Cooking with Acid

When Acidic Foods Are Fine in Vintage Iron, When They Aren't, and How to Handle Tomatoes, Wine, and Citrus Without Damaging a Restored Surface

If there's one piece of cast iron advice that gets passed around more than any other, it's this: never cook acidic foods in cast iron. Like most kitchen wisdom that gets repeated often enough, there's truth in it—but the full picture is more nuanced. A well-seasoned vintage skillet can handle tomatoes, wine, and citrus just fine within reasonable limits. Understanding what those limits are, and why they exist, lets you use your cast iron for the full range of cooking it was designed for.

The Chemistry: What Acid Does to Cast Iron

Cast iron is a reactive metal. When acidic ingredients come into contact with bare iron, a chemical reaction occurs: hydrogen ions in the acid react with iron atoms, releasing iron molecules into the food. This is the source of the metallic taste people associate with cooking tomato sauce in cast iron—you're literally tasting dissolved iron.

Seasoning acts as a barrier. The polymerized fat layer that makes cast iron nonstick also protects the underlying iron from direct contact with acidic ingredients. A well-built seasoning layer prevents the acid-iron reaction—up to a point. Given enough time, acid will begin to break down the seasoning itself, exposing the iron beneath and starting the leaching process.

This is why cooking time matters more than whether you use acidic ingredients at all. A quick deglaze with wine takes thirty seconds. A slow-simmered marinara takes three hours. The chemistry is the same; the exposure time is completely different.

The pH Scale: Not All Acids Are Equal

Acidity is measured on the pH scale, which runs from 0 (highly acidic) to 14 (highly alkaline), with 7 being neutral. The lower the number, the more acidic the ingredient. Here's where common cooking ingredients fall:

•         Lemon juice: pH 2.0–2.6 (highly acidic)

•         Vinegar: pH 2.4–3.4 (highly acidic)

•         Wine: pH 3.0–4.0 (moderately acidic)

•         Tomatoes: pH 4.0–4.5 (moderately acidic)

•         Cherries and stone fruits: pH 3.2–4.5 (moderately acidic)

•         Coffee: pH 4.5–5.0 (mildly acidic)

The practical takeaway: lemon juice and vinegar are significantly more aggressive than tomatoes. A splash of lemon to finish a dish is a different proposition than simmering a pan in straight vinegar. Tomatoes, despite their reputation, are actually on the milder end of culinary acids.

The 30-Minute Rule

America's Test Kitchen conducted a definitive experiment on this question. They simmered tomato sauce in both seasoned and unseasoned cast iron skillets, tasting the sauce at fifteen-minute intervals and sending samples to a lab for iron content analysis.

The results: at fifteen minutes, tasters couldn't detect any metallic flavor in either pan. At thirty minutes, the sauce from both pans had developed a noticeable metallic taste—though the sauce from the unseasoned pan was far worse. Lab analysis confirmed that the unseasoned pan leached nearly ten times as much iron as the seasoned pan.

This gives us a practical guideline: in a well-seasoned pan, you have roughly thirty minutes before acidic ingredients begin to cause problems. That's plenty of time for most cooking applications—sautéing, pan sauces, quick simmers—but not enough for long braises or all-day tomato sauces.

What's Safe in Cast Iron

Given the thirty-minute threshold, here's what you can confidently cook in a well-seasoned vintage skillet:

Deglazing

This is the most common use of acid in cast iron, and it's completely safe. After searing a steak or browning chicken, splashing wine, stock, or even a little lemon juice into the hot pan to lift the fond takes less than a minute. The acid is in contact with the seasoning for seconds, not hours. Deglaze without hesitation.

Pan Sauces

A classic pan sauce—wine reduced with shallots, finished with butter—typically takes five to ten minutes from deglaze to plate. That's well within safe limits. Steak au poivre, chicken piccata, pork chops with wine and mushrooms: all fine in cast iron.

Quick Tomato Dishes

Shakshuka, eggs in purgatory, a quick skillet of tomatoes and sausage—anything where the tomatoes cook for fifteen to twenty minutes poses no problem. The key is keeping the simmer time under thirty minutes.

Finishing with Citrus

Adding lemon juice to a dish just before serving is safe. The acid barely touches the pan. Squeeze half a lemon over your sautéed greens, add a splash of lime to your carnitas—no problem.

What to Avoid—Or Move to Another Pan

Some applications genuinely don't belong in cast iron, regardless of how well-seasoned it is:

Long-Simmered Tomato Sauces

A proper Sunday gravy or Bolognese that simmers for two, three, four hours will damage even excellent seasoning. Use enameled cast iron (Le Creuset, Staub) or stainless steel for these applications. The enamel coating is nonreactive—it's effectively glass—and doesn't interact with acids at all.

