this process are two critical auditory markers known as the first crack and second crack. These "cracks" are pivotal moments during roasting that indicate chemical and physical transformations in the beans. Understanding these stages helps roasters fine-tune their craft, allowing them to create coffee profiles that cater to various taste preferences.

The Science of Roasting Coffee Beans

Before delving into the cracks themselves, it’s important to understand what happens during the roasting process. Green coffee beans are essentially seeds, containing water, sugars, proteins, lipids, and other compounds. When heated, these compounds undergo complex chemical reactions, including the Maillard reaction and caramelization, which contribute to the beans' final flavour and aroma.

As the beans are roasted, the heat causes them to expand, lose moisture, and develop internal pressure. This pressure eventually leads to audible cracks, which serve as milestones indicating key stages of the roast. The first and second cracks represent distinct points in the roasting timeline, each with unique implications for the bean’s development.

What is First Crack?

The Physical and Chemical Changes During First Crack

The first crack is a crucial moment in the roasting process, marking the transition from the drying phase to the development phase. This stage occurs when the internal temperature of the beans reaches approximately 196–205°C (385–401°F), although the exact temperature may vary depending on the type of beans and roasting equipment.

At this point, the following changes take place:

1. Moisture Evaporation: The water content within the beans, which starts at around 10–12%, has been steadily decreasing. By the time first crack begins, much of the remaining water vaporizes explosively, creating internal pressure.

2. Structural Expansion: The beans expand significantly, nearly doubling in size. This expansion weakens the cellular structure, leading to the characteristic cracking sound.

3. Audible Pop: The first crack is often described as a popcorn-like popping sound. This sound signifies that the bean's outer structure has fractured due to internal pressure.

4. Flavour Development: First crack marks the beginning of significant flavour transformation. The sugars in the beans start caramelizing, and the Maillard reaction intensifies, creating complex compounds that contribute to the coffee’s sweetness, acidity, and body.

Significance of First Crack

For most roast profiles, first crack is the point where the coffee begins to shift from underdeveloped to properly developed. Before first crack, the beans are usually too green or grassy in flavour. Once first crack occurs, roasters can decide whether to stop the roast (for a light roast) or continue developing the beans further.

What is Second Crack?

The Physical and Chemical Changes During Second Crack

The second crack occurs at a higher internal temperature, typically between 224–230°C. This phase represents a deeper level of roast development and is characterized by different physical and chemical phenomena:

1. Structural Breakdown: The beans experience further expansion and weakening of their cellular structure. This time, the cracking sound is caused by the fracturing of the bean’s cell walls, as oils begin to seep to the surface.

2. Oil Migration: At second crack, the oils within the beans start to break down and migrate outward, resulting in a glossy appearance. This is a hallmark of dark roasts.

3. Audible Crack: The second crack produces a subtler and more rapid series of cracks compared to the first crack. It is often likened to the sound of Rice Krispies in milk—a quieter, more consistent popping.

4. Flavour Transformation: The flavours at this stage move toward darker, more robust profiles. Acidity diminishes significantly, while bitter and smoky notes become prominent. This phase is ideal for those who enjoy bold, intense flavours in their coffee.

Significance of Second Crack

The second crack is a turning point for creating dark roasts. If the roasting process is stopped shortly after second crack begins, the beans will exhibit characteristics of a "full city roast" or "Vienna roast," retaining some complexity alongside bold flavours. Prolonging the roast beyond second crack leads to darker profiles, such as French roast or Italian roast, where the coffee takes on pronounced bitterness, smokiness, and a charred taste.