沙漠研究 19-1, 93- 96 (2009 )Journal of Arid Land Studies“Desert Technology IX ”
Refereed Paper

Changes in the Biochemical Composition of the Tataouine Virgin Olive Oils
During Thermal Oxidation
Imen OUESLATI1, Faouzia M. HADDADA1, Issam NOUAIRI1, Maria Z. TSIMIDOU2, Mokhtar ZARROUK1
Abstract: An experimental investigation was carried out to evaluate the quality of virgin olive
oils from varieties (Chemlali Tataouine, Fakhari Douirat, Zarrazi Douirat, and Dhokar Douirat)
grown in the harsh conditions of the arid region of Tataouine, using accelerated oxidation anddeep frying studies at 180 ˚C from 0 to 10h. The experimental data showed the resistance ofthe studied oils to the oxidation especially Chemlali Tataouine and Zarrazi Douirat who
maintained their high stability and their natural biophenol contents during 2.5 and 5h,
respectively, under frying temperature. A positive correlation was observed between theinduction time and the total polar phenol contents during the thermal oxidation. The capacityof the heated and unheated oils to quench the free radicals has been also studied.
Keywords: Antioxidant, Frying, Oxidation, Virgin olive oil

1. Introduction
Recent studies suggest that the phenolic compounds naturally contained in virgin olive oil (VOO)
improve its resistance to oxidative deterioration (Gordon et al., 2001). In fact, they interrupt the initiationand propagation stages of the oxidative chain reaction since they act as chain breakers by donating a radicalhydrogen to alkylperoxyl radicals formed during the propagation step of lipid oxidation and subsequently
forming a stable radical (A•) through the well-known reaction (Frankel, 1998): ROO• + AH → ROOH + A•,
and therefore improve the oxidative stability of virgin olive oil and extend its shelf life. In southern
Tunisia, the region of Tataouine (latitude 32˚55′ north; longitude 10˚09′ east) is Characterized by a harshpedoclimatic conditions (chalky grounds, low rainfall (88-157 mm year-1) and a high temperature (max 45˚C). Despite these severe conditions, the cultivation of the olive tree being the principle agriculturalcommodity in this region. In fact, because of the lack of rainfall and the limited resources of undergroundwater, growing takes place in depressions to obtain the maximum of water. In this region, there are somevirgin olive oils from autochthonous varieties who showed an excellent natural antioxidant composition(Oueslati et al., 2009). To discover the degree of protection of such compounds to the oils and to better
understand how the quality of olive oil is affected over time, an accelerate thermal oxidation of VOOs has
been done at deep-frying temperature. So, the aim of this study was to establish the chemical changes
occurring in the VOOs from Tataouine varieties after exposure to high temperature (180˚C).
2. Materials and Methods
2.1. Oil samples
Four samples of VOOs were analysed from the following varieties: Chemlali Tataouine (CHt),Fakhari Douirat (FD), Zarrazi Douirat (ZD), and Dhokar Douirat (DD). All the varieties were grown in
the locality of Douirat in the region of Tataouine (southern Tunisia). Oil extraction was performed using
an Abencor laboratory oil mill, kneading the olive paste at 28˚C for 30 min. All samples were stored in
the freezer in darkness in amber glass bottles until analysis. A comparison with the main Tunisian varietyChemlali (CH), grown in the region of Sfax (35˚00’N; 10˚33’E; 128 m asl; ~ 230mm rain year-1; mean
temperatures: 15-29 ˚C, slightly alkaline sandy soil (pH ~7.5)), was also carried out.
2.2. Heating experiment
From the oil samples, 70g was placed into 100mL open glass beakers (7 cm high, 5 cm i.d.).
1

The

Laboratoire Caractérisation et Qualité de l’Huile d’Olive, Centre de Biotechnologie, Technopole de Borj-Cédria,
B.P. 901, 2050 Hammam-Lif, Tunisia Fax no. 0021679412948, tel. no. 0021679412948, email:mokhtar.zarrouk@cbbc.rnrt.tn
2
Laboratory of Food Chemistry and Technology, Department of Chemistry, Aristotle University of Thessaloniki,54124, Thessaloniki, Greece.

