Pudding and pastry cream

Why

Pudding and pastry cream are thickened with cornflour or flour, which thicken fully only at the boil (see the Thickening with starch card), and the starch also protects the yolks, so a cream with yolks can take a brief boil without curdling (see the Tempering egg yolks card). Yolks, however, contain the enzyme amylase, which breaks down starch: if the cream does not get close to boiling, the amylase survives and a cream that looked thick on the hob turns runny in the fridge; according to America's Test Kitchen the cream is done at about 93 °C, when the first bubbles burst on the surface and the cream is thick and glossy. Lumps come from starch that was not stirred into cold liquid, or from cream that thickens at the bottom, where it is hottest, before it is stirred in; that is also where it scorches first. The skin on top is a dried layer of proteins and starch from the hot surface, from which water evaporates, so the surface is covered with film straight away. A cream of milk and eggs is nourishing and moist for bacteria, so it is cooled quickly in a shallow dish and kept in the fridge.

How to do it

• Whisk the starch or flour smooth with the yolks, sugar and part of the cold milk; only then pour in the hot milk while stirring constantly.
• Return the mixture to a heavy-based pan and stir it over medium heat with a spatula or whisk without stopping, including the corners at the bottom.
• Cook a cream with yolks until it thickens and bubbles burst on the surface, then keep stirring for 1–2 minutes; with a thermometer, aim for at least 93 °C.
• A pudding without eggs made from custard powder only needs boiling as the packet says; stir that to the bottom too.
• Strain the hot cream into a shallow dish, press film directly onto the surface and put the cream in the fridge within 2 hours at most.
• Briefly whisk the cooled cream smooth before use; beat it into a butter cream a spoonful at a time, when it is the same temperature as the butter.

If it goes wrong

Whisk out lumps, strain the cream through a sieve or blend it briefly. Do not stir scorched cream up from the bottom any more: pour the top part into a clean pan and leave the scorched layer behind; if the cream tastes burnt, do not use it. Cream that has turned runny in the fridge will not thicken by itself: return it to the pan, add a spoonful of starch stirred into cold milk and cook it briefly until it bubbles again.