How do I use gelatine and other setting agents?

Setting agents are the part of dessert-making with the least room for approximation. Too little and nothing sets, too much and it turns rubbery, and a few common fruits stop gelatine working altogether.

Step by step

1. Soak leaf gelatine in plenty of cold water.Five minutes until floppy, then lift it out and squeeze firmly. It carries its own water with it, so it does not need measuring separately.
2. Bloom powdered gelatine in cold liquid.Sprinkle it over a few tablespoons of cold liquid and leave five minutes until it looks like wet sand. Adding powder straight to hot liquid gives permanent lumps.
3. Dissolve it gently, never boil it.Stir the bloomed or soaked gelatine into warm liquid — around 50 to 60°C is plenty. Boiling weakens its setting power measurably.
4. Use the right quantity.Roughly 4 to 6 platinum-grade leaves, or 10 to 12g of powder, per litre for a soft wobbly set. Double that for something you need to unmould and slice.
5. Know the fruits that break it.Fresh pineapple, kiwi, papaya, mango, figs and fresh ginger contain enzymes that digest gelatine. Tinned versions are heat-treated and are fine.
6. Chill it long enough.Four hours minimum, overnight for anything you want to unmould. Gelatine continues to firm for a long time after it first sets.
7. Use agar for a vegetarian set.Agar-agar is seaweed-derived and must be boiled for one to two minutes to activate. It sets firmer and more brittle than gelatine, sets at room temperature, and does not melt in the mouth the same way.
8. Use cornflour or eggs where a soft set suits.Custards, pastry cream and some panna cottas set with starch or egg rather than gelatine, which gives a softer, creamier texture.
9. Add gelatine to a small amount first.Dissolve it into a ladleful of the warm mixture, then stir that back into the bulk. Adding it to a cold bulk gives stringy threads.

Tips

• Never boil gelatine. It is the most common reason a set fails despite the right quantity.
• Tinned pineapple is fine; fresh pineapple destroys the set completely.
• Under-set is easier to fix than over-set. Err on the light side and chill longer.

Common mistakes

• Adding gelatine powder straight to hot liquid — The outside of each granule swells and gels instantly, sealing dry powder inside, and those lumps never dissolve. Blooming in cold liquid first lets every granule hydrate evenly.
• Using fresh pineapple or kiwi in a jelly — Both contain protein-digesting enzymes that break gelatine down, so the mixture simply never sets. Tinned fruit has been heated enough to destroy the enzymes and works perfectly.

If it doesn't work

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