Wrapping My Cold Groceries in a Warm Insulating Grocery Bag

This happened in August in the D.C. area. I left Costco with my groceries, which included cold milk, orange juice, yogurt, ice cream, and similar. Facing a 20-minute ride home, I wrapped the cold stuff inside an insulated food storage bag, which has a reflective Mylar interior. Problem is, I had left the bag inside my parked car, whose interior was probably 120 degrees F. When I opened the bag, it was maybe 85 F inside, so I was unsure whether to bundle the cold items inside it or not, thinking the warmth of the bag and its insulated construction would actually rapid warm my groceries, instead of keeping them cold. Yes, I ran the car’s AC but wondered if that would make much difference, as my cold items were now insulated from the increasing coldness of my car. (Next time shopping, I took my insulating bag inside the store.)

Any ideas?

There’s not much thermal mass to the bag compared to the soon-to-be-contents.
I would bring a separate (and better insulated) bag and freezer blocks for something like ice cream.

As a pre-George Constanza Jason Alexander reminds us, insulation keeps the “hot side hot and the cool side cool”. In theory, the warm mass of the air inside should be mostly displaced by the contents, and the thermal content of the actual bag should be pretty small; Mylar certainly doesn’t have much thermal mass, and the fabric probably doesn’t,either. However, those fabric bags are mostly effective in just preventing circulation and unless you pack them with ice or cold packs they aren’t really going to keep things cold for very long, and probably not any better than just sticking the cold items directly in front of a vent.

If you were going for any significant distance I would suggest something like this: 20L the Ultimate Portable Dual Zone Fridge Freezer | ICECO 21QT GO20 – www.icecofreezer.com

You can precool it on the drive to the store, and then keep it cool all the way home. Of course, it is not inexpensive and doesn’t compact down like a bag, so it may no be your ideal solution.


If you can remember to put some sort of cold pack inside the bag before leaving home, that’ll solve the problem.

If I put ‘take ice!’ on the lists (shopping and daily to-do), sometimes that helps. If I remember to look at the lists before leaving.

Once when I got fish at Costco but told them I had a long drive, they told me to grab several plastic bags from the meat/fish department and then fill the bags with ice from the soda dispensers at the front. Pack it all in a box. It worked well.

One would think that the heat trapped in the insulated bag would neutralize any positive effect after cold items were placed in it. Did the OP note whether the cold groceries were still cold on arrival at home?

*the title of this thread could be recycled for a sleazy blues song.

Thank you, one and all, for your very helpful information. To answer one of your questions, yes, the cold stuff was acceptably cold by the time I got home, although the ice cream was starting to get a bit soft.

I have several of those mylar-lined insulated bags, and they work quite well for short periods of time – I’d say for up to about an hour if well packed with cold and/or frozen items. I’ve never had cold groceries noticeably warm up, nor frozen stuff thaw. But as others have said, for lengthier time periods an ice pack of some kind would be a good idea.

The two things I do to help keep things cold in the summer are (1) I bring the bag(s) with me into the grocery store so they don’t heat up in the car, and (2) I give the cashier all the cold and frozen stuff first to pack in the insulated bags, and make sure that no room-temperature stuff is packed in there.

Thank you so much for this suggestion, wolfpup. I will definitely do it this way when summer weather returns. Happy Thanksgiving!

Here’s a tip. Ideally, you could put a freeze-pack from home in the bag before you head to the store. Failing that, head to the frozen aisle first, and grab a bag or two of frozen peas or corn to suck up the heat in the insul-bag. Then do your room-temp shopping. When you go back to the frozen aisle just before you check out, put the bags of veggies back where you got them, and put the cold stuff you need in the chilled bag.

Face it, your ice cream might still be soft by the time you get home, but you got your edge.

That’s a nasty thing to do to whoever winds up buying the now-thawed stuff you just put back in the freezer. Yes, it’ll freeze back up and not be poisonous; but it’ll freeze back up in a non-pourable and non-separable clump, and quality will deteriorate.

And if the store realizes it’s been out of the freezer, they may be required by law to throw it out.

So don’t.

It’s not a big deal with flash-frozen sturdy veg like corn and peas, and they’re still mostly frozen after a short ride in your insul-bag. It’s probably happened to you before, and you didn’t know it. Skids of frozen foods spend time on loading docks and in back rooms before the stockers can get to them. Not all your food makes that ideal, no-delay journey from the FroFoo factory to your cart without some unplanned hiccup.

It’s certainly happened to me before, whatever the reason was, and I did know it, as soon as I opened the package; at any rate, I’ve certainly opened packages that had clearly been thawed longer than was plausible for what would have happened between my taking them out of the freezer case and my getting them into my own freezer. And sometimes the contents were in pretty crappy shape.

Because such things can happen accidentally doesn’t make doing it deliberately OK.

Completely agree. Furthermore, if sometimes frozen foods go through a thaw-refreeze cycle in the course of handling, doing it a second time is just going to make it that much worse.

I’m surprised no one suggested I use liquid nitrogen. :slightly_smiling_face:

As things such as milk, juice and yogurt aren’t going to suffer from being put momentarily into a hot car before the AC brings the air temperature down, I wonder if you wouldn’t be better just using the insulating bag for frozen goods.
From the point of view of the ice cream, these other cool foods are warmer and would contribute to the heat loss of the ice cream. While they would help bring the air temperature inside the bag down somewhat, once the air temperature got to that of the cool foods, it wouldn’t get any colder.
It would be an interesting physics problem to see which is more effective.

Of course, having freezer blocks or getting some ice from the store would be better.

If I put them into the trunk of the car/the far back section of a hatchback on a hot day, I think it’s going to take some time before the air conditioner gets that area down even to around 70; and it’s certainly not going to get it to 40 (F). For a short drive, that’s not an issue; but if one lives far from the store in question, it can be.

One solution to that is two separate insulated bags – one for the refrigerator stuff and a different one for the frozen. Plus ice packs. If I’m heading out for a major shopping run two counties over, that’s what I’ll plan for. If for a routine run someplace twenty minutes away, I’m liable to put the refrigerator and freezer stuff in together; the refrigerator items are indeed warmer than the freezer ones, but they’re a good bit colder than the rest of the car.

If the idea is to keep ice cream from warming up, then putting it in the trunk doesn’t seem to be a productive idea to me.

The question of optimizing this would be an interesting student project for someone.

I suppose I could put it in the passenger seat; although even that’s going to take a while to cool down, and is only going to be cooled to the upper 60’s. I don’t think the air conditioning can even be set to the 40º that refrigerated non-frozen items are supposed to be held at. But what I actually do is put cold items in a cooler, which usually rides in the back area of the hatchback. If the weather’s hot and I remembered, there’ll be an ice pack in the cooler.

Okay, this is risky, but I’m going to talk about what to do when you get to your hot car in the summer. The sun will have heated it to 140 deg. F or more. That includes all the air in the air-conditioner ducts. So, let’s say it’s 80 deg F outside.
Open all the doors, to let the 140 deg air rise out, replaced by 80 deg air. Start the car, and turn on the A/C full blast. Even after opening the doors, the body and seats are still super-heated, so don’t put it on recirculate just yet. The best your A/C can do is about a 40-45 deg drop between the source and the output vent, so if your car is still 95 inside, you’re better off working from 80 than 95.

I’ll brace myself for the naysayers now.