Cooking science

Free starch and gluey texture

Starch inside an intact cell thickens nothing. Starch released into the surrounding liquid is glue. The difference between fluffy mash and wallpaper paste is entirely whether the cells were opened.

Inside the cell, or outside it

A cooked potato is a mass of cells, each packed with swollen starch granules. Handled gently, those cells separate from one another and stay intact — which gives a mash of discrete soft particles that reads as light and fluffy.

Handled roughly, and especially by a blade, the cell walls rupture. The swollen starch spills out into the liquid between the cells, where it behaves exactly as starch in liquid always does: it thickens, and in this concentration it turns sticky and elastic.

The same starch is present either way. What changed is which side of the cell wall it is on.

What ruptures cells

In descending order of destructiveness.

A food processor or blender

Blades shear cell walls open in seconds. The single worst thing you can do to a potato.

A stick blender

The same problem in a smaller package.

Over-mashing or over-beating

Even by hand. Each stroke damages more cells.

Waxy potatoes

Smaller, denser cells that rupture more readily. Floury varieties separate more cleanly.

Boiling hard

Agitation knocks the pieces about and starts the damage before you have touched them.

Mashing while wet

Free starch needs liquid to turn into glue. Drier potato is more forgiving.

Where else it happens

• Gluey risotto, if it is stirred far too aggressively or cooked too long. Some starch release is the point; too much is paste.
• Sticky rice that was meant to be separate, where over-stirring or too much water releases surface starch.
• Cloudy pasta water, which is the same starch release and is useful rather than a fault.
• Gummy potato gratin, where sliced potato is rinsed too little or too much depending on the effect wanted.
• Slimy soup, when a starchy soup is blended too aggressively or for too long.
• Gluey bread dough with over-worked rye or very high-starch flours.

Avoiding it in mash

1. Use a floury potato — Maris Piper, King Edward, Rooster, Russet.
2. Cut into even pieces and start in cold water so they cook evenly.
3. Simmer rather than boil hard.
4. Drain thoroughly and return them to the dry pan over a low heat for a minute to steam off surface water.
5. Use a ricer, a mouli, a masher or a fork. Never a blade.
6. Warm the milk and soften the butter first, so the mash does not need working to combine them.
7. Fold gently and stop as soon as it is smooth.

Where the released starch is wanted

Everything that makes gluey mash is the same mechanism that makes several sauces work, and the difference is only whether it was intended.

• Pasta water. The starch that comes off pasta is what makes a sauce cling. It is the single most useful thing this mechanism does.
• Risotto. The whole texture is starch released from the rice by stirring. Wash the rice and there is no risotto.
• Congee and rice porridge, where prolonged cooking releases everything.
• Thickening a soup with potato, blended, which is the same gluey texture used deliberately at a lower concentration.
• A roux and a slurry, which are the controlled versions — starch released into liquid on purpose.
• Bread dough, where damaged starch from milling feeds the yeast and contributes to the crumb.
• Rice noodles and rice paper, which depend on it entirely.
• The test is always concentration. A little released starch thickens; a lot turns elastic and glue-like.

Common questions