Why Fresh Pineapple Stops a Jelly From Ever Setting

Somebody makes a bowl of bright red jelly for a birthday, stirs in a generous handful of fresh pineapple chunks, and slides it into the fridge. Four hours later the family gathers round with spoons. The plain jelly made alongside it wobbles beautifully. The pineapple one is still a bowl of sweet red soup with fruit floating in it, and no amount of waiting will change that.

Nothing went wrong with the recipe. The pineapple did it. Packets of jelly often carry a small warning against adding certain fresh fruits, and pineapple is the first name on the list.

What holds a jelly up

The wobble comes from gelatine, which is made from collagen, a structural protein found in animals. Scientific American's kitchen science series describes what happens in the bowl: the gelatine proteins form a tangled mesh full of tiny pockets that trap the water, and they stay tangled as the mixture cools. A set jelly is mostly liquid, held in place by a net.

How pineapple cuts the net

Fresh pineapple contains an enzyme called bromelain. It is a protease, an enzyme whose job is to cut other proteins into pieces, and the magazine compares it to a pair of scissors. The University of Oxford's chemistry department, which uses this as a classroom experiment, explains that the pineapple enzyme breaks down the collagen in the gelatine. Strands that have been snipped short cannot tangle into a net, so the water is never caught.

Pineapple is not alone. Kiwi has a protease called actinidin, papaya has papain, figs have ficin and ginger root has zingibain. Apples, oranges, strawberries, raspberries and blueberries have none, which is why they sit so happily in a fruit jelly. The pineapple and papaya enzymes are also sold in meat tenderiser, where cutting up protein is the whole point.

What people often get wrong

The usual guess is that pineapple is too juicy or too sour, and the jelly is simply watered down. But oranges are sour and juicy too, and they are on the safe list. The Oxford notes add a neat test: vegetarian jelly, which is usually set with carrageenan from seaweed and not with a protein, is not affected by the pineapple at all. A second mistake is to think the freezer will tame the fruit. It will not. Freezing does not damage the enzyme, the Oxford chemists note, so frozen pineapple spoils a jelly just as well as fresh.

How heat switches it off

Heat is what works. An enzyme is a protein folded into a precise shape, and enough warmth bends it out of that shape, so the scissors no longer close. In 2010 two food scientists at Kasetsart University in Bangkok, Rungtip Jutamongkon and Sanguansri Charoenrein, heated pineapple fruit extract and measured how much of the enzyme's activity survived. At 40 degrees Celsius it lost nothing in an hour. At 80 degrees it was almost entirely gone after 8 minutes.

• 40 °C no loss of activity after 60 minutes
• 50 °C about 83% of the activity still left
• 80 °C almost all of it gone after 8 minutes

So the fix is simple. Boil or steam the pineapple for about five minutes, let it cool, and the jelly will set around it as if it were a strawberry. The Oxford classroom version uses a minute in the microwave. Which means the soup in the fridge was never a cooking failure. It was a tiny enzyme, doing in a dessert bowl exactly what it was built to do, although nobody had asked it to.