Cooking oil is expensive which is why we re-use it and this Ramzan we’ll need more of it for those samosas and pakoras at iftari. The only problem is that constantly reheating and reusing a batch of oil can break it down to release cancer-causing chemicals.
During cooking, proteins and carbohydrates in the food react with the hot oil. Oil-soluble flavours in the food and its seasonings are therefore released into the oil you are using. If the oil is heated to a very high temperature, as when you fry, chemical reactions take place.
In frying, food is heated to temperatures typically between 149 degrees Celsius and 215 degrees Celsius. The fatty acids in the oil break down into harmful trans fats. Additives, such as food colouring, preservatives and metal compounds found in food, react with the fatty acids from the oil to produce toxic aldehydes. Repeated or high-temperature frying makes the levels of trans fats and aldehydes in the oil go up each time.
The toxic aldehydes are organic compounds. And just how toxic they are can be judged from the fact that aldehyde-based compounds are used as disinfectants in operation theatres, says intestinal-gastroentologist Dr Irfan Daudi.
These chemical compounds are carcinogenic — meaning they are potentially cancer-inducing. Repeated use of the same cooking oil irritates the lining of the digestive tract. These chemicals interact with the cells in the lining of the food pipe, the stomach and the intestines and alter their genetic makeup, thereby killing the cells. They have been linked to different types of cancer and neurodegenerative diseases such as Alzheimer’s and Parkinson’s.
“In our part of the world, the incidence of certain cancers like oesophageal cancer, stomach cancer, gall bladder cancer is very high,” Dr Daudi says. “There is also a huge association of high cholesterol with certain cancers such as breast and colon cancer.” Hence, his advice is to throw away that cooking oil after a single use.
There are certain cholesterol-free vegetable oils, which should not be heated at all. Research by a team of food science experts at the University of the Basque Country, Spain discovered that heating sunflower, flaxseed and olive oil leads to degradation of fatty acids into aldehydes. Another study by the University of Minnesota’s Food Science and Nutrition department found similar results with soybean oil when it was heated up to 185 degrees Celsius.
For a long time, vegetable oil has been considered healthy compared to animal fat-based oils such as butter and desi ghee because they contain healthy unsaturated fats. But cardiologist Dr Khawar Kazmi says that when heated extensively, vegetable oil also produces trans fats, which are even more harmful than saturated animal fats. They are potentially atherogenic, meaning they lead to an increase in ‘bad cholesterol’ and clog up arteries.
There are two things we do wrong when cooking: the amount of oil we consume and the number of times we reuse our cooking oil or subject it to extensive heat. “The recommended amount of oil to be consumed is about 1 litre per person per month. But here an average person is consuming three times more,” says Dr Kazmi.
There are certain vegetable oils which do not break down into trans fats as swiftly as the others because they are more heat-stable. Dr Kazmi recommends palm oil for the cooking of Pakistani cuisine. Olive oil should ideally only be used in salads.
Signs your cooking oil needs to be discarded
When the oil darkens or smells of food that is fried in it.
The oil becomes more viscous, that is, it pours slowly.
Food particles remain suspended or collect at the bottom of the oil container or frying pan
When smoke appears on the oil’s surface
When foam appears consistently while frying
How to reuse oil safely
Ideal temperature for frying is 190 degrees Celsius. Tip: to test the heat drop a piece of bread into the oil; if it sizzles and bubbles in a few seconds, it’s a good temperature. Do not fry in cold oil for the risk of soaking it up, nor heat the oil so much that it smokes.
Do not mix different types of oil.
Store oil in a cool, dark place.
Published in The Express Tribune, Sunday Magazine, July 14th, 2013.
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