Not all plastic containers or bags are created equally, with some containing dangerous chemicals including bisphenol A, phthalates and dioxin, which could migrate into foods when heated in the microwave. Steam-in bags don't contain these chemicals, making them safe to use in the microwave for the short periods of time as directed by the manufacturer, according to the Fruits and Veggies -- More Matters website. Most plastic steam-in bags are designed for no more than 15 minutes of cooking time in the microwave and for a single use. Using the bags incorrectly, such as for longer times than recommended, may negate their safety.

Cooking vegetables in the microwave is one of the most beneficial way to prepare them because it helps to retain the majority of the vitamins and minerals they contain, advises the Harvard University "HealthBeat" magazine. Boiling your veggies causes them to leach away their nutrients into the water when they are cooked on the stove. Unlike boiling, microwavable steam-in bags need little to no water added to them to cook the veggies inside, which helps them retain their nutrients. You can also add any sauces or seasonings to the veggies that you desire directly in the bag after cooking, but not during the cooking process. These bags aren't designed to cook veggies in sauces, especially high-fat or sugary ones.

Prior to cooking your vegetables, wash them thoroughly with running water and remove any damaged, bruised or rotten areas, recommends the U.S. Food and Drug Administration. Before microwaving your vegetables, store them directly in the steam-in bag in the refrigerator for up to two days or two weeks in the freezer. You can cook frozen vegetables safely in a steam-in bag without defrosting them beforehand. After cooking, discard the bag and store leftovers in a separate container. Cook vegetables separately from raw meats to prevent cross-contamination from pathogens in the same bag. For even cooking, chop your veggies into small, equal size pieces.

Foods cooked in steam-in bags are very hot. Use oven mitts when handling the bag and allow it to sit for a minute before opening it slowly to let the hot steam out. Rotate the bag halfway through the cook time and shake the contents to ensure even steaming. Only use the bags in the microwave, not the oven, to cook your vegetables. Some bags are meant to lay flat to cook your veggies, while others are designed to stand up; follow the manufacturer's instructions for the particular bag you are using. Cook your veggies in the steam-in bag to 135 degrees Fahrenheit, recommends the Iowa.gov website.