Deep-frying is the most common and one of the oldest methods of food preparation worldwide, and since cooking oils are expensive, there is an economic incentive to reuse these oils.
I’ve seen my mom and grandmother deep fry some snacks on a cold rainy day and once the cooking was done, pour all the remaining oil into a container to recycle for another time. I’ve heard McDonald’s workers talk about how they make fries in the same oil they have been using for the last few weeks. I’ve been told that deep fried food wasn’t healthy, that it was detrimental to health, but I never really understood the extent until the second year of my food science degree.
It was in an afternoon lab session where we had to perform a simple tit-ration. We acquired a bit of a yellow coloured oil sample from a commonly seen brand and poured an amount into a conical flask. We set up our burette with a strong acid and commenced the experiment. Tit-ration is a technique where a solution of known concentration is used to determine the concentration of an unknown solution, commonly observed by a colour change in the conical flask. A normal tit-ration generally uses somewhere from 15-25mL of acid to observe the colour change.
So we began by slowly adding some acid into the oil, initially only adding 1mL at a time till we got to about 28mL; we were certainly confused but kept adding more and more acid hoping to see a colour change soon. 35mL and nothing!
We inquired our lab demonstrator and only got instructed to “keep trying”, so we did exactly that, we added more. We added more till the entire burette was empty. 50mL of a strong acid and no change whatsoever.
We went back to the demonstrator and questioned her again; what were we supposed to see? What are we aiming for? Why isn’t it working? All these questions but the same response every time, “keep trying!”
Now with slight frustration and even more motivation than ever to go home early, we filled up our burette yet again and added more acid. We were desperate to see that yellow change to something else, anything else!
But to our dismay, another 50mL and no difference even now, except that our conical flask was full of a yellow solution to the point we couldn’t possibly add more.
Aggravated now with almost 3 hours wasted on what we thought would be a swift experiment, we went back to the lab demonstrator, this time adamant to get an answer from her. Something besides “keep trying”!
Finally, an answer; not what we had hoped for or wanted from her but a good change from what we had heard all through the lab. “It probably won’t work because it’s expired oil”.
Why didn’t it work?
Well, there are two types of oils: saturated and unsaturated. Two words I believe we have come across frequently, yet might not know their meaning accurately. Saturated oils only have single bonds between their carbon atoms, i.e. they are ‘saturated’ with hydrogen atoms. Some of them are vegetable oils which are generally solid at room temperature. While unsaturated oils, which tend to be liquid at room temperature, consist of one or more double bonds and are useful for frying food. Unsaturated fats are thought to be a healthier option in the diet than saturated fats, but that doesn’t make frying the oil a better option.
The dilemma with reusing oil is that each time you reuse it, it gets more and more destabilized until it decomposes. The oil’s physical appearance and fatty acid composition get altered because heating it causes the oil to undergo a series of chemical reactions like oxidation, hydrolysis, and polymerization. Cross-links occur between different fat molecules, combining them, resulting in a substantially much bigger and more viscous substance. This means the number of double bonds in the structure increase making the oil stronger and harder to break down.
In addition to the double bonds, a toxin called 4-hydroxy-trans-2-nonenal (HNE) forms when oils such as canola are reheated. Once absorbed in the body, HNE reacts with DNA, RNA, and proteins affecting basic cellular processes.
Its consumption has been associated with increased risks of cardiovascular disease, stroke, Parkinson’s disease, Alzheimer’s disease, Huntington’s disease, various liver disorders and cancer.
It only takes one use to create HNE in the oil, and reusing oil at too high of a heat i.e. frying foods at or above 375°F/190°C can lead to accumulation of HNE.
It is also advisable to use oils low in linoleic acid –olive oil and canola oil as researchers have found that HNE is more likely to build up in oils with high levels of linoleic acid. These include safflower oil, grape seed oil, sunflower oil and corn oil, so staying clear of them is beneficial for health.
In addition to the reused oil developing strange flavours and odour by going rancid, it may also contain carcinogenic free radicals. These are molecules capable of damaging cells and leading to increased risk of cancer, as well as affect the quality of your food. Free radicals are easily absorbed into fried food and once ingested, they attach themselves to healthy cells and lead to diseases i.e. cause atherosclerosis which can lead to increase in bad cholesterol levels, blocking the arteries.
What to take away from this?
These are only a few of the numerous issues caused by reheating your oil. Just imagine if 100mL of a strong acid could not break down a small amount of expired oil, how difficult would it be for your own body to do so.
Also remember, restaurants are under no obligation to use fresh oil every time since it’s not economically favourable for them, so if your health means something, it would be advantageous to avoid deep fried foods in a restaurant or takeaway.
Therefore, it is advised to not heat cooking oil more than once due to its possible deleterious effects on health. Rather dispose it after every use by placing it in a container and throwing it in your trash. Don’t pour it down the drain; it could cause more complications later. It is also more suited to use fresh oil every time you cook rather than trying to save that dime.
About the Author
Saipriya Shahi holds a Bachelor’s of Science in Food Science from the Auckland University of Technology. With a goal to change the future of food with her innovative yet quality product ideas, while being obsessed with photography on the side.