The presence of food showed a protective effect on the oils, with oil samples processed without food showing a higher level of oxidation than the oil samples processed in the presence of food. All polyphenolic components of olive oils decreased in concentration with the thermal treatment and this decrease was dramatic in the presence of food. During processing, two new compounds were found in olive oil samples and their concentration was higher for samples containing a higher initial polyphenolic content. The content in tocopherols was not so dramatically affected by the thermal treatment as was the polyphenolic content. Moreover, a sparing effect of food was, however, observed with the tocopherol content of samples which probably contributes to the better oxidative stability of these samples.
The suitability of different commercial olive oil categories for domestic frying was investigated. Oil samples were taken every 3 h of frying and evaluated for free acidity, peroxide and p-anisidine values, specific extinction coefficients, oxidative stability, fatty acids, vitamin E, β-carotene and total phenols, until the total polar compounds achieved the maximum legal value (25%). All olive oils were fried during more time than the commercial vegetable oil blend taken for comparison (from 24 to 27 h, against 15 h).Extra virgin olive oil could protect food during cooking,