Cooking with Extra Virgin Olive Oil by Mary Flynn - PhD, RD

Extra virgin olive oil is the safest oil to use for cooking, as clearly illustrated in a study published in 2018 by de Alzaa, et.al. The study tested the effect of heat and time for the cooking oils of extra virgin olive oil, virgin olive oil, olive oil, canola, rice bran, grapeseed, coconut, peanut, sunflower, and avocado.^1 

A number of experiments were performed which tested the oils for their ability to withstand heat. The tests were done with the oils blindly labeled so the researchers would not know which oil they were testing. Each oil was subjected to two different heating trials. One trial was a gradual heating of a sample of about a cup of each oil in a pan fryer from 25°C (77ºF) to 240°C (464ºF), collecting samples as the oils reached a range of temperatures that would reflect ones that could be used in home cooking. The second trial used a sample of 3 liters (about 3 quarts) of each oil and heated the oil in a deep fryer at 180ºC (356ºF), which is the highest recommended temperature for deep frying, with samples collected over 6 hours.

Smoke Point

One of the tests was for “smoke point”. Smoke point is a test that somehow became a talking point by laypeople for assessing the healthfulness of oils for cooking. However, cooking is not done at a smoke point temperature so it has limited value for assessing an oil. Also, in the past, smoke point was thought to be related to the stability of the oil under heat with a high smoke point thought to be best, although the studies used to support this are limited and did not support the value of smoke point when assessing home cooking conditions. One of the tests in the de Alzaa study was for the production of polar compounds, which are unhealthy products made when oil is breaking down while being heated. Polar compounds are not healthy to ingest as some polar compounds have been related to the development of cancers and neurodegenerative diseases, such as Parkinson’s and Alzheimer’s disease. For this reason, a number of health agencies have identified an upper limit of polar compounds in frying oil of at most 24 - 27%.

De Alzaa and colleagues showed that smoke point was positively correlated with the production of polar compounds, which means a high smoke point did not mean the oil was healthy to cook with and in fact meant it was unhealthy to cook with. Extra virgin olive oil produced the least number of polar compounds when heated, indicating that it was the most stable oil for cooking. Interestingly, despite a relatively high MFA content, canola oil demonstrated a rapid increase in polar compounds when heated from 150ºC (302°F) to 240ºC (464°F), reaching 27.5% polar compounds, which is above the limits permitted for human consumption. This would mean that canola oil becomes unstable when heated.

Oxygen is the most harmful element

A study done at UC Davis looked at the effect of oxygen, temperature, and heating time on extra virgin olive oil and also some of the phenols in EVOO using home cooking conditions for temperature and time.^2 The results indicated that oxygen was the most harmful to the oil as EVOO on top of the stove at a fry temperature (around 350°F) had a greater phenol loss compared to EVOO heated in the oven at 400°F. This study also gives support to the advice to limit oxygen exposure of EVOO by quickly replacing the cap on the EVOO bottle when using it at home, not leaving EVOO in open bowls on the table (use something that can be covered), and cover the pan on the stove or put food immediately into the oil when cooking with EVOO. For the testing of phenols, oleocanthal was the most stable phenols tested with very little lost with heating, which would mean it would still be available for providing health benefits after cooking with oil high in oleocanthal. EVOO that starts with a high level of phenols would have less loss of total phenols with heating compared to an EVOO with a lower level of total phenols.

The stability of EVOO for cooking is partly due to the high monounsaturated fat content, which means it will not easily oxidize, and the phenols it contains that act as antioxidants, which work to decrease the oxidation of the oil. In addition, extra virgin olive oil contains the form of vitamin E that acts as an anti-oxidant, further decreasing the chance of oxidizing the oil. This would not be the case with refined olive oil. Refined olive oil has a very small level of phenols and contains more free fatty acids so it would not be stable for cooking. The same would be true for vegetable seed oils such as corn, safflower, soybean, and sunflower. These oils are all high in polyunsaturated fats, which easily oxidize, and do not contain phenols. While these oils naturally contain some vitamin E, the vitamin E in the oil is needed to help de-crease the oxidation of the oil so it is not available for absorption into the human body when these oils are consumed.

Summary:

· EVOO is the most stable oil for cooking.

· Oxygen exposure seems to be the most damaging to the EVOO and decreases phenol content.

· EVOO with higher phenol content such as ZOI or Olio Nuovo will have more phenols left after heating compared to a lower phenol content EVOO.

· Seed oils which contain proportionally more PUFA compared to EVOO will easily oxidize.

· Canola oil, despite the relatively high MFA content, produces high levels of polar compounds when heated, making it unhealthy for home cooking.

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1. de Alzaa F GC, Ravetti L. Evaluation of chemical and physical changes in different commercial oils during heating. Acta Scientific Nutritional Health 2018;2.6:2-11.

2. Xueqi L, Bremer, G. C., Connell, K. N., Ngai, C., Pham, Q. A. T., Wang, S., Flynn, M. M., Ravetti, L., Guillaume, C., Wang, Y., Wang, S.C. . Changes in chemical compositions of olive oil under different heating temperatures similar to home cooking. Journal of Food Chemistry and Nutrition 2016;4(1):7-15.