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We shouldn't cook acids, alkali and concentrated salts on teflon. But what are exact numbers? Can I put a sour dairy stuff for on it example, like kefir (it's good to put it in pancakes) ? Where is safe pH range?

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We shouldn't cook acids, alkali and concentrated salts on teflon.

This is incorrect. Teflon (PTFE) itself is one of the most non-reactive substances you can use on cookware, in some ways better than ceramic. To quote Wikipedia:

It is nonreactive, partly because of the strength of carbon–fluorine bonds, and so it is often used in containers and pipework for reactive and corrosive chemicals.

So why is there so much advice on not cooking high-acid foods in teflon pans? As far as I can tell, it's because teflon pans scratch easily, and acids can get into scratches. This has two potential effects:

  1. On pans with poor bonding between teflon and the backing metal, the acid can dissolve some of the adhesion between PTFE and pan, causing it to flake off.
  2. Many teflon pans have the teflon backed by aluminum, and aluminum gives off flavors to acidic foods. So even the small amount of aluminum exposed by the scratches can cause the high-acid food to taste "off", especially if it is slow-cooking.

Given the above, however, I think you can see that there is no specific pH that's bad for teflon pans. The lower the pH the worse the two above effects can potentially be, but a lot more depends on the manufacture of the pan, how scratched or abraded the teflon is, and even how long and how hot the food is in the pan. And if your teflon pan is brand-new and high-quality, it's one of the best vessels for making high-acid foods you have.

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    The backing metal on Teflon pans is often aluminum. If the Teflon has any holes to bare metal, pH 4.2 (tomato sauce or so) will corrode the surface and enlarge the hole. pH 9 and up will also corrode aluminum, but we don't usually eat foods that basic. Boiling sodium carbonate solutions in an aluminum Teflon pan would be a bad idea. – Wayfaring Stranger Nov 26 '19 at 0:01
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    Yeah, and potassium and magnesium flourides are also a really bad idea, but I'd be more concerned about the eater than the pan. – FuzzyChef Nov 26 '19 at 2:10
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    -FuzzyChef: I nixtamalize corn in Calcium hydroxide. That'd be the death of an aluminum pot, Teflon or not. en.wikipedia.org/wiki/Nixtamalization – Wayfaring Stranger Nov 26 '19 at 3:01
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    Indeed, one of the earliest uses of Teflon was as a chemically inert coating in uranium hexafluoride plants during the Manhattan Project. – David Richerby Nov 26 '19 at 10:42
  • @WayfaringStranger I don't know about calcium hydroxide, but as far as I remember, I have successfully held lye (sodium hydroxide) in a teflon pot for the purpose of soap making. – rumtscho Nov 26 '19 at 15:30
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I don't know where your information came from about "We shouldn't cook acids, alkali and concentrated salts on teflon", but it is incorrect. Teflon is the least chemically reactive material you are likely to encounter in daily life. There probably are a few substances that can react with it, but if you have a way of sourcing these, you hopefully won't be asking about this on the Internet.

While I can't give you exact numbers, anything that you can eat is safe to use on teflon by a very wide margin. Go ahead and cook it.

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    "Teflon is the least chemically reactive material you are likely to encounter in daily life, that's its whole selling point." I agree that Teflon is very unreactive but its "whole selling point" is that things don't stick to it. That has nothing really to do with its chemical reactivity. – David Richerby Nov 26 '19 at 10:36
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    @DavidRicherby Maybe the sentence is not perfectly formulated. The way I learned it, it is exactly the inertness that makes Teflon nonstick. So, at least for my old chemistry teacher, it being inert and it being nonstick was kinda the same thing, and he never differentiated in language between the two. I can remove that part of the sentence, to make it more precise. – rumtscho Nov 26 '19 at 15:34
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    @DavidRicherby Teflon's chemical inertness was in fact its first commercial use. Nonstick pans came later. – barbecue Nov 26 '19 at 16:48
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    @barbecue Yes, I already commented on the use of Teflon in the Manhattan Project. But we’re in the context of cooking, not chemical engineering. If somebody is selling you a Teflon pan, their whole spiel will be about how stuff doesn’t stick to it. – David Richerby Nov 26 '19 at 16:56
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    As @rumtscho pointed out, it's really the same thing. Extremely strong C-F bonds make PTFE very reluctant to engage in any kind of bonding, which is what's involved in both reactions and friction. – barbecue Nov 26 '19 at 17:06

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