A custard is an age old dessert made with eggs, sugar, and cream, cooked together into silky smooth bliss. The flavor of a custard, sweet and a little eggy, is rarely considered without the velvety texture that accompanies it. So much so that “custard-y” is a reference point for other silky rich textures, like the prehistoric relic, the paw-paw, a fruit native to America often called a custard apple.
At it’s simplest, a custard is sweet cream thickened with cooked egg yolks. Custard can be cooked over the stovetop, to make stirred custards that are used as sauces like the famous Crème Anglaise, or as the base for frozen custard, the richest of all ice creams. When the custard is baked in the oven, it creates the king of all desserts, the crackly caramel topped crème brulee, or it’s naked cousin, the pot de crème.
The egg yolk is the workhorse of custard, responsible alone for the increased viscosity that transforms sweet cream into thick sauces, spoonable puddings, and sliceable tarts. Inside each egg yolk are tiny bundled proteins, like little balls of tangled yarn. When heated, the proteins in the egg yolk begin to change their nature and unravel, or “denature.” Once denatured, the proteins in the egg yolk begin to overlap with each other, and everywhere these long strands touch, they form a cross-link. When enough of the proteins cross-link, they interrupt the flow of the water carried into our custard by the milk and cream. This interruption is called coagulation, meaning to change from a liquid into a semi-solid or solid state. Visually, you’d call it “thickening.” When the custard is stirred as it cooks, these cross links are partially broken as they are created, resulting in a custard that retains fluid properties. Simply put? A stirred custard flows when poured.
If we leave the custard to cook without agitating it, for example still-baking a custard in a dish, the denatured proteins cross link until they form a gel that looses its fluidity, becoming solid. These still-baked custards should be chilled before serving, allowing the butterfat in the custard to firm up, creating a silky custard we can spoon into our mouths. If we serve the sweet still-baked custard as is, it is considered a pot de crème, and if we sprinkle the top with sugar and use a torch to caramelize it, we have made a crème brulee. In spain, a similar dessert is served, Crèma Catalan, the sugar caramelized by a disk of iron that has been heated in a wood fire.
Should you place the caramel in the baking dish before the custard is baked, the dessert is served inverted, released from it’s baking vessel, and called Crème Caramel, or in Spanish speaking parts of the world, Flan.
Savory custards are also prized by chefs for their velvety texture, particularly on the breakfast table where eggs reign supreme. Flavored with herbs, or rich lobster, and often inclusive of cheese, savory baked custards are as delicious as their sweet brethren. Savory custards often hide inside pastry shells, where they go by the name Quiche. Japanese chefs have long placed their savory custards in steamers, creating an even more delicate custard called Chawanmushi.
Great care must be taken when cooking a custard, as the texture is entirely dependent on the point to which we cook the proteins in the eggs. If the eggs aren’t cooked enough, proteins don’t cross-link enough to disrupt the flow of water, and the custard is thin and runny. Should they be overcooked, the proteins in the eggs begin to coagulate tightly, forming small curds. These curds are wonderful for scrambled eggs, but are unwelcome in a custard.
A baked custard, contained in a heat proof baking dish called a ramekin or casuela, must be baked in a water bath. The surrounding water insulates the baking dish allowing a slow, even bake for the custard inside. Without the water bath, the baking dish will transfer too much heat from the oven to the custard, causing it to curdle against the edges. A stirred custard requires low heat and constant agitation with a wood spoon or rubber spatula. This ensures the heat that is introduced to the bottom of the pot is distributed evenly through out the custard as it cooks, and prevents the custard from curdling on the pot’s hot bottom and sides.
Because the proteins in egg yolks begin to coagulate at 155°F, and become solid at 165°F, you can use a thermometer to monitor the temperature of a stirred custard, ensuring you never heat it past 165 degrees F, and contemporary recipes often instruct you to bring your custards to this temperature. Traditionally, stirred custards are cooked to “nape.” This term describes the flow of the custard on the back of a spoon. To test for Nape, dip a large spoon in the custard, then swiped with your finger across the back of it following the same vertical line as the handle, splitting the spoon into two hemispheres. Hold the spoon flat, handle to the side of the spoon head, but with the spoon head flipped vertically, to allow gravity to pull the custard downward the newly wiped space. This old fashioned trick allows you to easily see the viscosity of the thickening custard. Proper nape is thick, and smooth, and barely moves when gravity pulls it down, like the flow of latex paint. Once this thickness is achieved, the custard is immediately poured from the pan into a bowl, which is placed in an ice bath and frequently stirred to stop the proteins from coagulating any further.
Baked custards require far less hands-on cooking, but do require you to check the texture of the baking custard frequently as it nears completion. A baked custard is finished when a gentle shake produces a wiggle that is like jell-o. The center will be the last part of the custard to coagulate, so as you give your custards a little shake, watch the bulls-eyes to see if it shimmy’s. If the outside jiggles, but the inside looks soupy, bake the custards for 10 more minutes. This checking-and-cooking stage can feel excruciatingly long at times. Once you open the oven door, and remove the foil cover from the water bath, the whole set-up needs to recover the lost heat before the custards begin to start cooking again. Over-checking can cause your custards to stop baking all together. Often, if many baked custards are cooking together, some will be done before others, and must be removed from the batch as the remainder finish cooking.
A water bath is easy to set up, simply fill a large baking dish with 2 inch walls with the ramekins that hold your custard, then add enough hot water that the water level reaches half way up the sides of the ramekins. If you use cold water it will take your oven more time to warm the water bath, hot water is a better choice. Once the water is added, cover the whole pan with foil, and cut 3 inch vents in it to allow steam to escape. I like to do this as close to the oven as possible, moving the water filled pan can be a little tricky, and balance is necessary to keep any water from splashing into the custard filled ramekins.
It takes many tries to be able to pinpoint the exact moment when the egg yolks have cooked to their velvetiness moment, and it is the true mark of an experienced pastry chef. Luckily, custards are so delicious, you’ll find no complaints as you try and try again, narrowing down that moment of custardy perfection for yourself.