Oil is an integral part of cooking, especially if it is for Indian cooking. For quicker results, frying remains one of the popular methods of food preparation. Ready to eat food consumption too has increased many folds. Restaurants and even at our home, oil is reheated and reused until it is discarded and replaced with fresh oil.
Surprisingly, how many of us are actually aware of the potential health hazards of reheated oil!!!
How does Reheating cause Bad Effects?
Reheating of oil undergoes series of chemical changes, producing some harmful substances which can cause ailments in the long run.
Along with minor complications like heartburn, acidity, irritable throat (due to inhalation), recycling cooking oil can make it rancid or spoiled.
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In addition to having strange flavours and odours, reheating of oil generates Free radicals. Free radicals can attach themselves to healthy cells and lead to diseases. These free radicals can be carcinogenic i.e. can cause cancer and also atherosclerosis which can lead to an increase in bad cholesterol levels and blocking the arteries.
Reheating vegetable oils like sunflower or corn oil has been found to release high concentrations of chemicals called Aldehydes, which have been linked too with illnesses including cancer, heart disease and neurodegenerative changes like Alzheimer’s and Parkinson’s disease and dementia.
Experts say that on heating even the best quality of edible oil several times, releases toxic aldehydes and the more times the oil is reheated, the concentration of the toxic chemicals become higher making it more injurious to health. These toxins then react with human proteins, enzymes and hormones, which can lead to serious health problems.
When cooking oils are heated, reactions such as oxidation, hydrolysis, isomerisation and polymerisation occur, resulting in the formation of a variety of volatile compounds and monomeric and polymeric products some of which are potentially toxic. Some of these oxidised volatile products (eg acrolein and other α,β-unsaturated aldehydes) are known to be responsible for the off-flavour and negative effects on human health.
Studies have also found a toxin called 4-hydroxy-trans-2-nominal (HNE) forms when such oils as canola, corn, soybean and sunflower oils are reheated which reacts with DNA, RNA and proteins affecting basic cellular processes.
Reheating increases trans fatty acids content in the oil. Trans fats are even worse than saturated animal fats, once absorbed in the body. While Saturated fats increase the concentration of LDL (Bad cholesterol) in blood, trans fatty acids behave like a double-edged sword, on one hand, they increase LDL in the body and on another hand they also decrease the amount of HDL (Good cholesterol).
Trans fat is found to be associated with increased risks of cardiovascular disease, stroke, Parkinson’s disease, Alzheimer’s disease, Huntington’s disease, various liver disorders and cancer.
Factors affecting Damage by Oil during Reheating
There is no set number to how many times one can reuse the oil as it depends on factors like which oil was used, how long frying was done, the temperature of oil while frying, the smoke point of oil, storage conditions.
The oil you use:
- The smoke point is the temperature at which a fat or oil produces a continuous wisp of smoke and is a useful indicator of an oil or fat’s suitability for frying. Beyond a smoke point, oil starts to burn or begins to smoke producing harmful chemicals.
- Basically, oils with high smoking point do not break down at high temperature. Not only they are safe but can be reused for frying, deep frying.
- Fats with a smoke point below 200 °C are not suitable for deep frying.
- Products like extra virgin olive oil contain a great deal of non-fat components, like the anti-oxidants, so it tends to have a lower smoke point. Very refined oils have a higher smoking point.
- Vegetable oil has a high smoke point than oil of animal origin. Refined oils have a higher smoke point than unrefined oils.
- Every time the oil is reheated, smoke point dips.
- Oils with high smoking point-Sunflower, Safflower, Soybean, Rice Bran, Peanut, Sesame, Mustard and Canola oil.
- On the other hand oils with a low smoke point like olive oil should be used for sautéing, steaming, stewing and as a salad dressing.
- Choose an oil low in linoleic acids, such as olive oil and canola oil. HNE is more likely to build up in oils with high levels of linoleic acid: safflower oil, grape seed oil, sunflower oil, and corn oil. So avoid these oils for deep frying.
- Don’t mix different types of oils out of proportion.
The temperature of Oil while cooking:
- The ideal temperature for frying is 375°F. Frying foods at or above 375°F can lead to the accumulation of 4-hydroxy-2-trans-nominal (HNE) in the oil.
- So, when heating oil to very high temperatures, use a thermometer to ensure that you’re not heating the oil above 375°F or 190°C.
- Turn off the heat after you are done the cooking. Exposing oil to prolonged heat accelerates rancidity.
- Avoid iron or copper pots or pans for frying oil that is to be reused. These metals also accelerate rancidity.
- Hot oils tend to polymerize—their molecules join together into much bigger molecules that give the oil a thick, gummy consistency and a darker colour, can be seen settled down at the bottom of a container after the oil cooled down.
Storage of oil:
- Fats are susceptible to oxidation when they are exposed to light, heat and oxygen so try and keep your oils tightly sealed in a cool, dry cupboard.
Points to be kept in Mind for usage of Reheated oil
- Try to limit the quantity of oil used, use only required a quantity of oil. Fresh oil every time is the best way to avoid future complication, it is not really practical due to economic reasons.
- If well-trained, properly stored and not overheated on first use, these oils should be safe to reuse.
- Minimize food contamination while frying
- Food particles in the oil can spoil the oil much sooner than expected.
- Avoid adding salt to food before deep-frying, because salt lowers an oil’s smoke point. Once oil smokes, it is no longer safe nor desirable to reuse.
- Try to minimize the amount of oil you use and eat. Soak up excess oil with a paper towel.
Straining used oil:
- Cool down the leftover oil and then transfer into an airtight container through a strainer/cheesecloth. Use this oil within a month. Bacteria feed on food particles left in the oil.
Storing used oil:
- Refrigerate used oil. Refrigerating or freezing used oil retards bacterial growth. Un-refrigerated oil becomes anaerobic and leads to the growth of Clostridium botulinum, which causes botulism, a potentially fatal food poisoning.
- Before reusing, check the used oil for the colour and thickness. If it is dark in colour and is greasy/sticky than usual it is time to change the oil.
- Discard oil that has got Rancid — meaning old and stale –
- If oil is smokey, smell like fish on heating much, discard this batch.
- Proper ventilation in kitchens is also beneficial in reducing the impact of these potentially toxic volatile compounds.