Fat is bad for you
No, fat is a simple carrier of energy. All the accounts saying fat is bad for you focus on obesity. If you are at a normal weight and you burn as many calories as you consume there is nothing bad about fats. In fact, you need a little amount of fat to be able to take up vitamins A, D, K and E, along with certain Phytochemicals (miscellaneous chemicals found in plants, of which some, dubbed ‘phytonutrients’, are thought to be good for you).
But, then, at least saturated fats are bad for you, right?
Yes, saturated fats raise the amount of low density lipoprotein (LDL) in your bloodstream. Cholesterol by itself is not bad for the body, in fact, the body makes cholesterol itself. But when bound to LDL in excess, a lot of cholesterol will enter the bloodstream instead of being send to the liver (what happens if it gets bound to high density lipoprotein, HDL) where the LDL might react with free radicals and attach to damaged blood vessel walls.
Nonsaturated fats raise the amount of HDL in the blood and thus prevent heart disease. But the effect of regular excercise, losing excess weight and stopping with smoking will have a greater total effect on the HDL/LDL ratio.
So, using nonsaturated fats, like olive oil, are good for cooking?
No! If you use highly nonsaturated fats that means they are amazing for cold dishes. Think about e.g. extra virgin olive oil, and linseed oil. But nonsaturated fats are much more reactive than their saturated counterparts, so as soon as you heat them they will start to oxidize, polymerize and to hydrolize. Also, nonsaturated bonds can switch from cis- to transconfiguration (where the latter is thought to increase LDL).
Both hydrolysis and oxidation will create free fatty acids that readily polymerize. “Polymerisation reactions are particularly significant in frying oils where prolonged use leads to high molecular weight compounds which cause foaming and increased viscosity. Discarded oils are sometimes found to contain as much as 25% of polymerised material.” – Food: The Chemistry of Its Components
What kind of oil is best for deep frying then?
For deep frying you would want a fat that is as saturated as can be, which at the same time has a high smoking point, the oil also has to be cheap and relatively tasteless. The two oils that have a high smoking point, are relatively cheap and tasteless are difractionated palm oil, and semirefined sunflower oil. Where the latter is the general choice of oil for deep frying. But palm oil is a lot more saturated, so probably a better choice. Soybean oil also fits with the requirements except for being very unsaturated. This makes it good for wok dishes, where the oil is directly consumed after cooking or thrown away.
Conclusion & directions
Use one of the oils listed in the section above
Regularly change the oil. If the oil turns thick or frothy, you are way overdue. Also, when you change the oil, throw ALL the oil away, and get rid of any crumbs and what not. Any oxidized remains, crumbs and salt kickstart the oxidation process and will significantly decrease the life span of your fresh oil. Best to clean the entire pan with soap and rinse well with water.
Buy a frying pan with a powerful heating element. When you add your fries to the heated oil, the oil will decrease in temperature. This decrease in temperature will make it easier for the fat to seep into your fries. When you have a powerful heating element in your frying pan, the temperature will be back up to 180 C (356 F) in no time, searing close the crust of the fries. (This is also why making pan fries works so amazingly in a cast iron pan. Cast iron heats up really slowly and unevenly, but when properly heated it will have a shitton of heat trapped in it, so the temperature of the pan will not drop much even when you throw frozen fries in it.)
Another way of keeping the temperature steady is filling your oil up to the max as more oil will hold more heat and thus will cool down less.
Should you use thawed fries then? I once asked this to my professor but he said that the thawed fries were a bit higher in fat content and more sompy. This is probably because fat cannot penetrate the frozen fry, and thus the frying pan has more time to sear close the crisp of the fry before the interior properly thawes and heats up.
Don’t put your frying pan higher than 190 C (374 F). A lot of research on this matter has already shown that 180 C (356 F) is the best temperature for frying a lot of things and every degree above that will make the oxidative process exponentially worse.
If possible, obtain a frying pan with a lid that can be used to close the oil tank airtight, so no oxygen can dissolve into the oil while you are not using it. I don’t know exactly how effective this is, but it will at least help a little.
I hope this gave you a guideline on how to deepfry as healthy as possible.