Q:Is it advisable to re-use a cooking oil or fat after deep-frying a food item in it? Are all cooking oils and fats equally good for frying?

GKC, Calcutta

Oils and fats used for cooking are of four varieties — saturated fats, mono-unsaturated fats (MUFA), poly-unsaturated fats (PUFA) and hydrogenated fats.

When deep-frying food, the oil or fat reaches very high temperatures. At these temperatures, oils and fats can undergo oxidation as well as alteration. However, different fats oxidise at different rates. While saturated fats like butter, clarified butter (ghee), coconut and palm oils as also the mono-unsaturated fat groundnut oil oxidise very little, others MUFA oils like olive, rapeseed and mustard oxidise to a slightly greater extent. However, the most unstable are the fragile PUFA oils like sunflower and safflower (also known as kardi oil), corn, soyabean, cottonseed, sesame and rice bran. These PUFA oils oxidise very quickly. When oxidised, oils and fats generate ‘free radicals’, i.e. oxygen fragments that cause widespread damage to the cells in the body. They also lead to over-production of the ‘bad eicosanoids’. The combined effect of these two helps promote blood clotting, arterial damage, altered immunity and degeneration, leading to heart attacks, cerebral strokes, painful joints, asthma, skin allergies, wrinkled skin, premature greying of hair, cataracts and development of cancer — to name a few.

The greatest harm is caused by PUFA oils. When these oils are heated to deep-frying temperatures, a part of their unsaturated fatty acids undergoes changes called ‘polymerisation’. This results in the formation of highly toxic compounds and chemicals that have cancer-producing potentiality besides damaging the arteries and causing heart diseases. Further, under the impact of high temperatures, a part of the unsaturated fatty acids undergoes transformation into trans fatty acids (TFA) — fats capable of deranging the body’s hormonal system and the immune balance and affecting the cell membranes leading, once again, to widespread degenerative diseases including heart problems and cancer.

When an oil or fat is re-used after deep frying, the above mentioned changes of oxidation, polymerisation and transformation become accentuated. The more number of times an oil is used for re-frying (as in most commercially-sold deep-fried food and restaurant grub), the more pronounced the changes, making the oil or fat dangerously toxic. Cooking oils and fats are expensive and it may not be economic to throw them away after deep-frying in them just once. However, from the point of health, it is best not to re-use them.

As we see from the above discussion, it will be wise to keep the PUFA oils out of the frying pans and the deep friers. On the other hand, hydrogenated fats (like vanaspati or margarine) are loaded with dangerous TFAs and should be avoided.