Vol. XXV · Cakes, Pastry and Sweets Plate 9 of 12pastry cream, crème pâtissière, custard, cornstarch, egg yolk, amylase, boiling, curdling, lemon curd, 卡仕达酱, éclair filling, tart fillingMH

Pastry Cream and Curds.

A plain custard curdles at a boil. Pastry cream, with a spoonful of cornflour, must be boiled — or it turns to soup in the fridge by morning.

Pastry cream (crème pâtissière, 卡仕达酱) is milk, sugar and egg yolks thickened with cornstarch or flour, cooked until it boils, then enriched with butter and vanilla. Two ideas make it different from the custards of XVII.5. First, starch protects the egg: swelling granules thicken the liquid and get between the egg proteins, so they cannot bond into curds even at 100 °C. A pastry cream can therefore be boiled, which a crème anglaise cannot. Second, it must be boiled: egg yolks contain amylase, an enzyme that digests starch (V.9). If the cream only reaches 80 °C, the starch has thickened but the amylase survives, and overnight it cuts the starch chains and the cream turns runny.

So the method is: whisk the yolks, sugar and starch together; pour on the hot milk; return to the heat and whisk constantly until it thickens, then keep whisking at a gentle boil for one to two minutes until it is glossy and no longer tastes of starch. Lemon curd uses the same pair of thickeners in different proportion — eggs, sugar, lemon juice and butter, sometimes a little starch — cooked gently to about 80 °C; its acid and fat help it set, and with no starch it need not boil. The workshop cooks a pastry cream.

Below: the thickness of a pastry cream right after cooking and the next day, from the highest temperature it reached and how long it was held there (teaching model: cornstarch thickening by the course starch kinetics; yolk amylase destroyed steeply near the boil and, if it survives, thinning the cream overnight).

What you will learn
Workshop
Teaching model: thickening = the course cornstarch setting fraction after a minute at the top temperature (VII.1); surviving amylase = exp(−k × seconds) with k = 0.02 s⁻¹ × 10^((T − 85) ÷ 5); next-day thickness = thickness × (1 − 0.8 × surviving amylase). Shaped on published accounts of yolk α-amylase.

Thick is not enough: boil it

In this teaching model the starch has fully thickened the cream by 80 °C, but held a minute there the yolk's amylase is barely touched (0.89 of it survives), and next day the cream has thinned to 0.29. Brought to a boil and held a minute, it keeps 1.00. The cream looked identical both times when it came off the heat — which is why so many pastry creams fail overnight, and why recipes insist on a full boil.

Method for 500 mL: heat 400 mL milk with a split vanilla pod; whisk 4 yolks, 80 g sugar and 35 g cornstarch until pale; whisk in the hot milk; return to medium heat and whisk constantly, reaching into the corners, until it thickens and bubbles, then boil gently 1–2 minutes, still whisking; off the heat, whisk in 25 g butter; strain into a shallow dish, press film onto the surface to stop a skin, and chill quickly (XII.7). Beat smooth before using in éclairs (XXV.7), tarts and 菠萝包 fillings. Lemon curd: whisk 3 eggs, 150 g sugar, 120 mL lemon juice and zest over a water bath to about 80 °C until it coats a spoon, then whisk in 100 g butter.

At the stove

Make two small pastry creams: take one off the heat as soon as it thickens, boil the other for a minute. Chill both overnight and compare.

The fact that yolk amylase survives a gentle cook and ruins starch-thickened custards was worked out by food scientists in the mid-twentieth century; cooks had long known the rule, "boil it or it weeps", without the reason.

Terms

Pastry cream
Custard thickened with starch and egg, boiled.
Amylase
An enzyme in egg yolk that digests starch (V.9).
Lemon curd
Eggs, sugar, lemon and butter cooked to a thick cream.
Starch protection
Swollen starch keeping egg proteins from curdling.
Pastry cream = custard + starch. The starch protects the egg so it can boil — and it must boil for a minute to destroy the yolk's amylase, or it thins overnight. Lemon curd sets gently near 80 °C.

Sources

  1. A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.
  2. H. McGee, On Food and Cooking: The Science and Lore of the Kitchen, 2nd ed., Scribner, 2004.