Microwave Popcorn Doesn't Work Without the Bag, and the Bag Is the Actual Invention

US4691374 and its relatives cover the microwave popcorn bag, and specifically the part of it that does the work: a susceptor, a small panel of metallized polyester film laminated into the bottom of an ordinary paper bag.

Popcorn has an unusually long association with microwaves. When Percy Spencer discovered microwave heating at Raytheon in 1945, popcorn was among the first things he deliberately tried, and it is depicted in his original patent. Popcorn is nearly the perfect microwave food: each kernel is a sealed pressure vessel with its own water supply, and it announces its own doneness audibly.

For about forty years, the results were still bad.

Why plain bags fail

A microwave oven does not heat evenly. It produces a standing-wave pattern with hot spots and cold spots, which is why turntables exist. Kernels sitting in a cold spot never reach the roughly 180 psi of internal steam pressure needed to burst.

You end up with the familiar bad outcome: a layer of scorched popped corn where the hot spots were, and a substantial bed of unpopped kernels everywhere else. Adding time burns the popped corn before it rescues the rest.

Oil helps couple energy into the kernels, but oil alone does not fix the spatial problem.

What the susceptor does

The susceptor is a very thin layer of aluminum — thin enough to be partly transparent to microwaves rather than reflecting them like foil.

Microwaves passing through it induce currents in the metal, and because the film is so thin and resistive, that energy dissipates as heat in the film itself. The panel becomes a hot surface, quickly reaching temperatures well above what the food alone would achieve.

So the bag heats the kernels two ways at once: microwave energy heats the water inside each kernel directly, and the susceptor conducts heat into the kernels sitting on it, evening out the cold spots and giving the bottom layer the extra push it needs.

The rest of the bag is engineered around that. The paper is folded so it expands as steam builds, vents enough to avoid bursting, and holds a pool of solid fat and salt that melts and distributes. The whole assembly is a single-use cooking appliance that costs a few cents.

What it enabled

General Mills introduced shelf-stable microwave popcorn in the early 1980s, and it became one of the fastest-adopted food products of the decade. It is frequently credited with driving microwave-oven sales as much as the reverse — a genuine reason for households without one to buy one.

Which makes the paper bag a rare object: a piece of disposable packaging that is simultaneously the cooking vessel, the heating element, the serving bowl, and the reason people bought the appliance.

See the original

The full text and figures of US4691374 are on patents.us.