Microwave safe, food grade, and what FDA authorizes

Last night’s curry is in a plastic clamshell from the restaurant. You set the lid on loosely, press two minutes, and eat it out of the container.

Something in that sequence was authorized by the federal government before it was allowed to touch food. Something else in it was never authorized for what you just did. Telling the two apart is easier than it sounds, and the first clue comes from a word search.

Title 21 of the Code of Federal Regulations holds every FDA food rule there is. Search its full text for the phrase “microwave safe” and you get zero results. Search for “dishwasher safe” and you get zero again. Search for “food grade” and you get 51 hits, every one of them describing the grade of some specific ingredient, like the food-grade wax on a waxed apple, rather than defining a claim a package is allowed to make.

That is not a scandal. It is a clue about where the real rules live, and they are more specific and more useful than the words on the lid.

The legal move at the center of all of this

One sentence of regulation sets up everything else. FDA’s definitions for food additives, at 21 CFR 170.3, fold into the legal meaning of the word “food” anything that moves out of a container and into what the container is holding.

So federal law does not treat your dinner and the box around it as two separate objects. Whatever leaves the box becomes part of the dinner. And anything that becomes part of food needs permission to be there.

That is what an indirect additive is. Parts 174 through 179 of FDA’s food regulations sit under the heading Indirect Food Additives, and the word “indirect” carries the meaning: nobody added the substance to the food on purpose. It got there because the food sat in something. Citric acid stirred into a drink is a direct additive. Whatever comes off the bottle while the drink sits in it is an indirect one.

A component of a food container can be lawful by five routes, which FDA’s page on determining the regulatory status of a food-contact material lays out. It can be named in a regulation in parts 174 through 179, in which case no further review is needed. It can be covered by an effective food contact notification. It can be exempt under the threshold of regulation at 21 CFR 170.39, the provision for substances expected to migrate at such low levels that FDA does not require a full review, which FDA’s page puts at 0.5 parts per billion in the diet. It can be generally recognized as safe, and FDA points out that not every substance with that status appears in its regulations. Or it can be prior sanctioned, meaning FDA or USDA wrote a letter before 1958 saying it had no objection to one specific use.

The notification route is where most new materials go now. Congress created it in the Food and Drug Administration Modernization Act of 1997 to replace the slower petition process, and the fine print is the instructive part. An effective notification is specific to three things at once: the company that filed it, the substance itself, and the conditions of use described in the filing. FDA treats notifications as proprietary to that company. A competitor using the identical chemical for the identical purpose has to file its own.

Hold on to that phrase, conditions of use. It is where the microwave question gets answered.

“Food grade” and “food safe” are assertions, not standards

Those 51 hits are worth a closer look, because they show what the phrase is for. In 21 CFR 101.4, the ingredient labeling rule, food-grade animal wax shows up as an optional way to describe a produce coating. In 21 CFR 101.83, food-grade fatty acids appear inside the specification for a plant sterol mixture. The phrase modifies a particular ingredient inside a particular rule. It is never defined, and it never appears as a claim a package may make.

“Food safe” does even less. Two hits in all of Title 21, both inside FDA’s general manufacturing and preventive-controls rules, neither of them a definition.

Sorting label vocabulary into categories makes this manageable. Some claims are defined in regulation. Some are run as government programs. Some are private third-party certifications. And some are undefined, which means the company saying it is making its own assertion and backing it with its own reputation. “Food grade” and “food safe” are in that last group. A supplier calling a resin food grade is usually telling you something true, that the resin has a food-contact authorization. The phrase itself has no definition, no test and no agency behind it.

Undefined does not mean unenforceable, though, and pottery shows the difference. FDA’s November 2010 guidance on imported traditional pottery says ceramicware that releases extractable lead under FDA’s test method should not be labeled lead free, and that if it is, FDA may treat the labeling as false and misleading and the article as misbranded. So an undefined phrase can still get a product seized when it is untrue. The enforcement hangs on the deception, not on a standard the words have to meet.

