In an ongoing series, Borough Market regular Jenny Chandler explores eggs in all their various forms. This week: custard
Words: Jenny Chandler
Image: Paul Thompson
The breakfast egg is more often than not a simple affair: whether it be fried, scrambled, poached, or French toast (eggy bread in my household), the egg steals the show. Yet in the kitchen, it’s the egg’s ability to thicken, set and enrich other ingredients that make it the cook’s best friend.
It all comes down to “protein coagulation” in the words of Harold McGee (On Food and Cooking, 2004). A whole beaten egg will thicken at about 70C and set at 73C (the whites set at a slightly lower temperature than the yolks when separated) but once the proteins are diluted by milk and sugar (in a classic custard, for example) then you’re talking closer to 80C for the liquid to thicken.
Not only does the mix need a higher temperature to coagulate, the dispersed proteins make up a much more delicate framework and a tiny bit of extra heat will tip the balance into a curdled disaster. The proteins have bonded so closely with each other that the liquid gets squeezed out between them, resulting in firm lumps and a watery mess. You’ve surely been there at least once!
Denser, thicker and richer
When it comes to custard—and I’m talking crème anglaise—the traditional ratio of one yolk to 250ml of full fat milk will thicken to a silky pouring custard, but if you’re after a more substantial custard for a trifle perhaps, then no amount of cooking and stirring will get you there. You need to up the protein to water ratio, by using more egg yolks to milk or substituting some cream for some of the milk (more fat, less liquid) and you will create a denser, thicker and richer result.
All is not lost should your custard begin to curdle—tip it straight into a cool bowl and then whizz with a stick blender, strain, and though you’ll lose a little density, your custard will be serviceable.
Starch, in the form of flour, arrowroot or corn flour, completely changes the game. The starch stops the proteins from binding too closely and thickens the liquid at the same time—hey presto, we have the more compliant custard that’s regularly stirred up in homes, schools and canteens up and down the country (albeit from a packet in most cases).
The custard rules
In this case we throw all the custard rules out of the window: it’s not just that we can allow the custard to reach a boil, we need it to boil in order for the starch to swell. It’s alchemy. Just a tablespoon of flour (twp teaspoons of corn starch or arrowroot) is enough to stop 250 millilitres of custard from curdling. The more starch we add, the thicker the cream.
Crème pâtissière is the classic thickened custard you find in fruit tarts, cream puffs and doughnuts (ever tried a Bread Ahead doughnut? My favourite ever was a glorious rhubarb and custard creation).
The same rules apply to set custards that are baked in the oven rather than stirred on the hob. If you’re looking for a slicing consistency such as a lemon tart or quiche, a generous amount of yolks will do the trick, although it’s customary in many recipes to use some whites too. The white tends to create a firmer gel-like set (think rubber, once over cooked!) which becomes handy in a dish such as crème caramel that we want to turn out of a mold with no fear of collapse.
Long, slow cooking is key to the perfect silky texture and a bain marie (water bath) is used to temper oven heat and prevent curdling. Throw in some flour and you’re in more forgiving territory at once—think clafoutis or, another stunning French dish, far Breton which both bake in a hot oven. A little more flour in the batter and we’re on to Yorkshire pudding, where we start playing with another of the egg’s magical properties—the ability to trap air. But that’s another blog post.