TLDR; – A study heated the ten most common cooking oils and found that Extra Virgin Olive Oil (EVOO) contained the fewest toxic compounds at the end, even though it has one of the lowest smoke points.
The Prevailing Theory of Cooking Oils
For years, my oil selections in the kitchen have followed the common guidelines passed around in health-focused circles. These consist of:
- Use extra virgin olive oil (EVOO) or butter for dressings or low-heat applications.
- Use regular/refined olive oil for medium heat.
- Use coconut oil, avocado oil, or ghee for high heat.
The core principal behind these guidelines was to always balance the applied heat with the smoke point of the oil. Once an oil hits its smoke point, it starts oxidizing, breaking down, and releasing toxic compounds and free radicals into your food.
The theory was that EVOO, being largely unfiltered, contained a greater number of compounds that would be damaged by the high heat and thus its health value would drop off rapidly when used for cooking.
Conversely, it was assumed that heavily processed oils such as canola oil, grapeseed oil, and sunflower oil, while being bad for you for many other reasons, at least were better at high temperatures due to their high smoke points (the better option for this category being avocado oil). In fact, most of the top articles out there about healthy oils for cooking still list canola as one of the top choices😱.
A 2018 study from down under has turned this entire theory on its head.1
Study DesignRather than just measuring the smoke point of an oil and assuming that this correlated with toxic compounds released, researchers measured the toxic compounds themselves. The results were surprising. Click To Tweet
Two tests were performed:
- Oils heated gradually (over 20 minutes) from 25 to 240°C / 77–464°F (higher than domestic cooking temperatures) and sampled every 30˚.
- Oils heated at 180°C / 356°F for 6 hours (longer than most slow cooking methods), collecting samples at 30, 60, 180 and 360 minutes.
All heated samples were cooled at room temperature (25 ± 1°C, 77 ± 1ºF) and then stored until chemical analysis.
For reference, deep frying is generally carried out between 170-180°C and shallow pan frying tends to be uncontrolled with temperatures between 180-230°C, whereas eggs will cook at ~70˚C/160˚F.
Ten Oils were tested, listed here along with their starting smoke points (in ˚C):
- Extra virgin olive oil (EVOO) – 206.7 +/-2.5
- Virgin olive oil (VOO) – 175.3 +/- 0.6
- Olive oil (OO) – 208.3 +/- 1.5
- Canola oil (CO) – 255.7 +/- 0.6
- Rice bran oil (RO) – 237.0 +/- 1.7
- Grapeseed oil (GO) – 268.0 +/- 1
- Coconut oil (CoO) – 255.7 +/- 0.6
- High oleic peanut oil (PO) – 226.3 +/- 2.1
- Sunflower oil (SO) – 254.7 +/- 1.5
- Avocado oil (AO) – 196.7 +/- 0.6
NOTE: These smoke point values vary considerably from some of the more commonly reported numbers.
What They Found
In each sample, they measured the degree of oxidation, the current oxidative stability, the smoke point, fatty acid profile (including trans fats), and other toxic byproducts called polar compounds.
Many of these polar compounds have been associated with various forms of cancer and neurodegenerative diseases such as Alzheimer’s and Parkinson’s Disease.4
At this point trans fats shouldn’t really need much explanation. They are bad. They increase your risk of developing diabetes, heart disease, cancer, etc. The studies showing this are too numerous to count. While this portion of the study was not particularly surprising, it just goes to enforce the fact that olive, coconut, and avocado oils are a step above the seed oils.
Next up we get to some of the more surprising results. (Apologies for the low-quality images–I wasn’t able to procure the raw data, and converting this type of graph is almost impossible.)
Unsurprisingly, coconut oil remained at the very bottom of the charts due to its extremely high saturated fat content (~90%).
Why are saturated fats stable at high temperatures?
For an oil to be oxidized, chemical bonds must be broken. This process requires energy–the precise amount of which is determined by the specific combination of fatty acids that the oil is made up of. The bonds in saturated fats typically require more energy to break than those in polyunsaturated fats, with monounsaturated falling in between. The higher the temperature, the more energy is present for the breaking of these bonds and subsequent formation of free radicals. 5
What was more surprising from these results is how EVOO, full of monounsaturated fats, stayed neck-in-neck with the coconut oil. And yet, upon further reflection, this does indeed make sense. The activation energy of lipid oxidation is higher in the presence of antioxidants. This means that for oils rich in antioxidants, more energy is required to break the bonds, and thus the rate at which toxic by-products are formed is lower. This also explains why both virgin and refined olive oil underperformed the EVOO across the board: increased processing and filtration means fewer antioxidants.
