Mashed potatoes

Why

A cooked potato is a cluster of cells full of swollen starch, held together by pectin that softens as it cooks. Gentle mashing separates the cells whole and the mash stays fluffy; a blender or vigorous whisking tears them open, the swollen starch escapes and, together with the liquid, forms a sticky, gluey mass. Floury potatoes have more starch and less water, their cells come apart easily once cooked and soak up butter and milk well, while firm waxy potatoes break down less readily and give a dense, pasty mash. Warm milk doesn't cool the mash and is usually absorbed more easily, so you can generally stir for less time – and the less work, the fewer torn cells.

How to do it

• Choose floury potatoes for mash, or all-rounders at a push.
• Cut them into evenly sized pieces, put them in cold salted water and cook until tender.
• Drain them and let them steam dry in the hot pan for a moment so leftover water doesn't thin the mash.
• Mash them with a masher or press them through a ricer; don't use a blender or stick blender.
• Stir in the butter first, then gradually the warm milk, and stir only until combined.

If it goes wrong

Gluey mash can't be made fluffy again – torn cells don't knit back together. Use it another way: bake it with cheese, add flour and egg to fry potato cakes, or use it to thicken a soup. Next time, only mash the potatoes and warm the milk first.