Monday, October 1, 2012

Apple #605: Microwave Steam Bags

I have a request!  Daily Apple reader Rodolfo wants to know about those plastic bags that are specifically used for steaming frozen vegetables.  He wants to know how they work.  He suspects they're different from a typical plastic storage bag, but how, exactly?

Rodolfo was even kind enough to send me a picture of the type of steamer bag he uses.
(Photo from my friend Rodolfo [his name has been changed to protect his innocence.])

I suspected the answer to Rodolfo's question would require a patent search, and I was right.  But, never fear, when it comes down to it, how those bags are put together is pretty easy to explain.  And the best way to explain how they work is to explain it in terms of how they are different from a regular old sealable (or zippable) plastic storage bag.

Regular plastic storage bag on the left, zippable plastic steamer bag on the right.
(Photo from Cross My Heart)

  • I should admit right up front, I've never actually used plastic steamer bags.  I steam my vegetables the way my mom taught me, in a pot of water with a steamer basket.  But in order to answer Rodolfo's question, I read a lot of people's descriptions about how they work.  I'm a little nervous generalizing on the basis of read-it-only, but I'll do my best.
  • First, the plastic in the steamer bags is made of materials that won't leach toxins into your food when microwaved.  Whew!  Specifically, the Ziploc bags are made from polypropylene (PP #5) and polyethylene (PET #1).  
  • The bad plastics we've heard about recently are bisphenols (BSPs).  The steamer bags do not contain those kinds of plastics.  
  • So, the fact that the steamer bags don't have the bad plastics in them doesn't seem to make them all that different from a typical storage bag.  So how else are they different?
  • If you've ever compared a freezer storage bag to, say, a sandwich storage bag, you would have noticed that the plastic in the freezer storage bag is much thicker.  In order to withstand the temperatures in the freezer, and to resist getting torn or broken if the bag gets knocked around, the plastic is made of several layers of plastic.  There might be two or three layers of plastic, depending on the manufacturer, and the layers might be of different types of plastic that are tolerant of various temperatures.  The outside layer, for example, might be able to withstand colder temperatures than the inside layer.

Thicker freezer bag on the left, thinner sandwich bag on the right.
(Freezer bag photo from Made in, sandwich bag photo from Industrial Laboratories)

  • The same is true about these steam bags.  The plastic is thicker and it's usually made of layers.  The difference is with the steamer bags is the inside layer has to withstand a melting point, because the stuff inside is going to get really hot when it's heated in the microwave. 
  • Fortunately, since cooking times are relatively short, it doesn't have to be able to stand the heat for very long, but it does have to hold up to high temperatures.
  • If the bags are designed to go from freezer to microwave, then the bag is probably going to have more layers, or the plastics will be a little different because they will have to be able to hold up under the cold temperatures of the freezer, and not get torn or broken during transport.  
  • But the upshot is, the steamer bags have multiple layers of plastic, and the layer inside has to be able to withstand higher temperatures.
  • The next big difference is the steam bag has to have a way to release the steam.  If you put those vegetables in the microwave and sealed them up tight with no way for the steam to get out, the package would explode, and you'd have pulverized broccoli all over the inside of your microwave. 

The problem for steamer bag makers: how to let out the steam so the bag doesn't explode, but not let out so much that the vegetables don't actually steam?
(Photo from dipity)

  • If you told the consumer to puncture the bag before microwaving, as we all have done with frozen dinners, that still wouldn't be ideal because the hole would allow too much moisture to escape during cooking, and the vegetables wouldn't really get steamed.
  • So the plastics people came up with a pretty ingenious invention: the vent.  The Ziploc bags have a vent across the top, other types of bags have vents that appear across one side of the bag, and still others have the vent set up differently.  But they mostly use the same general idea.
  • The clever thing is, the vent doesn't open up until the package is being microwaved.  Exactly how the vent works depends on the manufacturer and the patent, but here's a description of how one company does it.  This isn't from the patent for Ziploc's bag, so how theirs works might be a little different than this.  But it's probably close.
  • The plastic in this steamer bag has two layers, and outer, protective layer, and an inner, heat-friendly and sealable layer.  The inner layer is doubled up and sealed to itself, so the inner layer now has two layers.  The outer, protective layer is on top of the inner, doubled-up layer.
  • The vent is made by making an opening in the seal of the doubled-up layer.  That could be done either by not sealing it in one spot when they double it up, or by cutting a slit in it after it's been sealed.  Either way, there's a slit in the doubled-up inner layer.  The outer, protective layer is still on top of the inner layer.
  • When the bag gets put in the microwave and heated up, the inner, doubled-up layer is not bothered by the heat because the plastic was made to withstand high temperatures.  But the pressure that builds up inside the package pushes the plastic from the inside so that it bulges, which, in turn, makes the vents in the inner layer open up.  Because the outer protective layer is not as good at holding up to the heat,  the outer layer opens and, voila, the steam escapes.

