Why did my jelly or panna cotta not set?

A dessert that refuses to set has a small number of possible causes, and one of them catches people out repeatedly because the ingredient seems entirely innocent.

Step by step

1. Check for the enzyme fruits first.Fresh pineapple, kiwi, papaya, mango, figs and fresh ginger contain protease enzymes that break gelatine down. It will never set. Tinned or cooked versions are fine, because heat denatures the enzyme.
2. Get the ratio right.Roughly 6 to 7 sheets of leaf gelatine, or about 12g of powder, per litre of liquid for a standard set. Softer desserts use less; anything to be turned out needs a firmer set.
3. Bloom it properly.Soak leaves in cold water until floppy, or sprinkle powder over cold liquid and leave it to swell. Adding dry gelatine straight to hot liquid gives lumps that never dissolve.
4. Dissolve without boiling.Gelatine dissolves at around 50 to 60°C. Boiling it degrades the protein and weakens the set, sometimes destroying it. Warm, not simmering.
5. Squeeze leaf gelatine out.Wring the soaked sheets before adding them, or the water they carry thins the mixture and softens the set.
6. Mix it in evenly.Add the dissolved gelatine to a little of the warm base first, then combine. Pouring it into a cold mixture sets it into threads instantly.
7. Allow for acid and alcohol.Very acidic mixtures and high alcohol content both weaken a set. Increase the gelatine slightly for a wine jelly or a very citrus-heavy dessert.
8. Watch fat and sugar too.High fat, as in a rich panna cotta, and very high sugar both interfere. Follow a tested recipe rather than scaling one heavily.
9. Give it long enough.Four hours minimum, overnight for anything deep or to be turned out. A shallow set is much faster than a deep one.
10. Check the fridge is cold enough.Gelatine sets below about 15°C. An overfull or warm fridge, or a door left ajar, is a genuine and overlooked cause.
11. Rescue an unset mixture.Warm it gently, add more bloomed gelatine dissolved in a little of the liquid, mix thoroughly and re-chill. This works provided no enzyme fruit is present.
12. Use the alternatives knowingly.Agar sets firmer, sets at room temperature and is unaffected by those enzymes, but it must be boiled to activate and gives a more brittle texture. It is not a direct swap.

Tips

• Fresh pineapple, kiwi, papaya, mango, figs. Any of those and gelatine will never set — use tinned, or use agar.
• Never boil gelatine. Warm enough to dissolve is all it needs, and boiling weakens it.
• An unset dessert usually rescues: warm gently, add more bloomed gelatine, re-chill.

Common mistakes

• Using fresh pineapple or kiwi with gelatine — Their protease enzymes digest the gelatine protein, so it can never form a gel. Tinned fruit is fine because the canning heat destroys the enzyme.
• Boiling the mixture to dissolve the gelatine — It degrades the protein and significantly weakens or destroys the set. It dissolves at 50 to 60°C.

If it doesn't work