Waxy vs Starchy Potatoes: Which for Mash, Roast or Salad?

The difference between waxy and starchy potatoes comes down to how much starch and water each one packs into its cells. Starchy (or floury) potatoes like russets are high in starch and low in water, so their cells swell and separate when cooked, giving fluffy mash and crisp fries. Waxy potatoes like red or fingerling potatoes have less starch and more water, so their cells stay glued together, perfect for potato salad and anything that needs to hold its shape.

What's actually inside a potato?

A raw potato is about 75–82% water. Most of the rest is starch, stored in tiny granules packed inside each cell, with a little protein, fibre and sugar. The cells themselves are held together by pectin, a sort of natural glue in the cell walls.

The key number is dry matter, the fraction that is not water. Floury potatoes sit at around 20–24% dry matter, most of it starch. Waxy potatoes are closer to 16–18%. That gap of just a few percent changes everything about how they cook.

You will sometimes read that waxy potatoes contain a different kind of starch, more amylopectin and less amylose. Varieties do vary a little in their starch makeup, but the evidence suggests it is a smaller factor than overall starch and water content, plus how tightly the cells stick together. Think of it mostly as a starch-and-water story.

What happens to potato starch when you cook it?

Starch granules are tightly packed, almost crystalline bundles of glucose chains. When you heat them in water above about 60–70°C (140–158°F), they soak up water, swell to many times their size and soften. This is starch gelatinisation, the same process that thickens gravy, as explained in how flour thickens a sauce.

Here is where the two types split.

In a starchy potato

Each cell is crammed with starch. As the granules swell, they press outwards on the cell walls. At the same time, heat softens the pectin glue between cells. The result: cells pop apart from their neighbours into individual, starch-filled little balls. That is the mealy, crumbly, fluffy texture of a baked russet. Break one open and it falls into dry, snowy flakes that soak up butter like a sponge.

In a waxy potato

There is less starch to swell, so there is less pressure on the cell walls, and the cells stay stuck together. Waxy potatoes often have stronger cell-to-cell bonds too. The texture stays firm, smooth, moist and creamy, and the potato keeps its shape even after slicing, tossing and dressing.

Chef Fizz says: Starchy potatoes are a crowd that scatters. Waxy potatoes are a team that holds hands.

Which potato types are which?

Potatoes sit on a spectrum, and the middle is crowded with all-rounders.

New potatoes are any variety harvested young, before they have built up much starch. That makes them waxier than the same variety would be at full size.

Which potatoes make the best mash?

For fluffy mash, choose starchy potatoes. Their separated cells absorb butter and milk easily and give a light, cloud-like result. Yukon Gold makes a creamier, denser, naturally more golden mash that many people love even more.

What you want to avoid is gluey mash. Each cooked cell is like a balloon full of gelatinised starch. Mash gently and the balloons stay mostly intact. Overwork them, especially with a food processor, blender or vigorous electric mixer, and you burst the cells, releasing sticky free starch that turns the whole bowl into paste.

Waxy potatoes are prone to gluey mash because their cells are tougher and need more force to break apart, so you end up working them harder. If you love the flavour of waxy potatoes in mash, crush them roughly instead of trying to get them smooth.

A few rules for great mash:

• Start the potatoes in cold, salted water so the outside does not overcook before the centre is done.
• Cook until a knife slides in with no resistance, about 15–20 minutes for 5 cm (2 inch) chunks.
• Drain well and let them steam dry for a minute or two.
• Use a ricer, food mill or hand masher.
• Add warm butter first, then warm milk or cream. Cold dairy cools the potatoes and makes them gummy.

Which potatoes are best for roasting and fries?

Starchy potatoes win for crunch. Their high starch and low water mean less moisture to drive off before browning starts, and their fluffy surface cells form a rough, crunchy crust. Roast potatoes and fries made from russets or Maris Pipers have that classic crisp-outside, soft-inside contrast.

For the craggiest roast potatoes, parboil chunks for about 8–10 minutes in salted water with 1/2 teaspoon of baking soda per 2 litres (about 2 quarts). The slightly alkaline water breaks down the surface pectin faster. Drain, shake them in the pot to rough up the outsides into a starchy paste, then roast in hot fat at 220°C (425°F). All those bumps turn into extra crispy bits. For more on getting a crisp finish, see why roasted vegetables go soggy.

Waxy potatoes roast perfectly well too. They just give a smoother, creamier inside and thinner, more delicate skin rather than a shattering crust. Smashed potatoes made with small waxy potatoes are a great middle ground.

Which potatoes should you use for potato salad?

Waxy, every time. They stay in neat cubes or slices through boiling, cooling and tossing, while starchy potatoes crumble and turn the salad into rough mash.

Two tricks for better potato salad:

• Dress them warm. Toss the drained, still-warm potatoes with vinegar or some of the dressing. Their warm cells absorb flavour far better than cold ones.
• Salt the cooking water well. About 1 tablespoon of salt per litre seasons potatoes all the way through.

Cooked and chilled potatoes also undergo starch retrogradation, in which gelatinised starch slowly reorganises into firmer, more crystalline structures. It is the same process that makes bread go stale, and it is part of why cold potato salad feels firmer than warm boiled potatoes.

The restaurant trick for firmer potatoes

Cooks sometimes give potatoes a gentle pre-cook at about 60–70°C (140–158°F) for 20–30 minutes before boiling them normally. At those temperatures, an enzyme called pectin methylesterase activates and modifies the pectin so it binds more strongly with calcium in the cell walls. The result is potatoes that hold their shape better, even starchy ones, without getting tough. Hold a steady temperature with a thermometer, then boil as usual.

Want a quick refresher on picking the right potato? The Cooking Scientist app has a two-minute lesson on waxy and floury potatoes, with a mini game.

Try it at home

You can sort mystery potatoes with a salt water float test, which measures their density.

1. In a large jug, dissolve 1 part table salt in 11 parts water by volume, for example 1/4 cup salt in 2 3/4 cups water. Stir until clear.
2. Drop in one potato of each type you have, such as a russet, a Yukon Gold and a red potato.
3. Watch which ones sink and which float.

What you'll see: Starchy russets usually sink, waxy red potatoes usually float, and all-rounders like Yukon Gold may hover in the middle or bob near the surface.

Why: Starch is denser than water, so potatoes with more starch and less water are denser overall. The salty water is just dense enough to float the low-starch, waxy ones while the starchy ones sink. Results vary with growing conditions, so treat it as a fun guide, not a lab test.

Bonus round: boil one chunk of each for 20 minutes, then mash each with a fork. The sinkers turn fluffy; the floaters stay smooth and dense.

Common mistakes

• Using waxy potatoes for mash. They need so much work that they turn heavy and gluey. Choose starchy or all-purpose potatoes, or crush waxy ones roughly.
• Using starchy potatoes in salad. They fall apart into crumbs once dressed. Use red, new or fingerling potatoes.
• Blending mashed potatoes. Food processors and blenders burst starch-filled cells and turn mash to glue. Use a ricer or masher.
• Dropping potatoes into boiling water. The outside overcooks and crumbles before the middle is tender. Start them in cold, salted water.
• Skipping the steam-dry. Wet boiled potatoes give watery mash and soggy roasties. Let them sit in the hot pot for a minute after draining.
• Storing them somewhere warm or bright. Light turns potato skins green with chlorophyll, a sign that bitter, toxic solanine may be building up, and warmth makes them sprout. Keep potatoes cool and dark.