Olive oil holds much better than vegetable oils during frying, study finds
180°C for 6 hours.
The first trial consisted of gradually heating a sample of 250 mL of each oil in a pan fryer from 25°C to 240°C, collecting samples as the oils reached 150°C (302°F), 180°C (356°F), 210°C (410°F) and 240°C (464°F). The overall time to reach the highest temperature was approximately 20 minutes.
In the second trial, a sample of 3 L of each oil was heated in a deep fryer at 180°C (356°F), which is the highest recommended temperature for deep frying, for 6 hours collecting samples at 30, 60, 180 and 360 minutes.
All heated samples were cooled at room temperature (25 ± 1°C, 77 ± 1°F) and then stored until chemical analysis".
Trans-fat content of oils before and after heating
Interestingly enough, vegetable oils had higher content of trans-fats while extra virgin olive oil (EVOO) and virgin olive oil (VOO) had the least before heating, which the authors attributed to the production method of the oils.
"There is a remarkable difference between initial trans-fat content in refined oils and non-refined oils. Grapeseed showed the highest amount of initial trans-fat content while EVOO and VOO showed the lowest. These results are consistent with the oil production method, as refined oils are bleached and heated during the industrial process, and virgin oils such as EVOO, VOO and avocado (only produced with mechanical processes) maintain a naturally lower level of trans fats." Figure 1
Effect of time on evolution of polar compounds
After heating all oils for 6 hours at 180°C, it was found that
"For all oils tested, the formation of polar compounds tended to increase with time. Higher values, after 6 hours of heating, were obtained in refined seed oils: So sunflower oil (21.75%), grapeseed oil (20.24%), canola oil (17.32%) and rice bran oil (15.66%). The lowest values were obtained in EVOO (10.5%) and coconut oil (9.68%)." Figure 2
Effect of temperature on evolution of polar compounds and oxidative stability
On increasing the temperature from 150°C to 240°C, canola oil showed a rapid increase in polar compounds, which was well above that permitted for human consumption.
"Canola oil demonstrated a rapid increase in polar compounds from 150oC to 240oC (Figure 6), with its highest value of polar compounds (27,5%) above the limits permitted for human consumption, followed by grapeseed (19,3%) and rice bran (13.0%) oils.
Furthermore, extra virgin olive oil (EVOO) and peanut oil showed the highest oxidative stability, while seed oils, such as canola, grapeseed, sunflower and rice bran oils showed lower oxidation stability.
The authors of the study concluded that extra virgin olive oil (EVOO) was the most stable oil when heated, followed closely by coconut oil and other virgin oils such as avocado oil. They also noted that an oil's smoke point doesn't predict its performance when heated.