Cooking science

Tenderising with enzymes

Some fruits carry enzymes that cut protein chains rather than merely unfolding them. That is a genuinely different mechanism from cooking, it works at room temperature, and it does not stop until something destroys the enzyme.

Cutting, not unfolding

Heat denatures protein — it unfolds the chain and lets it link to its neighbours. The chain itself stays intact.

A protease does something else entirely: it cuts the chain into shorter pieces. Structure that depended on those chains being long simply ceases to exist, and it cannot be rebuilt.

That is why a fresh pineapple stops gelatine setting — gelatine is a protein network, and the enzyme cuts it apart faster than it can form. And it is why an enzyme marinade left too long turns the surface of meat to paste rather than tenderising it.

Which foods carry them

Pineapple

Bromelain. Strong, and present throughout the fruit and especially in the core and stem.

Papaya

Papain. Strongest in the unripe green fruit and in the latex.

Kiwi

Actinidin. Widely used in Korean cooking as a marinade.

Fig

Ficin. Less familiar and genuinely effective.

Ginger

Zingibain. Milder, and part of why ginger appears in so many meat marinades.

Melon and mango

Weak activity, enough to soften a fruit salad over hours.

Destroyed by heat

All of them. Tinned and cooked fruit carries no active enzyme, which is why tinned pineapple sets in a jelly perfectly.

What it means in practice

• Fresh pineapple, papaya, kiwi and fig will not set with gelatine. Cook the fruit briefly first, or set the dessert with agar instead.
• They curdle dairy over time. A fruit salad with fresh kiwi and cream separates within an hour.
• They soften other fruit in a salad. Add them last.
• In a meat marinade, minutes rather than hours. Half an hour is usually plenty; overnight turns the surface to mush.
• They work from the outside in, so they tenderise a thin cut evenly and a thick one only at the surface.
• A tingling mouth when eating fresh pineapple or kiwi is the enzyme acting on soft tissue. It stops.
• They keep working in the fridge, more slowly. Cold does not stop them; heat does.

Using them deliberately

1. Purée or grate a small amount of the fruit — a spoonful is plenty for a marinade for four.
2. Combine it with the other marinade ingredients and coat the meat thinly.
3. Set a timer. Thirty minutes for thin cuts, up to an hour for thicker ones.
4. Rinse or wipe the meat before cooking if the marinade is heavy, so the surface can brown.
5. Do not reuse an enzyme marinade or leave meat in it "just a bit longer".
6. For jelly and set desserts, either cook the fruit for a few minutes first or use agar, which is a carbohydrate and unaffected.
7. For fruit salads containing them, dress and serve promptly rather than making them ahead.

Common questions