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Journal of Oleo ScienceCopyright ©2012 by Japan Oil Chemists’ SocietyJ. Oleo Sci. 61, (6) 303-309 (2012)

DSC evaluation of extra virgin olive oil stability under accelerated oxidative test: effect of fatty acid composition and phenol contentsLorenzo Cerretani1, Alessandra Bendini2, Massimiliano Rinaldi3, Maria Paciulli3,Stefano Vecchio4 and Emma Chiavaro3＊

1 Dipartimento di Economia e Ingegneria Agrarie, Università di Bologna, P.zza Goidanich, Cesena (FC), I-47521, Italy2 Dipartimento di Scienze degli Alimenti, Università di Bologna, p.zza Goidanich 60, I-47521 Cesena (FC), Italy3 Dipartimento di Ingegneria Industriale, Università degli Studi di Parma, Parco Area delle Scienze 181/A, I-43124 Parma, Italy4 Dipartimento di Scienze di Base e Applicate all'Ingegneria, via del Castro Laurenziano 7, I-00185 Roma, Italy

1 INTRODUCTIONExtra virgin olive oil（Evoo）is widely consumed in the

countries of the Mediterranean Area. The presence of high percentages of oleic acid and minor components with anti-oxidant activity（i.e. tocopherols, phenolic compounds）in an ideal ratio makes its use very desirable for an increasing number of the European consumers who value the func-tional and nutritional values of food1）. In this context, a health claim on the protection of low-density lipoprotein（LDL）particles from oxidative damage by polyphenols in olive（olive fruit, oil and mill waste waters as Olea euro-paea L. extract and leaf）was recently approved（April 8th 2011）by the EFSA Panel on Dietetic Products, Nutrition and Allergies2）.

Evoo presents a relatively high stability to autoxidation（from 12 to＞18 months）, but this stability differs in rela-tion with the high variability of its composition, and in par-ticular to the content of oleic acid（55-83％）and phenolic compounds（50 to 1000 mg/kg）3）. In addition, there is little control of shelf life in many worldwide retail markets for

＊Correspondence to: Emma Chiavaro, Dipartimento di Ingegneria Industriale, Università degli Studi di Parma, viale Usberti 181/A, I-43124 Parma, ItalyE-mail: emma.chiavaro@unipr.itAccepted December 13, 2011 (received for review October 25, 2011)Journal of Oleo Science ISSN 1345-8957 print / ISSN 1347-3352 onlinehttp://www.jstage.jst.go.jp/browse/jos/　　http://mc.manusriptcentral.com/jjocs

Evoo bottles stored at room temperature, which can develop high levels of oxidation leading to undesirable flavour. Thus, new, simple and fast methods are needed for the evaluation of oxidative status of Evoo, avoiding expen-sive and time consuming techniques and assuring correct classification among legally defined commercial catego-ries4）.

Differential scanning calorimetry（DSC）is a well known thermoanalytical technique that is used in several applica-tions in the field of vegetable oils5, 6）. In recent years, several publications have emerged on the use of DSC for the assessment of Evoo quality7－9）. These have studied the cooling and heating thermal properties and established the composition of Evoo in terms of triacylglycerols, fatty acids and minor components10, 11）, also using statistical approach-es12, 13）.

DSC was also found to be a valuable tool for assessing the oxidative deterioration of vegetable oils14）. Its applica-tion to evaluate the degree of thermo-oxidation after con-ventional heating has been reported for Evoo15）as well as

Abstract: Three extra virgin olive oils having different fatty acid compositions and total phenol contents were submitted to an accelerated storage test at 60℃ for up to 21 weeks. Their oxidative status, evaluated by peroxide values and total phenolic content, was related to differential scanning calorimetry cooling profi les and thermal properties. Changes in crystallization profi les were consistent starting from 12 weeks for the two oil samples (B and C) that had a higher content of linoleic acid and medium/low amounts of phenols, respectively, whereas they became detectable at the end of the test for the remaining oil (sample A). Decrease of crystallization enthalpy and shift of transition towards lower temperature were also evident at 4 weeks of storage for samples B and C, whereas the same changes in the transition profi le were noticeable at 12 weeks for sample A. Differential scanning calorimetry appears to be suitable for the discrimination of oxidative status of extra virgin olive oils with widely different fatty acid composition.

