I f you dehydrate your own backpacking food, you probably package individual servings in quart-size freezer bags. Not only are Ziplocs a lightweight, low-bulk way to pack food, but they also make for mess-free cooking. Just add hot water directly to the bag and wait for your food to soften.
Many backpackers say the method is perfectly reliable—but is cooking dinner in a freezer bag actually safe?
Sarah Kirkconnell literally wrote the book on freezer bag cooking (FBC). As the author of Freezer Bag Cooking: Trail Food Made Simple and Freezer Bag Cooking: Adventure Ready Recipes (among others), she is widely credited with inventing the convenient cooking method.
“When I first started backpacking the concept of ‘FBC’ (Freezer Bag Cooking) wasn’t a term yet,” Kirkconnell told The Trek via email. “It was coined on the old school Backpacker forums I was a member of due to my recipe developing… I was a single mom and didn’t have a lot of spare money, but I was going on so many trips so I started looking at old trail cookbooks and came across a few vintage ‘turkey roasting bag’ recipes and that got my mind working.”
Kirkconnell hoped to develop a cheaper, tastier, more convenient alternative to commercial backpacking food. “I wanted to be able to hike long miles and not be cleaning up pots in the dark, or wasting fuel… I wanted to be able to eat meals I liked and not go broke.”
The idea caught on. Many backpackers today still love the technique’s simplicity. Since hot water won’t melt freezer bags, hikers can store and cook their food in one container sans dirty dishes.
Kirkconnell pointed out that back in the early 2000s, “Mountain House meals actually came in a thick turkey roasting bag inside the mylar outer bag, and had an awful cardboard collar to keep the bag shut. I just updated it.”
Kirkconnell isn’t personally concerned about the safety of cooking in a freezer bag. “Even when I was backpacking constantly, I was only eating one meal a day out of a bag, and that was for, say 2-3 weekends a month or a five-day trip here and there. People drink and eat out of plastic nearly daily, not realizing it,” such as the plastic linings in disposable coffee cups and many canned foods and beverages.
However, she also believes that it’s down to each individual to determine their comfort level with the technique. Her books provide alternate instructions for those who prefer cooking in a pot. They also recommend heating water to just 180˚F—somewhat below boiling—for freezer bag cooking.
As Kirkconnell points out, Ziploc’s website indicates that their bags “meet the safety requirements of the U.S. Food and Drug Administration (FDA) for temperatures associated with defrosting and reheating food in microwave ovens,” implying that they can handle boiling water.
Unlike the oven roasting bags Kirkconnell referenced, which are made of heat-resistant nylon thermoplastic, Ziplocs are food-grade polypropylene. Polypropylene is a flexible plastic also common in products like yogurt cups and ketchup bottles. Notably, it doesn’t contain bisphenol A (BPA), a toxic component of many rigid plastics—including, as Kirkconnell pointed out, the linings of metal food and drink cans, plastic bottle caps, and more. (The FDA has stated that the trace amounts of BPA in these products don’t pose a risk to human health.) Ziplocs are also phthalate- and dioxin-free.
Companies like SC Johnson, the makers of Ziploc bags, don’t have to disclose every ingredient in their products. For that reason, we can’t know exactly how all components in a freezer bag might react to hot water, even though the EPA considers polypropylene generally safe. Further complicating matters, some studies indicate that polypropylene toxicity can vary based on the manufacturing process.
Dr. Stewart Lonky, a board-certified pulmonologist and author of Invisible Killers: The Truth About Environmental Genocide, says polypropylene won’t leach toxic chemicals into human tissue under usual circumstances. He points out that the plastic is even used in some non-absorbable surgical sutures. “Even when heated slightly, there is very little leaching, if any, of plasticizers into the contents of a ziplock bag,” Lonky told The Trek via email.
But most backpackers are doing more than just slightly heating a Ziploc. What happens if you add near-boiling water to one? “There is the possibility of polypropylene breakdown at these temperatures, with more leaching of plastic byproducts… into foods, particularly fatty foods. We don’t know what the long term effects of these compounds are in humans,” said Lonky.
And while Ziplocs don’t contain BPA, we don’t know whether they contain other harmful bisphenols, such as bisphenol S (BPS).
“Furthermore, recent research in surgical ‘mesh’ products made of polypropylene has shown that there is talc in the polypropylene matrix, containing stabilizers such as BHT (Butylated hydroxy-toluene) and BHA (Butylated Hydroxy-anisole), both of which have been shown to leach from polypropylene at moderate to high temperatures. BHA is a known hormone emulator while BHT can induce tumor growth in numerous animal species.” To be clear, we don’t know if these substances are found in freezer bags or not, only that they are present in some polypropylene products.
Lonky said adding boiling water to a Ziplock could leach “very small amounts” of these chemicals into food. These compounds could accumulate in human tissue over time. So while FBC may not be dangerous if done sparingly, he warns against making a regular habit of it.
With that in mind, thru-hikers planning to dehydrate and mail most of their food for a five or six-month trip may want to consider an alternative approach, such as cold-soaking or cooking in a pot, oven bag, or a product like the Cnoc Buc.
However, for health-conscious hikers, the nutritional content of the food itself should be just as relevant as the packaging. Kirkconnell encouraged hikers not to let safety concerns discourage them from packing out homemade meals. “Use the bag or don’t. The recipes are what matter in the end.”
Featured image via Katina (Arachne) Daanen.
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Oh hey there! I'm Ibex, the lead writer and content editor of this site. I mostly spend my spare time scheming new adventures and inadvertently setting my hair on fire while cooking. Appalachian Trail 2018 Wonderland Trail (+ a dozen or so other NPS trails) 2019 Colorado Trail 2020 Colorado Trail 2021.
This article is unnecessarily alarmist without providing anything approaching proof. Suppositions and a lot of “we don’t know” statements do not add up to anything actionable. Opinions do not equal science. You can write better articles than this (and you have). This one is disappointing.
Very general and not well researched, sorry. Ziploc brand bags usually use PE (LDPE, LLDPE, rarely HDPE) and not PP. Some other bags use PP, but PE is way more common (especially in freezer bags) since its a good thermoplastic and highly chemically resistant. There are also a lot of factors that go into how these plastics breakdown beyond “heating just below boiling is mostly safe”, oxidizers, oils, UV exposure, and prolonged temperature exposure contribute to them breaking down. LDPE really shouldnt be heated beyond 65C for any extended period of time, the 90C/195F high temp resistance is for very short exposures. Leaching is also not the only concern, general degradation (mechanical and chemical) releases microparticles and can promote leaching too. There are also things in there beyond the plastics themselves, like dyes and inks, which are regularly being discovered to be not safe. You also cant assume you know what bag you have, some contain PVC and PC which are not really safe at all.
If someone wants to do something like this, use silicone and stop being stupid and wasteful. its not perfect but its way better. Keep in mind, there is a reason SC Johnson tells you not to use their bags like this or for sous vide, they know its likely dangerous and are avoiding blame. This is the same company the regularly sells products they know can cause cancer with little to no warning, so when they tell you not to, you really probably shouldnt.
No offense, but leave plastic safety articles to people who are knowledgeable of plastic safety. articles like this only act as a disservice and spread misinformation and conjecture. and if you are knowledgeable of plastic safety you really should have done better research here, this is pretty basic chemistry.