How anyone knows what migrates, and what the test stands for

Migration is driven by four things, and three of them are in your kitchen’s control: how hot the food is, how long it sits, how fatty it is, and how acidic it is. A 2025 systematic review in Reviews on Environmental Health pulled together 67 studies covering 5,378 food samples and found exactly that pattern for phthalates and bisphenols coming off packaging. Temperature, contact time and food composition drove the rates, with high-fat and acidic foods at the top. Those authors also found too few studies on PFAS migration to draw any conclusion, which is its own useful finding, and PFAS in general belongs to another article here.

FDA’s testing framework is built around the same variables, and seeing it once changes how you read a container. The agency maintains two reference tables. One sorts food into nine types by what it does chemically: nonacidic water-based, acidic water-based, water-based with free oil or fat, dairy, low-moisture fats and oils, beverages by alcohol content, moist baked goods with and without surface fat, and dry solids with and without surface fat. The other sorts use into ten conditions, running from high-temperature heat sterilization, for which FDA’s example is above 212 degrees Fahrenheit, down through hot filling above and below 150 degrees, room-temperature filling, refrigeration and freezing.

Two of those ten conditions matter here more than the rest. Condition 8 covers frozen or refrigerated ready-prepared food meant to be reheated in its own container at the time of eating. Condition 10 covers cooking above 250 degrees Fahrenheit. Everything else on the list is about holding food, not heating it.

The testing itself uses stand-in liquids rather than real food. FDA’s December 2007 chemistry guidance for food-contact submissions calls for 10 percent ethanol for water-based and acidic foods, 50 percent ethanol for high-alcohol products, and a food oil for fatty ones. A room-temperature application is tested at 40 degrees Celsius, about 104 Fahrenheit, for ten days, with the results extrapolated out to 30 days for glassy polymers like PET and polystyrene. A retort application gets two hours at about 121 degrees Celsius followed by 238 hours at 40.

Here is where a widespread belief needs correcting. People often say manufacturers test under deliberately exaggerated conditions. FDA’s guidance says close to the opposite about the test itself: match the most severe condition the material is meant for, and do not go beyond it, because an inflated exposure estimate triggers a larger toxicology package than the use requires. The conservatism lives somewhere else, in the exposure arithmetic. FDA generally assumes a new substance captures the entire market for its intended use, and converts a concentration into an estimated daily intake using a flat 3 kilograms of food per person per day. A dietary concentration of one part per million becomes 3 milligrams a day for everyone.

So the safety margin is built from pessimistic assumptions about how much of the stuff a population eats, not from cooking the test sample harder than anyone would at home. Which means the protection depends on the material being used the way it was tested.

“Microwave safe” is a claim about one condition of use

Now the phrase that sends people to the internet at dinnertime. No federal regulation defines it. FDA’s own microwave oven page does not define it either. What the page does say is that glass, paper, ceramic and plastic are used in microwave cooking because microwaves pass through them, that containers can still get hot from the food inside, and this: “Some plastic containers should not be used in a microwave oven because they can be melted by the heat of the food inside.” For which containers to use, the page sends you to the oven’s instruction manual.

So what is a manufacturer telling you? Roughly three things. That the material has a food-contact authorization that covers reheating in the container, FDA’s condition 8. That the piece will hold its shape and not melt or warp at microwave temperatures. And that the company has done its own testing and will stand behind the claim.

What it is not telling you is longer. It does not certify that nothing migrates, only that what migrates was evaluated for a particular use. It does not cover any power level and any duration. It does not distinguish a bowl of oatmeal from a bowl of oily leftovers, even though fat is one of the strongest drivers of migration. It says nothing about the hundredth cycle, or about a surface that has been scratched by a fork and a dishwasher. And it does not transfer to a different container that merely looks similar.

Four situations, settled on the actual rules

1. Reheating in a takeaway container. The clamshell was probably authorized for hot filling or for refrigerated holding, FDA’s conditions 3 through 6. It was built to survive hot food going in and a night in the fridge. Reheating in it is condition 8, a different authorization the maker may never have sought. Tip the food onto a plate. It takes four seconds.

