Frying and sautéing vegetables can be healthier than boiling, say scientists at Spain's University of Grenada. There's a catch, though — you have to use extra-virgin olive oil (EVOO).

Frying common Mediterranean diet veggies — potato, tomato, eggplant, and pumpkin — increases the phenolic compounds naturally found in the vegetables.

Phenolic compounds, which have strong antioxidant properties, help prevent chronic degenerative diseases such as cancer, diabetes, and macular degeneration.

The antioxidant and phenolic content of foods can be increased or decreased depending on how it is processed. The researchers found that frying or sautéing vegetables in EVOO increases their antioxidant abilities and the amounts of phenolic compounds.

To determine which method of cooking produces the most beneficial compounds, researchers cooked 120 grams — a little over four ounces — of potato (Solanumtuberosum), pumpkin (Cucurbitamoschata), tomato (Licopersicumesculentum) and eggplant (Solanummelongena), all of them without seeds or skin.

In controlled experiments, the vegetables were fried and sautéed in EVOO, or boiled in water, or boiled in a mix of water and EVOO. Of the three cooking methods, the amounts of phenolic compounds contained in the raw vegetables were increased by cooking in olive oil.

"Oil as a mean of heat transfer increases the amount of phenolic compounds in vegetables, opposite to other cooking methods such as boiling, where heat transfer is done through the water," said professor Cristina Samaniego Sánchez.

"When the phenolic content of the raw vegetable is high, the total content of phenols is increased even more if EVOO is used in the process," she says.

"Therefore, we must stress that frying and sautéing conserve and enhance the phenolic composition. Hydrothermal cooking methods can be recommended when the food is consumed together with the cooking water, as the addition of EVOO improves the phenolic profile and compensates for the deficiencies of the raw food," she says.

Sánchez warned, however, that using olive oil in cooking increased the caloric content of vegetables.

The research was published in Food Chemistry magazine.