Vinegar-Based Braises

Adobo, sauerbraten, anything that simmers in vinegar for an extended period: these belong in nonreactive cookware. Vinegar's low pH makes it more aggressive than tomatoes, and braising times of an hour or more compound the problem.

Reduction of Straight Citrus or Wine

Reducing a cup of wine or lemon juice to a concentrated sauce keeps acid in contact with the pan for an extended period at high concentration. Diluted acids (wine in a braise with stock, tomatoes with other vegetables) are safer than concentrated acids alone.

Storing Acidic Food in Cast Iron

Never leave acidic food sitting in a cast iron pan after cooking. Don't make chili in the Dutch oven and refrigerate it overnight still in the pot. Transfer to a storage container immediately after cooking. Overnight exposure will strip seasoning and leave you with rust spots and metallic-tasting food.

Seasoning Quality Matters

Everything above assumes you're working with a well-seasoned pan. Seasoning quality dramatically affects how your cast iron handles acid.

New or lightly seasoned pans: Avoid acidic ingredients entirely until you've built a solid foundation. Lodge recommends waiting until your pan has been used regularly for several weeks before introducing tomatoes or wine. Cook fatty foods—bacon, sausage, fried chicken—to build seasoning first.

Well-seasoned pans: A pan with months or years of regular use can handle more acid and longer exposure times. The seasoning layer is thicker, more durable, and more resistant to breakdown. My daily-driver skillets have years of seasoning built up; they handle deglazing and quick tomato dishes without any visible effect.

Freshly restored pans: If you've just stripped and reseasoned a piece, treat it like new. Even if the pan has decades of history, you're starting the seasoning from scratch. Build it up before challenging it with acid.

If Damage Occurs

Suppose you forget about the marinara and let it simmer for two hours, or you stored last night's salsa in the skillet overnight. What now?

First, don't panic. Cast iron is nearly indestructible. You haven't ruined the pan—you've just set back the seasoning. Here's the recovery process:

1.       Clean immediately. Wash the pan thoroughly with hot water and a stiff brush. A little dish soap is fine here—you're already rebuilding the seasoning.

2.      Inspect for damage. Look for dull gray patches where seasoning has been stripped, or any orange rust spots. If you see bare metal or rust, proceed to step 3. If the seasoning looks intact (still black and relatively smooth), skip to step 5.

3.      Remove any rust. If rust has formed, scrub the affected areas with steel wool or a rust eraser until you're back to clean gray iron. Wash again to remove residue.

4.      Dry completely. Towel dry, then heat on the stovetop for several minutes to evaporate all moisture. This is critical—you don't want new rust forming.

5.      Reseason. Apply a very thin layer of oil to the entire cooking surface. Heat in the oven at 450–500°F for an hour, or season on the stovetop until the oil smokes and bonds. Repeat two or three times to rebuild a protective layer.

6.      Cook fatty foods. For the next several uses, cook bacon, fry chicken, sauté in butter—build the seasoning back up before reintroducing acidic ingredients.

Field Company puts it well: you really can't ruin cast iron. Even severely stripped seasoning can be rebuilt with regular cooking. The pan has survived worse than a long-simmered tomato sauce.

A Note on Iron in Food

Some iron leaching into food isn't necessarily bad. Cast iron has been used for centuries partly because it contributes dietary iron—the same form found in leafy greens and legumes. A study by the American Dietetic Association found that pasta sauce cooked in cast iron increased in iron content by nearly 1,000%, while less acidic foods like cornbread increased by only about 28%.

For people with iron-deficiency anemia, cooking in cast iron can actually be beneficial. For most people, the small amounts of iron that leach during normal cooking are harmless and may even be helpful. The issue isn't health—it's taste. Nobody wants their tomato sauce to taste like a penny.

Practical Summary

•         Under 30 minutes: Safe in a well-seasoned pan. Deglaze freely, make pan sauces, cook quick tomato dishes.

•         Over 30 minutes: Use enameled cast iron or stainless steel. Long braises, all-day sauces, and vinegar-based dishes belong in nonreactive cookware.

•         pH matters: Vinegar and citrus are more aggressive than tomatoes and wine. Be more cautious with concentrated acids.

•         Seasoning matters: New and freshly restored pans need time to build seasoning before handling acid. Well-seasoned pans are more forgiving.

•         Never store: Transfer acidic foods to storage containers immediately after cooking. Overnight contact will strip seasoning.

•         Clean promptly: After cooking acidic foods, wash the pan soon rather than letting residue sit.

•         Damage is fixable: If you do strip seasoning, the pan isn't ruined. Clean, reseason, and rebuild.

Vintage cast iron has been cooking tomatoes, deglazing with wine, and finishing with lemon juice for a hundred years. The key is understanding the limits—and having the right pan for the job. Keep an enameled Dutch oven for your Sunday gravy, and let your seasoned skillet do what it does best.