Recently people have been asking me a lot of questions about cooking foods in plastic bags. Reasons for doing so may include the convenience factor, such as when you are camping or otherwise trying to prepare meals over a campfire, in which case cooking omelets in plastic bags apparently works great:
(see link here: http://allrecipes.com/recipe/85107/omelet-in-a-bag/ or
These omelets are made by mixing all the ingredients in a Ziploc bag, and then putting that bag into boiling water for 13 minutes. This is certainly longer than it would take to cook an omelet the traditional way, however, using this method up to 13 different omelets can be cooked at the same time! What could be wrong with this picture?
Quite a bit, actually. Let’s start with the Ziploc bags themselves. Manufacturer instructions on the Ziploc website make it abundantly clear that they should not be subjected to the high temperatures of boiling water (212 degrees Fahrenheit, or 100 degrees Celsius), and that doing so can cause the plastic to start to melt and/or for chemicals to end up leaching into the food. This is true even though most (all?) of Ziploc bags are labeled as BPA-free, which removes one of the major chemicals of concern from the bags.
But if it’s BPA-free, that means it is safe, right? Also no. All that the label BPA-free means is that the particular product does not contain BPA. But, it turns out, there are lots of other chemicals that are very similar in structure to BPA and also have beneficial materials properties, so manufacturers have started to replace BPA with these analogues (BPS and BPF are two of the most common alternatives). These compounds aren’t regulated at all, yet, at least in this country, and so the Ziploc bags can be full of these chemicals. More concerningly, there is relatively little research done on the toxicities of these compounds, and most of the research that has been done indicates that they are at least as toxic (if not more toxic) than BPA itself.
What about bags that are made especially for cooking, though, and what about cooking at much lower temperatures? Let’s tackle the second question first. Cooking in vacuum-sealed plastic bags at a relatively low water temperature is a method that is called “sous-vide,” which is French for “under vacuum.” This method of cooking, in which the food in placed in a plastic bag, vacuum sealed, and then cooked in relatively low temperature water for a long time, has become more popular recently with the popularity of the Anova Precision Cooker (see website here: https://anovaculinary.com/what-is-sous-vide/). Unlike using boiling water, the water temperature used in sous-vide cooking can be anywhere from 130 degrees Fahrenheit to 170 degrees Fahrenheit, all of which are well-below temperatures at which plastics would actively start to melt and contaminate the food inside the bag.
Even though the plastics are not melting, though, other chemicals can be leaching into the food when they are cooked in water, and probably they are. Are these chemicals bad for you? Unclear, at least until there are any systematic studies done on the chemical contamination in food cooked sous-vide, and on the toxicities of those chemicals. Does the food taste better being cooked this way? Possibly, although this is clearly an issue of personal preference. For me personally, I think I’ll go with some more traditional baking, cooking, or frying (yuuummmm…frying). Can’t go wrong with that! (Or can you? Stay tuned for the next post of chemistry of cooking oil, coming next week).