Despite the importance of first crack and second crack to roasters, the causes of the two ‘pops’ during roasting are not well understood. First crack is the result of steam pressure building up inside the beans, eventually overcoming the strength of the beans’ structure and causing them to pop open, releasing a burst of steam and carbon dioxide from the beans.
This is often described as analogous to the way popcorn pops: a sudden, explosive release of steam bursting the structure of the kernel open. However, there is an important difference: the expansion of coffee beans during roasting doesn’t occur in a sudden explosion, as it does for popcorn; instead, the beans expand continuously throughout the roast.
Modelling First Crack
A better explanation for first crack is linked to the glass transition that takes place during roasting — and to the evaporation front. Because the bean loses moisture from its outer layer first, the centre of the bean undergoes the glass transition at a different temperature to the outside of the bean. This difference could be the cause of the mechanical stresses in the bean that lead to first crack.
In a series of papers, researchers at the University of Oxford and Jacobs Douwe Egberts built a mathematical model to describe the behaviour of moisture inside the bean during roasting (Fadai et al 2019). Their model predicts what happens inside the beans as the evaporation front moves from the outside to the core of the bean.
According to the model, throughout most of the roast, the temperature at the core of the bean is fairly similar to the temperature at the edge, but the moisture content is very different. The outside of the bean dries out quickly, and therefore the glass transition temperature in this part of the bean is high.
In the core, however, the moisture content remains high because the steam can’t escape until the evaporation front reaches the core. The glass transition temperature in this part of the bean is therefore much lower.