What Is Custard? Eggs Thickening a Liquid Without Turning Into Scrambled Egg
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
Custard is a liquid thickened by egg proteins unfolding and linking together as they heat. The whole technique consists of getting them to link enough to thicken and not enough to clump, which is a narrower window than most preparations allow.
What the heat does
Egg proteins are folded chains held in compact shapes, and heating gives them enough energy to unfold, exposing regions that were tucked inside. Those exposed regions attach to each other, forming a network that traps liquid and thickens the mixture. Continuing to heat tightens that network further, squeezing out the liquid it was holding and producing lumps of protein in a watery medium, which is curdling. The two outcomes are stages of one process rather than different processes, so the difference between a smooth custard and a ruined one is a few degrees and a few seconds. Yolks set at a lower temperature than whites, which is why yolk-based custards are made in a narrower range, and the presence of sugar and of other liquid raises the setting temperature and widens the window usefully.
The controls
Several factors widen the margin and each is deliberate:
- •Sugar, which interferes with the proteins linking and raises the temperature at which the mixture sets
- •Dilution with milk or cream, which separates the proteins and slows the network forming
- •Starch, which further interferes and makes the mixture nearly impossible to curdle, which is why pastry cream can be boiled
- •Gentle heat, applied over water or at low direct heat, which prevents the bottom passing the target while the top is still cold
- •Constant stirring, which distributes heat and breaks up any network beginning to form locally
- •Tempering, adding hot liquid to the eggs slowly before returning everything to the heat, which avoids a shock
The main kinds
Custards divide by whether they are stirred or baked and by what is added. A stirred custard is cooked in a pan with constant agitation and never sets solid, remaining pourable, which is how sauce custards and the base for ice cream are made. A baked custard is set in a dish in a gentle oven and is not stirred, so the network forms undisturbed and holds the whole thing in a soft solid, which covers baked puddings, quiches and set desserts. Starch-thickened custards use flour or cornflour alongside the eggs, are far more robust and are the basis of pastry fillings. A water bath surrounding a baked custard keeps the outside from exceeding the target before the centre reaches it, which addresses the same problem that stirring solves in a pan.
Where else the principle applies
The same protein behaviour governs a range of preparations that are not called custard. Scrambled eggs are the same reaction taken deliberately past the point of separation and then stopped, with the difference between creamy and rubbery being exactly the difference discussed here and the remedies being the same, namely low heat, constant movement and removing the pan before it looks done. Hollandaise and its relatives are egg yolk emulsions held just below setting, which is why they split when overheated. Cheesecake is a baked custard with cheese. Quiche is a savoury one. Bread pudding and several set desserts are custards with something in them. Recognising the common mechanism means that a cook who understands one of these understands all of them, which is more useful than learning each recipe as a separate procedure.
Fixing and preventing
The failures have known remedies and known causes. A curdling stirred custard caught early can sometimes be rescued by removing it from the heat immediately, plunging the pan into cold water and beating hard, or by blending, which breaks the clumps small enough to pass unnoticed. Straining catches whatever remains, and straining is worth doing routinely rather than as a rescue. A baked custard with a weeping watery layer and a coarse texture was cooked too hot or too long, and there is no repair. Bubbles on the surface of a baked custard come from air beaten in during mixing and are avoided by mixing gently and resting the mixture. A skin forming on a cooling stirred custard is protein drying at the surface and is prevented by covering it with the cover touching the surface.
The takeaway
Heat unfolds egg proteins, which link into a network trapping liquid, and continued heating tightens that network until it squeezes the liquid out, so thickening and curdling are stages of one process. Sugar, dilution, starch and gentle heat all widen the window between them. Starch makes the mixture essentially uncurdlable, which is why pastry cream can be boiled and a sauce custard cannot.