Week 1: Pastry Cream

Pastry cream or crème pâtissière is a staple filling. It can be used as a filling for tarts, cream puffs, eclairs, donuts, cakes, and other pastries. Pastry cream itself is thick and holds its shape when piped, but it is often combined with other ingredients to make different fillings. For example, it can be folded with whipped cream to create a lighter filling called crème légère or a whipped cream stabilized with gelatin to make crème diplomat. It can be combined with meringue to make crème chiboust or combined with whipped butter to make crème mousseline. It is extremely versatile and a key recipe for any pastry fan.

Pastry cream is usually comprised of milk, eggs, sugar, butter, and starch. Much like lemon curd, we again have an emulsion of fat (butter + milk fat) in water stabilized by the emulsification and coagulation of eggs and further thickened by butter as it cools. For more details on all the cool science in play in emulsions, see the first week of my lemon curd adventures. Unlike lemon curd, there is not a highly acidic environment, so the coagulation process goes more slowly. The milk and any flavorings provide the main flavor for the pastry cream. The eggs in pastry cream are typically yolks only. As we learned in my lemon curd experiments, yolks give a thicker, richer texture, and pastry cream is all about richness. Sugar plays a similar role regulating the coagulation of the eggs as it did in lemon curd. The sugar molecules physically block proteins in the eggs from bonding together, slowing the coagulation process, and raising the coagulation temperature.

Unlike lemon curd, pastry cream also uses a starch (usually flour or cornstarch) to further thicken the mixture. A fluid's viscosity or thickness largely depends on its volume fraction. The volume fraction is given by the ratio of the volume of molecules in the mixture to the total volume of the mixture (which is the volume of molecules in the mixture + the volume of water in the mixture). Starches thicken fluids in three ways. First, when heat is applied to starch granules, they absorb water and swell, taking up drastically more volume. Additionally, starch granules clump together. This traps water inside of the structure, which means that the total volume of the structure is the volume of the starch plus the water, further increasing the volume fraction. Finally, starches leak long chains of polymers as they are heated. The polymer chains tangle together to form a network that further thickens the mixture. All these factors combined give pastry cream a rich, luxurious texture. But which starch to use? My literature review showed that flour often leaves a floury flavor in the pastry cream, so I decided to use only cornstarch for my recipe testing. Cornstarch needs to reach a temperature of 203˚ F to absorb water. The nearest visual marker around this temperature is boiling, which occurs at 212˚ F. This is why most recipes call for reaching a boil before removing the mixture from the heat.

Generalized Technique

The general technique I will use for all of my recipes will be to mix the cornstarch and sugar together to prevent the cornstarch from clumping. Then I will whisk the sugar and cornstarch vigorously with the eggs for ~1-2 minutes until it is pale yellow. I will then heat the milk and salt on medium-low in a medium, heavy-bottomed saucepan, stirring regularly until it comes to a simmer. Next, I will temper the mixture by adding a small amount of milk to the eggs at a time and whisking constantly until the milk and eggs are fully combined. Note: You could just mix the eggs, sugar, and cornstarch into the milk at the beginning, but then you need to stir the pot constantly as it heats. Because of this, I prefer the tempering method, but there isn't a strong reason to go with either unless you are already heating milk first to infuse flavor (e.g. vanilla beans).

I will then return the mixture to the pan and heat it over medium, stirring constantly, until the mixture begins to boil. The pastry cream at this point is thick and boiling will look like a few bubbles forming and popping like lava bubbling. Once it starts boiling, I will cook it for 1 minute, whisking constantly before removing it from the heat and straining into a bowl. Finally, I will slowly whisk in the butter, one small piece at a time, until it is fully incorporated. I will then transfer the pastry cream to an airtight container, place a layer of plastic wrap over the top to prevent a skin from forming, cover it, and refrigerate it for at least 4 hours.

There are a large number of pastry cream recipes with a wide amount of variation. Next week, I will look at these recipes to identify any major trends and try out a number of recipes to try to find a reference recipe.