Why Does a Jelly Melt in Your Mouth? It Sets Just Below Body Heat

The raw material is collagen, the fibrous protein that makes up skin, tendon, cartilage and the organic part of bone, and which holds animal bodies together. Collagen molecules are three long protein chains wound around each other in a tight rope, and prolonged heating in water unwinds those ropes and breaks them into separate chains, which dissolve. What is left when that solution is dried is the product in question. Commercially it is made from pig skin, cattle hide and bone, and the same thing happens unintentionally whenever a stock made from bones sets solid in the fridge.

The behaviour on cooling and warming is what makes it useful:

• •Dissolved chains move freely in hot water, so the liquid stays liquid
• •On cooling, chains begin rewinding around each other in short stretches
• •Those junctions link chains into a network trapping all the water
• •The network holds at fridge temperature and is quite weak
• •Warming to around thirty five degrees unwinds the junctions again
• •The whole process reverses completely and can be repeated

Several other substances set liquids and none melts in the mouth, which is the property that matters to a cook. Setting agents from seaweed form networks held by much stronger associations that survive well above body temperature, so a jelly made with them stays solid as it is eaten and reads as firm rather than as melting, which is why they suit vegetarian and vegan versions but change the experience. Starch thickens by swelling rather than by forming a network and gives a quite different, pasty texture. Pectin needs acid and sugar and gives the set of jam. The melting point here is an accident of protein chemistry and happens to sit a few degrees below the human body.

The product is sold by a strength rating rather than by weight, and the test behind that rating is one of the odder standard measurements in food science. A solution of fixed concentration is chilled under fixed conditions, and a plunger of a specified diameter is pressed into the surface until it depresses it by four millimetres, with the force required recorded in grams. That figure is the rating, and commercial grades run from around fifty to around three hundred. A higher figure means less material is needed for the same set, and it also correlates with a higher melting point and a cleaner flavour. Sheets sold for domestic use vary in weight between grades so that one sheet always sets the same volume.

The network is delicate and several common ingredients destroy it, which accounts for most kitchen failures. Fresh pineapple, kiwi, papaya, figs and fresh ginger contain enzymes that cut protein chains, so a jelly made with them never sets at all, and the remedy is to cook the fruit first, since heat destroys the enzymes. Tinned versions are already heated and work fine. Too much sugar, alcohol or salt competes for the water and weakens the set. Boiling the solution for too long breaks the chains shorter and reduces its strength. And the material must be softened in cold liquid before being warmed, since adding the powder to hot liquid makes lumps with dry centres.

Heating collagen unwinds its triple ropes into separate chains, and cooling lets those chains partially rewind into junctions that trap the water in a network. That network melts around thirty five degrees, a few degrees below body temperature, which is what produces the melting sensation and what no seaweed, starch or pectin setting agent reproduces. Enzymes in fresh pineapple, kiwi and ginger cut the chains and prevent a set entirely.