Finely ground freshly roasted coffee beans, preferably Arabica, have ~92°C hot water pushed under high pressure (9 bar) for a relatively short time (30sec). This provides a concentrated coffee with minimal bitter acid (tannins) and sufficient caffeine (slower extractions provide higher caffeine kicks) as well as maximum extraction of flavor chemicals and oils. At the same time, it creates a pleasant reddish-brown foam with aesthetic values as well as the taste sensation provided by a foam filled with aroma molecules; this also works to reduce the evaporation loss of flavor molecules between sips.
Obviously coffee doesn't contain large amounts of good foaming surfactants, so it's relatively difficult to get a good crema, especially as we'll see, two key aspects of foaming actively fight against its stability. Surfactants are a complex mixture of polysaccharides and brown protein complexes (containing melanoidins). The surface tensions of the respective components are ~60 and ~46 mN/m - so the polysaccharides are hardly surfactants, but they provide foam stability and the proteins are fairly weak surfactants, although they are good enough to produce modest amounts of foam. This surfactant mixture can be extracted with normal hot water, but the resulting coffee cannot then be encouraged to produce a good foam. The key to crema is 9 bar pressure and CO2, a chemical produced during coffee roasting and (mostly) locked into the beans before grinding. Plenty of CO2 is lost during grinding, and the rate of CO2 loss from the fine particles immediately after grinding will be high – so espresso needs to use ground beans as soon as possible. Probably a problem with very fine grinding (apart from possible blocking of the espresso flow) is that the CO2 loss during grinding is too high. During extraction, the CO2 is dissolved under high pressure, so when the coffee comes out of the machine, the bubbles instantly come out of the solution and form foam. The disadvantage of this process is mentioned in the Ostwald chapter; CO2 has a high solubility in water and therefore diffuses rapidly through the foam walls so Ostwald ripening is rapid. Anything in the system that introduces air into the foam instead of CO2 will create a foam that is more stable to maturation and therefore more stable to evacuation which is faster for larger bubbles as we know from the Drain section. However, artificial attempts to create air (cheap espresso machines) will tend to produce larger bubbles, giving a foam that will drain quickly. Another message from the Ostwald modelers (and it makes sense intuitively) is that the narrower the original distribution of bubble sizes, the less maturation there is - the fewer large bubbles in the distribution, the less pressure gradient there is to power the diffusion process. . Anything in the system that introduces air to the foam instead of CO2 will create a foam that is more stable to maturation and therefore more stable to discharge which is faster for larger bubbles as we know from the Drain section. However, artificial attempts to create air (cheap espresso machines) will tend to produce larger bubbles, resulting in a foam that will decrease rapidly.
As we know from the Antifoam section, if oil droplets or solid particles are too small, they will tend to be swept from the foam walls to the PB and knots, where they will only act as "slow" antifoams when significant drainage has taken place. . This effect is beautifully illustrated in the image from the Illy newspaper.
A classic way to make a fine emulsion is to force the oil and water under high pressure through a narrow hole - a process no different from the espresso process. So again, high pressure will contribute to a good cream by minimizing the size of the oil droplets and making them less effective antifoaming. Of course, high Pc values from rigid protein/saccharide interfaces will also help resist the antifoaming effect. Over-roasting the coffee can reduce saccharides and produce a less stable foam.
We know from the rheology section that smaller foam diameters lead to higher modulus and yield stress - i.e. a "tighter" foam. For espresso, the "sugar test" is used to find out how long a known weight of sugar is supported in the cream. It is certain that higher pressures and fresher coffee combine to produce the hardest crema because they produce a large number of small diameter bubbles.
What size is a good espresso bubble? This is important for both rheology and drainage, because small is nice for both. A representative Arabica espresso from Illy paper has bubbles in the 50 µm range, which is either integrated or leads to Ostwald ripening from a smaller size. it has developed. These small dimensions are satisfactorily accurate for good modulus/yield voltage and low drain. There is much debate about the effects of free fatty acids in coffee. To the extent that they form insoluble oil droplets, they can act as defoamers. To the extent that they migrate to the surface to form a solid wall (see previous discussions on "Gilette" foams), they can stabilize the foam against both maturation and drainage. Perhaps the fatty acid effect is entirely negative, as polysaccharides seem to dominate stabilization (low levels give foam with low stability). It can be difficult to tell with such a complex system. But that's the joy of this scientific field: important scientific ideas combined with the pleasure of a delicious cup of coffee.
milk foam
Probably a cappuccino foam needs a thicker foam as it should float on the coffee.
The surface tension of milk is not far from that of water - so it is not full of free surfactants. Presumably the surfactants (mostly proteins) are wrapped in other parts of the milk, so they are not present on the milk surface. In other words, frothing milk is quite difficult, and when you froth the result is a generally coarse froth that dissipates quickly for reasons we know well. The magic of latte is the process that produces lots of little bubbles.
Sometimes bubbles are said to form around the steam. This must be wrong - steam is water vapor and as soon as the water condenses, a steam bubble should collapse. So a latte foam is an air foam. Steam is a source of heat (raising the temperature of the milk), but also the kinetic energy that draws air into the milk. If the steam wand is inserted too deep into the milk, you will get warm milk and no foaming. With a simple wand, the skilled barista or an amateur with a clever wand (like me) will know to insert it deep enough into the milk to draw in the fine air, but not so shallow that it will produce an unnecessarily large froth.
The main surfactant is a protein, as in cream, which is (probably) relatively slow to migrate to the interface (and perhaps slow to be released from what binds it before foaming starts) and is therefore a weak frother. when it gets there it gives a strong interface, therefore a stable foam. If the bubbles are small, the drainage rates are smaller, and the foam is relatively firm (it feels luxurious) without being too stiff to pour for latte art. If you've made a foam similar to that from whipped cream (at higher oils), the foam will thin out strongly, but it's impossible to pour at low shear speeds.
It is well known that it is much easier to froth low-fat milk. We can guess why - milk fat globules are a reasonable antifoam. The 4% fat froth in whole milk is the hardest to get, but the resulting drink isn't rich enough for a proper latte - skimmed milk. At a higher oil percentage foaming becomes easier, but more like whipped cream, where other effects are replaced - for example, the o/w emulsion takes on a viscosity that can change drainage. The exact shape of the oils available seems to make a big difference. While whole milk is relatively standardized in overall composition, some types are weak frothers and "type" can mean different types of fresh milk or milks that change "type" over time. Presumably this is due to subtle changes in oil composition that can change the balance of oils from ineffective to effective antifoamers; these changes may be in size and/or hydrophobicity. Only a small change in the balance between triglycerides and diglycerides is required to shift the balance between antifoam and non-foam.
Everyone says cold milk is needed for the best foam. At least in skim milk, the optimum frothing temperature is around 40-60°C, where "optimal" is a mix of stability and (small) bubble diameter. So, if one is frothing through an isothermal process, the perfect latte uses milk that has been preheated to the optimum serving temperature of around 60°C. Probably because foam formation is quite slow and inefficient and the frother (steam) increases the temperature, you need to start cooling to allow time for the milk to reach ~65°C when the proteins start to denature and froth is lost. You cannot froth the milk that has reached this temperature again. At this higher temperature, further changes occur to reduce the roundness of the milk's flavor (it has a "boiled" flavor). ) so staying in the right spot is vital. The optimum foaming temperature seems to be due to the fact that caseins (they are useless frothers themselves) undergo a transition that makes them less good as stabilizers of foams formed via protein surfactants.
It is often stated that heated milk is sweeter. Sometimes this means that lactose (which has about 1/3 the sweetness of sucrose) becomes more soluble at higher temperatures.