Vol. VII · Starches and Sugars Plate 5 of 11potatoes, floury, waxy, dry matter, starch, mash, chips, salad, glueMT

Waxy and Floury Potatoes.

A floury potato falls apart into fluff; a waxy one holds its shape. The difference is starch content and how firmly the cells are glued.

Potatoes differ most in their dry matter — the solids left after the water, mostly starch. Floury (mealy) varieties, such as russets and Maris Piper, are about 20–22 % dry matter; their cells are packed with starch, swell when cooked and come apart from each other, giving fluffy mash, light baked potatoes and chips with crisp outsides. Waxy varieties, such as many salad and new potatoes, are about 16–18 %; their cells stay together, so slices hold their shape in salads, gratins and stews.

The swollen starch can also become a hazard. Mash a floury potato gently and the cells separate intact; work it hard — in a blender or with too much beating — and the cells burst, releasing gelatinised starch that turns the mash gluey. The workshop shows how dry matter sets the use, and how working the mash raises the glue.

Below: starch per 100 g and the uses suited to a potato of the dry matter you choose, and a teaching index of glueyness against how hard the cooked potato is worked.

What you will learn
Workshop
Starch taken as about three-quarters of dry matter. Glueyness: a teaching index rising with the share of cells burst by working, faster for floury potatoes.

Choose the potato, then treat it gently

A potato of 21 % dry matter carries about 15.8 g of starch per 100 g: floury, for mash, chips and baking. Pressed gently through a ricer it scores 0.21 on the glue index; a waxy potato, with less starch to release, only 0.16, though its cells cling together and it mashes dense rather than fluffy; the same floury potato in a blender, 0.79 — wallpaper paste. Mash with a ricer or a hand masher, add warm butter and milk, and stop.

Cooks without labels test potatoes in brine: in water with about one part salt to eleven, floury potatoes (denser, more starch) sink and waxy ones float. Floury potatoes also make the best chips because their dry, starchy surface crisps (IX.6) and the best roast potatoes, roughed up after parboiling so the gelatinised starch on the surface fries into a crust. Waxy potatoes are for potato salads, gratins, crisp shredded stir-fries such as 酸辣土豆丝, Spanish tortilla and anything that must hold its slices.

At the stove

Boil a floury and a waxy potato side by side. Mash half of each with a fork and blend the other halves for thirty seconds. Compare the four textures; the blended floury potato will be stringy and gluey.

Antoine-Augustin Parmentier promoted the potato in eighteenth-century France, where it had been viewed with suspicion; dishes "à la Parmentier" still carry his name.

Terms

Floury (mealy) potato
High dry matter and starch; cells separate when cooked.
Waxy potato
Lower dry matter; cells stay together, slices hold their shape.
Dry matter
The solids in a potato after its water; mostly starch.
Gluey mash
Mash whose burst cells have released gelatinised starch.
Floury potatoes (more starch, cells that separate) for mash and chips; waxy for salads and gratins. Work mash gently.

Sources

  1. A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.
  2. H. McGee, On Food and Cooking: The Science and Lore of the Kitchen, 2nd ed., Scribner, 2004.