Key words: Cooling, Differential scanning calorimetry, Extra virgin olive oil, Fatty acids, Total phenols

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for different commercial categories of olive oil after micro-waving16）. The capacity of DSC to discriminate among Evoo samples having different origins and chemical composition after microwaving was also recently explored17）. On the other hand, DSC application for the evaluation of autoxida-tion phenomena has not been widely studied. Changes in selected thermal parameters have been evaluated on Evoo samples treated by forced light exposition up to 8 weeks18）

or after exposure to air bubbles19）. Variations in cooling transition profiles and related thermal properties were also studied on a Evoo sample in the presence and in the absence of a phenolic fraction during accelerated storage treatment carried out at 60℃ for up to 4 weeks20）.

The aim of the present investigation was to evaluate the application of DSC cooling curves and related thermal properties for the assessment of the oxidative status of three Evoo samples chosen for their different fatty acid composition and total phenol content, and which are rep-resentative of the Italian oil market. Evoo samples were subjected to an accelerated storage test at 60℃, which is generally considered to be the optimal temperature to mimic oxidation that develops during normal storage3）. Only the cooling transition was analyzed, as it is well known that the crystallization of oils is influenced by chemical composition.

2 EXPERIMENTAL PROCEDURES2.1 Samples and storage

Three Italian samples of Evoo obtained from olives handpicked in 2009 and produced using continuous cold extraction systems were employed: sample A from Emilia Romagna（blend of Leccino, Frantoio, Moraiolo and local varieties）; sample B from Apulia（blend of Ogliarola di Lecce and Cellina di Nardò varieties）; sample C from Emilia Romagna（blend of Frantoio, Leccino, Pendolino and Moraiolo varieties）. The samples were selected considering three estimated levels of total phenol content（high, medium and low, respectively）.

Each sample was divided in 4 aliquots（43.7 mL, 40 g）and kept in the dark at 60℃ in a thermostatic oven（ISCO, Milan, Italy）for 21 weeks. Each aliquot was stored in an in-dividual open glass bottle of 250 mL（i.d.＝5 cm; surface area exposed to air 19.6 cm2）. Bottles were removed from the oven after 4, 12 and 21 weeks and immediately ana-lyzed for their chemical composition and DSC thermal properties.

2.2 Chemical analysis

Fatty acid（FA）compositions were determined on fresh samples22）as fatty acid methyl esters by capillary gas chro-matography（GC）analysis after alkaline treatment. The results were expressed as area normalization in percent

（％）. FA were expressed according to their degree of un-saturation as saturated（SFA）, monounsaturated（MUFA）and polyunsaturated（PUFA）fatty acids. Peroxide value（POV, expressed as mEq O2/kg lipids）was evaluated ac-cording to the official methods described in annex III of EEC Regulation 2568/9123）. Total phenol content,（ex-pressed as mg gallic acid/kg oil）, was measured using a spectrophotometric assay as previously reported24）. POV and total phenols were determined on fresh samples and after 4, 12 and 21 weeks of storage. Three replicates were analyzed per sample.

2.3 DSC analysis

Samples of oil（8-10 mg）were weighed in aluminium pans, covers were sealed and analyzed with a DSC Q100（TA Instruments, New Castle, DE, USA）. Indium（melting temperature 156.6℃, ΔHf＝28.45 J/g）and n-dodecane（melting temperature －9.65℃, ΔHf＝216.73 J/g）were used to calibrate the instrument, and an empty pan was used as reference. Oil samples were equilibrated at 30℃ for 8 min and then cooled at －80℃ at a rate of 2℃/min. Dry nitrogen was purged in the DSC cell at a flow rate of 50 cm3/min. DSC cooling curves were analyzed with Uni-versal Analysis Software（Version 3.9A, TA Instruments）to obtain enthalpy of crystallization（J/g）, peak temperature（Tp1 and Tp2 for the major and the minor exothermic peaks, respectively）, onset temperature（Ton,℃）and offset tem-perature（Toff,℃）of the transitions. The range of transitions was calculated as the temperature difference between Ton and Toff. Three replicates were analyzed per sample.

