The Slow Cooker Liner Is Not a Sandwich Bag

The short answer

The instinctive reaction to a slow cooker liner is that you are simmering food in a plastic bag for eight hours, and that this must be roughly the worst thing you could do in a kitchen. It is a reasonable instinct and it gets the material wrong.

The bag in the box is nylon. It is the same family of polymer as a turkey oven bag, and it is there precisely because nylon tolerates heat that other films do not. A polyethylene bag — the material of a Ziploc — would deform long before a roasting bag noticed. Reynolds rates the liner material to 400 °F. A slow cooker on high tops out around boiling. In engineering terms the film is loafing.

So the honest question is not “will it melt”. It is what comes off a polyamide film held against hot, fatty, slightly acidic food for the better part of a working day. And that question has a real answer, a real regulatory limit, and a genuine hole in the middle of it.

What the manufacturer actually says

We take brand claims from the brand’s own materials rather than from third-party listings, so here is Reynolds, verbatim: “The liners are made of a special BPA-free material suitable for cooking foods in slow cookers on high, low, or warm settings. Reynolds has safely used this blend of nylon for over 30 years.” And on compliance: “Slow Cooker Liners are BPA-free and FDA-compliant for cooking.”

Two things in that are worth sitting with. The first is that BPA-free is close to meaningless here, and not because the claim is untrue — it is true. Bisphenol A is a polycarbonate and epoxy-resin issue. Nylon was never going to contain it. Saying a polyamide film is BPA-free is like advertising that a wooden spoon is gluten-free, and we have written about how much work that phrase does elsewhere.

The second is the handling instruction, which is a structural warning rather than a chemical one: “At no time should the liner be lifted from the bowl with food inside.” Reynolds also distinguishes the liners from its oven bags, noting they are “specifically tested and designed to guarantee performance only when used as a slow cooker liner” — which is a useful correction to the widespread claim that the two products are simply different sizes of the same thing.

The gap: which nylon is it?

This is the part of the story that has not been told, and it is the reason this page exists.

“Nylon” is not one material. The two that dominate food contact are chemically unrelated at the monomer level:

• Nylon 6 is polymerised from a single monomer, ε-caprolactam. Residual caprolactam can migrate out of the finished film, and the EU sets a specific migration limit of 15 mg/kg for it.
• Nylon 66 is made from hexamethylenediamine and adipic acid. There is no caprolactam in it to migrate. It also absorbs less moisture and holds up better at temperature, which is why it is often preferred where hot water is present.

Reynolds says “a blend of nylon”. Industry descriptions of oven bags say they are made from food-grade nylon, often nylon 6 or nylon 66. Nobody in the consumer-facing chain states which, and a blend could mean both.

The consequence is unusual and worth stating plainly: you cannot determine from any available information which migration question applies to the product in your kitchen. If it is nylon 66, the caprolactam literature is irrelevant to you. If it is nylon 6, it is the most relevant thing there is. That is not a scandal — undisclosed polymer grades are ordinary in food packaging — but it does mean anyone giving you a confident number for slow cooker liners is working from an assumption they have not told you about.

What has actually been measured

There is no published measurement of a slow cooker liner. What exists is the closest analogue: food sold packaged in nylon-6 and then heated inside that packaging.

A survey published in Food Additives & Contaminants analysed 50 retail foodstuffs packaged in nylon-6. Caprolactam was confirmed in nine of the 50, in the range 2.8–13 mg/kg. It was indicated in a further 15 samples at 0.8–11 mg/kg, though those did not satisfy all five of the confirmation criteria the authors applied.

Every result — confirmed and unconfirmed — fell below the European specific migration limit of 15 mg/kg. The average across all 50 samples, counting non-detects at half the limit of detection, was 2.6 mg/kg with a standard deviation of 3.1. Quoting the top of the range without that compliance result would be technically accurate and thoroughly misleading, which is why it appears here attached to it.

The finding that transfers most directly to a liner is this one: every sample with detectable caprolactam migration came from an application involving heating the food in the packaging. Sausage meat heat-processed in nylon casing. Pouches boiled, microwaved or roasted. In one case, half of the extractable caprolactam moved out of a nylon-6 sausage casing into the scalded sausage, arriving at 4.2 mg/kg.

Cold storage in nylon did not produce detectable migration. Heat inside the pack did. A slow cooker liner is a heat-inside-the-pack application by definition.

Where the analogy runs out

Two variables in a slow cooker sit outside anything in that survey, and honesty requires flagging both rather than extrapolating through them.

Duration. Scalding a sausage takes minutes. Boiling a pouch takes minutes. A slow cooker runs six to ten hours, sometimes overnight. Migration is a diffusion process and time is one of its inputs, so the honest position is that the published numbers describe a much shorter exposure than the one you are asking about.

Fat and acid. The foods that end up in slow cookers — chilli, pulled pork, tomato-based stews, curry — are fatty and mildly acidic, and both push migration upward for most polymers. A separate strand of the caprolactam literature deals specifically with packaging intended for fatty foods, which tells you the industry considers fat the harder case.

Set against that, one variable runs the other way: a slow cooker is not hot. On high it sits near 100 °C against a film rated to roughly 204 °C, and a good deal of the published nylon migration work involves roasting temperatures far above anything a crock will reach. It is a long, warm, greasy exposure rather than a short, extreme one, and there is no published data on that exact combination.

The crock is not the problem

Worth saying clearly, because a page like this can easily read as an argument against slow cooking: the appliance is one of the better surfaces in a kitchen.

A slow cooker insert is glazed ceramic or stoneware — a vitrified, non-porous surface that does not shed polymer, does not have a nonstick coating to degrade, and does not care about acid. It is the same reason ceramic ranks where it does in our cookware guide. If you cook directly in the crock, there is no plastic in the food path at all. The lid gasket, where one exists, is silicone rather than a polyolefin, and it sits above the food line.

Which puts the whole question in proportion. The liner is an accessory sold to save you washing up. Skipping it removes the entire uncertainty for the price of a soak.

How the options compare

What we would actually do

• Default to no liner. Not from alarm, but because the uncertainty is free to eliminate and the benefit it buys is a rinse. If a liner is what makes the appliance get used on a weeknight, that is a real benefit and it counts.
• Do not substitute a food storage bag. This is the one genuinely wrong move available. Polyethylene is not a cooking film, and the whole reason liners are nylon is that the cheap alternative does not hold.
• Follow the lifting instruction. Reynolds says never to lift the liner out with food in it. That is about seams and hot liquid, and it is the most likely way this product actually hurts someone.
• Do not store leftovers in the liner. Decant into glass or steel once it has cooled. Long contact with fatty food is the variable nobody has measured, and there is no reason to add hours to it after the cooking is done.
• Do not re-use one. They are single-use films and a second cycle stacks another six hours onto material that has already been through it.
• Ignore “BPA-free” on this product. It is true and it is empty. Nylon was never a bisphenol A material, so the claim rules out something that was never in play.

Related reading: microplastics in sous vide, vacuum sealer bags, do Ziploc bags release microplastics, what “microwave safe” means, nylon as a material, and our slow cooker liner entry.