94beakers were heated at 180 ˚C for 10h in an oven. Aliquots of the heated oils (10 g) were taken every 2.5 hand stored at -20˚C in sealed dark glass vials under nitrogen atmosphere until analysis.
2.3. Determination of induction time
Induction time was evaluated by the Rancimat apparatus (model 743, Metrohm Co., Basel,Switzerland). Stability was expressed as the oxidation induction time (h) using 3.5g of oil. Thetemperature was set at 100 ˚C, and the air flow rate was 10 L h-1.
2.4. Colorimetric determination of total phenolics content
Total phenols were measured in the polar fraction extracted from 2.5 g of oil using methanol/water(60:40 v/v), and the determination was based on the Folin-Ciocalteu method (Tsimidou, 1998). Results
were expressed as caffeic acid (CA) equivalents (mg CA kg-1 oil).
2.5. Antioxidant capacity (AOC) of the VOO samples
The olive oil samples were analyzed for their capacity to scavenge the stable 1,1-diphenyl-2picrylhydrazyl radical (DPPH•) using the method of Kalantzakis et al. (2006) .
3. Results and Discussion
The induction time of the unheated oils, showed wide variation among the different tested VOOs (Fig.
1). The oil stabilities ranged from a minimum of 30h for Dhokar Douirat to 100h for Chemlali Tataouine.
According to these results, the olive varieties used in this study could be considered representative
examples of low- and high-stability olive oils. In accordance with the overall mechanism proposed forthe autoxidation process and the induction time calculated for the VOO samples, we can differentiate twogroups of oils. The more stable VOOs are Chemlali Tataouine, Zarrazi Douirat and Fakhari Douirat withan induction time of 100, 83 and 63 h, respectively, whereas the less stable oils are Dhokar Douirat whichshowed an induction time (30h) similar to that of Chemlali (34h). As shown in figure 1, the induction
time of Chemlali, Fakhari Douirat and Dhokar Douirat decreased rapidly by 43, 34 and 27%, respectively
during 2.5 h of heating at 180 ˚C. Whereas, Zarrazi Douirat maintained its induction time during 2.5 h ofheating. Except the VOO of Chemlali Tataouine, a drastic decrease in the induction time in all theremaining varieties after 2.5 h of heating. These results are similar to those found in the heated VOOs
from Spanish varieties Picual and Cornicabra which are the most stable oils (Salvador et al., 2001).
Concerning Chemlali Tataouine VOO, it is interesting to underline its very high capacity to maintainits induction time during 5 h. In fact, there are considerable changes in the induction time of its unheatedand heated VOO (100-84 h, respectively) during 5 h. Chemlali Tataouine VOO showed a decrease by
50% in its induction time at 7.5 h of heating and maintained this stability, which is considered high (>50 h),
for a long period, even after 10 h of heating. This exceptional potential may be due to the conjunction ofhigh phenol content and high levels of monounsaturated fatty acids in such variety (Oueslati et al., 2009).

Fig. 1. Change in the induction time of the Tataouine VOOs during heating at 180 ˚C as compared to ChemlaliVOO with CHt, Chemlali Tataouine; ZD, Zarrazi Douirat; FD, Fakhari Douirat; DD, Dhokar Douirat
and CH, Chemlali.

95

The results given in figure 2 indicate that the unheated VOOs of Chemlali Tataouine, Zarrazi Douirat
and Fakhari Douirat are with remarkable high contents of total polar phenols (> 450 mg CA kg-1).
Chemlali Tataouine showed the highest total polar phenols with an amount reaching 802 mg kg-1.
Whereas, Dhokar Douirat and Chemlali are with the lowest total polar phenol contents (< 400 mg CA kg-1).
During the heating, the total polar phenol content of Dhokar Douirat VOO declined drastically by 78%
after 2.5 h of heating at 180 ˚C. This result is in agreement with that reported by Gómez-Alonso et al.
(2002), who found that the concentration of hydroxytyrosol and its secoiridoid derivatives of a VOO
sample decreased after 2.5 h of heating at 180 ˚C. In opposite, all the remaining studied samples,
although they showed a diminution by 50% in their total polar phenol content after 2.5 h of heating, they
showed a relatively high phenol content (173-400 mg kg-1). It is interesting to mention that Chemlali
Tataouine showed a high polar phenol content (> 372 mg CA kg-1) even after 5 h of heating.
A correlation between polar phenol compounds and the induction time is well-established in the casesof unheated and heated oils (R2 > 0.8) (Fig. 3A). Dhokar Douirat VOO, which had the lowest total polar
phenol content, showed the lowest oxidative stability, whereas, Chemlali Tataouine which showed thehighest total polar phenol content, showed the highest induction time.
It can be seen from the results of figure 4 that the AOC of unheated Chemlali Tataouine, Zarrazi
Douirat and Dhokar Douirat virgin olive samples were similar and higher than those of unheated VOOs ofFakhari Douirat and Chemlali, probably due to the much lower tocopherol content of the latter oils
(Oueslati et al, 2009). Fakhari Douirat and Chemlali VOO samples practically lost their AOC after 2.5 hof heating at 180 ˚C. The other studied samples seemed to maintain some of their AOC for a longer
period, even after 10h of heating. It is interesting to stress that Chemlali Tataouine showed the highest
AOC during heating, in fact, it maintain all its capacity to scavenge the free radicals during 5 h of heating;Fig. 2. Change in the total polar phenol contents of the Tataouine VOOs during heating at 180°C as compared
to Chemlali VOO with CHt, Chemlali Tataouine; ZD, Zarrazi Douirat; FD, Fakhari Douirat; DD,Dhokar Douirat and CH, Chemlali.

