Covering foods in your microwave serves much the same function that it does on the stovetop: It holds in moisture and heat and lets foods cook more quickly. Also, trapped steam aids in tenderizing foods, and the cover prevents splatters. (When microwaved foods are uncovered, moisture will evaporate and liquids thicken -- but not as much as in conventional cooking.)
Here's a brief primer on some different ways to cover microwaved dishes:
*Plastic wrap: This provides a good seal for trapping steam and creates the ideal environment for cooking vegetables and fish. When using plastic wrap, make a small slit or turn back one corner to vent steam and prevent wrap from splitting.
When a recipe asks you to "cover tightly," use either a matching casserole lid or plastic wrap (you can also use a microwave-safe dinner plate that fits the casserole). Plastic wrap gives the tightest seal, and polyvinyl chloride plastic wrap will expand like a bubble without splitting, so it does not need to be vented or slit. Inside the bubble, trapped steam cooks foods quickly and evenly.
Foods high in fat and sugar can become very hot in your microwave. Such foods should not come in contact with plastic wrap or they might melt it. When covering container, leave at least 1 inch between plastic wrap and surface of food.
Nylon cooking bags are another plastic product perfect for microwaving. Meats can be marinated right in the bag and then transferred to your microwave for cooking.
If bag has a foil strip on the end, remove and discard it. If necessary, cut a 1/2 -inch wide strip from mouth of bag to use as a tie, or use string (never use a metal twist tie). Tie open end loosely or cut a small X in top to vent steam while microwaving.
Be sure to use cooking bags specifically manufactured for microwave cooking; other plastic bags can melt at high temperatures.
*Waxed paper: Helps to hold in heat and prevent splatters, without steaming food (it retains some steam, but not as much as plastic wrap). Waxed paper helps to equalize cooking of foods of uneven shape, like chicken parts -- but it does not tenderize, as does steaming under plastic wrap. To use, wrap waxed paper loosely over dish. Circles of waxed paper can also be cut to fit the bottoms of pans when you bake cakes, for easy removal.
*Paper towels: These don't hold in moisture, like plastic wrap and waxed paper; instead, they absorb it. They prevent splatters and soak up fat, and for this reason they're excellent for cooking bacon. Place bacon between several layers of paper towels to microwave, and it emerges more crisp because towels soak up moisture and fat. You can also dry herbs between layers of paper towels, but check frequently -- overheated portions could cause paper towels to flame.
Paper towels are ideal for reheating foods like baked goods and sandwiches, since towels will absorb excess moisture and prevent bottoms from going soggy. A wet paper towel, with most of the moisture squeezed out, can be placed over a dish during microwaving to help steam tender fish fillets or scallops. A lightly damp paper towel wrapped around corn tortillas can also help to soften them during microwaving.
While I always do my best to buy recycled paper products, this is one case where you want new: Towels made of recycled paper can have small flecks of metal in them. Also avoid towels containing nylon, which could overheat and ignite paper. Plain white (no pattern) towels are the best choice for microwaving.
*Foil: Some recipes will ask you to shield portions of food with foil (it's called shielding because foil shields food from microwaves -- they can't pass through metal). You may need to shield the fin and head ends of a whole fish, wing and drumstick tips of poultry, or the smaller end of a roast. Take care to keep foil smooth, to reduce risk of arcing. Recipes usually call for foil to be removed at some point during cooking to let the shielded portion "catch up."
Check your manufacturer's instructions first, to make sure that small pieces of foil may be used in your oven. Most microwave ovens manufactured since 1980 come with specially protected magnetron tubes that will not be harmed by the use of small amounts of foil.