Extra virgin olive oil (EVOO) is well known as a source of bioavailable phenolic compounds that provide various health benefits across many diseases — cancer, cardiovascular disease, neurodegenerative disorders and others.
Olive oil is a monounsaturated fat that is higher in bioactive compounds compared to other vegetable oils.
While it is well known to be an all round healthy oil, maintaining the vast array of nutritional qualities under cooking processes is often questioned.
So let’s evaluate what the research shows surrounding OO and various domestic cooking processes — deep frying, pan frying, boiling and roasting.
The most recent study, published in Food Chemistry, 2015, aimed to study the effects of different domestic cooking processes on the phenolic composition of various Mediterranean foods, potato, tomato, pumpkin, eggplant, cooked in EVOO.
Interestingly, Pan frying results in slightly faster degradation of EVOO compared to deep frying.
Authors of a review, published in Food Research International, 2013, suggest that this is likely due to “higher food: oil contact surface, higher exposure to atmospheric oxygen, and lower temperatures under processing”
However, they also note that “in comparison with other vegetable oils, the fried food is enriched with olive oil antioxidants, as long as the olive oil is not extensively heated.”
A study, published in Food Science and Technology, 2010, used a sample of EVOO and OO with carrots, onions, and potatoes, to evaluate the effect of boiling.
Vegetables were boiled for 60 minutes with 60 g of each OO added at the beginning or 15 minutes before the end of the boiling process.
As expected, the results showed that boiling does not result in oxidation. Tocopherols and all polyphenolic components decreased in concentration.
However, adding OO to the boiling process just 15 minutes before the end of the boiling process increased “content of both oleanolic acid derivatives, 3,4-DHPEA-EA and 4- HPEA-EA, and hydroxytyrosol acetate.”
A study, published inFood Chemistry, 2010, looked at the behaviour of olive oil phenolic compounds during roast processing.
According to the study, “Samples included extra-virgin olive oil (EVOO), virgin olive oil (VOO), olive oil (OO), sunflower oil (SFO), soy oil (SO), corn oil (CO) and peanut oil (PNO). Beef (150 g, cube shape) or 150 g of potatoes (six quarters of potatoes).”
The vegetables were processed in 60 g of each oil in an oven at 180 °C until the inner temperature of meat reached 180 °C, which took an average of 60 minutes.
Sunflower and seed oils oxidized and failed to maintain antioxidant capacity compared to olive oil, which did not oxidize during roasting, the authors suggesting due to a higher tocopherol content.
Clearly, the unprocessed oil samples including olive oil, contained higher levels of phenolic compounds, which after roasting was significantly reduced in all samples.
For example, “a dramatic loss of 3,4-DHPEA-EDA (98percent) and 3,4-DHPEA-EA (70 percent)” was found in OO samples. However, the radical activity was higher for OO samples, compared to other vegetable oils.
Overall, OO degradation due to processing is a complex issue determined by many different factors. Regardless of some benefits being shown toward deep frying, high temperatures of deep frying do induce chemical changes such as oxidation, polymerization, cyclization, and hydrolysis.
However, compared to other vegetable oils, OO is a better choice for cooking, regardless of the method, as it is more resistant to the effects of oxidation and free radical production.
According to the authors of the study published inFood Chemistry 2015, “These chemical reactions are influenced by the type and quality of the oil, the food properties, and the food/oil ratio, among other parameters.”
In conclusion, OO and in particular EVOO is considered a healthy cooking oil overall, especially in comparison to other vegetable oils. Minimizing cooking time and not reusing oils helps reduce oxidation and loss of phenolic compounds. However, to maintain the full benefits and bioactive compounds in OO the best way to consume it is raw, in its original unprocessed state.