Science of Custards & Creams …and a Fruit Tart
While making pastry cream for the first time, I decided to delve into how it all works and was intrigued by the science behind this.
Pastry cream involves one of my favorite ingredients and one of my favorite words…Eggs and Coagulation!
Again, Mr. McGee and his lovely book, On Food and Cooking, had helped me with this endeavor.
Coagulation means to turn a liquid into a solid or semi-solid. The tricky part is not curdling your creation. Curdling is forming semi-solid lumps in a liquid. No one wants a curdled pastry cream, right?
So, let’s talk about coagulation and how to make our custards and creams moist, delicate and consistent in texture.
As we all are aware, eggs are protein but when we look closer eggs are little clumps of proteins surrounded by water. The proteins are compact and negatively charged so they repel each other and stay separated in the water, especially in the egg white. In the yolk, there is fat and some of the proteins bind with the fat. To turn our liquid eggs into a smooth and creamy semisolid, the proteins must be unfolded and dispersed amongst the water molecules. They must be forced to bond with each other, creating a solid network where the water is now mixed in with the proteins. There are three main ways to break these protein bonds; heat, acid or mechanically breaking them apart.
When heating eggs they should be cooked just to the point were the proteins will start to coagulate, DO NOT OVERCOOK THEM! Depending on the part of the egg, coagulation occurs between 145 to 180 degrees Fahrenheit. Depending on what the egg is diluted with (salt, acid, cream), this temperature starts to change. Regardless, coagulation occurs below the boiling point (212 degrees Fahrenheit at sea level, about 205 degrees Fahrenheit here in Denver). Remember, the more ingredients the eggs are diluted with, the more delicate the coagulation process becomes and more easily disrupted by overheating.
Acids such as cream of tartar, lemon juice, vegetable juices lower the pH and therefore lessen the negative charge on the protein. Salt also neutralizes the negative charge of the proteins as well. This causes the proteins to coagulate earlier during cooking before they can completely unwind and therefore makes the eggs tender.
Creams (and custards) are created when the egg is diluted with about 4 parts liquid. Custards are made in the same dish that they are served, such as with quiche, flan, and cheesecake. Custards are baked so the heating process is somewhat controlled. Creams, although similar to custards, are cooked by constant stirring over heat and then poured into a dessert shell like this fruit tart I made.
When the proteins are surrounded by sugar and cream, the temperature at which coagulation occurs increases to about 175 to 185 degrees Fahrenheit. If the temperature is exceeded, curdling will occur. Resist the urge to crank up the heat to hasten the coagulation process. I will say I did this with this tart and fortunately got minimal clumps. Don’t despair, if you do this too. The clumps can be strained out if your cream isn’t too thick. I left mine alone and the cream was still wonderfully tasty!
Some more helpful tips…
Always add hot ingredients to cold ingredients (notice with this pastry cream the warmed half and half is added to the egg and sugar mixture) and using cornstarch can help prevent curdling. The cornstarch molecules swell in liquid as they are heated and slow the binding of the proteins.
French Fruit Tart
adapted from Gourmet or Gourmand?
1 1/2 c. all-purpose flour
1/2 c. powdered sugar
1/4 t. salt
9 T. cold unsalted butter, cut into 1 inch chunks
1 large egg yolk, slightly beaten
Put all of the dry ingredients in a food processor and pulse to combine. Add the butter and pulse until the mixture looks like corn meal. Add the egg yolk and mix until the dough forms a rough ball and the food processor starts to resist.
Butter a 9 inch fluted tart pan with a removable bottom. Press the dough evenly over the bottom and up the sides of the pan. Freeze the crust for at least 30 minutes, I froze mine for 2 days before baking.
Preheat the oven to 375 degrees Fahrenheit and butter a piece of aluminum foil and place buttered side down over the crust. Put the tart pan on a baking sheet, bake the crust for 25 minutes, then remove the foil. Continue baking until the crust turns golden brown about 10 minutes more.
Let cool and pour pastry cream in immediately or store the shell in the refrigerator for several days before assembling with cream and fruit.
3 c. half and half
6 large egg yolks
1/2 c. sugar
1/3 c. + 2 T. cornstarch
1/4 t. salt
1 t. vanilla extract
If necessary, have a fine mesh sieve ready over a bowl. Warm the half and half in a saucepan. In a bowl, whisk together the egg yolks, sugar, cornstarch and salt until well combined. Slowly pour and whisk the warm half and half into the egg mixture and then pour this mixture back in the saucepan. Cook the entire mixture on medium heat, continually whisking until thickened. Remove from the heat and whisk in the vanilla. Strain the pastry cream through the sieve and then press plastic wrap onto the top of the pastry cream to prevent a skin from forming until it cools and is poured into the pastry shell.
Once the pastry cream is spread evenly in the shell, decorate the top with sliced kiwi, blueberries and strawberries. Combine a few tablespoons of fruit jam with a few tablespoons of water. Heat over low heat and once smooth and glossy, let cool slightly and then brush onto the fruit to create a shiny glaze.
Chill until ready to serve. Serve within 3 days. After that the tart becomes a little soggy.
April 3, 2012
This entry was posted in baking, brunch, desserts, Science of and tagged coagulation, creams, custards, eggs, fruit tart, pastry cream, science of.