The critical factor in potatoes, he says, is water content. A lot of his research for roasties overlaps with his quest for the perfect chip. “I want the centre to be moist and fluffy and the outside to have that glass-like crunch,” he says. “If your water content is too high, your potatoes aren’t going to crisp. If your dry content is too high, the potatoes get crumbly and fall apart. Some potatoes are not fit for purpose. Things like this make people think they can’t cook.” (In his restaurants, Blumenthal currently uses Agria potatoes though he recommends Albert Bartlett Rooster, Burgundy Red or Maris Pipers as good all-rounders.)
He experimented with different methods of drawing excess water out of the potatoes – pin-pricking them, vacuum cooking them – only to conclude that the crucial element is the boiling. “That first cooking stage in the water is crucial,” he says. You want to peel them, cut them into quarters or eighths to maximise corners, and rinse them, before an intense boil.
“People say ‘parboil’, but you want to cook them quite heavily until they start to break up, skimming off any excess starch that builds up. It might sound counterintuitive to dry potatoes out by simmering them in boiling water, but with all the cracks and fissures there’s a much bigger surface area for crispness, and the oil and fat can run into them.”