Why is my crème pâtissière runny?

Two things about crème pâtissière are counter-intuitive. First, unlike a custard, it must boil — the cornflour or flour that thickens it only gelatinises fully at boiling point, and the egg yolks are protected from scrambling by the starch. Second, egg yolk contains alpha-amylase, an enzyme that digests starch, and it is only destroyed by a couple of minutes at boiling point. A pastry cream that set beautifully and then went soupy in the fridge overnight was not boiled long enough — that is the enzyme at work, not a mistake in the ratio.

What you'll need

• A heavy-based pan
• A balloon whisk
• Cling film

Step by step

1. Whisk the yolks, sugar and starch to a smooth paste first.No lumps of cornflour. Adding a splash of the cold milk to loosen it makes tempering easier and prevents dry pockets that become lumps later.
2. Temper with the hot milk.Pour the hot milk onto the yolk mixture in a thin stream while whisking, then return the whole lot to the pan. Pouring cold yolks into hot milk scrambles them.
3. Bring it to a full boil, whisking constantly.Not a shiver, not a simmer — large slow bubbles breaking the surface. The mixture will thicken suddenly and dramatically as it approaches the boil. Keep the whisk moving into the corners of the pan.
4. Hold the boil for one to two minutes.This is the step almost everyone skips. It fully gelatinises the starch and destroys the enzyme in the yolks that would otherwise thin the cream overnight. Whisk hard throughout — it catches easily at this stage.
5. Get the starch quantity right.Roughly 40–50g of cornflour per litre of milk for a piping-firm cream, less for a pouring one. Flour thickens less per gram than cornflour and gives a slightly heavier result.
6. Cool it with film on the surface.Spread into a shallow tray, press cling film directly onto the surface, and chill fast. A skin is not just unpleasant — it means the surface has dried and it will not whisk back smoothly.
7. Whisk it smooth before use.Chilled pastry cream sets into a solid, slightly rubbery block. That is correct. Whisk or beat it for thirty seconds and it becomes smooth and pipeable again.

Tips

• Crème diplomate — pastry cream lightened with whipped cream — is more forgiving to pipe and lighter to eat. Fold the cream in only once the pastry cream is completely cold and whisked smooth.
• Cornflour gives a cleaner flavour and a glossier finish than plain flour; a mixture of the two is traditional in some recipes and gives a softer set.
• Vanilla is best infused into the milk as it heats, then left to steep off the heat for ten minutes before you carry on.
• Pastry cream is a food-safety-sensitive item — eggs, milk, no acid. Chill it fast in a shallow tray, keep it refrigerated, and use it within three days.

Common mistakes

• Taking it off the heat the moment it thickens — It thickens before it boils, so it looks finished a minute early. Stopping there leaves the starch under-gelatinised and the enzyme alive.
• Adding more cornflour to a runny cream — The problem is almost always heat, not quantity. Extra starch gives a pasty, dull cream once you do boil it properly.

If it doesn't work