Phenols and the antioxidant capacity of Mediterranean vegetables prepared with extra virgin olive oil using different domestic cooking techniques
Introduction
The Mediterranean diet of the Spanish population is characterized by a high intake of vegetables and extra virgin olive oil (EVOO). These are both important sources of biofunctional compounds that have been linked to the prevention of chronic degenerative pathologies (Hernández-García, Wood, Castro-Obregón, & Covarrubias, 2010), not only because they contain nutritive antioxidants such as vitamins C, E, and ß-carotenes, but also important amounts of non-nutritive antioxidants such as phenolic compounds (Poljsak, 2011). When the supply of these alimentary compounds is guaranteed in the diet, they complement the activity of endogenous antioxidant defenses and counter the excess of free radicals caused by pathological conditions (Finley et al., 2011). Phenols are one of the most significant types of antioxidants. They are found in EVOO and vegetables typically consumed in Spain, such as eggplant, potato, tomato and pumpkin, with qualitative and quantitative compositions specific to each type. Measurements by ABTS, DPPH, FRAP and ORAC have found a close positive correlation between the phenolic contents in extracts of various vegetables and their antioxidant capacity, with the highest contribution made by some phenolic acids such as chlorogenic acid (Kalogeropoulos et al., 2007, Dini et al., 2013, Luthria, 2012, Martínez-Hernández et al., 2013). The profile of the phenols and antioxidant capacity are altered during the domestic procedures foodstuffs are subjected to before consumption, and it is generally thought that the effects are destructive. In some case, however, the parameters actually rise in comparison with the raw foodstuffs (Bunea et al., 2008, Bellail et al., 2012, Chiou et al., 2009). Cooking techniques have different effects depending on whether they are carried out in polar or apolar media. Hydrothermal processes have a drastic effect on the contents of hydrosoluble antioxidants, such as phenols, but, on the contrary, when apolar media such as deep or surface frying are used, the phenol concentrations in the vegetable matrix are less affected (Miglio, Chiavaro, Visconti, Fogliano, & Pellegrini, 2008). It has been suggested that losses during culinary treatment are related to other factors such as the vegetable/water proportion, cooking time, and the surface area exposed to leaching (Volden, Borge, Hansen, Wicklund, & Bengtsson, 2009). As a result of all the transformational processes prior to consumption, the eventual heterogeneity of the foodstuffs means that the databases on antioxidant contents are still far from being exhaustive, and even tables as extensive as Phenol-Explorer have limitations, such as the lack of data on cooked foodstuffs (Tarascou, Souquet, & Mazauric, 2010). The aim of the present research is to study the effect of different domestic cooking techniques on the qualitative and quantitative phenolic composition of representative Mediterranean foods.
Section snippets
Chemicals
All chemicals used were analytical reagent grade, unless otherwise stated, and water was obtained from a Milli-Q purification system (Millipore, Bedford, MA). Gallic, p-hydroxybenzoic, p-hydroxyphenylacetic and vanillic acids and pinoresinol, quercetin, luteolin, apigenin and 6-hydroxy-2,5,7,8-tetramethyl-chroman-2-carboxylic acid (Trolox) standards were supplied by Sigma–Aldrich (Milan, Italy). 3,4-dihydroxybenzoic, chlorogenic, caffeic, syringic, p-coumaric, o-coumaric and ferulic acids,
Moisture, dry matter and fat content
In Table 1, Table 2, Table 3, Table 4 we can see that the highest loss of moisture and highest fat gain in the vegetables occurred during deep frying, whereas sautéing (at <100 °C) maintained percentages of moisture similar to those of the raw vegetables and the vegetables boiled by both methods, but with lower percentages of fats. With boiling, the trend was to increase (p < 0.05) or not significantly change (p > 0.05) the moisture content, while the percentages of dry extract and fat fell or were
Conclusions
Each cooked vegetable developed a specific profile of phenols, moisture, fat, dry matter and antioxidant activity determined by the original characteristics of the raw vegetables and by the cooking technique applied. The choice of cooking method depends on the raw ingredient. When the phenol contents of the raw vegetable are high, the TPC rises even more if EVOO is included in processing and the application of hydrothermal treatments does not affect the final concentrations. When this is not
Conflict of interest
The authors declare that there are no conflict of interest.
Acknowledgments
This article on the results of the first author’s doctoral thesis. We wish to express our gratitude to PROMEP/SEP, México UDG-598, and to Robert Ian MacCandless Carrey for his assistance in improving the English language version of the manuscript.
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