Some stuff I learned about cooking oils recently.
I would consider this to be extremely important and everyone should at least take into consideration for maintaining health.https://besynchro.com/blogs/blog/7770981-oxidized-oils-is-your-cooking-oil-toxic
I can't say much for this specific website, but it checks out my sniff test based on my soap making experience, my long time cooking enthusiast self, and cross-referencing with other websites on the matter.
Oils processed from various foods (olives, seeds, nuts, grains...) break down/oxidize/go rancid over an amount of time, generally on the order of months to a few years. How fast they go rancid depends on temperature, humidity, exposure to light, and probably what they are being used to cook with.
All of these oils are composed of various different fatty acids that do different things at different temperatures or exposed to different chemicals (like when making soap or cooking with garlic or whatever). Some oils will last longer than others or start burning at higher temperatures because their fatty acid chemical makeup is different. It is impossible to get an oil that is composed of only one of these fatty acids, so we just have to make due with overall oil composition.
Common street knowledge is that deep fried shit is shit for your health.
Most of the time this is correct. The main reason is that restaurants reuse the oil over and over again at relatively high temperatures. They typically use canola oil, corn oil, or some other similar vegetable oil that won't burn at the higher temps. They (likely) won't change the oil out until the oil is far past its rancid state. It is oil that has gone rancid that is very unhealthy, not strictly because something is deep fried. Oil goes rancid much quicker at higher temperatures.
How do you know if oil has gone rancid?
As near as I can tell the only way most people test is if it smells bad. I'd also say throw the oil away if its past the "best by" date. Or maybe burn the oil in an oil lamp or your diesel truck or generator instead of throwing it in the dumpster. Companies processing the oils test for the totox value, or probably other similar tests.http://www.andersonintl.com/oil-oxidation-how-to-measure-it-and-why-it-matters/https://www.sciencedirect.com/science/article/pii/B9780081004357000162
What happens when you consume rancid oil?
Cancer, among other things.>Plant-based oils, such as safflower oil and sunflower oil, contain plenty of polyunsaturated fatty acids (PUFAs) which oxidize easily in the presence of light, heat, or oxygen in the air. Many of the oxidation products of PUFAs have been reported to have cytotoxic (toxic to cells) and mutagenic (capable of changing the DNA) effects. Cytotoxic and mutagenic substances are commonly known to increase the risk of cancer, and indeed, a 2002 study published in the journal Anticancer Research reported that rancid oils not only appear to be involved in tumor promotion but also in tumor initiation. This study was carried out on mice, and rancid corn oil was used as the source of spoiled fatty acids.https://www.healwithfood.org/bad-for-you/rancid-oil.php
Technically my research into this subject started when I started making soap. One of the first things I came across was the "dreaded yellow spots" that appeared in my bars of soap. Turns out those come from rancid oils being used, and the soap making process does not preserve the oils in anyway either. So I learned of the relationship between temp, humidity, and age of oils to how quickly they go rancid. I just didn't make the connection of rancid soap to rancid cooking oils until recently, because I'm a dumbass.
In the soap making world, canola, sunflower, safflower, grapeseed, and corn oils are considered crap to use because they go rancid rather quickly. You can often find these oils in beard oils too, not because they are good for the skin or hair, but because they are cheap filler oils used to make more money with.
Once upon a time, crysco was developed as an attempt to make cottonseed oil last longer for the soap manufacturers at the time. Since then, they instead moved away from cottonseed oil and instead focused on a vegetable shortening product to replace lard with for baking. Lard and tallow are extremely long-lasting oils that are high in saturated fatty acids, along with most other oils that are solid at room temperature.
One time I had an FDA paid "expert" dietitian try to tell me that saturated fatty acid oils like coconut oil was unhealthy, specifically calling out coconut oil. These people want you to get cancer and die. But (you) already knew that, right?
As near as I can tell, "saturated fats are unhealthy" is a hit to promote the canola oil and vegetable oil industry paid for by companies like pillsbury (they make crysco) to the FDA.
Even though olive oil is really healthy, at this point I would not recommend using it for pan frying much of anything because of how easily it burns and breaks down. Grapeseed oil is really healthy too but goes rancid rather quickly, but at least it does not burn at lower temps. I would recommend frying everything in either coconut oil or lard. Lard is really good because you can make sure your guests are not islamist shitheads or kikes.
tl;dr don't buy cooking oils that are a few months away from the "best by" or expiration date. Throw away smelly oils. The shit causes cancer.