Also worth noting is how canola oil and grapeseed oil, both with high smoke points and supposedly stable at high temperatures, showed a massive spike in polar compounds produced after surpassing 150˚C. Even the refined olive oil + avocado oil showed a small jump as temperatures surpassed 180˚C.
Smoke point itself is not a stable number. As an oil is heated, the smoke point drops both with time and with temperature.
Why is eating fried food in a restaurant so much worse than making it at home?
Even healthy oils like EVOO start oxidizing and producing toxic polar compounds when heated for more than 25 minutes. In home cooking, most frying will happen within this window. In restaurants, they are limited by economics. Not only will they tend to use cheaper (and less healthy) oils whenever possible, but even if they do use a healthier oil they are almost certain to fry multiple batches of food in the same pot of oil before throwing it out. This can easily extend into the 360+ minute cook time seen in the graph above.
- EVOO performed best across the board, followed extremely closely by coconut oil. Given all of it’s other health benefits (anti-inflammatory6, cholesterol lowering7, etc.) I would recommend using EVOO in any situation where the flavor is not detrimental (the same going for coconut oil as the second choice).
- For situations where a neutral flavor is needed, I find avocado oil to be the best choice from a flavor perspective. In most aspects of the study it performed similarly to the refined olive oils.
- The smoke point of an oil was inversely correlated with both the degree of oxidation and the polar compounds produced.
NOTE: This study was conducted on oils by themselves. When food is added to the oil, as the moisture is released from the food the oil undergoes a process known as hydrolysis, wherein the oil is broken down into glycerol and free fatty acids–compounds which can have their own negative health impact.
WARNING: Not All Olive Oil’s Are Created Equal
The health properties of olive oil can vary drastically based on the variety, the region, and even the freshness of the oil. One study analyzed the phenolic compounds and anti-oxidant capacity of 55 different EVOO’s from 9 different countries and 4 continents, finding that the total reducing capacity ranged from 40 – 530 mg/kg. The oils produced from the Italian coratina variety possessed the highest amount of phenolics and the sample produced from French cayon contained the lowest amount of total reducing capacity 8.
Another study (industry funded) found that the total polar material (TPM) percentage in different types of olive oils heated at 180˚C also varied significantly. Some of the lowest quality oils showed an even worse performance than canola oil. 9
A significant portion of the olive oil sold in the United States is imported from Italy. It was found several years ago that that almost 70% of that is actually fake, having been mixed with cheaper oils like canola oil by the Italian Agromafia.10
Even when we are buying pure olive oil, it is often more than a year old, having been stored first in some combination of warehouses and shipping containers and been exposed to varying light and temperatures. I have heard about this for years, and only recently decided to get myself a bottle of high quality stuff to see if I could perceptually determine a difference. Boy was I surprised.
I signed up for the Fresh Pressed Olive Oil Club, where they go and source oils from small farms and ship them to the U.S. via jet. I also went and bought a medium-fancy bottle of olive oil from Whole Foods, along with their store-brand EVOO. When the FPOOC bottle arrived, I did a blind taste test of 1 tsp of each.
I needn’t have bothered.
The FPOOC oil, from a brand named Colli Etruschi (shown below), was like no oil I’ve ever tasted. It was FRESH! It was even more bitter than the others, indicating a higher polyphenol content (oleuropein in particular is quite bitter), but it also tasted both fruity and herbal. I definitely can’t do the description justice (lacking a suitably pretentious palate and vocabulary), but the word I keep coming back to is that it tasted “green.”
Too bad the stuff is so expensive. I’m currently debating whether or not I can justify a regular subscription to the FPOOC–as part of my supplement budget given the clear added health boost.
I will no longer buy any olive oil other than EVOO, and I will be using that for all culinary applications in the future where the flavor doesn’t NOT work.
What are you experiences with Olive Oil? Anyone have any brands that they love, and can get at a reasonable price point?