Short answer: far less than the internet tells you, and almost certainly not in your fridge.
Here is the long answer, including the part that isn't settled.
Where the scary number comes from
Nearly every article claiming steel leaches metal into your food traces back to one study. Oregon State, 2013, published in the Journal of Agricultural and Food Chemistry.
It's a good study. It found real numbers. After six hours, nickel in tomato sauce rose up to 26-fold. By the tenth run, about 88 micrograms of nickel per serving.
Read the method, though.
The sauce was cooked. Simmered. Two to twenty hours, at boiling point, in a saucepan.
That is not a container in a fridge. That is a stockpot on a burner for most of a working day.
The study is quoted constantly by people selling glass. The word “cooking” is in its title.
The part that surprises people
Here's what the same body of research found, and what almost nobody repeats:
Steel leaches most when it's new, and less every time you use it.
Release dropped with each cooking cycle and levelled off after the sixth. The reason is that the chromium oxide film on the surface doesn't wear away with exposure — it enriches. The surface becomes progressively more chromium-rich and more corrosion-resistant over time.
The material gets tougher the longer you own it.
That is the exact opposite of what a plastic container does, and it is the entire reason we put ten years on the steel.
So what about pH?
The chemistry is real and worth understanding.
Lemon juice sits around pH 2.0–2.6. Vinegar, 2.4–3.4. Tomato sauce, 3.5–4.5. Coffee, 4.8–5.1. Chicken, 5.6–6.0.
Lower pH means more available hydrogen ions, which means more potential to react with metal. That part is true, and it's why the cooking study found what it found — heat and acid and hours, together.
Take away the heat and you take away most of the reaction. Chemical reactions slow dramatically as temperature drops. Your fridge is roughly 96 degrees colder than a simmer.
What we don't know
We looked for a study measuring 304 steel against acidic food at fridge temperature over several days.
We couldn't find one. As far as we can tell, nobody has run it.
So we're not going to tell you it's proven safe. What we can tell you is that the research people cite to frighten you is cooking research, and that the chemistry gives every reason to expect cold storage to be a different order of magnitude entirely.
If someone claims certainty here in either direction, they're ahead of the evidence.
Would 316 help?
No — and we've written about this separately.
316 adds molybdenum for saltwater and chemical plants. Boat fittings. Marine air. When seven common food-contact grades were tested against the Council of Europe's release limits, all of them passed.
316 isn't the safer grade. It's the marine grade. Your fridge is not the North Sea.
Is glass the answer instead?
Glass is genuinely inert. That's a real advantage and we're not going to pretend otherwise.
It also breaks, it's heavy, and in the Netherlands you cannot recycle a glass food container — it doesn't go in the glasbak, it goes in the restafval, and it's incinerated. That's the government's own guidance, not ours.
Pick whichever you like. Just pick it for a real reason.
If your Thursday chili tastes off
Consider the more likely explanation before you blame the box.
It's Thursday. The chili was cooked on Sunday. Four days is a long time for cooked food, and flavours in a chili shift on their own — fat oxidises, alliums turn, acid keeps working on everything in there.
If you want to be careful with a very acidic dish across a full week, the sensible move isn't a different container material. It's eating it earlier in the week, or freezing the back half on Sunday.
That's not a steel problem. That's a Sunday problem.
What we'll say plainly
Every surface your food touches is 304. The walls, the lids, and the inside of the bottle cap — 304 food-grade stainless steel, with one food-grade silicone ring.
We print the grade because most brands don't, and the ones that write “food grade” without a number are usually hoping you won't ask.
Store your tomato sauce in it.
Food should be stored without doubt.