Most food is fine in a steel container. That's the boring truth, and it makes for a short article, so the internet has invented a lot of rules that aren't real.
Here's the honest version: a long list of things that are fine, a short list of things that need a thought, and one that's a genuine materials issue.
The fine list
These need no thinking about at all. Roasted vegetables. Rice and grains. Pasta and pasta salad. Chicken and other cooked proteins. Overnight oats. Chia pudding. Hummus and dips. Cut fruit. Leafy salads. Soup and stew, with a couple of centimetres of headspace if it's going in the freezer. Yoghurt. Cheese. Nuts. Chocolate. Bread and baked things.
That's fifteen, and it's most of a normal week. Steel doesn't stain, doesn't take on smells, and doesn't hand last Tuesday's curry to today's rice.
The acid question
Tomato sauce, citrus, vinaigrette, marinades. You'll read that these pull metal out of steel and need moving to glass.
We've written the long version of this, and the short version is: the research that found measurable nickel transfer is about cooking — hours of simmering — and no study we can find measures a box of sauce sitting in a fridge for two days. We won't tell you steel is inert, because it isn't. We will tell you that the fridge is not a stockpot, and that we've never found a reason to move leftovers to glass.
The one that's real: heavy brine
This is the exception, and it's not a myth.
Chlorides — salt — are the one thing that genuinely troubles 304 steel. Given enough concentration, enough time and a stagnant surface, chloride can pit the passive layer: small localised marks rather than a general dulling. This is precisely why 316 exists and why it's called a marine grade — the molybdenum in it resists chloride specifically. Seawater is the problem 316 was designed for.
Note what that does and doesn't mean. 316 is not a safer grade or a purer one, and we're not going to pretend our 304 is an upgrade on it. On chloride, 316 is genuinely better. We don't sell it.
In practice this affects almost nothing you'll do. A portion of olives, pickles, feta in brine, kimchi or sauerkraut in a container for a few days in a fridge is fine — pitting wants concentration, time and warmth, and a cold container for a week has none of them. What we'd avoid: leaving brine standing in steel for months, and letting a salty container dry out with the residue still on it. Rinse it, dry it, and the question doesn't arise.
And if you're fermenting a crock of sauerkraut over six weeks — that isn't what a lunch container is for, in any material.
The two things steel won't do
It won't keep anything hot or cold. Ours is single-wall, no vacuum. Coffee will not stay warm in it, a salad will not stay cold in it, and any guide telling you plain steel holds temperature is describing a vacuum flask, not a container.
It won't go in a microwave. Ever. Food comes out onto a plate. If that's a dealbreaker, it's a dealbreaker, and we'd rather you knew now.
What's left
Two materials, both named: 304 stainless steel and one food-grade silicone seal. That's the whole list, lid included. You can read exactly what touches your food, gaps and all.
Most food is fine. The rules are fewer than the internet suggests, and the one real exception is a jar of brine you were going to leave alone for a year anyway.
Food should be stored without doubt.