2.4 Statistical analysis

Data were analyzed using SPSS（Version 17.0, SPSS Inc., Chicago, IL, USA）statistical software. SPSS was used to perform one-way-analysis of variance（ANOVA）and Tukey’s honest significance difference post-hoc test at a 95％ con-fidence level（p ≤ 0.05）to identify differences both among storage times for each sample and samples at each storage time.

3 RESULTS AND DISCUSSION3.1 Chemical analysis

As shown in Table 1 and Fig. 1a, the three Evoo samples subjected to the accelerated oxidative test were character-ized by significant differences in terms of fatty acid compo-sition and phenolic content. In fact, EvooB presented the lowest content in oleic acid and the highest in palmitic acid and linoleic acid, and consequently was the poorest in MUFA category and the richest in SFA and PUFA classes. On the other hand, EvooA exhibited significantly high per-centages of MUFA and low of PUFA due to the specific contents of oleic acid and linoleic acid. EvooC showed a

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fatty acid composition between EvooA and EvooB. These compositional data are completed by the differences in total phenol composition among the three Evoo samples（storage time 0, Fig. 1a）. In fact, this content can be con-sidered particularly elevated for EvooA（740 mg gallic acid/kg oil）, moderate for EvooB（505 mg gallic acid/kg oil）and low（261 mg gallic acid/kg oil）for EvooC, showing signifi-cant differences among samples. The initial POV values（9.2 meq O2/kg lipids）were widely below the legal limit for the extra virgin category for all oils showing an unaltered oxi-dative status4）.

Taking into account the favourable chemical characteris-tics of EvooA in terms of the high ratio between oleic acid and linoleic acid, as well as the considerable phenolic content25）, it could be expected that this sample was the most resistant to oxidative changes. In reality, until the end of the 12th week of thermal stress, the increase in POV of EvooA was very limited and a consistent rise was seen for both EvooB and EvooC（Fig. 1b）. Only at the 21th week of the accelerated storage test EvooA showed a higher value of peroxide compounds. Thus, EvooA preserved the highest phenolic content for the entire time of the test, even if the decrease in these antioxidant compounds was already significant after 4 weeks, as previously observed20）, as for the other samples（Fig. 1b）. It can be hypothesized that EvooA achieved the phase of accumulation of primary oxidation products later than EvooB and EvooC, as shown in Fig. 1b, whereas the increase of POV proceeded slowly for this sample. This was already observed for samples with analogous FA composition and high phenol content under similar oxidation conditions26）. After 12 weeks of accelerat-ed storage, EvooB reached a higher amount of peroxide compounds than EvooC due to its lower oleic/linoleic ratio27）. At week 21, both samples significantly decreased in POV likely as a consequence of the conversion of the primary into secondary oxidation products as well as vola-tile compounds that were responsible for the off flavour of Evoo during oxidative deterioration27）.

3.2 DSC analysis

The patterns of crystallization profiles during storage are shown in Fig. 2a, b and c for EvooA, EvooB and EvooC, re-spectively. At time 0, all samples showed the classical crys-tallization profile of Evoo with two exothermic events, a major（peak 1, insert a of Fig. 2）peak at lower temperature and a minor（peak 2, insert a of Fig. 2）at higher tempera-ture regions of the DSC curves, as described in previous studies8, 9, 12, 28）. Changes in crystallization profiles became evident for oil samples at different storage times. In fact, none of the samples showed evident modifications at the first time of storage, in accordance with a previous study where an Evoo sample was stored up to 4 weeks under the same conditions20）. A small decrease in the height of peak 1 was observed for EvooB and EvooC, however. In addition,

Table 1　 Main FA percentages and their classes based on the unsaturation degree (SFA, saturated; MUFA, monounsaturated; PUFA, polyun-saturated) for the Evoo samples

