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Microwave Safe Containers

by Michael Chu
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In the United States, food grade containers are containers manufactured with materials that the Food & Drug Administration (FDA) has approved as safe for use in food preparation and storage. Food grade materials are generally known to not leach harmful substances or react with food in a harmful manner.

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 & Ceramics
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.

Plastics
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:
foodsafety.gov
Food Grade Containers for Brining
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Written by Michael Chu
Published on May 13, 2005 at 10:42 AM
47 comments on Microwave Safe Containers:(Post a comment)

On May 14, 2005 at 03:03 PM, Tz'Akh (guest) said...
Subject: an interesting story
I work for a company that is a plastics compounder - which means they getplastic directly from the nuclear power plants that cook it out of petroleum, mix it into different forms, and then ship it out to other companies that make products. The chemists that design the melts (plastic recipe runs) refuse to use 'microwave safe' or any other plastic containers in a food settling where the container gets hot.

The plastics that are labeled 'safe' for such conditions are the same plastics that would normally be unsafe, mixed with a strengthening additive. When, over the course of time and exposure to heat & other radiation, the plastic itself does not break down rapidly, but instead the additive 'takes the hit' and comes out of the container (with small quantities of plsatic).

The problem is that the strengthening additives are highly toxic - with various kinds having known connection with organ failures and nervous system problems. The safety tests done on the plastic compounds I've been able to research don't have any awareness of whether the small quantities of an additive that comes off from the plsatic container are stored in our body or pass through it. No sense of whether there is a internal system threshold for our body, even though it's known that people can reach thresholds through job exposure to some of them. ('Threshold' meaning once you're exposed to a certain amount of it you experience dramatic physiological reactions (vomiting, loss of consciousness, etc) when exposed further.

To wrap up the rest of this disclosure; I am highly sensititive to many chemicals - what is safe and/or unnoticable to other people can make me sick. For the month or two that I worked in the plastics plant (before I began telecommuting) there would be days that I would become nauseous, couldn't stay awake, sharp headaches, etc. My arguement is that the notion of a "food grade" plastic is probably best taken with a small footnote of caution (probably provided by me via this post ;)

-Zak


On May 14, 2005 at 05:05 PM, Jason (guest) said...
I thought nuclear power plants don't use petroleum - radioactive uranium or plutonium, steam, turbines, sure, but petroleum? Why did you even start witht hat "tidbit" of (mis?)information?

It makes me suspect your whole story...


On May 14, 2005 at 06:55 PM, lena (guest) said...
Interesting article, this is exactly a topic I am interested in at the moment.
Quote:
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.


I have a large (25 kg) bag of whole wheat, that I mill at home. Any suggestions on how to store this? Food grade containers that I can find are way too expensive. Large ones do not even seem to exist in my country, and multiple small ones would cost hundreds of dollars. I now keep them in the original paper bag, and cover that with a plastic trash bag. So the grains do not actually touch the plastic. I wonder if the mill where I bought the grains stored them in food grade containers, by the way.

Any suggestions would be much appreciated. The solution I now have is not ideal because mice will still eat through the trash bag, of course.


On May 16, 2005 at 03:59 PM, shpoffo (guest) said...
Subject: OT: to note on "(mis?)information"
Quote:
petroleum..... It makes me suspect your whole story...


Nuclear power plants have a "waste heat" cycle, where the system is producing heat that they can't use for turbines, etc. Rather than let that heat be wasted (dissipated into the atmosphere) they use it to heat giant vats of petroluem. The heat causes the petroleum to separate into layers of material; gasoline, ployethene, other plastics, sludge, etc.

I never said the nuclear reactors use petroluem for their fuel source. I shared it because it is a fun tidbit that reveals part of the way our system works. Thanks for being polite enough to make it a question, though...


Zak


On May 16, 2005 at 04:03 PM, shpoffo (guest) said...
Subject: large food grade containers
lena, had you considered finding a wooden barrel? You could inquire into a local winery as to their source of barrels, as you can then be sure the barrel is not being treated with any dangerous chemicals (and barrel-makers are hard to find outside of the wine industry these days)

Otherwise look for a large food-safe plastic bag to use as a liner in and otherwise inappropriate container.



