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https://cooking.stackexchange.com/a/5863/6168

If you put any acid in cast-iron, you are harming your seasoning, and leeching iron into your food. This will affect the taste of your sauces, I find pan sauces taste metallic when made in cast iron.

Indian dishes often require tomatoes in most of the dishes. I am not intending to cook any tomato sauces in the Kadhai. Will I never be able to cook any Indian dishes requiring tomatoes in a well seasoned iron Kadhai (assuming I'll re-season again)?

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Aquarius_Girl
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  • duplicate: http://cooking.stackexchange.com/questions/11547/would-a-cast-iron-pot-work-well-for-chili-if-so-why the answer there applies to this question as well. – Aquarius_Girl Jan 01 '13 at 07:07

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The PH scale ranges from 0 - 14 with the lower numbers being more acidic than the higher number. Canned tomatoes are generally around 3.5-4.5 on the acidic scale, in other words just above the half way point below neutral 7.

As long as your tomato based sauce is not left in your seasoned Kadai (as Kristina rightly points out) for a long timeframe and your kadhai is washed out immediately after use no damage to the seasoned coating should occur.

Just remember to wash out your Kadai (with warm water and a cloth) immediately after use to preserve the seasoned coating.

spiceyokooko
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    pH is a logarithmic scale, so pH 3.5 is not "twice" as acidic as pH 7. It's about 5,000 times more acidic. – baka Jan 01 '13 at 02:57
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    @baka Useful to point out, but I don't know why you think I said ph 3.5 was twice as acidic as 7, because I haven't. – spiceyokooko Jan 01 '13 at 10:32
  • @baka Nitpicking maybe, but I get the figure to ~3000 times more acidic. Or, since a pH of 7 is neither acidic nor alkaline, the concentration of hydrogen ions is ~3000 times greater. – Chris Steinbach Jan 01 '13 at 14:33
  • @spiceyokooko: " in other words just above the half way point below neutral 7" – baka Jan 01 '13 at 17:28
  • @ChrisSteinbach: Sorry, I was rounding, and not actually doing the math. – baka Jan 01 '13 at 17:28
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    @baka Yes, but that's not saying how much more acidic a ph of 3.5-4.5 is over a ph of 7. It's saying where it lies on the ph scale for the benefit of those who may not know or understand the ph scale, specifically which side is acidic and which side is alkaline. – spiceyokooko Jan 01 '13 at 23:29
  • what about the PH level of fresh tomatoes? – Aquarius_Girl Jan 03 '13 at 05:40
  • @AnishaKaul Fresh tomatoes generally have a slightly higher ph than canned, depending on ripeness, variety, growing conditions etc. They generally fall in the 4-5 ph range. – spiceyokooko Jan 03 '13 at 12:36
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I've made tomato-based dishes in my seasoned cast iron pans including spaghetti sauce and chili with no perceivable off-taste or damage to the pans' seasoning as long as I removed the food soon after cooking since prolonged exposure, from my experience, will affect the seasoning of the pan, if not also the flavor of the food.

Kristina Lopez
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According to this site,

For pH values below 4.0, ferrous oxide (FeO) is soluble. Thus, the oxide dissolves as it is formed rather than depositing on the metal surface to form a film.

Once ferrous oxid (i.e. iron oxid) becomes soluble, your body is able to metabolise it. Your body is able to regulate iron levels to some extent, so while it doesn't seem likely that this, on its own, would lead to iron poisoning, anyone taking iron supplements might want to avoid cooking canned tomatoes in cast-iron. Fresh tomatoes have a pH between 4.3 and 4.9 according to this online source and should be alright.

Note also, from the site in my first link, iron corrosion increases drastically below a pH of around 3.8. If spiceyokooko is correct, and the pH value range for canned tomato is between 3.5 and 4.5, I think you would do well to measure the pH value of your tomatoes before using cast-iron. There are various inexpensive pH test kits and instruments available. It's quite likely that your pharmacy stocks alkacid paper or similar.

I also recommend you read this excellent answer about cooking wine and vinegar in cast-iron.

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Chris Steinbach
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