Cooking science

Surface area to volume

Cut a potato into chips and you have not just made it smaller — you have multiplied the surface area enormously while the total volume stays the same. Almost everything that happens at a surface happens more, and almost everything that happens inside happens less.

Cutting multiplies surface, not volume

A whole potato has one skin. Cut into chips, the same potato has many times the surface area — every new cut creates two new faces — and the same amount of flesh.

Everything that happens at a surface is now amplified: browning, crisping, water loss, salt uptake, oil absorption. Everything that depends on the interior is diluted: the ratio of soft inside to crisp outside falls.

This is why a baked potato is mostly fluffy interior with a little skin, and chips from the same potato are mostly crust. It is the same food, cut differently.

Where it decides the outcome

• Chips against a baked potato. More surface means more crust and less soft middle.
• Diced against whole vegetables in a roasting tin. Smaller pieces brown more and dry out faster.
• Mince against a steak. Mince has an enormous surface area, which is why it browns readily, why it dries out easily, and why it is handled differently for food safety.
• Grated against chopped. Grating exposes vastly more surface, which is why grated onion is harsher and grated potato weeps.
• Thin slices in a dehydrator. Drying speed depends almost entirely on this ratio.
• Small joints against a big one. More surface per unit of meat means more crust and more moisture loss.
• Ice cubes against a block. More surface means faster melting, which is why crushed ice dilutes a drink so quickly.

Using it deliberately

Want more crust

Cut smaller. Chips, roast potato chunks with roughed edges, diced vegetables.

Want a moist interior

Cut larger, or leave whole. A whole roasted cauliflower against florets.

Want faster drying or curing

Cut thinner. Salt and air reach more of the food.

Want slower cooking

Leave it whole and accept a longer time.

Want less oil absorbed

Cut larger. More surface means more oil taken up in frying.

Want faster infusion

Cut or bruise. More surface releases aromatics faster.

Deciding how to cut something

The question is what you want more of.

1. Decide first whether the dish is about the crust or about the interior.
2. For crust — chips, roast chunks, diced vegetables, thin slices. Cut smaller and accept more browning and more moisture loss.
3. For interior — a whole roasted cauliflower, a baked potato, a large joint. Cut larger or not at all, and accept less crust.
4. For something in between, cut large and rough up the surface. A parboiled potato shaken in the pan has the volume of a large piece and the surface of a small one.
5. For frying, remember that more surface means more oil absorbed. Larger pieces come out less greasy.
6. For infusing or curing, cut smaller or bruise, because the process happens entirely at the surface.

Common questions