Gelatin, agar and panna cotta

Why

Gelatin is a protein derived from collagen: it swells in cold water, dissolves in warm liquid and, as it cools, forms a network that sets the liquid; it sets slowly, over several hours in the fridge, and melts at around body temperature, which is why panna cotta melts in the mouth and softens in the warm. Prolonged boiling breaks its chains and the jelly then sets more weakly. Fresh pineapple, kiwi, papaya and figs contain protein-splitting enzymes (bromelain, actinidin, papain, ficin) that break gelatin down, so creams and jellies won't set; in canned fruit, or fruit you boil for a few minutes, heat has destroyed the enzymes. Agar, by contrast, is a carbohydrate from red seaweed: it dissolves only in boiling water, sets at 32–43 °C and melts again only above 85 °C, so it holds up even when warm and fruit enzymes don't affect it. Gelatin is made from animal skins and bones, mostly pork and beef, whereas agar is plant-based and suits vegetarians too.

How to do it

• For 500 ml of liquid, the manufacturer Dr. Oetker allows 6 sheets or 1 sachet of powdered gelatin; strength varies between brands, so follow the packet.
• Soak sheets one by one in cold water for about 5 minutes and squeeze them out; stir powdered gelatin into 6 tablespoons of cold water and let it swell for 5 minutes.
• Dissolve the swollen gelatin in hot but no longer boiling liquid, or on its own over low heat; don't boil it.
• Don't pour it straight into a cold cream: first stir about 4 tablespoons of the cream into the dissolved gelatin, then stir it into the rest.
• Stir agar into cold liquid, bring it to the boil and simmer as the packet directs; it sets even at room temperature, so pour it straight away.
• Chill jellies and creams covered in the fridge for several hours, ideally overnight; add fresh or frozen pineapple, kiwi, papaya or figs only boiled for a few minutes, canned, or use agar.

If it goes wrong

If a cream didn't set because there was too little gelatin, warm it gently until it melts, stir in more dissolved gelatin and chill again. If a jelly didn't set because of fresh fruit, bring it to the boil and simmer for a few minutes to destroy the enzymes, cool it until warm and stir in fresh gelatin; a whipped cream will lose its airiness, and a dairy cream with acidic fruit may curdle. Melt strands and lumps of gelatin by warming gently, then strain the cream. Boil agar that didn't set once more.