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Comment by lithocarpus | Hacker News Reader

> 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.

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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.

Analogous is the label on vodkas that says “distilled N times”, as if they were making it in batches using pot stills.

Vodka is made almost exclusively in industrial-scale continuous fractionating columns. The concept of it being “distilled N times” is farcical. (But technically defensible because you can map the output of the fractionating column to any arbitrary N via equivalence relationships.)

That's interesting and nice to know.

Good point that industrial use might not strictly be "reusing" - that might apply more to restaurants where they have big vats of frying oil and keep dipping food in it for multiple days. Even worse I've seen small scale home fryers where people neglect to change the oil for who knows how long - but that's probably not super common.

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...

Absolutely agree. I don't think I'd want industrially prepared stuff fried in tallow either.

There is a chemical distinction in that tallow is mostly saturated fat which doesn't oxidize nearly as easily, so it might be less bad to fry with or re-fry with. But still.

saturated fat is bad however:

> A 2020 Cochrane meta-analysis of roughly 59,000 participants across 15 randomized controlled trials found that replacing saturated fat with polyunsaturated fat reduced combined cardiovascular events by 21%.

…about same difference as taking statins (per the article)

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