What to use instead of cornstarch
It depends entirely on what it's doing in the dish. Here's the job, what else does it, and what to skip.
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Thickening a fruit filling or a pan sauce
The granules swell and burst in hot liquid and trap water in a gel. It sets clear, which is why pie fruit stays bright instead of going muddy.
All-purpose flourRan out
2 tbsp flour per 1 tbsp cornstarch, and boil 2 min longer
Cloudy instead of glossy, and it tastes pasty until it has properly boiled. Fruit colour dulls noticeably.
Potato starchGluten-free
1:1
Thickens at a lower temperature and thins out again if you hold it on heat. Stir it in at the end and take the pan off.
Tapioca starchGluten-free
2 tbsp per 1 tbsp cornstarch
Sets glossy and slightly stringy. Right for a fruit pie, gummy in a savoury pan sauce.
ArrowrootGluten-free
1:1, stirred in off the boil
Clearest set of the lot. Goes slimy with dairy and breaks down on reheating, so use it for something eaten once.
What doesn't work: Doubling the cornstarch to make it set faster. Past about 2 tbsp per cup of liquid the gel turns chalky and mutes the fruit. Give it a full minute at a real boil instead.
Crisp coating on fried chicken, shrimp or tofu
No gluten, so it fries into a thin brittle shell rather than a bready crust, and it browns slowly enough that the coating stays crunchy after saucing.
Potato starchGluten-free
1:1
Crunchier and more shattery, and it browns even less. This is the Korean fried chicken texture.
Rice flourGluten-free
1:1
Sandier, slightly grainy bite. Holds up in a wet sauce longer than cornstarch does.
Tapioca starchGluten-free
1:1
Crisps hard out of the fryer then turns chewy within 10 minutes as it cools.
All-purpose flourRan out
1:1
Browns faster and tastes toastier, but the crust is thicker and goes soft a few minutes off the heat.
What doesn't work: Baking powder used as the coating itself. It belongs in a dredge at 1-2% to raise the pH and help browning. As the coating it makes a bitter, soapy skin with no crunch.
Velveting film on stir-fried meat
A thin starch coat on each slice holds moisture in and keeps the wok's direct heat off the protein, so the meat stays soft in a screaming-hot pan.
Potato starchGluten-free
1:1
Slicker film and a whiter finish. Slices separate slightly more easily in the pan.
Tapioca starchGluten-free
1:1
Tackier surface, so the sauce clings harder to each piece.
What doesn't work: All-purpose flour. Gluten grabs the hot pan and the slices weld into one doughy clump instead of separating, and the coating tastes raw.