Nigerians are known to be lovers of tasty meals and globally, cooking oil continues to play a crucial role in the preparation of such meals.

Cooking oils are integral part of a human diet as they are used in almost all types of food preparations including frying, baking, and extrusion cooking. However, they are either consumed fresh or thermally oxidized- i.e when the fresh form of cooking oil is heated at high temperatures during various food preparations to increase palatability. This is a usual domestic practice in Nigeria, and is not only limited to various homes; but also, a regular practice in restaurants and the commercial food industry.

Understanding the Science of Cooking-oils

Cooking oils are usually extracted from seeds, kernels, or mesocarp (middle layer) of the fruit of plants. The initial crude forms of the oils are usually refined, bleached, and deodorized before being marketed as a golden yellow liquid product—the color preferred by most consumers.

However, fat molecules known as triglycerides(TGs) form the bulk (95%–99%) of cooking oils, with micro components (such as carotenoids, vitamin E, and other antioxidants) forming the remaining 1%–5%.

"Cooking oils are of different types. There are saturated oils (which are usually solid at room temperature) and unsaturated oil which you see in the liquid form. However, the unsaturated oils are healthier than the saturated ones,"says Dr Atta ur Rehman Madni, a clinician nutritionist.

"[Moreso], there are different types of unsaturated oils - the polyunsaturated and monounsaturated oils. But the best type of oils are the ones rich in monounsaturated fatty acids,"he added.

On a nutritional viewpoint, cooking oil can contribute up to two-thirds of the total fat in our diet and therefore form the major dietary fat as well as an important source of energy. Most importantly, cooking oils and other edible fats provide us with the essential fatty acids that our body cannot synthesise.

Oil and Heat: A complicated relationship

The major use of cooking oil is in frying, where it functions as a heat transfer medium and contributes flavour and texture to foods.

"All fats, including the oils you typically use in the kitchen, have a smoke point, the point at which they will begin to produce smoke. Specifically, it’s when anything in the oil — fats, proteins, sugars, and other organic material — starts to interact with oxygen and burn," Joseph Provost, a chemistry and biochemistry professor at the University of San Diego who co-wrote The Science of Cooking: Understanding the Biology and Chemistry Behind Food and Cooking highlighted in The Washington Post.

Smoke point refers to the temperature at which an oil starts to burn and smoke. "At the smoke point, the oil gives off smoke, fat molecules begin to break down, evaporate and settle down on kitchen walls and utensils. Besides adversely affecting the taste and flavor of the cooked product, its chemical composition changed, thereby making it toxic," he added.

"Any type of cooking that requires repeating or reusing cooking oils multiple times, makes the properties of the oil become deteriorated. The metabolic change and other chemical reaction that takes place makes the quality of the oil poor." Madni said

Health Implications of thermally oxidized cooking oils

Studies have shown that continuous heating and reheating of oil aids its transformation into trans fats, which not only raises the bad cholesterol - low density lipoprotein (LDL) - levels in the body, but also lowers the good cholesterol - high density lipoprotein (HDL) levels.

"The chemical changes that take place in reheating of cooking oils will increase the percentage of trans fats - a form of fat which is harmful to the body- thereby leading to an increase in the bad cholesterol profile known as low density lipoprotein (LDL),"madni said.

The cholesterol usually formed from this fat travel to different parts of the body system through the blood, alongside the transportation of vitamins and minerals. However, once there's an increase in the bad cholesterol (LDL), it builds up on the walls of the blood vessels, thereby forming what is known as plaque,which overtime narrows the blood vessels leading to different health risks especially, heart related diseases.

"The practice of reheating oils, which changed the chemical composition of the oil into trans fats, leads to increased bad cholesterol, decrease of good cholesterol (HDL), blockage of blood vessels as well as higher risks of dietary related diseases such as diabetes, stroke, heart attack, high blood pressure amongst others,"says Dr Folu Olatona, a nutrition and wellness expert, cum a consultant public health physician at the college of medicine, University of Lagos, Nigeria.