FA (％) EvooA EvooB EvooC

Palmitic acid 13.0B 15.5A 13.1B

Stearic acid 2.4A 1.8B 2.1A

Oleic acid 73.7A 64.9C 71.6B

Linoleic acid 6.2C 10.0A 8.1B

SFA 15.9B 17.7A 15.7B

MUFA 77.3A 71.7C 75.6B

PUFA 6.9C 10.7A 8.7B

A, B, C The same superscript letters within each row are not signifi cantly different (n＝3, p < 0.05). RSD ≤ 2.5％

Fig. 1　 Changes in phenolic compounds (a) and POV (b, normalized data) for Evoo samples A, B and C at different storage times (weeks). Error bars represent ＋/－ 1 standard deviation, (n＝3). Bars with the same capital letters within each sample at different storage times were not sig-nifi cantly different (p < 0.05).

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the cooling transition did not show marked changes for EvooA up to 12 weeks of storage. At 21 weeks, the major cooling peak（peak 1, insert a of Fig. 2）shifted towards lower temperature and became broader, also enlarging the range of transition. Similar changes were evident for EvooB and EvooC after a shorter time of storage（4 weeks）, al-though the shift in peak 1 appeared to be more consistent than EvooA（at 12 weeks）in both samples. The profile of peak 2 was also altered by oxidation as it became less evident and flattened, especially for EvooC, at the same time of storage. Thus, changes in the transition profiles became evident when lipid oxidation noticeably increased, in accordance with the POV trends shown in Fig. 1b. All these changes in the DSC crystallization profile have already been observed for Evoo to conventional15）or micro-wave16, 17）thermo-oxidation and are related to the formation of lipid oxidation products that may weak and/or hinder in-termolecular bonding between TAG molecules leading to the formation of more irregular crystals as well as to the in-crease of the oil viscosity15, 29）. Crystallization profiles dra-matically changed at the end of storage for EvooB and EvooC, as peak 1 disappeared and the profiles of peak 2 doubled showing a shoulder peak at lower temperature（at about －14.9℃ and －17.2℃ for EvooB and EvooC, re-spectively）than the original peak. These significant modifi-cations of the DSC crystallization profiles of Evoo have not been previously reported, and may be attributable to the high content reached by oxidized lipid molecules（both primary and secondary lipid oxidation products）.

Cooling thermal properties obtained for the three Evoo samples at 0 time and during storage are summarized in Table 2. Thermal properties of unheated oils did not show a trend increasing phenols as EvooB, which exhibited mod-erate phenol content and also showed significantly lower enthalpy and a larger range of transition（higher Ton, Tp2 and lower Toff and Tp1）than the other samples. These results confirm previous hypotheses about the absence of a direct role of phenolic compounds in influencing the DSC crystallization profile, as no significant differences were re-cently found among cooling thermal properties for a Evoo sample in the presence and absence of phenols20）. In con-trast, another study11）found an effect of variety on cooling thermal properties such as Ton and Tp1 for Evoo comparing oils before and after partial phenol removal. Otherwise, higher Ton, Tp1, Tp2 and lower Toff were previously found for Evoo samples with both a high content of palmitic and lin-oleic acids（as shown by EvooB in Table 1）, and high statis-tical correlations were obtained among the thermal proper-ties and amounts of these compounds12）.

All samples showed a significant decrease in enthalpy of crystallization during storage, which became consistent at different storage times for EvooA（12 weeks）in comparison with EvooB and EvooC（4 weeks）. A significant shift in transition towards lower temperature was also observed

-0.4

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Heat Flow (W/g)

Exo Up Temperature (°C)

a

0 weeks

4 weeks

12 weeks

21 weeks

b

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c

0 weeks

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21 weeks

Fig. 2　 Representative DSC cooling curves of Evoo samples A (a), B (b) and C (c) at different stor-age times (weeks).

