How Microwave Crisping Sleeves Work

General Mills' 1978 patent on using a thin, metal-coated plastic wrap that converts microwave energy into intense surface heat to crisp and brown food like Hot Pockets.

What it covers

This patent describes a packaging material that browns and crisps the surface of microwaved food. It uses a flexible plastic sheet (the dielectric substrate, such as polyester) coated with an extremely thin layer of metal (like evaporated aluminum). When placed in a microwave, this metallic layer acts as a semiconductor with an electrical resistance between 1 and 300 ohms per square. This specific resistance causes the metal to absorb a portion of the microwave radiation and convert it directly into thermal heat, reaching temperatures high enough to sear or crisp the food's surface. Meanwhile, the remaining microwave energy passes straight through the film to cook the inside of the food. For example, this is the technology behind the grey, metallic-looking sleeves used to crisp microwave pies or pocket sandwiches.

What it doesn't cover

• —Does not cover rigid microwave browning dishes or trays that do not conform to the shape of the food.
• —Does not cover susceptor materials with a surface resistance outside the 1 to 300 ohms per square range.
• —Does not cover packaging that completely blocks microwave energy from reaching the inside of the food.
• —Does not cover non-flexible, non-wrapping heating elements that cannot be wrapped around or draped over a food item.

The clever bit

Instead of completely blocking microwaves with thick metal foil, this design uses an incredibly thin, evaporated metal layer. This thinness creates just enough electrical resistance to turn some of the microwave's electromagnetic fields into heat, while letting the rest of the waves pass through to cook the food's interior.

Why it matters

Before this invention, microwave ovens could only heat water molecules inside food, leaving crusts soggy and pale. This patent solved the 'soggy crust' problem, enabling the entire modern industry of microwaveable pocket sandwiches, pot pies, and pizzas. It allowed food companies to sell convenient, crispy-crust frozen meals that cook in minutes.

Real-world examples

• 1.Hot Pockets crisping sleeves
• 2.Microwave pot pie crisping trays
• 3.Microwave pizza crisping discs