Ziploc steam bag after cooking. The thing has bulged and filled with steam, only some of which has escaped. The rest has stayed in the package and steamed the vegetables.
(Photo from Cyber Shape Up)

  • That's why it's important to put the bag in the microwave with one side in particular facing up.  That's the side with the vent in it.  You put the vent-side facing down, you might block the vent, and then you'd get pulverized broccoli.
  • That vent-in-the-inner-layer method is not how everybody does it.  Some people cut slits through all the layers from the get-go, and they make the slits small and spaced far enough apart that they think they can keep too much moisture from escaping.
  • There's still another method of using a breathable patch in the plastic.  The patch is, itself, made of plastic, and it lets the steam out, but it's not porous enough to let water or anything else out.  In that case, you would peel away a protective sticking covering over the patch before microwaving.
  • But as far as I can tell, the inner layer with a slit in it might be the more popular method.
  • Now, I can't resist saying this: these steamer bags are made of plastic, which is made from petroleum.  You use the bag once and you have to throw it away.  I suppose you could try to use it again, but the plastic might degrade with the increased use and heat, and the vent system might not work properly, and nobody's tested these things for multiple uses to determine if you'd get toxins in your food, so it's just a bad idea to use them several times.  They're meant to be disposed of after one use, so if you use them, that's what you should do.  

That plastic steam bag came from petroleum.
(Image from Highlights)

  • I use a steamer basket to steam vegetables.  You might say, Aw, but that takes time!  Dude.  You cut up the broccoli while you boil the water.  You put the broccoli in the steamer basket, put that in the pot of water, put the lid on top.  Two minutes later, it's done.  Same amount of time's passed.  And you haven't used any plastic!  You've saved that petroleum for your car!

Steamer basket. You can buy one like this for $4.95 from Amazon, or you can find them hanging around in any grocery store, or at any store like Target or Wal-Mart, or any store that sells any kitchen stuff.

  • If you don't like the environmentally friendly argument in favor of using a steamer basket, how about this one: the food and plastics companies are ripping you off.
  • I used to research the agriculture and food manufacturing industry, and I can tell you that food processors are always looking for a way to get you to spend more money on food.  There's no margin of profit on a bunch of asparagus.  But if they can cut up the asparagus for you, freeze it or can it, and put it in a pretty package, they can get you to pay a lot of extra money for that asparagus. 
  • You might say, But I'm not buying the Birds Eye pre-packaged stuff, I'm just buying the bags and freezing and steaming my vegetables myself!  To that I say, Ziploc is just trying to get in on the action.  You have to buy the steamer bags every time you want to steam something.  You buy the steamer basket once and you're done.
  • You might say, But I don't know how to use a steamer basket! Dude.  It's easy. It's so easy, about 15 people have written how-to pages about it. Here's one such page of instructions.  The only question is, how long do you steam different kinds of vegetables?  After a couple minutes, lift the lid and poke 'em with a fork.  If they give, they're done.  If they still feel crunchy, they're not. Put the lid back and wait a couple minutes more. When you can smell the vegetables, lift the lid and poke 'em again.  They're probably done.
  • I'm all in favor of convenience.  I think this vent invention is really pretty dang clever, and I applaud whoever came up with it.  But it seems to me, in this case, the convenience isn't that much of a time savings, and it's certainly not a financial savings.  I think we as a country have bought this steamer bag scam, hook, line, and sinker.
  • That's just my opinion.

The people who make these things look at this steamer bag trend and think, "Cha-ching! What an easy way to make some extra bucks!"
(Green Giant vegetables, Red skin potatoes with free microwave steam bag! photo from Packaging Digest, Birds Eye vegetables and rice, Kashi's entire meal in a steam bag photo from Supermarket News, Ziplog steam bag photo from The Nibble)

Dr. Weil, Plastic Steaming Bag Danger? February 26, 2009
Florence Williams, Is It Safe to Heat Food in Plastic? Good Housekeeping
Lauren Keith, Two Microwave Steam Bags--Which is Better? KFVS
Joe Terrell, Ziploc Zip 'N Steam: "Does It Work?" KLTV, July 31, 2007
Steaming Splendor: Easy, Convenient, Tasty Steamed Vegetables, The Nibble
Su, Jau-Ming and Wolak, Paul Z., Freezable/microwavable packaging films and venting package, US PAT 7,812,293, October 12, 2010
Mita, Kozo et al., Sealable package for heating in a microwave oven, US PAT 6,596,355, July 22, 2003
Bemis Introduces Hermetic, Steam-in-Bag Package Technology for Meals that Include Proteins and Sauces, Bemis Company, October 10, 2008


  1. I agree that steamer bags (and ANYTHING designed to be used once--unless we're talking biohazard stuff) are a waste and should be avoided! Hear hear, Apple Lady!! I have a follow-up question: so, how much water are you supposed to put in the bottom of the pot that the steamer doohickie goes in? I usually use my rice cooker to steam vegetables, but sometimes I want to steam more than one thing at a!

  2. Hi Renee,

    You don't need much water in the pot at all. You don't want it to touch the bottom of the steamer basket. So if you want to eyeball a measurement, look at the height of the feet on your steamer basket. The water in the pot should be slightly less than that height.


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