A marshmallow is a syrup that has been taught to hold air

A foam needs a liquid that will whip and then stop moving

Aerated confectionery is easy to describe and awkward to make, because the two requirements pull against each other. The liquid has to be thin enough that beating can drive air into it and divide that air into small bubbles, and then it has to become rigid enough that the bubbles cannot rise, merge or escape. Any liquid meets one of those conditions at a time. The whole craft consists of arranging for the change to happen at the right moment.

A hot sugar syrup is the answer because its viscosity is a function of temperature and concentration, both of which you control. At the point of whipping it is fluid enough to aerate. As it cools it thickens steeply, and by the time a setting agent has done its work the foam is fixed. Marshmallow, nougat, divinity and the softer nougat-style bars are all variations on that single sequence, differing mainly in how much water is left and what does the setting.

The temperature of the syrup is most of the recipe

The syrup is cooked to a specified temperature for one reason, which is that temperature is the readable proxy for how much water remains. A marshmallow syrup is taken to the soft-ball region and keeps enough water to stay soft and yielding for weeks. A nougat syrup goes considerably further, into the range that gives a firm chew, and a harder Italian-style nougat further still. The eating quality is decided in the pan, before any air has been introduced.

Sucrose alone won’t do the job, because a concentrated sucrose solution wants to crystallise and a crystallised foam is grainy and short-lived. This is why these recipes always contain glucose syrup, invert sugar, honey or an acid that inverts some of the sucrose in the pan. The mixed sugars get in the way of orderly crystal growth and they also hold water, which keeps the finished sweet from drying out. Leaving them out is one of the more reliable ways to produce something with the texture of sand.

Gelatine and egg white lock the foam by different means

Two setting agents dominate and they behave nothing alike. Gelatine dissolves in the warm mixture, disperses through the foam and then forms a network as the whole thing cools below its setting temperature, so the foam is held by a thermally reversible gel. That reversibility is why a marshmallow softens in a warm room and melts pleasantly in the mouth, and why it can be toasted into liquid without falling apart entirely.

Egg white sets by an entirely different route. Its proteins unfold at the air interface during whipping and are partly coagulated by the hot syrup poured into them, and the structure that results doesn’t melt back. Nougat built on whipped white is therefore stable at temperatures that would ruin a gelatine marshmallow, and it dries progressively rather than softening. Some recipes use both, which gives a foam that sets quickly on cooling and then holds its shape as it dries.

Nougat is the same process with the water pushed lower

Move along the moisture axis and the family reveals itself. A marshmallow is a wet foam with a soft gel holding it. A chewy nougat is the same foam with substantially less water, a firmer sugar phase and usually nuts or fruit folded through it, and the chew comes from the sugar itself rather than from any gelling agent. The harder styles are drier again, and the difference between a soft torrone and a brittle one is largely a difference of a few degrees in the syrup.

Fat plays a supporting part that is easy to underestimate. A small quantity of butter or cocoa butter beaten into a nougat shortens the chew and makes it release from the teeth, whereas too much destabilises the foam, because fat is an efficient antifoaming agent and will collapse bubbles it comes into contact with. That is the same reason a trace of yolk ruins a meringue, and it is worth remembering whenever a recipe insists that the fat goes in last and sparingly.

These sweets go stale by drying rather than by staling

A marshmallow left out does not go off in any microbial sense, because there is far too much dissolved sugar for that. It dries. Water leaves the surface, the outer layer becomes tough and eventually the whole thing turns leathery, which is why they are sold in sealed bags and why the dusting of cornflour and icing sugar on the outside is doing more than preventing stickiness — it also buffers the surface against the air.

The reverse happens in a humid room. These confections are hygroscopic, and a nougat that takes on moisture goes sticky, loses its cut edges and eventually weeps. Neither failure is reversible in any useful way. Storage is therefore about excluding air rather than about cold, and a fridge is usually the wrong instinct: it is a drying environment, and the cold also firms a gelatine foam into something noticeably less pleasant to eat.