A macaron shell is a foam that has been deliberately deflated

Two contradictory requirements in one batter

A macaron shell has to do two things that pull in opposite directions. It has to flow, so that a piped mound settles into a smooth disc with no peak and no ripples, which requires a batter loose enough to level itself. And it has to hold gas, so that the shell rises and develops its characteristic ruffled base, which requires a batter stiff enough to trap air. A properly whipped meringue satisfies the second condition and fails the first completely.

So the method deliberately destroys part of what was just built. Almond flour and icing sugar are folded into the meringue and the mixture is then worked further, past the point where anyone would ordinarily stop, until enough air has been driven out and enough of the foam has been broken down for the batter to flow slowly and continuously. That is macaronage, and it is the step that decides almost everything about the result.

The end point is a rheological description, not a count

Recipes describe the finish in terms of behaviour rather than time, because time isn’t the variable. A batter that falls from the spatula in a thick unbroken ribbon and sinks back into itself over several seconds is ready. One that breaks into lumps hasn’t been worked enough; one that pours freely has been worked too much. The number of folds depends on the meringue, the almond flour, the bowl and the hand, and quoting a figure is close to useless.

The two failures are visibly distinct. Under-mixed batter holds its piped shape too well, keeps the nipple left by the nozzle, bakes with a rough surface and often cracks as gas escapes through a top that never levelled. Over-mixed batter spreads into thin flat discs, has too little gas left to lift a foot, and frequently leaks oil from the almonds as it bakes. Between those lies a window that is narrower with a French meringue than with an Italian one, which is why professional kitchens tend to use the latter.

The skin has to form before the oven does anything

Piped shells are left to stand until the surface is dry to the touch, and that skin is a structural component rather than a courtesy. It is a thin, set layer of sugar and protein that has lost enough water to become rigid, and it seals the top of the shell. How long it takes depends almost entirely on the humidity of the room, which is why the same recipe takes a quarter of an hour on one day and well over an hour on another.

Without a skin the shell has no sealed top and the gas escapes upwards, cracking the surface and giving no foot at all. With too heavy a skin the top is thick and dull, and the shell can lift away from its own interior. It is one of the few places in baking where the correct instruction really is to touch the thing and decide.

The foot is gas taking the only route left

In the oven, the air in the batter expands and the water in it turns to vapour, and all of that gas has to go somewhere. Upwards is closed by the skin. Downwards is closed by the baking sheet. So the batter is pushed outwards and downwards at the boundary between the two, and the material that squeezes out around the base sets there as the ruffled band called the foot.

That is why the foot is diagnostic. No foot means the gas escaped elsewhere, usually through an unformed skin, or there was too little gas to begin with because the batter was over-worked. A foot that spreads outwards rather than standing proud means the batter was too loose. A hollow shell — a dome with nothing under it — means the top set and rose while the interior was still moving, generally an oven running hot at the top.

Almond flour brings its own oil to the argument

Almonds are around half fat, and that fat is a permanent hazard in a foam. Ground almonds warm during grinding and release oil, and oil is an efficient antifoaming agent, so a flour that has begun to release it will collapse a meringue quietly and thoroughly. This is why almond flour intended for macarons is ground with icing sugar, which absorbs the oil and keeps the particles separate, and why blitzing it too enthusiastically at home turns it towards paste.

The grind also decides the surface. Coarse flour gives a speckled, uneven top that no amount of technique corrects, so the mixture is sieved and the coarse fraction discarded. And because the finished shells are two-thirds sugar by weight and hygroscopic throughout, they soften in a damp room and firm up in a dry one — which is why filled macarons are matured overnight in the fridge on purpose, letting moisture move from the filling into the shell until the texture is right.