2. Reusing a yogurt tub. The Agriculture Department’s Food Safety and Inspection Service puts margarine tubs, cottage cheese containers and yogurt cartons in a category it calls cold storage, and says of them: “These materials are not approved for cooking and chemicals can migrate into food.” Those thin-walled tubs are fine for leftovers in the refrigerator and wrong for reheating. The same page rules out foam trays, foam cups, brown paper bags, and takeout cartons with metal handles.

3. Cling film touching hot food. The same agency’s guidance is specific and almost nobody follows it: leave enough space between the food and the top of the dish that the wrap does not touch the food, and vent it so steam escapes. Film pressed against hot, oily food combines heat, fat and direct contact, the three conditions that push migration up fastest.

4. A plastic container in the dishwasher’s heated dry cycle. “Dishwasher safe” appears nowhere in federal food regulation. It is a durability claim: the piece will come out the shape it went in. Set it against FDA’s own benchmark. The standard migration test for a room-temperature application runs at 104 degrees Fahrenheit. A heated dry cycle runs hotter than that, with whatever oil did not rinse off still on the surface. A top-rack position, away from the element, is the cheap fix.

Glass, steel, ceramic, silicone and the composite bowls

Plastic gets the attention, and some of the sharper findings are elsewhere.

Glass and stainless steel are the boring answer and a good one. In a 2024 study in Toxics that tested 46 bowls and cups bought in China against glass and ceramic controls, the glass and ceramic pieces released the least of what was measured, by a wide margin. Neither material is magic. Glass breaks, and steel is a poor match for long storage of very acidic food. For heat plus fat plus time, they are the materials with the least to give up.

Ceramic glazes and lead are the real hazard in this group, and the hazard is concentrated in handmade and imported pieces fired at uncontrolled temperatures, where lead in the glaze never gets fully locked into the glass. FDA’s 2010 guidance names imported traditional pottery, including Mexican terra cotta from specific manufacturers, some of it labeled lead free while releasing extractable lead. Acidity is the trigger. A 2025 study in the Journal of Public Health Management and Practice tested glazed pottery sold in Reynosa, Mexico, and found acidic dishes cooked in it carried a median 103.4 milligrams of lead per kilogram against 11.19 for nonacidic dishes, about nine times more. Seven of the eight items tested against Mexico’s own leaching standard failed it.

There is a labeling rule here that almost nobody knows about. Under 21 CFR 109.16, ornamental ceramicware sold since July 1994 that looks like it could hold food is legally presumed to be for food use unless it carries two things: a conspicuous stick-on label with letters at least an eighth of an inch tall warning that it is not for food, and the same warning molded, fired or permanently painted onto the exterior base. A hole bored through the food-contact surface also works, which tells you how seriously the rule takes the problem. FDA has found pieces where the warning appeared only on the packaging, which goes straight in the trash.

Silicone bakeware and lids are regulated as rubber articles intended for repeated use, under 21 CFR 177.2600, and the rule is more demanding than people assume. Silicone elastomers are named in it, and a finished article has to pass extraction limits in both distilled water and n-hexane at reflux: for water, no more than 20 milligrams per square inch of food-contact surface in the first seven hours and 1 milligram in the next two. The rule also says to wash the article thoroughly before its first use with food, which is a step worth taking literally.

Bamboo and melamine composite dishware is the one to look at twice. Melamine-formaldehyde resin is authorized at 21 CFR 177.1460 as the food-contact surface of molded articles used to handle, package, transport or hold food, mixed with refined woodpulp. Bamboo fiber is not the filler the regulation names, and the authorization is about holding food, not heating it. The 2024 Toxics study found bamboo-filled pieces released more melamine and its derivatives than plain melamine pieces, which in turn released more than wheat straw, which released more than glass. That study used European test conditions and a worst-case scenario, and it looked at products on the Chinese market, so treat the numbers as a signal rather than a measurement of your bowl. The signal points one way, and these bowls are sold as eco-friendly, which is an undefined word.

Can linings are a separate question with its own article here, since bisphenols are the issue and the regulatory story around them is long.

