Baking

Cornstarch Conversion

Cornstarch weighs 120g per US cup.

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Cornstarch's 120g per cup happens to match all-purpose flour's density almost exactly, which is a coincidence worth knowing about since the two are sometimes discussed as if they were interchangeable thickeners — they're not, despite the similar cup weight, because cornstarch's thickening power per tablespoon is roughly double flour's.

In practice, almost no recipe calls for a full cup of cornstarch — it's typically used in tablespoon quantities as a thickener for sauces, pie fillings, and gravies, where a little goes a long way. The cup figure exists mainly so a recipe that does specify a larger quantity (a batch of gluten-free baking blend, for instance) converts accurately.

Cornstarch thickens through a different mechanism than flour: its starch granules swell and gel when heated in liquid, producing a glossy, translucent finish, versus flour's more matte, opaque result — which is why a pie filling thickened with cornstarch looks noticeably different from one thickened with flour even at a chemically comparable thickening power.

Because cornstarch is used almost exclusively in small tablespoon-or-less quantities as a thickener rather than a bulk ingredient, its 120g cup figure functions mostly as a reference point for scaling a larger batch (a gluten-free flour blend, for instance) rather than a quantity most home cooks will ever measure directly.

Cornstarch loses its thickening power if boiled too vigorously or for too long after it's been added, which is why most recipes call for bringing a cornstarch-thickened sauce to a gentle simmer rather than a rolling boil once the slurry goes in.

Mixing it with cold water first, before adding to a hot liquid, prevents the clumping that happens if the dry powder hits heat directly.

It's also used to keep shredded cheese from clumping in the bag, a food-industry use unrelated to its role as a sauce thickener.