The microwave oven, the electrical appliance that has brought life to leftovers, is warming America's hearts in a new way. It isn't just for heating last night's spaghetti anymore.

New microwave specialty products - many focusing on potatoes - are keeping pace with a fast-moving society.Consumers now are able to buy frozen items marketed specifically for microwaves - hamburgers, sandwiches, casseroles, French fries, mashed potatoes - even milk shakes.

And with an estimated 70 percent of American households owning a microwave, the items are catching on fast.

Simplot, Ore-Ida in Boise and Idaho Fresh-Pak in Lewisville are nationwide leaders in microwave food products.

Golden Valley Microwave, which jointly owns Lamb-Weston with ConAgra Inc., is among businesses trying to master the microwave market. Lamb-Weston gained national recognition by supplying fast-food restaurants with French fries.

Ore-Ida and Simplot did likewise, using Idaho spuds to gain contracts with companies such as McDonald's and Burger King. But in about 1985, after noticing the popularity of microwave frozen products, they expanded their horizons. In came a line of frozen French fries that could be heated in a microwave and eaten in minutes.

About 3 percent of Ore-Ida's frozen potatoes are now used for microwave products. Simplot has gone one step further, introducing a variety of products - including milk shakes - under the MicroMagic brand name.

Idaho Fresh-Pak's Idahoan brand of instant mashed potatoes, introduced just this year, is reportedly the only such microwave product on the market in the U.S.