J. Kenji López-Alt’s roast potatoes are different from every other roast potato you’ve made, and the difference comes down to a single step: adding baking soda to the parboiling water.
The alkaline water breaks down the potato’s outer structure faster than neutral water would, creating a thick, starchy paste on the surface when you rough the potatoes up after draining. That paste dehydrates in the oven’s heat and then fries in the surrounding fat, building a crackly, shatteringly crisp crust that’s unlike anything a thin-skinned parboiled potato can produce.
The technique in plain language: boil with baking soda → drain and shake until the surface looks ruined → tip onto screaming-hot fat in a preheated pan → don’t move them for 20 minutes → flip and finish.
Everything else — the fat you choose, the aromatics you add, whether you use Russet or Yukon Gold — is secondary. The baking soda and the aggressive roughing-up are the two steps that cannot be skipped or softened.
Why the Baking Soda Works
At neutral pH, potato pectin holds the cell walls together firmly. Raise the pH with baking soda and those same walls begin to dissolve faster during the parboil, releasing a slurry of broken starch that coats the outside of each chunk. That starchy coating is the raw material for the crust.
Without the baking soda, parboiling produces a thin, slightly sticky layer on the potato surface. With it, you get a thick, fuzzy, almost mashed-potato-looking exterior. More surface area = more contact with hot fat = more browning = more crunch.
Make-Ahead Notes
You can parboil and rough up the potatoes up to 8 hours ahead. After roughing up, spread them on a parchment-lined sheet pan and refrigerate uncovered — the exposed surface dries out in the fridge, which actually makes the final crust crispier. When you’re ready, preheat the pan and fat, and proceed from step 4. The cold-from-the-fridge potatoes will need an extra 5 minutes of roasting time.
The Fat
Duck fat or goose fat produces the crispiest, most flavorful crust and is traditional for British Sunday roasties. It’s sold in jars at most specialty grocery stores. Extra-virgin olive oil is the next-best option for most home cooks and works well at this temperature. Avocado oil (smoke point 520°F) is the most heat-stable and produces a neutral-flavored crust. All three produce excellent results; the technique is what makes the potato, not the fat.
