Family: Cookware
Stainless steel cookware
An iron-chromium alloy under a protective film: it releases nickel and chromium while cooking, most of all when new and in acid.
Overview
What makes stainless steel stainless is not the resistance of iron itself but a thin oxide film on the surface: direct surface measurements of 316L — the grade used for food-contact cookware — show that this film is enriched with chromium and molybdenum. The film is not absolute. Cooking in an acidic medium moves nickel and chromium from the steel into the food: in measurements on tomato sauce their concentrations rose tens of times over, most of all with new cookware, and fell from cycle to cycle, settling at a steady level after roughly the sixth cooking. Whether those amounts mean anything is decided not by the numbers themselves but by an agency reference: the tolerable daily intake of nickel is 13 µg per kilogram of body weight.
Quality indicators
- State of the evidence
The numbers come from a single 2013 study; the agency reference is current
More detail
All the figures for nickel and chromium release come from one work, and it is more than ten years old — the platform marks it as stale. No later measurements of the same subject were found. The reference against which those figures are judged is, by contrast, current: the 2020 agency assessment rests on more than 47,000 occurrence measurements.
Composition
- What the material is
The surface oxide film is enriched with chromium and molybdenum
More detail
On 316L steel, what is usually only described has been measured: the protective film is bi-layered, and the share of chromium and molybdenum within it grows. The measurement avoided any contact with air during transfer, so the film was studied as it had formed in solution.
Qualitative characteristics
- What changes the amount
Acidity, time, grade of steel and age of the pan
More detail
Most is released from new cookware; with each successive cooking cycle release falls and settles at a steady level after roughly the sixth. Long cooking increases release: over 20 hours nickel rose 34-fold and chromium about 35-fold relative to sauce cooked without steel. Different grades of steel give different amounts.
Health-related indicators
- What the pan adds to the food
Nickel and chromium — substances some people are sensitised to
More detail
In measurements on tomato sauce, nickel concentrations rose up to 26-fold and chromium up to 7-fold after six hours of cooking, depending on the grade of steel. At the tenth cooking cycle an average of 88 µg of nickel and 86 µg of chromium passed into a 126 g serving of sauce. The agency assessment names systemic contact dermatitis in nickel-sensitised people as the critical acute effect.
- Plastic particles in the food
In the experiment it added none: non-plastic cookware introduced no microplastics
More detail
In the same experiment non-plastic cookware introduced no microplastics into the food simulant at all — a measured zero, not an absence of data. The caveat: the work divides cookware into plastic and non-plastic and does not name a particular grade of steel.
Regulatory notes
- Agency reference
The tolerable daily intake of nickel is 13 µg per kg of body weight
More detail
The 2020 agency assessment sets a tolerable daily intake of nickel at 13 µg per kilogram of body weight; for the acute effect in nickel-sensitised people the figure given is 4.3 µg per kilogram. The figures apply to nickel from ALL dietary sources, not from cookware alone.
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