Zak


On May 16, 2005 at 04:47 PM, Johneegeek said...
lena wrote:
Interesting article, this is exactly a topic I am interested in at the moment.
Quote:
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.


I have a large (25 kg) bag of whole wheat, that I mill at home. Any suggestions on how to store this? Food grade containers that I can find are way too expensive. Large ones do not even seem to exist in my country, and multiple small ones would cost hundreds of dollars. I now keep them in the original paper bag, and cover that with a plastic trash bag. So the grains do not actually touch the plastic. I wonder if the mill where I bought the grains stored them in food grade containers, by the way.

Any suggestions would be much appreciated. The solution I now have is not ideal because mice will still eat through the trash bag, of course.

I think I remember that in Alton Brown's book: I'm Just Here For More Food (http://snipurl.com/exjv)
He uses a metal trash can (The fancy ones you put your foot on to open the top) to store flour. Of course this goes against the advice that you use food grade containers. :?


On May 19, 2005 at 04:12 PM, an anonymous reader said...
lena wrote:
Interesting article, this is exactly a topic I am interested in at the moment.
Quote:
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.


I have a large (25 kg) bag of whole wheat, that I mill at home. Any suggestions on how to store this? Food grade containers that I can find are way too expensive. Large ones do not even seem to exist in my country, and multiple small ones would cost hundreds of dollars. I now keep them in the original paper bag, and cover that with a plastic trash bag. So the grains do not actually touch the plastic. I wonder if the mill where I bought the grains stored them in food grade containers, by the way.

Any suggestions would be much appreciated. The solution I now have is not ideal because mice will still eat through the trash bag, of course.


You might try going to the pet store and looking at the containers they sell to store dog/cat food. They generally hold about 40lb of kibble and cost around $35-50. :)


On June 21, 2005 at 04:09 AM, Claudia (guest) said...
Subject: Large food storage container
Lena,
The Baker's Catalogue has a 22 quart container. Here is the page:
http://shop.bakerscatalogue.com/items/item6069.html


On August 26, 2005 at 10:54 PM, Julie (guest) said...
Subject: Storage container for milled wheat
What about using a galvanized steel can? I might be way off here, but it may be possible to store the grain in the bag inside of the steel can. Just a suggestion....good luck!


On September 21, 2005 at 08:45 PM, drsimpson (guest) said...
Subject: Large food grade containers
I stumbled on your fine forum searching for inexpensive containers for long term water storage (disaster preparedness). Since I like to cook I like the concept of Cooking for Engineers.

I have been using galvanized steel cans for storing bird seed and dog food on the back porch. I do use the food every day. Am I killing the wildlife and my dog?

Thanks for any feedback... :unsure:
Dianne


On July 26, 2006 at 01:03 AM, an0n (guest) said...
Subject: RE: Large food grade containers - water storage
You can find used food grade barrel suppliers at www.pharmacal.com/recond.htm


On August 01, 2006 at 03:43 PM, arete (guest) said...
Subject: largeish containers, metal bird seed, etc
One largish (20 qt) container that I use is a 5-gallon bucket. In particular, a 5 gallon bucket originally used to ship pickles to a fast food establishment, which they were happy to give me for free.

Considering the many plastic issues, even still I only use it for short-term cold temperature applications to reduce leaching. A wine barrel is an awesome idea. I have no idea how voluminous 25kg of wheat is.


Regarding your bird feed: I think your galvanized can is _probably_ fine, for several reasons:
1. Metals tend to be relatively nonporous and metals finishes (like galvanization) tend to be relatively robust and strong. I wouldn't necessarily sandpaper the finish and eat the dust, though.

2. You're not heating it, heating encourages everything (but especially plastics) to release all sorts of chemicals.

3. You're not soaking it. Filling something with a liquid vastly increases leeching from the container compared to storing solids, because it's easier to leech into the liquid and because the liquid will touch every last bit of the sides of the container. (A seed, for instance, will only touch a tiny piece of container but "block" a bunch more from being touched by other pieces; there are lots of tiny air pockets.)


