Often referred to as pre-infusion, pre-wetting or just wetting, the first phase of coffee extraction is where water meets a dry bed of coffee and slowly penetrates the mass of coffee. This phase is technically referred to as imbibition. It describes a natural process where water is absorbed by solids-colloids and swelling results (Imbibition, Wikipedia). Other than the miracle of coffee, it also describes how seeds absorb water to germinate.
The main processes occurring during the extraction are described in “Coffee secrets unveiled by in-silico coffee” (Coffee Knowledge Hub | Coffee secrets unveiled by in-silico coffee). But we would like to go deeper into the imbibition, since it sets the stage for the extraction “concert”. During imbibition, approximately the first five seconds of the coffee extraction, water is absorbed by the coffee grains. We try to picture the imbibition process and its effects within a single coffee grain (figure 1.). From a mathematical point of view, this phase introduces two processes: the dissolution and the diffusion. When hot water enters the single coffee particle, it dissolves the chemicals inside the grain and emulsifies oily substances. Then, these substances move into the grain and reach the outer surface of the coffee grain, through diffusion. The diffusion process continues for the whole extraction and we can better understand it by studying specific time intervals.
Figure 1.: Section of a coffee grain at two different intermediate times of the extraction process.
In more detail, Figure 1. shows the effects of the previously described processes on a single grain at two different times of the extraction process. We denote with T the duration of the extraction process. The left figure represents the grain at time t=1/4 T. It can be seen that the core of the particle is lighter than its exterior; in fact, the substances that were in the external layer has been dissolved in greater quantities than those in the core. The figure on the right shows the grain at time t=3/4 T. Here the grain is darker than in the previous time, since the water has penetrated the grain almost entirely and most of the substances in it have already been extracted. Furthermore, the color difference between the core and the outer layer is not as pronounced as in the previous case, since at this time a large part of the substances in the core was subjected to the extraction process.
The next question is whether all the grains are equal to each other and their behaviour is equivalent. During the grinding, the coffee bean is crushed until it is reduced into very small particles. How small? By analysing the coffee powder with the granulometer we can obtain curves that help us to answer this question. The grinding determines the particle size distribution of the powder (Figure 2). The distribution reveals the size of particles in the powder as well as the volume covered by each particle of the grind sample. Each grain is “seen” by the granulometer as a sphere.
Figure 2.: Typical granulometric curve of an espresso coffee powder.
Figure 2. shows a typical granulometric curve of an espresso coffee powder. It has the trend of a bimodal distribution, i.e., it has two modes. The mode is the value that appears most often in a set of data values. In other words, it is the value that is most likely to be sampled. Interested readers can read more here Wikipedia. The two modes in Figure 2 fall in correspondence of the two picks. This means that inside the coffee dose there are two particularly important types of grind sizes: the fine particles, whose diameter is smaller than 100 µm, and the larger particles, whose diameter is bigger than 100 µm. Usually, the mode on the left is chosen as the characteristic size for fines, while the mode on the right is for coarse.
In fines and coarse particle, the physico-chemical processes are the same, but how do the different particle sizes affect the evolution in time of these processes? Performing numerical simulations, based on the model shown in (Egidi et al., Cameron et al.), we can answer this question. Fine particles get wet faster than coarse particles. In fact, since their diameter is smaller, the water takes less time to reach the centre of the particle. In addition, the substances contained in them are all dissolved, or almost all, in a shorter time. At the other end of the spectrum, in the coarse particles, the water takes longer to reach the inner core. Thus, the substances that are in the core are less dissolved and begin the process of dissolution and diffusion later on. However, the substances close to the external surface of the boulders dissolve immediately. Therefore, after the fines have stopped releasing compounds, the boulders continue releasing and feeding the extraction till the end of the water’s contact time.
So it is clear that even extractions are a myth. No matter how level or evenly distributed your grinds are in the coffee basket, the granulometry means that during the wetting phase, saturation of the individual particles and the consequent extraction is variable over time within the coffee bed. The extraction yield during imbibation, the solubles and solids extracted from the original coffee dose, is a product of the average of the different particle sizes and the finite time water is in contact with each particle. Therefore a tasty espresso is a fine balance where you avoid too much overextraction of finer particles and too much underextraction of the coarser particles.
Egidi, N., Giacomini, J., Maponi, P., Perticarini, A., Cognigni, L., Fioretti, L., 2021. A reduced model for the coffee percolation and assessment of the extraction efficiency. Preprint.
Cameron, M. I., Morisco, D., Hofstetter, D., Uman, E., Wilkinson, J., Kennedy, Z. C., Fontenot, S. A., Lee, W. T., Hendon, C. H., Foster, J. M., 2020. Systematically Improving Espresso: Insights from Mathematical Modeling and Experiment. Matter 2.3: 631-648. https://doi.org/10.1016/j.matt.2019.12.019.
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