When a container should go, and when a person should make a call

A container is telling you something when it warps, clouds, goes sticky or chalky, develops a fine crackle in the glaze, holds a smell after washing, or shows a scratched haze where a fork has worked on it. None of those is a measurement, and all of them mean the surface has changed from the one that was tested.

Two situations call for a person rather than a decision in the kitchen. If handmade or imported glazed pottery has been in daily use for cooking or for acidic foods, especially with a young child or anyone pregnant in the house, ask a clinician about a blood lead test rather than guessing from the glaze. Lead exposure in children is managed by measurement, not inference. And if a container melted or burned and someone ate the food, or a child chewed a piece off, call Poison Help at 1-800-222-1222, which runs 24 hours a day. For a medical emergency, 911.

The sentence worth remembering

Dishwasher safe tells you the container will survive a dishwasher. Microwave safe tells you a manufacturer believes its material is authorized for reheating and will not melt. Food grade tells you nothing defined at all.

The rules behind all three are written around conditions of use: a temperature, a duration, a food type. So change the question you ask of a container in your hand. Is this safe has no answer. Is this the use it was built and tested for usually does, and the container itself, its label and the food you are about to put in it will tell you.

Keep exploring: plastic resin codes, household chemical safety and microplastics.

Sources

1. U.S. Food and Drug Administration. “Regulatory Background: Food Contact Substance Notification Program.” fda.gov
2. U.S. Food and Drug Administration. “Determining the Regulatory Status of Components of a Food Contact Material.” fda.gov
3. U.S. Food and Drug Administration. “Food Types and Conditions of Use for Food Contact Substances.” fda.gov
4. U.S. Food and Drug Administration. “Guidance for Industry: Preparation of Premarket Submissions for Food Contact Substances (Chemistry Recommendations), December 2007.” fda.gov
5. U.S. Food and Drug Administration. “Microwave Ovens.” fda.gov
6. U.S. Food and Drug Administration. “Guidance for Industry: Safety of Imported Traditional Pottery Intended for Use with Food and the Use of the Term Lead Free, November 2010.” fda.gov
7. U.S. Food and Drug Administration. “Lead in Food and Foodwares.” fda.gov
8. U.S. Food and Drug Administration. “Packaging and Food Contact Substances (FCS).” fda.gov
9. USDA Food Safety and Inspection Service. “Cooking with Microwave Ovens.” fsis.usda.gov
10. Electronic Code of Federal Regulations. “21 CFR 170.3, Definitions.” ecfr.gov
11. Electronic Code of Federal Regulations. “21 CFR Part 174, Indirect Food Additives: General.” ecfr.gov
12. Electronic Code of Federal Regulations. “21 CFR 177.2600, Rubber articles intended for repeated use.” ecfr.gov
13. Electronic Code of Federal Regulations. “21 CFR 177.1460, Melamine-formaldehyde resins in molded articles.” ecfr.gov
14. Electronic Code of Federal Regulations. “21 CFR 109.16, Ornamental and decorative ceramicware.” ecfr.gov
15. Electronic Code of Federal Regulations. “Full-text search of Title 21 for the phrases food grade, food safe, microwave safe and dishwasher safe.” ecfr.gov
16. PubMed. “Tanzer M, Boissiere-O'Neill T, Sly PD, Vilcins D. "Phthalates, bisphenols and per- and polyfluoroalkyl substances migration from food packaging into food: a systematic review." Reviews on Environmental Health, 2025. DOI: 10.1515/reveh-2025-0027.” doi.org
17. PubMed. “Liu S, Wang Y, Liu Z, et al. "Migration of Melamine and Its Derivatives from Melamine/Bamboo/Wheat Straw-Made Tableware Purchased from Internet Markets or Retail Shops in China." Toxics, 2024. DOI: 10.3390/toxics12020143.” doi.org
18. PubMed. “Castellanos-Carrizal C, Castillo-Ruiz O, Ramirez-Quintanilla LY, et al. "Enhanced Leaching of Soluble Lead by Cooking Acidic Food in Glazed Pottery Sold at the Mexico-US Border." Journal of Public Health Management and Practice, 2025. DOI: 10.1097/PHH.0000000000002305.” doi.org