On August 02, 2006 at 09:20 AM, guest (guest) said...
Subject: microwave safe ????
First all plastics regardless of name brand or what have you give of chemical products that they were made from as like all chemical formulations they break down over time. What disturbs me is the lengths that the industry goes to silence any leak of facts that show microwaves are not healthy. just look at what happened to the two swiss researchers who were prosecuted for exposing the dangers of microwaved food. it was claimed they were interfering with commercial trade. fortunately the european court lifted the prosecution on appeal.
From my point of view after indepth research is never to eat anything out of a microwave oven. Im able to readily do this as im in control of my free will to decide what i eat and how its cooked, its that simple. Remember you are what you eat (what you do is what you get back, second natural law).


On September 12, 2006 at 12:27 AM, Larry (guest) said...
Subject: Microwave safe
Letís get a grip. Microwaving food is a good thing. Microwaves are non-ionizing, and do not leave any residual radiation of any sort. Itís good because it kills the micro organisms in the food that you are cooking or defrosting. You know the ones that can make you sick (like salmonella). Donít use plastic, use Pyrex.

Larry


On September 23, 2006 at 04:23 AM, Amady (guest) said...
Subject: Re: Microwave safe
Is there any symbol used to understamp the container to show it is microwave safe?

Larry wrote:
Letís get a grip. Microwaving food is a good thing. Microwaves are non-ionizing, and do not leave any residual radiation of any sort. Itís good because it kills the micro organisms in the food that you are cooking or defrosting. You know the ones that can make you sick (like salmonella). Donít use plastic, use Pyrex.

Larry


On December 19, 2006 at 03:44 PM, Khathi (guest) said...
Quote:
Considering the many plastic issues, even still I only use it for short-term cold temperature applications to reduce leaching. A wine barrel is an awesome idea. I have no idea how voluminous 25kg of wheat is.

Wine barrels are also often burned or smoked inside, to impart special flavours to wines or spirits, so barrel should be new and non-treated. And better to soak it with clean water, as barrels tend to be not airtight when newly done. Most woods also leach into the content if it's liquid. That's why oak barrels are so sought after in brandymaking.

About 25 kg -- just remember one simple estimate: 1 kilo ~ 2 pounds. So 25 kg of wheat is about 50 or 55 pounds, a size of rather largish paper bag.

Quote:
Regarding your bird feed: I think your galvanized can is _probably_ fine, for several reasons:

If the container in question is steel, that I suspect, than it itself is rather safe, especially if laquered/enameled. But galvanization leaver the question of what coating is use. Often is is galvanized with some nonferrous metal that might be unhealthy itself, such as copper. Tin and zinc is usually best solution, but I still prefer enameled steel, as most enamels are in fact rather dense and nonporous glass compounds, are really durable an are generally food-grade from the start -- I never met non-food-grade enameled containers, in fact.


On July 09, 2007 at 07:13 PM, Katy (guest) said...
Subject: Microwave safe
Larry, Microwaves can be used to kill microorganisms but the food would have to be kept at boiling temperature for at least ten minutes to do so. Any food that had been microwaved for ten minutes after it had reached 210 F would be pretty uneddible.


On July 09, 2007 at 11:54 PM, GaryProtein said...
When you cook food, aside from making the flavors, textures and consistency more palatable, you kill microbes that would be found in normally healthy meat, poultry, fish and vegetables. We don't knowlingly eat contaminated or tainted food. The purpose of cooking is not to sterilize food. Sterilzed food (with the exception of soups that simmer a long time) would probably be overcooked, burned, dried out or just generally not very appetizing.


On August 04, 2007 at 10:16 PM, marge (guest) said...
Subject: lexan countertop ovens
I wonder just how safe are the polycarbonate countertop ovens, wondering if they leach toxins while heating and cooking food even tho' no contact is made with the food. Does anyone have an answer for me? Thanks


On August 06, 2007 at 07:14 PM, blagos@tycoint.com (guest) said...
Subject: Microwave Safe
I often see this term "Microwave Safe" used on containers, plates, etc. How is this determined? Is there some sort of standard test that products must follow in order to be labeled as "Safe"? Or is there no real maning to this term?

Thanks,

Bryan Lagos
blagos@tycoint.com


On August 06, 2007 at 08:05 PM, Michael Chu said...
Subject: Re: Microwave Safe
blagos@tycoint.com wrote:
I often see this term "Microwave Safe" used on containers, plates, etc. How is this determined? Is there some sort of standard test that products must follow in order to be labeled as "Safe"? Or is there no real maning to this term?

