Analytical, Nutritional and Clinical Methods
Changes in antioxidant concentration of virgin olive oil
during thermal oxidation
Michalis Nissiotis, Maria Tasioula-Margari*
Laboratory of Food Chemistry, Department of Chemistry, University of Ioannina, Ioannina 45110, Greece
Received 6 October 2001; received in revised form 3 January 2002; accepted 3 January 2002
Abstract
The concentration of a-tocopherol, hydroxytyrosol derivatives and tyrosol derivatives was measured in samples of virgin olive oil
during thermal oxidation at 60 and 100
C. Hydroxytyrosol derivatives are the first antioxidants that are lost during thermal oxi-
dation (until a peroxide value of 20–30 meq/kg). Tyrosol derivatives seem to be the most stable compounds, while a-tocopherol has
an intermediate rate of loss at low peroxide values and is destroyed at peroxide values from 20 to 50 meq/kg. This means that
hydroxytyrosol derivatives are the first compounds to be oxidized, providing therefore oxidative stability to the oil. a-Tocopherol
seems to be oxidized after a significant decrease on hydroxytyrosol derivatives content. Tyrosol derivatives are the antioxidants that
decrease with the lowest rate, providing the oil with the less antioxidant activity. # 2002 Elsevier Science Ltd. All rights reserved.
Keywords: Virgin olive oil; a-Tocopherol; Phenolic compounds; Heating degradation; Antioxidant activity
1. Introduction
Antioxidants are known to act at different levels in
the oxidative sequence involving lipid molecules. The
presence of important natural antioxidants in plant
food is attracting further interest because of their clear
benefits as anticarcinogenic agents and as inhibitors of
biologically harmful oxidation reactions in the body.
The evidence is accumulating that diets rich in plant
antioxidants derived from fruits and vegetables are
associated with lower risks of coronary heart disease
and cancer (Madhavi, Deshpande, & Salunkhe, 1996).
Extra virgin olive oil is particularly rich in poly-
phenols. A number of simple and complex phenolic
antioxidants have been identified in the polar fraction of
virgin olive oil. Both simple and complex phenols were
detected with the latter being the most abundant. 3–4-
Dihydroxyphenyl ethanol (hydroxytyrosol) derivatives
and p-hydroxyphenyl ethanol (tyrosol) derivatives
(Angerosa, d’Alessandro, Konstantinou, & Di Giacinto,
1995; Montedoro, Servili, Baldioli, & Miniati, 1992b;
Montedoro, Servili, Baldioli, Selvaggini, Miniati, &
Macchioni 1993; Tasioula-Margari & Okogeri, 2001b)
and lignans (Brenes, Garcia, Garsia, Rios, & Carrido,
1999; Owen, Mier, Giacosa, Hull, Spiegelhalder, &
Bartsch, 2000a) predominated among the complex phe-
nols.
Phenolic compounds have been reported as influen-
cing sensory quality (Brenes, Garcia, Garsia, Rios, &
Carrido, 1999; Esti, Cinquanta, & La Notte, 1998), as
having beneficial biological activity (Visioli, & Galli,
1998; Visioli, Bellomo, & Galli, 1998; Owen et al.
2000b) and oxidative stability of olive oil (Baldioli, Ser-
vili, Perretti, & Montedoro, 1996; Chimi, Cillard, Cil-
lard, & Rahmani, 1991; Papadopoulos & Boskou, 1991).
Several studies dealing with the antioxidant activity of
phenolic compounds and/or tocopherols of virgin olive
oil were performed under accelerated conditions invol-
ving elevated temperatures, after the addition of known
concentrations of antioxidants to an oil or model sub-
strate. The oxidative stability of virgin olive oils corre-
lates mainly with the concentration of hydrophilic
phenols and, in particular with the oleosidic forms of
hydroxytyrosol. The predominant phenolic compounds
of virgin olive oil have an antioxidant effect which
decreases in the order: hydroxytyrosol > oleuropein >
tyrosol (Chimi et al., 1991). A sinergistic effect between
0308-8146/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S0308-8146(02)00113-9
Food Chemistry 77 (2002) 371–376
www.elsevier.com/locate/foodchem
* Corresponding author. Tel.: +30-651-98345; fax: +30-651-
98795.
E-mail address: mtasioul@cc.uoi.gr (M. Tasioula-Margari).