The manual's guidance is correct. In general, it is not recommended to heat food or liquids in a completely sealed container with airtight lids in a microwave oven. This is because the buildup of pressure can cause the container to burst or explode, which can result in a dangerous situation.
However, there are certain cases when it is appropriate and even advisable to use lids or covers in the microwave:
Ventilation lids: Some microwave-safe containers come with special lids that have venting mechanisms. These lids allow steam to escape while keeping the food covered. They are designed to prevent splatters and messes inside the microwave while allowing controlled ventilation.
Microwave-safe covers: Microwave-safe covers, often made of microwave-safe plastic or glass, can be used to cover food or liquids loosely. These covers prevent splattering and help to retain moisture during cooking or reheating, without completely sealing the container.
Microwave-safe wraps: Microwave-safe plastic wraps or microwave-safe wax paper can be used to cover dishes or bowls to prevent splattering while still allowing steam to escape. These wraps should be placed loosely over the food or container, never sealed tightly.
Microwavable steam bags: Some commercially available steam bags designed for microwave use have built-in vents or perforations. These bags are safe to use, as they are specifically designed for microwave cooking and allow steam to escape.
In summary, always follow the manufacturer's instructions and guidelines for using lids or covers in a microwave. Avoid completely sealed containers, as they can create pressure build-up and pose safety risks. Instead, opt for microwave-safe lids, covers, or wraps that allow steam to escape and prevent splattering while heating or cooking food in the microwave.