In the U.S. that label can only be placed on containers that are constructed of materials that have been determined by the FDA to be safe for use in a microwave oven.


On November 03, 2008 at 02:44 AM, AlexR (guest) said...
Subject: Re: Zak
I have been unable to find any source which explains the use of petroleum as a coolant in nuclear power plants.

Could you cite one or two for me?


On December 03, 2008 at 07:35 AM, ponkan (guest) said...
Subject: nuclear plants and petroleum
He didn't say that nuclear plants cool with petroleum -- most use evaporative (water) cooling, which is the purpose of the familiar concave tower. The extra heat is used to power the distillation of petroleum into components. This method of using the waste products of whatever generation technique is called "cogeneration".

Put another way, power plants (coal, natural gas, nuclear) produce electricity and heat. The heat is usually just exhausted into the atmosphere. But if there is some process that requires heat to work -- petroleum distillation for example -- it makes sense to take that "waste" heat and put it to use. This way you get more out of your fuel, you can usually increase your power plant efficiency (this is important since they typically have an efficiency of about 30% or less).


On December 29, 2008 at 07:09 PM, aguest (guest) said...
Subject: food grade plastic and the FDA
The FDA is bought and paid for by the producers of the products it is supposed to regulate, namely the pharmaceutical and plastics industries. The fact that FDA endorses food grade plastics as safe means absolutely nothing to me.


On February 22, 2009 at 12:06 AM, an anonymous reader said...
An acquantance uses ice cream containers for reheating large amounts of food - some times a gallon at a time. He is not convinced there is any harm in this. I can't believe there isn't. My ``gooleing' efforts brought me here - is there documentation someplace I can show this fellow?
Thank you


On February 23, 2009 at 05:42 AM, K3DE (guest) said...
Subject: Pyrex utensils
Seems pyrex manufacturers are missing opportunity to make spoons etc. that can be left in larger containers of materials while being microwaved.

Obviously, using plastic or metal stirrers is awkward and time consuming.

Stirring, using pyrex (that remains in the food) would be very simply done!


On April 27, 2009 at 07:06 PM, Plasticsguy (guest) said...
Subject: LArge food storage container
You can use a 44 qt cooler to store your wheat, I believe the cooler material must be FDA listed.


On June 08, 2009 at 06:51 PM, an anonymous reader said...
Another option is to find a local supplier of bulk goods such as your local cooperative food store. . They often receive products in 5 gallon, food grade buckets. It is what I use to store flour.


On June 18, 2009 at 02:14 AM, Jk5552 (guest) said...
Subject: Gooleing!
[color=violet:8a0b9f3b86]Pyrex doesnt make spoons? OUTRAGEOUS!!! thats so gooleing!
im going to throw metal bird seed at Pyrex for that![/color:8a0b9f3b86]


On July 22, 2009 at 03:52 PM, Corsair (guest) said...
Subject: Microwave safe....real;y
I have purchased Chinese made ceramic plates and bowls at Target that are listed as Microwave safe. However the cookware heats up too hot to touch while the food remains barely warmed. Is this really microwave safe or have they just put on the label. Who checks to see if it is really safe. The cookware changes to a new style every few months, so the old cookware can not be tested by the seller.


On July 28, 2009 at 03:22 AM, jackmac22 (guest) said...
Subject: food grade plastic containers
Another inexpensive source for food grade 5 gallon buckets is your local donut chain, such as Dunkin' Donuts. They often receive their fillings in the buckets which they discard when empty since it is not worth the labor expense to clean and reuse them. We used to get them for a couple dollars each. It's extra income for them and it reduces their waste stream. You have to wash them, of course. But you would also wash any new food container before use.

One caution about storing grains in plastic containers. If you have a rodent problem, put the plastic containers in a galvanized trash can. When we raised chickens we learned the hard way that even the heavy duty Rubbermaid Roughneck cans don't slow the rats down.


