The complexity of espresso: a scientific perspective

The Italian word espresso (EC) suggests that this beverage must be quickly prepared on customer demands or rather extemporaneously prepared after the consumer orders. Another important characteristic of EC is the use of pressure. The beverage must be prepared not only with hot water, but also under pressure. Therefore, for the espresso definition, three essential features are required: extemporaneous preparation, fast brewing, and the use of pressure.

A typical definition of espresso is the following: a concentrated polyphasic beverage with a characteristic foam layer (crema) on the surface, prepared by forcing hot water (90 ± 5 °C) under pressure (≈ 9 bar) into a tamped coffee cake (roasted and grounded (R&G) coffee) for a short period of time (25 ± 5 s). The cup volume can change from 15 to 50 mL according to consumer preferences. To best exploit all of the coffee’s beans characteristics and to prepare a good EC with the desired sensory features, several variables can be controlled and adjusted. These variables can be classified under different families and in this report all variables are divided under machine setups, barista/consumer dependent variables and extraction time, which is a function of both (see Figure 1).

*Figure 1. *

The most studied variables influencing the EC preparation are temperature and pressure of water, and particle sizes of ground coffee, but scientists have not focused in depth on the influence of some tools, which can be installed and adjusted within the machine. A new study recently published in ‘Food Chemistry’ (2020) contributes to the understanding of how different variables, including various tools, can affect brewing (Khamitova, G et al., 2020). This work is the result of experiments carried out by a research team at the Research and Innovation Coffee Hub (RICH) and the University of Camerino. The final aim of the study was to optimize the brewing process to produce an espresso of high quality, while also lowering the amount of ground coffee used.

The brewing process was optimized by studying three variables connected to different components of the espresso coffee machine: the heights of the shower block, the designs of the filter basket, and the particle sizes of ground coffee. The filter basket is the metallic cylindrical support, that holds the coffee powder which is dosed into the basket and then tamped. Water passes through the coffee bed and thens flows out through small holes in the base of the basket. It filters most of the coffee solids. The shower block a cylindrical disc, with holes drilled through, assembled under each serving group, that together with the shower plate, delivers a homogeneous water diffusion over the surface of the coffee cake. The distance between the coffee cake and the shower plate can be adjusted by using shower blocks with different thicknesses. This can ensure there is an adequate empty space for the swelling of the coffee cake rising within the filter basket up to the level of the shower plate.

Two types of coffee were used for this experiment, 100% Coffea arabica (arabica) and 100% Coffea canephora (robusta); both coffees were supplied by ‘Le Piantagioni del Caffé’ roasting company. The equipment used for the research was composed of one Mythos 1 grinder, and one 2-group VA388 Black Eagle Victoria Arduino machine (Figure 2).

Figure 2.

The experiment was divided in two steps: the first to study the influence of different filter baskets (A: standard; B: small-sized holes; C: particular design) and different particle sizes (from fine to coarse particles, such as 200−300 µm, 300−400 µm, 400−500 µm, 500−1000 µm, and standard particle sizes (mixed particles)); the second step investigated the influence of various heights of the shower block (4, 5, 6 and 7 mm). Both experiments were performed using a standard amount of R&G coffee for Italian Certified Espresso (14 g) or lower amount (12 g). The quality of the espresso was investigated from a chemical point of view and, in particular, the total dissolved solid levels (TDS), the concentration of some bioactive compounds, such as caffeine, trigonelline and three chlorogenic acids, and the content of some volatile organic compounds were measured. In general, the ‘strength’ of the brewing can be associated with the content of total solids in the cup, while the selected compounds were those that occur in high concentrations in EC and influence the coffee flavor. Comparing different ranges of particle size, higher TDS levels were obtained in espresso coffee prepared with standard particles (mixed particles) and with finer grinds. This was most apparent when filter basket A and C were used. Similar results were measured for standard particles especially with filter basket A when decreasing the amount of the coffee powder to 12g. The best filter baskets were proven to be A and B (standard and small-sized holes, respectively). The content of bioactive compounds had similar TDS levels, and higher concentrations were measured by brewing espresso with finer and standard particles. These results are consistent with scientific literature. It has been reported that using finer particles generates a higher caffeine content (Andueza, S. et al., 2003). Figure 3 indicates the content of caffeine and chlorogenic acids (mg/L) in ECs prepared with different particle sizes using the standard filter basket.

Figure 3. Content of caffeine and total chlorogenic acids (mg/L) in ECs prepared with standard filter basket (A) at various particle sizes by using 14 g.

Several classes of volatile organic compounds (or aroma compounds) were detected in espresso coffee prepared at different conditions. The volatile compounds are responsible for the aroma profile of EC. Statistical analysis (Principal Component Analysis) was applied to understand the relationship among the different EC samples. The three filter baskets proved that lowering the amount of coffee powder by keeping the same particle sizes had little impact on the release of volatile compounds, therefore also a little impact on the coffee aroma. In detail, filter basket A was able to generate almost the same percentage of volatiles at 14 and 12 g of coffee powder. The results from different heights of the perforated disc demonstrated that using a lower amount of coffee powder could give the same brewing efficiency in terms of total solids and bioactive compound content, provided the height of the perforated disc was increased. The results of this project lay the foundation for studying in greater depth how different variables can be set on espresso machines to produce a high-quality EC with the desired characteristics. Moreover, all of these optimizations, quite simple to set up, can lead to an eco-friendlier consumption of EC by decreasing the amount of R&G coffee used, maintaining at the same time the same EC quality.

Espresso coffee is one of the most consumed beverages in the world, especially in southern Europe and Central America, but its consumption is also increasing in other areas such as Asia and Africa. Improving our understanding of the variables that can impact espresso coffee quality can support café operators globally by improving quality and reducing inventory costs.

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*References *

[1] Angeloni, S., Mustafa, A.M., Abouelenein, D., Alessandroni, L., Acquaticci, L., Nzekoue, F.K., Petrelli, R., Sagratini, G., Vittori, S., Torregiani, E., Caprioli G. Characterization of the Aroma Profile and Main Key Odorants of Espresso Coffee. Molecules 2021, 26, 3856. https://doi.org/10.3390/molecules26133856

[2] Khamitova, G., Angeloni, S., Borsetta, G., Xiao, J., Maggi, F., Sagratini, G., ... & Caprioli, G. (2020). Optimization of espresso coffee extraction through variation of particle sizes, perforated disk height and filter basket aimed at lowering the amount of ground coffee used. Food chemistry, 314, 126220.

[3] Andueza, S., De Peña, M. P., & Cid, C. (2003). Chemical and sensorial characteristics of espresso coffee as affected by grinding and torrefacto roast. Journal of Agricultural and Food Chemistry, 51(24), 7034-7039.