Research SectionPhenolic compounds and squalene in olive oils: the concentration and antioxidant potential of total phenols, simple phenols, secoiridoids, lignansand squalene
Introduction
High intakes of dietary fats have been implicated in the development of a number of diseases. In particular, positive associations between elevated dietary fat and cancer of the colon (Armstrong and Doll, 1975), breast (La Vecchia et al., 1998), prostate (Chan et al., 1998) and ovary (Risch et al., 1994), as well as atherosclerosis (Kuller, 1997) and coronary heart disease (Gerber, 1994) have been reported.
Recently, however, the epidemiologic data linking breast (Holmes et al., 1999) and colon cancer (Giovannucci et al., 1994), with total fat intake has not been upheld. In this regard, evidence is emerging that it is not only the amount, but also the type of dietary fat that is important in the aetiology of some cancers (Bartsch et al., 1999).
Confirmatory evidence for this hypothesis comes from a comparison between dietary habits and cancer incidence in Israel (Berry et al., 1991) and Greece (Katsouyanni et al., 1986). The daily fat intake in Israel is on average about 100 g (with 40 g animal fat and 60 g vegetable fat, mostly from sunflower oil, rich in polyunsaturated fatty acids), while in Greece the daily fat consumption is higher (140 g/day) than in Israel, with higher quantities of animal fats (60 g) and vegetable fats (80 g), the latter represented almost entirely by olive oil typical of the Mediterranean diet. In contrast with what is expected on the basis of total dietary fat intake, mortality from breast cancer is much higher in Israel compared to Greece (Rose et al., 1986). What could be the reason for this phenomenon?
Gerber (1994) has reported that while the vitamin E/tocopherol content of olive oil is low compared to vegetable oils, the amounts of other antioxidants such as squalene and various phenolic substances are relatively high. Although the antioxidant content of olive oil is well researched (Montedoro, Servilli, Baldioli & Miniati, 1992a, Montedoro, Servilli, Baldioli & Miniati, 1992b, Montedoro, Servilli, Baldioli, Selvaggini, Miniati & Macchioni, 1993), it was not until recently that a more detailed profile (Owen, Mier, Giacosa, Hull, Spiegelhalder & Bartsch, 1999, Owen, Spiegelhalder & Bartsch, 2000a) became available which may well explain the chemoprotective effects of this oil (Braga et al. 1998, La Vecchia, Negri, Franceschi, Decarli, Giacosa & Hepworth, 1995, Martin-Moreno et al. 1994, Trichopoulou, Katsouyanni, Stuver, Tzala, Gnardellis, Rimm & Trichopoulos, 1995). In addition to the already characterized simple phenols and secoiridoids, a further class of antioxidant phenolic compounds were detected in olive oil for the first time, namely the lignans (+)-1-acetoxypinoresinol and (+)-pinoresinol.
In our ongoing cancer chemopreventive studies, we have a particular interest in the health benefits of the Mediterranean diet, and therefore the aim of this study was not only to quantitate the phenolic and squalene content of a range of seasoning oils, namely (1) extra virgin olive oils (VOQ) from different production areas; (2) refined virgin olive oils (RVO) of different type; and (3) seed oils (SO) including corn, sunflower, peanut and mixed seed oils, but also to assess their antioxidant potential.
Section snippets
Oils
Thirty oils comprising VOQ (n=18), RVO (n=7) and SO (n=5) currently on the Italian market were studied.
Standard compounds
(p-Hydroxyphenyl)ethanol (tyrosol), catechol, syringic acid, p-hydroxybenzoic acid, vanillic acid, benzoic acid, 3,4-dihydroxy benzoic acid, homovanillic alcohol, o-coumaric acid, p-coumaric acid, gallic acid, Trolox and Fe2Cl3.6H2O were obtained from Sigma-Aldrich, (Steinheim, Germany). Ferulic acid, cinnamic acid, caffeic acid and EDTA were obtained from Fluka Chemie (Buchs, Switzerland).
Extraction of phenolics for HPLC analysis
Of the two methods compared, absolute methanol was superior to methanol–water (80:20, v/v) for the extraction of phenolic compounds from the oils (data not shown). After two extraction steps with absolute methanol as shown in Fig. 2, complete removal of the phenolic components was attained (Fig. 3). Consequently this method was adopted for the routine screening of the phenolic fraction in oils.
Chromatographic separation by HPLC
To assess the HPLC method, and, to evaluate the efficiency of extraction, a range of various simple
Discussion
The content of phenolic compounds and squalene in a range of olive and seasoning oils has been assessed and compared. Seed oils were devoid of the typical phenolic compounds detected in olive oils. It should be noted, however, that using a polar solvent such as methanol as extractant does not enable precise quantitation of vitamin E/α-tocopherol, and therefore the data presented here takes no account of the differences that may exist between olive and seasoning oils in the content of this
Acknowledgements
We are grateful to G. Schwebel-Schilling and G. Erben for the NMR spectra at 250 MHz and ESI-MS measurements, respectively, and to Roswitha Haubner for excellent technical assistance.
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