> There’s a secondary argument about oxidation — seed oils go rancid at high heat, producing potentially harmful compounds. This idea is chemically real and worth being thoughtful about (don’t reuse frying oil repeatedly). But the evidence that oxidation at home-cooking levels causes measurable harm in humans isn’t there.
Even if oxidation at home-cooking levels doesn't cause harm, which I suspect that it does though to a lesser degree, two thirds of seed oil market in the US is industrial or prepared food, much of which does go rancid or is reused frying oil.
I work in an area that involves, among many other things, analysis of cooking oil in factories. It might be hard to pin down the terminology of "reused" frying oil, because many of the frying processes are continuous. The raw material goes through a vat of hot oil on a conveyer and comes out the other end dripping with oil
The quality of the oil is continuously monitored, and new oil goes in while old oil goes out in the fried food itself. The crunchy and salty aspects make it palatable to eat oil. The oil doesn't actually spend a long time in the vat before coming out in the product.
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If that industrial prepared food switches to tallow I still have doubts about how much it is reused and what chemical changes it goes through...
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