Safety of Plastics Used in Sous Vide Cooking

Chris

A few people have asked about the safety of the plastics used in sous vide cooking. Here is our answer:

As most of us know, media reports have raised concerns about chemicals leaching into foods and drinks from their plastic containers. Many people are concerned that plastics used in sous vide packaging contain harmful chemicals that can leach into food during storage and cooking.

We’ve looked into this issue extensively. According to the latest research, the safest plastics for use with food are high-density polyethylene, low-density polyethylene, and polypropylene. Virtually all sous vide bags are made from these plastics (the inner layer of nearly all sous vide bags is polyethylene). Most name brand food storage bags and plastic wraps such as Saran Wrap are also made from safe plastics like polyethylene. It’s our opinion, after an extensive review of the scientific literature, that concerns about the safety of sous vide bags are misplaced.

Indeed, other plastics in your kitchen are where we focus our concern. Inexpensive, bulk plastic wraps are often not as safe. These products are still commonly made from polyvinyl chloride or polyvinylidene chloride, which can contain harmful plasticizers that have been shown to leach into fatty foods such as cheese, meat, or fish. Legitimate concerns exist about food exposed to these plastics at higher temperatures, such as when you microwave food wrapped in these plastics.

What should you do? Polyethylene-based plastic wraps are available at only slightly higher cost and do not raise such concerns. You can tell the difference because the PCV plastic wraps are very stretchy, whereas the safer polyethylene wraps are, frankly, not as good.

Another area for concern are the clear, rigid, plastic storage containers common in professional kitchens. These are made from polycarbonate. These plastics are a cause for concern because they contain bisphenol-A (BPA), a chemical that can disrupt hormone activity and can leach into food or beverages. Cracks and crazing due to wear and tear increase the rate at which BPA leaches out of polycarbonates. If you are concerned about BPA contamination, however, replace any polycarbonate containers that have cracks or crazing. Thankfully, newer polycarbonate containers are now, mostly, BPA-free as a result of the massive amount of media attention this controversy has received over the last few years.

Chris

GGPVBLIC

I think this the correct post to ask a question : I noticed in the pastrami video that the color of the cooking bath changed after 48h, showing that there was an exchange of molecules between the inside and the outside of the bag. I've experienced a similar thing at home, not with color but with flavor : I seasoned some potatoes with thyme essential oil, and guess what, at the end of the cooking process, the water bath had taken the thyme flavor.

I initially considered that the problem was coming from the quality of my bags (although they are clearly designed for sous-vide, a mention states they can go till 121°C) but seeing your video made me rethink.

So my question is the following : do you find it normal that these bags are not 100% hermetic and to experience such ins/outs?

thanks

Brendan Lee

I've had a similar experience as mentioned above and am too curious about that. 

Chris

All plastics will allow molecules to move through them. Smaller molecules will do this faster than larger molecules. Higher temperatures will tend to accelerate this diffusion. Aroma molecules tend to be very small, which is why they evaporate so readily, which is why we can smell them. They tend to move through the plastic bags over time. Pigments tend to be larger molecules and so they diffuse slowly, but they can do it over a very long cooking time at higher temperature. 

By extension, you should realize that oxygen and other gases will slowly seep through your sous vide bags over time. So, no, they are not perfectly hermetic. But at refrigerator temperatures they tend to be good enough for weeks or even months of storage.

Chris

GGPVBLIC

Thanks Chris for your feedback. Makes perfect sense.

Based on what you explain, don't you think that writing directly with a marker pen on the plastic bags - as I saw in some of your vids - is a controversial practice? One could easily consider that some of the chemicals from the ink could somehow go through the bag... 

Just sharing thoughts.

Jerome

Chris

Personally, I'm comfortable with the pens we use for a couple of reasons:

(1) The solvent for the ink is an alcohol, which will generally not penetrate through the bag because most of the alcohol evaporates as the ink dries. Indeed, we use Sharpie brand markers that have been certified by The Art & Creative Materials Institute (ACMI) as "safe and that are certified in a toxicological evaluation by a medical expert to contain no materials in sufficient quantities to be toxic or injurious to humans, including children, or to cause acute or chronic health problems."

(2) The pigments used in the ink are *very* large molecules and I would not expect them to diffuse through the packaging even after several weeks.

But if you are concerned, you could use labels.

Chris

victorwol

I was wondering the other day why I could smell the smoke of the pastrami coming out if the sous vide bag... I understand now... Thanks.

Jack Ostrowski

To clarify a few points. The aromatic (flavor) molecules in foods are organic nature i.e. ketones, aldehydes and such. They are not readily water soluble. However, they are readily dissolved in organic media i.e. acetone or alcohol. With that in mind plastic bags are organic in nature and not soluble in water. They may be dissolve in organic media. The plastic bags will readily permeate aromatic molecules. However, the plastic bag is impermeable to water and salts thus it will not pass water through even though water and salts are much smaller than many aromatic molecules.
As for the dyes in sharpie markers they are small. What holds the dye molecules to the bag is a resin. Depending on the nature of resin and dye molecules there may be a strong interaction that prevents the dye from leaching out of the resin. However, it may be possible for the dye in sharpie ink to diffuse into the bag at elevated temperatures.

Chris

@Jack — First off, welcome to ChefSteps. And thank you for adding some clarifying details.

I wouldn't go so far as to say that the plastic sous vide bags will readily permeate organic compounds, at least not with typical sous vide packaging, which is usually five layers of nylon and high-density polyethylene with resin tie layers. For most aromatic foods, even after days of cooking you cannot smell aromatics diffusing through the bags.

Smoked food is interesting, and specifically, it seems to be the eugenol (clove-aroma) and guaiacol (peaty) that most readily comes through the packaging. These are relatively low molecular weight compounds, but certainly not the lowest aromatics you'll find in food, so I'm guessing that these molecules tend to have some solubility characteristics that favors their migrate through the various plastics and resins used in the bags more than other volatile organic compounds.

As for the Sharpie, my understanding is that Sharpies don't use dyes at all. I believe they are basically an isopropyl alcohol as the carrier solvent, a resin (probably eurethane acrylic), and actual solid pigment particles (as opposed to a blend of dyes) that become bound in the resin when it polymerizes on the writing surface. I would be very surprised if the pigment particles are very mobile at all. Hence, my lack of concern with using Sharpies to write on the sous vide bags.

Tucker

just a question arent ziploc bags more expensive than buying chamber bags in bulk?

Brendan Lee

I think it depends on the size and how bulk of an order your buying. I think if you are going with the freezer bags they're like $9 for ~100 at my local big box warehouse

DiggingDogFarm

In this neck of the woods, Ziploc gallon freezer bags are 7.8 cents each (152 count) and the quart freezer bags are 4.96 cents a piece (216 count.)....that's at Sam's Club.

~Martin