Cooking acidic foods and reactive cookware

What is actually happening

Acidic foods - tomatoes, wine, vinegar, citrus, and to a lesser degree buttermilk and some fruits - dissolve small quantities of metal from a reactive pan surface. The dissolved metal goes into the food, where you taste it, and leaves the pan, where you see it as dulling, darkening or pitting.

That is the entire mechanism. Everything else on this page is which metals do it, how fast, and whether it matters.

The nonreactive materials, and why

Stainless steel

Stainless is stainless because chromium in the alloy forms a thin, self-repairing chromium oxide layer on the surface. Acid does not get through it in any timescale relevant to cooking. This is why stainless is the default for tomato sauces, pan sauces deglazed with wine, and anything with lemon in it - is stainless steel cookware safe covers the material in full.

Enamel

Enamel is glass fused to iron. The food never touches metal, so an enameled Dutch oven is fully nonreactive and will hold a tomato braise for as long as you want. This is most of why the shape and the finish became the standard for long-cooked acidic dishes - enameled cast iron care.

Glass, ceramic and coatings

Glass and glazed ceramic react with nothing. Nonstick coatings, PTFE and ceramic alike, are inert to food acids - a coated pan's limits are heat and abrasion, not chemistry - is nonstick safe.

The reactive materials

Bare aluminum: the main offender

Uncoated aluminum reacts readily. Simmer tomato sauce in a bare aluminum pan and you get a distinctly metallic taste and a pan that turns dull gray where the sauce sat. This is the main reason the light commercial aluminum pans professionals favor are a poor general-purpose choice at home - restaurant cookware for home kitchens.

Two important exceptions. Anodized aluminum has a hardened, sealed surface and is substantially less reactive, which is a genuine selling point of hard-anodized lines - best hard-anodized cookware. And the aluminum core of a clad pan never touches food at all, so a clad stainless pan is nonreactive regardless of what is inside it.

Unlined copper

Copper reacts strongly with acid and copper is not something to be ingesting in quantity. Culinary copper is therefore lined, historically with tin and now usually with stainless. Unlined copper has exactly one traditional food use, egg whites, and one common one, jam - both short, both specific. If the lining on a copper pan has worn through to bare copper, that pan needs re-lining before further use - best copper cookware.

Bare cast iron and carbon steel: the nuanced case

Seasoning is a layer of polymerized oil, and it is a genuine barrier - but it is a thin organic one, and acid degrades it. A quick pan sauce with a splash of wine in a well-seasoned skillet is completely fine and is standard technique. A two-hour tomato simmer is not: it will strip seasoning, expose bare iron, put an unmistakable metallic note into the sauce, and leave you re-seasoning the pan.

The practical rule is about thirty minutes on a well-established seasoning, and less on a new pan. Deglazing, which takes a minute, is never a problem. This is one of the most common causes of a sauce tasting wrong for no apparent reason - cast iron problems and fixes and carbon steel problems and fixes.

A new cast iron or carbon steel pan is far more reactive than a mature one, because the seasoning is thin. Save the tomato sauce until the pan has had a few months of ordinary cooking.

How acidic is acidic?

Time and temperature matter as much as acidity. Hot and long is what does damage; cold and brief does very little. Marinating in a metal bowl overnight is worse than a hot fifteen-minute simmer, which surprises people.

Is it dangerous, or just unpleasant?

For seasoned iron, the effect is culinary rather than a health concern - iron is a nutrient, and the quantities involved are small. People with hemochromatosis are the exception and should raise it with a doctor rather than with a cookware site.

For aluminum, the practical issue is taste and pan damage; the dietary contribution from cookware is small relative to other sources. For unlined copper, the caution is real and is the reason culinary copper has been lined for centuries.

The practical rules

1. Tomato sauce, chili, anything simmered long: stainless or enameled cast iron.
2. Deglazing with wine or vinegar: any pan, including seasoned iron. It is too brief to matter.
3. Lemon or vinegar dressings and marinades: glass or stainless, never bare aluminum, and never leave them standing in metal overnight.
4. Acidic desserts and fruit bakes: glass, ceramic or coated bakeware - baking dishes and casseroles.
5. Curds and custards with citrus: stainless, gently, ideally over water.
6. Storing leftovers: not in the pan, whatever it is made of. Acidic food standing cold in metal for two days is the worst case on this page.