On October 31, 2009 at 10:58 AM, an anonymous reader said...
Quote:
Larry, Microwaves can be used to kill microorganisms but the food would have to be kept at boiling temperature for at least ten minutes to do so. Any food that had been microwaved for ten minutes after it had reached 210 F would be pretty uneddible


Katy, how do you explain the fact that flash-pasteurization -- which involves heating foodstuffs to 71ļC/160ļF for 6 seconds -- is often performed using continuous-flow microwave ovens?


On December 12, 2009 at 04:52 AM, crasscrockery (guest) said...
My great grandpa has been microwaving his food in plastic containers since he was a wee lad and he's ok...


On December 20, 2009 at 10:02 PM, Umar (guest) said...
Subject: Does it mean plastics cannot be used to store food at all?
Based on all I ve read in this forum, does it mean plastic containers cannot be used for storing food at of any sort at all? Most portable food storage containers are made of these plastics and if these things are unsafe then wont it be wise for the FDA to halt the production of these unsafe ones? Millions of people around the world dont even know about the unsafe nature of these plastics and are using them. Please could anyone also tell me about the nature of the danger these plastics pose? what are the likely specific health problems do they cause? No one is saying.


On December 20, 2009 at 10:46 PM, Dilbert said...
Umar -

there are folks who get upset about a lot of things, don't worry to much about facts or real science, they hear something and immediately set about spreading the word that the sky is falling.

one can of course subscribe to the conspiracy theories that big [pick a name / business] has paid off the FDA, the USDA, along with every other world government agency, to 'suppress' the real dangers of "fill in the blank" - there's a lot of things touted as candidates for the blank.

one of my favorites is: canola oil will kill you - it's made from rapeseed - which in the mustard family - that's what they make mustard gas from! google mustard gas and see what it is really made from.

Bisphenol A has demonstrated health concerns in infants - hence the banned in baby bottle thing.

arsenic is toxic - no one debates that as "true"
one molecule of arsenic will not harm you.
ingesting many molecules will.

>>Please could anyone also tell me about the nature of the danger these plastics pose?
>>what are the likely specific health problems do they cause?

search engines will turn up all the information you every could need. just use your own brain and consider the source and science behind the claims.

>>No one is saying.
that should tell you something.


On January 04, 2010 at 10:08 AM, Bill (guest) said...
Subject: Microwave Safe Containers
Question, do microwaves pass through pyrex glass unattenuated? Can a pyrex glass cover be used to prevent microwaved foods from splatering all over the microwave and still heat the food safely?


On January 04, 2010 at 01:22 PM, Dilbert said...
Bill -

short answer is "yes" - heat proof glass tops are fine.

glass may become hot - that's heat conducted from the food itself and/or steam generated - but glass all by itself does not "heat up" due to microwaves. passing through anything will cause some attenuation - but it's very minor compared to the degree water absorbs microwaves.


On January 26, 2010 at 12:11 AM, guest (guest) said...
The Chinese plastic containers and plates from Target are likely melanine. I don't believe you're supposed to use that in the microwave. I've experienced them bubbling the cracking when heated too much.


On March 27, 2010 at 03:05 PM, Glenda (guest) said...
Subject: microwavable containers
Everyone ignors the fact that microwavable dinner plates over time get extremely hot without heating the food. Everytime this question is asked someone will say it is the food heating the plate...Wrong....I believe repeated heating in the microwave breaks the plates down or the combination of the dishwasher and microwave do it. We reheat leftovers fine on our dishes when new and over time (a few years) the plates and bowls are no longer usable for the microwave. I have bought dishes from different manufacturers and it doesn't matter. I'm looking for new dishes now. I cannot heat soups in my bowls because it will not heat the soup just the bowls. I have to use corning and then put into my dishware which makes the benefit of less work moot.


On March 27, 2010 at 03:55 PM, Dilbert said...
microwaves preferentially absorb by bi-polar molecules - water being the common example.

"glass" is a crystalline lattice, and (with a few exceptions) exhibits low bi-polar composition.

