Why did my lemon curd not thicken?

The mistake is nearly always stopping too early. Egg yolks begin to thicken around 65°C and reach the point where a curd holds its shape at roughly 80°C, and the change over those last few degrees is fast — the pan goes from liquid to coating in under a minute. People who have been warned about scrambling take the pan off early and end up with lemon sauce. The other half of the problem is judging it hot: curd roughly doubles in thickness as it chills, so it should look looser in the pan than you want on the spoon.

What you'll need

• A heavy-based pan or a heatproof bowl
• A digital thermometer (optional)
• A sieve

Step by step

1. Use a low, controllable heat.A heatproof bowl over barely simmering water is the safest method and is worth the extra ten minutes. Directly in a heavy pan on a low hob is faster and works, but the margin for error is much smaller.
2. Stir constantly and scrape the corners.A whisk or a silicone spatula, moving continuously, reaching the edges of the pan base where the mixture sets first. Stopping to do something else is how curd scrambles.
3. Cook past the point where it looks like nothing is happening.For a long stretch it stays thin. Then it thickens rapidly. Keep going — most failures are people giving up in the flat part of the curve.
4. Test properly.Lift the spoon and draw a finger across the back. The line should stay open with clean edges. On a thermometer, 78–82°C. Once you have that, take it off the heat immediately.
5. Sieve it while hot.Straight through a fine sieve into a clean bowl or jars. This removes the chalazae, any zest you do not want and any small scrambled bits, and it makes a marked difference to the texture.
6. Cool before judging.It will look loose in the pan. Chilled, it is roughly twice as thick. Do not add more egg or cornflour to hot curd on the basis of how it looks warm.
7. If it is genuinely too thin when cold, cook it again.Return it to the pan with an extra yolk beaten in and take it back to 80°C, stirring. This works — curd tolerates being recooked far better than most egg mixtures.

Tips

• Adding the butter at the end, off the heat, in small pieces, gives a glossier curd than cooking it in from the start.
• A teaspoon of cornflour is a legitimate insurance policy. It stabilises the curd and makes overcooking much less likely, at a small cost in silkiness.
• Curd is not a preserve. It keeps two to three weeks in the fridge and freezes well; it is not shelf-stable regardless of how it was jarred.
• The same method works for passion fruit, lime, orange and raspberry. Acidity varies, so taste and adjust the sugar rather than following a lemon recipe exactly.

Common mistakes

• Judging the set while it is hot — Hot curd is always thin. The set happens as it cools, and adding thickener at that point gives a stodgy curd.
• Boiling it to hurry it up — Above about 85°C the egg protein sets into visible curds and the texture is gone. There is no way back from a hard scramble.

If it doesn't work

Questions people ask