The focus of this week’s lesson – When eggs are exposed to high heat they curdle. But, starch releases amylose, coating egg proteins and hindering them from joining too tightly resulting in curdling.
As we learned inChapter 19 – Creme Brulee, custards need controlled temperatures and gentle heat to keep them from curdling. In this chapter we are not going to be able to be gentle and we are not using a water bath to control heat. In the case of pudding, quiche, pastry cream, and hot and sour soup eggs are exposed to high or prolonged heat and need a little help.
The Test Kitchen Experiment
Using a classic French Pastry Cream, the Test Kitchen heated half-and-half and sugar then tempered the mixture into five beaten egg yolks with a mixture of cornstarch and sugar. Once the eggs were tempered, they brought this mixture to a full boil and cooked it until thickened. They repeated this a second time and then ran two more tests without any cornstarch.
The pastry cream with added cornstarch was smooth and pudding-like. The pastry cream with out added cornstarch was as you expect, runny with curdled bits of egg in the liquid.
Starch
In the Test Kitchen Experiment, the recipe they based the test on was a French Pastry Cream that uses cornstarch. But when we looked at the recipe for Pastry Cream given in this chapter they used flour for the recipe. I wondered, why the change. Why not share the French Pastry Cream recipe? What happened and which starch shall we use?
Thankfully they gave us an explanation.
Cornstarch is a pure starch, whereas flour is 75% starch. Though cornstarch has the most thickening power. It can be fussy about temperature. The amylose in starch creates a gelatinous network with the egg protein creating a nice thickly set pudding. But, amylase in the egg yolks, if not destroyed by high enough heat weakens the starch gel that we are creating. If the custard is over heated the bonds of starch granules might also burst thinning out the resulting custard over time.
Flour contains starch, but it also contains proteins and lipids. Though we need more flour to thicken the custard than we do that of cornstarch, these extra components are useful as binders. They not only help trap liquid, but they keep the liquid trapped rather then allowing it to break down while cooling.
The Test Kitchen found that using flour as the thickener in Pastry Cream was much more reliable than cornstarch.
Cooking the Concept
Pastry Cream
Ingredients:
2 cups half-and-half
6 egg yolks, room temperature
1/2 cup granulated sugar
Pinch of salt
1/4 cup 1 1/4 oz) all-purpose flour
4 tablespoons unsalted butter, chilled and cut into 4 pieces and chilled
1½ teaspoons vanilla extract
Preparation:
Heat the half-and-half in a medium saucepan over medium heat until simmering. Meanwhile, whisk the egg yolks, sugar and salt together in a medium bowl until smooth. Add flour to yolk mixture and whisk until incorporated.
Remove half-and half from heat, and whisking constantly, slowly add 1/2 cup to the yolk mixture to temper. Whisking constantly, return tempered yolk mixture to half-and-half in the saucepan.
Heat over medium heat and cook, whisking constantly, until mixture thickens slightly, about 1 minute. Reduce heat to medium low and continue to simmer, whisking constantly, for 8 minutes. (Note: A lot of clumps began to form and worried us during this stage, but after whisking vigorously in the next stage the lumps incorporated into the cream.)
Increase heat to medium and cook, whisking vigorously, until bubbles burst on surface, 1 to 2 minutes.
Remove pan from heat; whisk in butter and vanilla until butter is melted and incorporated.
Strain pastry cream through fine-mesh strainer set over medium bowl.
(Notice how rich and thick the pastry cream has become.)
Press lightly greased parchment paper directly on surface and refrigerator until set, at least 2 hours or up to 24.
Use for filling wonderful baked goods, such as eclairs or Boston Cream cupcakes.