"plastics" - somewhat undefined - but yes they can contain bi-polar molecules which will heat up quite nicely via microwaves.

there is no "legal" definition to "microwave safe" - it is a marketing term that indicates the container will not "fail" when exposed to microwave energy. it does not mean the container will not get hot.

for example: "Pyrex" - which is a flavor of annealed boro-silicate glass - will heat up in a microwave. the boro-silicate composition makes for low expansion coefficients - which makes it microwave safe because when subjected to (relatively even) heating it does not break all too easily. the annealing process allows the molecules in the glass lattice to align - that makes 'stronger' but also it a bit on the bi-polar side, which is why it will get hot.

experiment: take a one cup pyrex measuring cup, fill it half (4 fluid ounces) with water, zap it in the microwave for 1 minute. use your finger to determine the temp difference between the "top" with no water) and the bottom (with water) - use care not to raise blisters on your fingers....

the top is warm, but noticeably less 'warm' than the bottom - some heat has indeed transferred from the water to the glass and up the sides. if you've got a more recent microwave oven that can running "empty" you can just put the empty measuring cup in and you'll notice it gets warm.

a microwave plate that gets hotter than the food simply means the Made-in-China of Who-Knows-What plastic is more readily absorbing the microwave energy faster than the water in the food.

"Corning Ware" is a bit problematic - from reading, the "Corning Ware" brand name was sold - not made by Corning Glass anymore, nor under their license or control. essentially a situation of "brand" made in China from who-knows-what to "no specifications applied" - if you've got 15 year old Corning Ware, you're golden. the new stuff may not live up to one's expectations.


On October 04, 2010 at 02:20 AM, Guest (guest) said...
Subject: 'Plastic' Container (Cups) Recycle Codes and Microwave Ovens
Read some where and have passed it on by word of mouth several times that a number '5' or higher in the three wide mobius arrows found on the bottom of many (most) coffee cups and the like (The resin coding system introduced in 1988 by the Society of the Plastics Industry (SPI)), correlates to the container being 'microwave safe.' Research suggests this is true but I don't find it 'advertised.'

Comments/concerns?


On October 04, 2010 at 12:46 PM, Dilbert said...
here's the list of SPI id's and what they mean:

http://www.scn.org/~bk269/plastics.html

the number is - per that article - not a good guide to safety in the microwave.


On September 26, 2011 at 01:19 PM, Anonymous Coward (guest) said...
Subject: Nuclear power plants and petroleum
Distilling petroleum is a complicated process. It requires more than just heat. Generally, it requires the facilities of a petroleum refining plant; there's a reason for distillation towers, cat crackers, and so on, plus the associated infrastructure. You can't just "heat up a vat" of petroleum and get anything except hot petroleum.

I have to assume that the OP mentioned nuclear power plants in order to make his story scarier to the average person. That "detail", however, which is not only contrary to logic but seems to exist nowhere else, throws the credibility of the whole post into question.

I do find it a little weird that it was posted on "Cooking for Engineers". But in any event, no, nuclear power plants aren't in the petroleum-refining business as a sideline.


On September 29, 2011 at 01:52 AM, Michael Chu said...
I never mentioned nuclear power plants. Please don't judge Cooking For Engineers based on what random people on the internet choose to leave as comments.


On October 19, 2011 at 01:24 PM, toddman (guest) said...
Subject: "plastic" compounds
So which plastics are safe for use in a microwave environment i.e. what are the technicals names for these... PTFE, PET, PEEK etc.

Thanks.


On January 09, 2012 at 10:58 AM, an anonymous reader said...
Subject: Back to the issue of containers that are microwave-safe
I have dishes that don't "pass" the test and do get quite hot when zapped along side 2 cups of water in a Pyrex cup for 2 minutes. The dishes say 'microwave safe'. My questions is whether they are safe for us to eat from, or whether there is a chance that they are leaching materials into the food. I have seen the coffee cups get hairline cracks in them (I heat a 1/4 cup of milk in my coffee cup for 45 seconds each morning for coffee.)

I don't care about them cracking - just about whether my family could be ingesting anything leaching out of the dishes. Michael, thanks for you website - I love it.


On May 16, 2012 at 10:03 AM, an anonymous reader said...
I personally don't use plastic containers. Over time you can litterally see the plastic breaking down which means you definately are ingesting it. I also personally think that that container imbues a plasticyyy taste into the food. It's not that much more difficult or expensive to just use pyrex food storage containers. Why take the risk? Why alter the taste of your food?

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