What is the difference in cooking oil before and after frying heavily?

[abdulbadii]

TL;DR Summary

The difference frying oil before and after heavy frying/cooking

What's actually details in difference of any cooking oil (vegetable, palm oil, etc) before and after exhaustive/heavy cooking ?
As only heard not clear or reliable folks' says, the many unsaturated bonds turns to be saturated afterward..
The need arose as it's quite affirmed that the former is so good as lubricating/penetrating oil, but not sure the difference if the same work performed by the used one, so need this scientifically explanation/arguments

 

[Bystander]

Summary:: The difference frying oil before and after heavy frying/cooking

What's actually details in difference of any cooking oil (vegetable, palm oil, etc) before and after exhaustive/heavy cooking ?
As only heard not clear or reliable folks' says, the many unsaturated bonds turns to be saturated afterward..
The need arose as it's quite affirmed that the former is so good as lubricating/penetrating oil, but not sure the difference if the same work performed by the used one, so need this scientifically explanation/arguments

See "pyrolysis."

 

[Rive]

I think pyrolisis is not exactly adequate for this. During usage the cooking oil absorbs some parts of the food (water, fats, small solid pieces) with quite diverse chemical properties and heat stability. What happens afterwards is a complex process of not only breaking down but quite amount of polymerization and combination.

The need arose as it's quite affirmed that the former is so good as lubricating/penetrating oil

If it's a run down bicycle where the proper lubrication would double it's worth, then maybe. But I would not use it for anything else.

The most creative 'raw' usage I have heard so far was for wood preservation. Somebody told me that it kills wood bugs some marvelously.
I've stopped buying from fry food stands after that conversation, though.

 

[jim mcnamara]

When cooking oil smokes during cooking it will create small amounts of acrolein which is undesirable from several viewpoints, so experienced cooks use oils with high smoke points to avoid this problem.

See:
https://en.wikipedia.org/wiki/Acrolein

Smoke point examples in deg C, oil names may vary by country:
Canola oil - 205
Olive oil- 240
Flax seed oil - 107 - not recommended for frying.

This discusses cooking oil reuse, mixing, smoke points, rancidity, etc. And lists most oils and some properties --
See:
https://www.seriouseats.com/cooking-fats-101-whats-a-smoke-point-and-why-does-it-matter.