Frying, using edible oils, is one of the most practiced food preparation methods as it gives characteristic aroma, texture and taste that is unmatched compared to other processes. Baking, also requiring specialty fats, is another process that can give hundreds of products, universally liked, though it needs high temperature heating equipment and industrially, bakery industry many be one of the biggest in food processing sector creating a variety of products like bread, biscuits, pastries etc. While baking is based more or less on wheat and its unique protein Gluten, frying has the advantage of using any raw material including cereals, pulses, vegetables, fruits, meat, fish and their combinations. Frying needs an oil which requires to be heated to temperatures varying from 160 C to 200 C for a few minutes to attain the desirable color and texture depending on the input material. Organoleptically fried food can be quite appealing and often addictive and health and nutrition experts always caution against its regular consumption that too in larger quantities because of its contribution to the obesity epidemic that is becoming a scourge in most countries causing diseases like diabetes, cardiac malfunctioning, hypertension and a host of others. Experts agree that calories derived from fat, both direct and indirect, should not exceed 30% of the total calories consumed in a typically normal diet working out to about an upper limit of 50 gm daily per capita.
While in the organized fried food manufacturing sector big capacity continuous fryers are used containing high volume of the frying oil, small and unorganized players use batch fryers with or without any temperature control. But invariably the oil is subjected to continuous heating for getting large quantities of products for supplying to the market. In house holds however the fryers, mostly shallow stainless steel pans, are used once or twice a week though the oil is repeatedly reused after each frying operation with occasional topping up. Usually frying oils are rarely thrown away unless it becomes too dark or sticky. There is no unanimity among scientists whether continuous heating does more damage than intermittent heating but all tend to agree that there will be heat damage to the oil to varying extent, depending on the chemical nature of the one used. More the degree of unsaturation in the fatty acid molecules higher will be the extent of damage. Higher temperatures and longer duration of frying can cause extensive damage, decomposition and interaction among the artifacts generated.
What will be the implications of damage to the oil, besides developing flavor problems to the products fried repeatedly in such oils? A matter of much concern is the effects of such oil damage to the human health. Till recently, frying industry was more concerned about oil damage because of loss of quality in the product besides economic losses incurred due to discarding of precious oil once it fails to meet the mandatory standards after repeated and long duration heating. As edible oils are made of organic chemical substances, mainly fatty triglycerides, they have their typical smoke point and flash point in terms of characteristic temperatures beyond which they decompose rapidly into hundreds of artifacts some of which are considered injurious to health. Among them aldehydic lipid oxidation products are considered highly hazardous to humans whether they are inhaled or consumed orally through foods. If so, those using oils vulnerable to generate toxic aldehydes during frying can be dangerous to both consumers as well as to the workers involved in the factories as the latter is exposed to volatile aldehydes continuously during the manufacturing operations. .
Toxic aldehydes like 4-hydroxy-(E)2 nonenal, 4-oxo-(E)-2 decenal and 4-oxo-(E)-undecenal, derived from alpha, beta unsaturated fats have been linked to many types of cancer and neurodegenerative diseases like Alzheimer's and Parkinson;s. These aldehydes, being highly reactive can combines with tissue proteins, enzymes and hormones affecting the functioning of many organs in the human body. It is because of this reason that fried foods made from oils with high amounts of alpha, beta poly unsaturated fatty acids are fraught with high risks for human consumption. According to available data which are rather limited in this area of concern, some of the frying oils found to be relatively safe are coconut oil, virgin olive oil, butter, lard and palm oil all of which contain predominantly saturated fatty acid triglycerides. It is interesting that oil derived from sunflower seeds happened to the worst performer, mostly attributed to the presence of more than 82% of unsaturated fatty acid triglycerides yielding toxic aldehydes at levels much higher than that recommended by the WHO to be safe.
According to some findings by scientists, a typical meal of fish and chips fried in vegetable oils contained 100 to 200 times more toxic aldehydes than the safe limit set by the WHO and the consumption such products regularly can lead to serious consequences to the brain functions in the long term. Some even compare such disastrous changes in human brains to the adverse climate changes the world is facing to day. How far such comparisons are realistic is a matter of debate though it definitely raises alarm among health experts who feel that more intense studies are required to be undertaken to unravel the mysteries of toxic aldehydes. One is troubled by the report that Sunflower oil is the oil targeted in these studies and the rationale is that it contains high levels of unsaturated fatty acids (UFAs), to the extent of 91%. If so what about oils like olive oil which is found to be a safe cooking oil though it contains 84% unsaturated fatty acids. How about other oils like mustardc oil (81% UFA), or peanut oil (82% UFA) ? Here again does the nature of UFA make any difference in generating toxic aldehydes during heating? No one knows for sure whether extent of MUFA or PUFA or their ratio in these oils determine the dynamics of toxic aldehyde formation. Also not clear is whether unprocessed vegetable oils containing many natural antioxidants and bioactive phytochemicals can generate toxic substances on being used as frying oils. A note worthy fact in the Olive oil experiment is that the scientists used virgin olive oil which is valued very much for its unique flavor profile to say it is a good frying oil which again raises the question regarding the role of natural antioxidants present in virgin oil compared to its processed counterpart.
While earlier studies used simulating experiments and model systems to find out the formation of toxic chemicals in frying oil at high temperatures for duration as long as 20-40 hours, some recent studies confirmed that even a few minutes exposure to frying temperatures would be sufficient to create sufficient levels of toxic chemicals when oils like Sunflower oil is used. A logical question that arises out of these studies is the influence of antioxidants and other oil protectants like scavengers of polar compounds added to frying oils by the processing industry on generation of toxic substances. Unfortunately no data seem to be existing to verify this crucial issue. On the face of it the recommendation that coconut oil is the best frying oil may be note worthy, especially when frying is done in households or restaurants using non stabilized frying oils. Also to be borne in mind is that unless more emphatic studies by divergent group of lipid scientists confirm these stray findings, all vegetable oils should not be condemned as unsafe for frying.