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for EvooA as Ton and Toff, and both exothermic peaks signifi-cantly shifted towards lower temperatures enlarging the range of crystallization in this sample. All these changes may be related to the formation of lipid oxidation products that could form mixed and disordered triglyceride crystals. These crystals require lower energy to crystallize, and also retard the phase transition30）. All these phenomena were also evident for EvooB and EvooC with the exception of Ton, which shifted towards higher temperature at the end of storage. The shift of transition towards higher tempera-ture has been previously attributed to the increase in diac-ylglycerol content in Evoo as consequence of lipid hydroly-sis due to auto28）- or thermo-oxidation phenomena16）, although a statistical correlation was not found in fresh oil samples where diacylglycerol amounts were low12）. In addi-tion, EvooB and EvooC did not show marked changes in thermal property（either for trend of changes or their extent）, although oxidative deterioration of EvooB ap-peared to be more consistent than EvooC at 12 weeks of storage（Fig. 1b）, in accordance with its fatty acid composi-tion. In a previous study carried out on microwave thermo-oxidized Evoo samples, modifications in thermal properties exhibited a different trend among oils, but the relationship of these changes with chemical composition and/or oxida-tion extent was not clearly established as several factors

appeared to be involved17）. Thus, the relation among Evoo cooling thermal properties and stability indices, not only from primary but also from higher oxidation stages（i.e. secondary and tertiary）, should be further investigated by evaluation of statistical correlations on a large set of oil samples taking into account both auto- and thermo-oxida-tion since the lipid kinetics of these phenomena are differ-ent.

4 CONCLUSIONSThe application of DSC appears to be suitable for the

evaluation of qualitative changes in extra virgin olive under temperature conditions that best mimic the kinetics of oxi-dation developing during storage at room temperature. This could be of interest for producers and traders of this high quality vegetable oil, since DSC has undisputable ad-vantages over classical analytical methods as it does not require sample preparation or solvent utilization, has a reduced environmental impact and avoids the use of time-consuming procedures.

Some thermal properties like onset and offset tempera-tures of crystallization as well as DSC peak temperatures represent an useful tool for the evaluation of degradation

Table 2　 DSC thermal properties obtained from the cooling thermograms of Evoo samples at different storage times

Storage time

(weeks)

Enthalpy of crystallization

(J/g)Ton (℃) Toff (℃) Range(℃) Tp1 (℃) Tp2 (℃)

Evoo A

0 57.9a A －11.2b A －44.7a A 33.5c D －38.3a A －15.4b A

4 56.1A －11.7A B －46.0B 34.1C －38.5A －15.4A

12 55.2A －11.7A B －46.9C 35.2B －39.0B －15.1A

21 36.4B －12.1C －68.9D 56.7A －60.7C －16.5B

Evoo B

0 56.1a A －10.3a B －48.7c A 38.4a C －42.3b A －12.8a A

4 52.4B －10.6B －50.6B 40.1B －43.2B －12.9A

12 36.2C －10.5B －74.0c C 63.5A －67.0C －13.7AB

21 n.d. － 8.8A n.d. n.d. n.d. －14.9B

Evoo C

0 57.2a A －11.1b C －46.5b A 35.4b B －39.4a A －14.9b A

4 53.0B －12.0C －47.7B 35.7B －40.2B －15.2A

12 31.6C －12.2B －73.1C 62.0A －66.7C －16.5AB

21 n.d. － 9.3b A n.d. n.d. n.d. －17.1B

a, b, c The same superscript letters within samples at 0 storage time are not signifi cantly differentA, B, C The same superscript letters within each sample at different storage times are not significantly

different (n＝3, p < 0.05). n.d.＝not determined. RSD ≤ 2％.

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of extra virgin olive oil since they were found to signifi-cantly change according to the different degree of oxida-tion. Otherwise, DSC cooling curves and related thermal properties seemed to discriminate among Evoo samples on the basis of the extent of thermal degradation only when these oil samples exhibited quite different oleic and linoleic acid percentages and/or phenol amounts（i.e. sample A from both B and C）. In addition, the results of this study showed that increasing amounts of phenolic compounds do not seem to have influence on crystallization profiles or related thermal properties of unheated extra virgin olive oil under the experimental conditions applied.

Finally, the application of DSC to the assessment of oxi-dation in extra virgin olive oil should be carried out by ex-amining naturally autoxidized samples and carefully evalu-ating the correlation among cooling thermal properties and molecules in the different lipid oxidation stages（volatile compounds, oxidized fatty acids and triacylglycerols）.

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