In most home kitchens, we'll find an assortment of different materials used in our containers ranging from glass to plastic to ceramic to metals. But are all food grade materials similar? Should we care if a container is marked Microwave Safe? Let's take a closer look at some common materials used in food containers and if they are microwave safe.
Glass containers are often marked microwave safe. These containers can be heated in a microwave without a problem. The issue with glass that is not microwave safe is that micro-air bubbles may be present in the glass and as the glass heats in the microwave oven, the bubbles may expand to the point where the glass breaks or shatters. (Obviously, you shouldn't eat food where it's glass container has broken.) Pyrex glassware is an excellent example of microwave safe, heat resistant glass that can also be baked. Even Pyrex glass cannot withstand the intensity of direct heat, such as a range or a broiler, for long, so don't use glassware with such heating methods. Also, all glass is susceptible to thermal cooling shock (rapid cooling, for example, dunking in cold water while hot) and may crack.
One popular method of testing if a particular glass is microwave safe is to microwave the container while it is empty for one minute. If the container is hot, then it is not microwave safe. If the container is warm, it should be fine for heating food. If the container remains cool, you can cook in the microwave with that container. Personally, I choose to either check to see if it's marked as microwave safe or ask the manufacturer before using in a microwave.
Any glass container with a metallic trim should never be microwaved. The electrical currents induced by the microwave radiation in the metals can cause sparking and pinpoint heating of the glass. Sometimes this can result in marring or even breaking of the glassware. Also, make sure any glassware with a colorful coating, finish, or stain should be marked for use with food or microwave safe before attempting to use in food preparation. The dyes, pigments, or stains may not be food grade. Almost always, decorative plates are not for use with food.
Food safe ceramic uses glazes that are made from harmless materials like silica, dolomite, kaolin, feldspar, ball clay, and others. In these glazes, the inevitable leaching that occurs is only a functional and aesthetic issue and has no health impact. Glazes that contain metals such as lithium, lead, or barium may present a health issue. Ceramic containers made with such glazes cannot be sold in the United States without either a permanent marking stating it is "Not for Food Use - May Poison Food" or have a hole in the container (presumably rendering it useless for food preparation). If you make your own ceramics, make sure you use a food grade glaze if you plan to use it in your kitchen.
Food grade plastics are made from a specific list of plastics approved by the FDA (which may include dyes and recycled plastic that have not been determined to be harmful to humans). Once a plastic container has been used to store non-food items (like detergent or paint), it can no longer be considered food grade. Plastics containers that are not food grade may leach plasticizers into food on contact. Due to the nature of plastics, they have a high affinity for fats. Plastics that come in contact with an oil-based substance will almost always be irrevocably altered and the plastic may never become truly clean once again. Contact to foods that are high in fat may cause leaching of the original oil-based substance into the food even if the plastic was originally food grade.
Microwave safe plastics are food grade plastics (which do not leach plasticizers) that are known to be able to withstand higher than normal temperatures. Plastics that are not microwave safe may leach harmful substances when heated in a microwave oven. (There was an internet e-mail scare/hoax that was passed around claiming the USDA or FDA and independent researchers showed that dioxin (a plasticizer) leached out of plastic wrap onto food being microwaved. This is untrue since all microwave safe plastics are dioxin free. Saran and Ziploc both maintain that their product lines are completely plasticizer free. The temperatures necessary to create dioxin (around 1500°F) are beyond the normal operating conditions of household and commercial microwave ovens.)
Do not microwave food in plastic containers or covered with plastic wrap that is not microwave safe.
Also, don't brine (or store) foods in containers that are not intended for food preparation - such as a "clean/brand new" mop bucket, plastic trash bag, or trash can.
Lexan is a food grade polycarbonate plastic that has gained a large following in the food service community. It is hard, durable, and resistant to reacting with oils resulting in a virtually stain and odor proof material. It is capable of handling a range of temperatures from below freezing to boiling. However, it is advisable to recycle and purchase new polycarbonate containers when they crack, chip or cloud up.
Recently, Sierra magazine published a report claiming that polycarbonate plastics leach an endocrine disruptor called Bisphenol-A (BPA). Unfortunately, the studies the article was based on cannot be directly related to use in the food industry since the tests were performed on non-food grade polycarbonate mouse cages (which affected the growth cycle of the mice). No evidence of food grade polycarbonates (such as Lexan) being a health hazard has been uncovered. Further studies are ongoing to determine if a potential for such a hazard even exists.
The S.C. Johnson Company says that the larger Ziploc brand bags are microwave safe. All Ziploc bags are made of microwave safe materials, but bags smaller than 1 quart size may be too thin to withstand the temperature of the food being microwaved. Be sure to vent Ziploc bags to allow steam to escape during microwaving. Please note that not all resealable plastic bags are microwave safe, you'll want to check with the manufacturer.}?>
Additional information can be found at:
Food Grade Containers for Brining