Milk Foam Versus Cold Foam: Two Techniques Explained

What actually holds a milk foam up

Foam is gas bubbles trapped in a liquid. What stops them merging and collapsing is a film of surface-active molecules around each bubble.

In milk, that job is done by proteins - principally whey proteins and casein. When milk is agitated, proteins unfold at the air-water interface and link to each other, forming an elastic skin around each bubble.

Fat works against this. Fat globules displace proteins at the interface and destabilise the film, and free fatty acids actively break foams. This is why whole milk foam collapses faster than skimmed milk foam, and why a trace of grease in a jug can stop milk foaming entirely.

So the general rule is: protein builds foam, fat breaks it.

Which raises an obvious question - why does whole milk make better cappuccinos? The answer is that hot and cold foaming are different situations, and the next two sections cover why.

Hot foam: why whole milk works and why 65 °C matters

When milk is heated, whey proteins denature - they unfold - which makes them substantially better at forming the elastic films that stabilise bubbles. Heat is actively helping.

That extra stabilising capacity is enough to overcome the destabilising effect of the fat, which is why whole milk steams into a glossy, stable microfoam that lasts several minutes, while skimmed milk makes a stiffer, drier, more meringue- like foam that separates quickly.

The temperature ceiling is around 65 to 68 °C. Above that:

• Proteins denature past the useful point and the foam stops forming.
• Lactose and proteins begin Maillard reactions, producing a distinctly cooked, slightly eggy flavour.
• Perceived sweetness drops sharply. Milk tastes sweetest at around 60 °C because heat makes lactose more detectable, and boiled milk tastes flat.

Technique for a domestic frother or steam wand:

1. Cold milk, cold jug. You want time to introduce air before the temperature arrives.
2. Air first, for the first three to five seconds, with the wand tip just below the surface - the hissing sound.
3. Then submerge and swirl for the rest, which breaks the large bubbles down into microfoam.
4. Stop at 65 °C - the jug will be uncomfortable but not painful to hold.

Cold foam: no heat, so the fat has to go

Cold foam gets no help from denatured proteins, because nothing has been heated. The proteins have to do the job in their native state, and the fat is working against them the whole time.

Which is why cold foam needs skimmed or very low-fat milk. Whole milk simply will not hold a cold foam for more than a few seconds.

The method: fridge-cold skimmed milk, a little syrup for sweetness and body, thirty seconds with a handheld frother. It should thicken visibly and hold soft ripples.

Vanilla sweet cream - the thing that goes on a cold brew - is a related but different product. It is two parts cream to one part milk, frothed cold, and it works because of the fat rather than in spite of it: at high fat concentrations the fat globules partially coalesce into a network, which is the same mechanism as whipped cream. It does not make a stable microfoam; it makes a thick, pourable cream that cascades through the coffee, which is the point.

So there are two cold techniques and they are opposites: skimmed milk for foam that sits on top, high-fat cream for a pour that falls through.

Plant milks, which vary enormously

Plant milks have little or no casein, so they foam by entirely different means - usually added proteins, gums and emulsifiers. Which is why "barista" versions exist and ordinary versions frequently do not work.

Curdling in hot coffee is a separate problem from foaming. It happens when coffee's acidity destabilises plant proteins, and it is worse with hot, acidic, lightly-roasted coffee. Three fixes: use a barista formulation, pour the coffee into the milk rather than the other way round, and use a darker roast, which is less acidic.

Barista oat milk is the most reliable all-round option and its natural sweetness suits flavoured drinks particularly well.