Cappuccini have been popular as a breakfast beverage in Italy and Europe since the early 1900’s when steam pressure espresso machines were first developed. Consisting of equal parts of espresso, warm milk and milk foam, it is perhaps the foam that adds the element of delight to something already delicious. In Italy, only a tourist would order one after mid-morning though.
Milk foams because proteins in the milk (casein and whey) can congregate on the surface of air bubbles and stabilize them for a little while. All the same, foaming milk entails a number of trade offs that need to be balanced for reproducible foaming, something baristas spend years perfecting.
Physical properties of milk and foam:
Full-cream (whole) bovine milk is about 3.5% protein in the ratio 80:20 casein to whey. These two proteins have different molecular structures that affect foaming: the casein promotes bubble creation, and the whey helps with stabilization once bubbles are formed.
Full-cream milk is about 4% fat. Fat competes with proteins, displacing them on the bubble surface. This decreases surface tension and destabilizes the bubbles. Therefore, skim milk foams more readily than full-cream milk. However, fat holds flavour and contributes mouthfeel, so the skim milk version won’t taste as interesting.
Gas dissolves better in a cold liquid. For example carbonated drinks go ‘flat’ as they warm. However, gasses that are more difficult to dissolve make more stable bubbles (the gas in the bubble doesn’t dissolve back into the liquid).
Thicker liquids help with bubble stability — thin liquid drains from between the bubbles and results in coalescing and bursting.
Keep in mind that milk is pasteurized (milk is sterile on secretion, but can become contaminated with exposure to the environment). The high-temperature short-time (HTST) method holds the milk at 71.7C for 15s. This temperature will not affect casein, but it will partly denature the whey protein, causing it to unfold. Unfolded proteins are better stabilisers because unfolding exposes hydrophobic and hydrophilic (water avoiding and attracting respectively) regions that seek out air-water interfaces.
Foaming milk with a steam wand:
Have some fat in the milk but not too much: 2% is a good compromise. Half and half full cream milk and skim milk is about right.
Start with cold milk. The fat is aggregated in small globules (homegenised) that will have little effect on bubble formation until the milk heats and the fat globules melt.
Consider playing with the milk. Add some whey protein isolate to increase the protein content (1% by weight). Perhaps thicken the milk slightly with a tasteless vegetable gum that hydrates without heating (xanthan, lambda carrageenan). Even flavour it (steep with crushed beans).
Try not to use old milk. With time, some of the fats can break down to simpler glycerides that are even more problematic for foaming.
The initial stage involves frothing air into the milk while it is cold creating numerous small seed-bubbles. The purpose of a thin-walled metal jug is to dissipate heat and slow milk warming. Bubbles need to be formed and stabilized by the proteins before the milk heats (protein-protein bonds on the surface will resist displacement by warmed fats). The nozzle is kept just under the surface to maximize air and minimize heating.
The milk is further heated with the nozzle under the surface. Air forced into the milk by the nozzle will not stay dissolved because the milk is warm, some will cross over into the seed bubbles and expand them. Stop when the milk temperature reaches about 65C. Further heating can result in a cooked flavour.
Heating the milk increases bubble size and foam volume, but reduces bubble stability because the fats will melt, and because the warmed liquid will drain more easily from between the bubbles.
The processes the proteins undergo when forming bubbles is not reversible. This means that foamed milk can not be cooled and re-foamed a second time.
Crema: An espresso foams of its own accord — the crema on the top. This is because when the beans were roasted, carbon dioxide was released as a byproduct of molecular breakdown at high temperature, and this is retained in the brittle spongelike structure of the roasted bean. Under high pressure (9 atmospheres in professional machines) the espresso becomes supersaturated with released carbon dioxide. Once it comes down to atmospheric pressure in the glass (but still hot), the gas is rapidly released and rises to the surface where it is trapped as bubbles by suspended solids and other molecules, forming the crema. This traps aromatics in the liquid, adding to the flavour of the first sip. Alternatively, removing the crema before serving is thought to sweeten the shot.
Boiling milk: It is the ability of milk proteins to foam that causes milk to boil over so suddenly in the pot. Just before boiling point is reached, gasses come out of the milk and briefly form bubbles that can’t break down quickly enough to avoid increasing the volume of milk in the pot.