Frying should be generally avoided. This is because it involves high heat that dissociates oils and fats, often rendering them carcinogenic or worse. But of course frying, like broiling and barbecuing, are such delicious ways of having some of our foods, that vague warnings are not especially useful. So let us be pragmatically specific.
You probably know that nutritionists advocate the use of unsaturated fats, since most of the processed foods we buy and eat are choked with saturated fats. A bit of balance never did anybody any harm. But what happens when we fry with unsaturated fats? Well, at temperatures of 150 deg C (300 deg F), unsaturated fatty acids become mutagenic, that is, they can damage our genes and these of our offspring and of our entire family. Above 160 deg C (320 deg F) trans-fatty acids begin to form. Above 200 deg C (390 deg F) trans-fatty acids are formed in substantial quantities. Above 220 deg C (428 deg F) trans-fatty acids form at exponential rates.
So what is wrong with trans-fatty acids, you'll say. Aren't even our best margarines, shortenings and other convenience foods made out of trans-fatty acids? They most certainly are. But this is a reason for avoiding them, not for becoming a manufacturer of trans-fatty acids in your own kitchen. For trans-fatty acids are twisted. As a well known writer on fats has written, imagine having the top part of your body twisted back to front. You'd never know whether you are coming or going. Trans-fatty acids are an example of this problem at the molecular level. Let us discuss these fatty acids a little further.
Trans-fatty acids are misfits in the scheme of things. The change in shape from the normal bent cis- form to the straight line of the trans- form, is a wrench thrown in the works. The trans- form only half-fits into enzyme and membrane structures of biological systems, taking up the space of cis- forms, but incapable of doing the work of normal fatty acids. They are not mere misfits; they are downright saboteurs of our wellbeing. Then, there is an undesirable stickiness about trans-fatty acids. For example, cis-oleic acid melts at 13 deg C (55 deg F) and is therefore liquid at room and body temperatures. Trans-oleic acid melts at 44 deg C (111 deg F) and is solid both at room and body temperatures. The liquid form of oleic acid changes into a sticky solid form. Our platelets become naturally stickier, and the likelihood of blood clots correspondingly greater, causing strokes, heart attacks, or circulatory occlusions in organs such as lungs, in our extremities, and various sense organs. Not a very brilliant way to prepare for a rigorous middle age and a healthy eldership.
Again our enzymes break down trans-fatty acids at slower rates than normal fatty acids. But the normal fuel of our heart is fatty acids, and a high consumption of trans-fatty acids necessarily and inevitably impairs its capacity. In situations of crisis, stress, or excessive energy demands, this could have fatal consequences. Are you still wondering why we are dropping dead from heart attacks, as if other forms of death were going out of style? Trans-fatty acids equally damage cell membranes, so that some molecules normally kept out of cells could now get in, while others required in the cell can now get out. This would necessarily diminish the vitality of cells. This also reduces our immune function, while it leaves the door open for allergic reactions. Are you aware of how fast our food allergies have increased during the past half-century?
One could go on and discuss how trans-fatty acids disrupt the normal and vital function of essential fatty acids (EFAs); how they change the molecular architecture of our bodies and interfere in energy flow; how they produce electrical short-circuits by being unable to take part in energy and electron exchange reactions, and so on. But surely the point has been made. Besides, this flogging of a dead horse was only to demonstrate the evils of frying with unsaturated oils.
So, should one then fry with saturated fats? Well, they behave somewhat better under conditions of high heat, but since our food supply is already choked with them, choosing consciously to eat more of the same is schizophrenic. We should try to limit saturated fats, not increase their consumption for the sake of a cooking method where the heat used is simply too high. When a food turns brown, this is not a sign that it is done, but rather a warning that it is burned. Proteins turn into carcinogenic acrolein. Starches and sugars too are browned through molecular destruction. Fats and oils turn to smoke by the destruction of their fatty acids and glycerol. So what is a safe frying method?
That is a contradiction in terms. But because monounsaturated oils are a lot more heat resistant than polyunsaturated seed oils, use olive oil for an occasional fry, which will deteriorate slowly but will not burn up to temperatures of 260-285 deg C (500-550 deg F). But remember that in many cases, frying may be replaced by the gentler stir-frying for a couple of minutes, through the use of the Chinese wok. Adding a bit of water to the wok at the start, will keep the temperature of frying appropriately low.