Foam is a complicated and difficult process that involves four main events: bubble production, bubble rise, drainage, and coalescence and disproportionation (Bamforth, 2004). Foam is a coarse dispersion of gas bubbles in a continuous liquid phase. The CO2 produced during coffee roasting, which is partially imprisoned within the cell structure, makes up the majority of the gas phase in espresso coffee. This continuous phase is an oil-in-water emulsion of microscopic oil droplets (less than 10 mm) suspended in an aqueous solution containing numerous coffee ingredients (such as sugars, acids, and proteins) as well as minute solid coffee cell-wall fragments (2e5 mm) (Illy and Viani, 2005). The bubble has a diameter of around 100 mm and looks to be covered in protein-rich material.
Espresso crema is categorized as a metastable froth with a defined lifespan (Dickinson, 1992). It can take up to 40 minutes for the crema to vanish in most circumstances (Dalla Rosa et al., 1986). The qualities of the crema change as it ages, from a liquid fine froth in freshly produced espresso to a dry polyhedral foam as it ages. Ideally, the crema should account for at least 10% of the total weight of the dish.
Espresso crema is categorized as a metastable froth with a defined lifespan (Dickinson, 1992). It can take up to 40 minutes for the crema to vanish in most circumstances (Dalla Rosa et al., 1986). The qualities of the crema change as it ages, from a liquid fine froth in freshly produced espresso to a dry polyhedral foam as it ages. With a foam density of 0.30e0.50 g/mL, the crema should make up at least 10% of the volume of an espresso (Illy and Viani, 2005). (Navarini et al., 2006). The authors of the latter study discovered a linear association between CO2 content and foam weight and volume.
Several attempts have been made to explain how crema forms in espresso coffee. The coffee oils are emulsified into the extracted liquid as water is pushed through the coffee under pressure. Furthermore, roasted coffee emits CO2 for a short period of time (degassing), and the longer the coffee is exposed to ambient pressure, the more CO2 it emits. This is why some espresso coffee packaging materials are designed to keep overpressure and CO2 out of the coffee matrix. During extraction, the leftover CO2 is released.
Despite the fact that CO2 has been suggested as the gas phase responsible for espresso coffee foaming, coffee scientists have yet to fully examine the bubble formation mechanism. On the other hand, the relationship between CO2 chemistry and foam generation has been investigated. Indeed, research reveals that the bicarbonate carbonic acid equilibrium is involved in the dynamics of the espresso extraction’s brief phase (Fond, 1995, Chapter 16).
The initial wetting step of brewing espresso, when hot water seeps into the spaces in coffee particles and interand intraparticle gas are concurrently driven out of the coffee bed, is known as the transient phase of extraction (Petracco and Liverani, 1993). This mass transfer between coffee particles and water happens at the same time, and the bicarbonate ions in the extraction fluid help to speed up the process.
Chemical reactions occur at high temperatures in the coffee bed as a result of the displacement of its equilibrium due to the pH evolution during brewing (from 7.0 to 7.5 to 5.5e5.0). The coffee is compacted by water pressure, the swelling of the coffee grinds, and CO2 degassing, which results in the foam and emulsion that gives espresso its much-loved crema.
According to other research, CO2 supersaturation conditions in coffee may be a driving reason for the creation of espresso coffee froth (Navarini et al., 2006). More specifically, CO2 solubilization (which occurs in the coffee bed) in high-pressure, high-temperature water may result in supersaturation conditions in the final cup, leading to the nucleation of small bubbles.
To elaborate, during CO2 extraction at high pressure and at a water temperature of 100°C, the CO2 concentration in water may be lower than the CO2 solubility. At 1 bar and 70°C, however, it may be above solubility. When there is heterogeneous nucleation and bubble rise following a phase transition (from high pressure to ambient pressure) when the high pressurized water left the coffee bed and enters the cup, these conditions are compatible with foaming. Champagne also exhibits this phenomenon, which is known as effervescence. In this scenario, effervescence could refer to the effect that occurs right after the espresso is prepared. Espresso’s micronic solid particles and submicronic cell-wall fragments may act as nucleation sites. Furthermore, due to the small volume of an espresso beverage, bubble rise is limited to around 1.5e2 cm, resulting in distinctively small bubbles in espresso foam (Illy and Navarini, 2011).
Foam instability normally occurs as a result of three occurrences once the foam has been produced (Prins, 1988). When the layer separating the bubbles collapses, there is first a coalescence among the bubbles. The second type of ripening is Ostwald ripening, which occurs when the foam sizes are polydispersed. Pressure differences between various sized bubbles disperse the smaller bubbles into the larger ones during this phase. Finally, gravity pulls the liquid away from the air bubbles. As a result, the coating thins, resulting in coalescence and Ostwald ripening.
Although similar events may be observed in any foam system, the high temperature of crema, as well as the way it cools from the top-down, adds another layer of complexity to understanding the physical phenomena that contribute to the development and stability of espresso crème. According to a prior study, the beverage’s relatively high temperature may have a negative impact on foam stability. The idea was that as the water evaporated, the thickness between bubbles decreased, causing the foam to collapse (Navarini et al., 2006).
Another study found that the amount of lipid in the foam can alter its stability. Total lipids in a regular espresso (25 mL) range from 45e146 mg for Arabica to 14e119 mg for Robusta (Petracco, 1989; Maetzu et al., 2001). Because pure Arabica espresso has a larger overall lipid content than Robusta espresso, the likelihood of lipid-induced froth instability is higher for Arabica.
Because espresso coffee contains emulsified lipids, lipid-induced foam instability could also occur as a result of oil spreading at the air-beverage interface (Schokker et al., 2002). This is in line with the fact that pure Arabica espresso has a lower surface tension than pure Robusta espresso (Petracco, 2001).
Solid particles, according to studies, have an impact on foam stability. Figure 17.4 depicts such particles in a pure Arabica dry espresso froth. They’re at the plateau’s edge, implying a tendency to be unattached. Unattached particles primarily follow the net motion of the liquid during drainage, implying that solid particles within the crema play a stabilizing effect. Hydrophobic particles can cause dewetting of the surface by destabilizing the foam through film bridging. Although the wetting nature of the solid particles found in espresso coffee has yet to be explored, the “tiger skin” effect seen in Arabica espresso could indicate that the solid particles have a foam stabilizing role (also known as the Pickering effect). The antifoaming action would otherwise be quite rapid (Kralchevsky et al., 2002).
However, the differences in foam adhesion found between the two coffee species could be attributed to distinct liquid-to-dry foam transition rates. The solid-like appearance of Robusta espresso foam, for example, could be due to a faster drainage rate, which appears to be a requirement for adhesion. The rheology of Arabica espresso foam, on the other hand, appears to be liquid-viscous for a longer period of time.