ABB

Fig. 3. Correlations between (A) induction time and total polar phenols, and (B) the AOC and total polar
phenols during heating of the studied VOOs with CHt, Chemlali Tataouine; ZD, Zarrazi Douirat; FD,Fakhari Douirat; DD, Dhokar Douirat and CH, Chemlali.

96

at 7.5 h of heating it lost 65% of its capacity to scavenge the free radical [DPPH•]. The capacity ofChemlali Tataouine VOO to quench the free radicals is much more higher than that found in heated Greekand Spanish virgin olive oils (Kalantzakis et al., 2006). Practically, Chemlali Tataouine and Dhokar
Douirat maintained about 30% of their AOC after 10 h of heating. This may be due to their highest initial
total tocopherol content (Oueslati et al., 2009) and total polar phenol content. But except the oil ofChemlali Tataouine, good correlations were observed between the AOC and the total polar phenols in all
the remaining studied oils with R2 > 0.850. In Chemlali Tataouine, although the phenol content showed adecrease in the three first times of heating (0-5 h), the AOC was maintained. This fact may be due to thehigh contribution of the tocopherols by comparison with that of the phenols in the AOC of such variety andas a consequence, the correlation between the AOC and the total polar phenols is not very high (R2 >0.609) (Fig. 3B).

Fig. 4. Change in the antioxidant capacity (AOC) of the Tataouine VOOs during heating at 180°C as
compared to Chemlali VOO with CHt, Chemlali Tataouine; ZD, Zarrazi Douirat; FD, Fakhari Douirat;
DD, Dhokar Douirat and CHs, Chemlali.

4. Conclusions
According to the literature, VOO rapidly losses its valuable natural antioxidants (polar phenols andα-tocopherol) during heating (Brenes et al., 2002) or in the early stages of frying of potatoes(Andrikopoulos et al., 2002), although it keeps its thermal stability for a longer time. These results should
be taken into consideration when VOO is recommended for frying. But, in the case of Zarrazi Douirat
and especially Chemlali Tataouine, this study showed their high potential when they are recommended forfrying. These oils have a remarkable stability, total phenol contents and as a consequence high AOC atfrying temperatures.
References
Andrikopoulos N.K., Kalogeropoulos N., Falirea A., Barbagianni M.N. (2002): Performance of virgin olive oil and vegetable
shortening during domestic deep-frying and pan-frying of potatoes. International Journal of Food Science and Technology, 37:177–190.
Brenes M., García A., Dobarganes M.C., Velasco J., Romero C. (2002): Influence of thermal treatments simulating cooking processeson the polyphenol content in virgin olive oil. Journal of Agricultural and Food Chemistry, 50: 5962–5967.
Frankel N. (1998): Lipid oxidation. Dundee, Scotland: The Oily Press Ltd..
Gómez-Alonso S., Fregapane G., Salvador M.D., Gordon M.H. (2003): Changes in phenolic composition and antioxidant activity ofvirgin olive oil during frying. Journal of Agricultural and Food Chemistry, 51: 667–672.
Gordon M.H., Paiva-Martins F., Almeida M. (2001): Antioxidant activity of hydroxytyrosol acetate compared with that of other olive
oil polyphenols. Journal of Agricultural and Food Chemistry, 49: 2480–2485.
Kalantzakis G., Blekas G., Pegklidou K., Boskou D. (2006): Stability and radical-scavenging activity of heated olive oil and othervegetable oils. European Journal of Lipid Science and Technology, 108: 329–335.
Oueslati I., Anniva C., Daoud D., Tsimidou M.Z., Zarrouk M. (2009): Virgin olive oil (VOO) production in Tunisia: the commercial
potential of the major olive varieties from the arid Tataouine zone. Food Chemistry, 112: 733–741.
Tsimidou M. (1998): Polyphenols and quality of virgin olive oil in retrospect. Italian Journal of Food Science, 10: 99–116.
Salvador M.D., Aranda F., Fregapane G. (2001): Influence of fruit ripening on ‘Cornicabra’ virgin olive oil quality. A study of four
successive crop seasons. Food Chemistry, 73: 45–53.