review - uclm · 2011-02-23 · review 1 introduction virgin olive oils, being mechanically...

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Eur. J. Lipid Sci. Technol. 104 (2002) 639–660 639

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1 Introduction

Virgin olive oils, being mechanically extracted from olivefruits (Olea europaea L.), retain volatile and non volatilecompounds, which are mainly responsible for their typicalflavour that makes them highly appreciated by consumersnot only in the countries of the Mediterranean basinwhere the olive oil production is concentrated. Thesalutistic properties of olive oil such as its high nutritionalpower, excellent digestibility, high oxidative stability evenwhen used for cooking, strong capacity of prevention ofheart and vascular troubles [1] do not explain the reasonsfor the increased popularity of the olive oil also in coun-tries where it was a relatively underused commodity com-pletely. The large increase in demand for high quality oliveoils is thought to be related to their peculiar organolepticcharacteristics that play an important role in human nutri-tion. Volatile aromatic compounds and also some nonvolatile compounds, strongly affecting sensory receptors,can decisively influence the food acceptability, direct thepreference of consumer and, in a word, determine thequality of life to a great extent.

The sensory attributes of the olive oil perceived by con-sumers arise from the stimulation of the gustative and ol-

factive receptors through a large number of volatile [2-4]and some non volatile compounds [5, 6], such as pheno-lic substances. The latter mainly elicite the tasting per-ception of bitterness [7]. In addition, they stimulate thefree endings of the trigeminal nerve located in all thepalate and also in the gustative buds giving rise to thechemesthetic perceptions of pungency astringency andmetallic attribute [8, 9].

All the other sensations experienced during the virginolive oil tasting are attributed to the stimulation of theolfactory epithelium by a large number of volatilecompounds present in the oil aroma in very low amounts.

Franca Angerosa

Istituto Sperimentale per laElaiotecnica, Città S. Angelo(PE), Italy

Influence of volatile compounds on virgin oliveoil quality evaluated by analytical approaches and sensor panelsVolatile compounds, retained by virgin olive oils during their extraction process, are re-sponsible for the oil aroma. Approximately one hundred and eighty compounds, whosestructure was assigned by means of gas chromatography-mass spectrometry, werefound in virgin olive oil aromas. The analytical approaches to their determination arebriefly discussed, considering the problems related to the volatile compound collectionand to adsorbents used for their trapping.

The sensory methodology for the evaluation of the organoleptic characteristics of thevirgin olive oils are reported and typical flavours and off-flavours are described. Com-pounds responsible for flavours, including factors affecting the volatile fraction, and foroff-flavours are carefully examined, also considering the causes that give rise to off-flavours. Relationships between volatile compounds and sensory characteristics,found by various researchers, are reviewed.

Keywords: Olive oil, volatile compounds, sensory attributes, relationships betweenvolatiles and the sensory characteristics.

Correspondence: Franca Angerosa, Istituto Sperimentale per laElaiotecnica, Contrada Fonte Umano, 65013 Città S.Angelo(PE), Italy. Phone: +39-085-95212, Fax: +39-085-959518; e-mail: [email protected]

Tab. 1. Characteristics shared by volatile compounds re-sponsible for virgin olive oil aroma.

• low molecular weight (<300 Da)

• high volatility so that a suitable number of moleculescan reach the olfactory epithelium as molecular dis-persion, transported by the air streams due to inhala-tion and expiration

• sufficient hydrosolubility to diffuse into the mucus thatcovers the sensitive olfactory cells

• fair liposolubility to dissolve in membrane lipids con-tiguous to proteins of receptors

• chemical features to bond specific proteins

2 Composition of volatile fraction

Approximately one hundred and eighty compounds be-longing to several chemical classes were separated fromthe volatile fractions of different quality virgin olive oils.Even though they belong to several chemical classes,they share the characteristics described in Tab. 1.

640 Angerosa Eur. J. Lipid Sci. Technol. 104 (2002) 639–660

Tab. 2. Volatile carbonyl compounds identified in virginolive oil aroma by MS.

Compound Ref

Aldehydesacetaldehyde [10]propanal [11]butanal [11]pentanal [10, 11]hexanal [10, 17]heptanal [10, 11]octanal [10, 11]nonanal† [10, 11]decanal [21]acrolein† [19]2-butenal [19]pentenal (cis-2?) [10, 11]trans-2-pentenal [10, 11]cis-2-hexenal [10, 11]trans-2-hexenal [10, 17]cis-3-hexenal [12, 20]heptenal (cis-2?) [10, 11]trans-2-heptenal [10, 11]trans-2-octenal [10, 11]cis-2-nonenal [21]trans-2-nonenal [10, 11]cis-3-nonenal [21]trans-2-decenal [10, 11]trans-2-undecenal [10, 11]2-methylbutanal [10, 17]3-methylbutanal [10, 13]2-methylbut-2-enal [10, 14]2,4 hexadienal [10, 12]2,4 -heptadienal (isomer A) [10, 12]2,4 heptadienal (isomer B) [10, 12]2,4-nonadienal [10, 21]2,4-decadienal (isomer A) [10, 20]2,4-decadienal (isomer B) [10, 20]benzaldehyde [10, 11]phenylacetaldehyde [21]

Ketonesacetone [11]2-butanone [14]2-hexanone [10]2-heptanone [12]2-octanone [10, 12]2-nonanone [10, 12]3-pentanone [10, 17]3-octanone [4, 10]3-metylbutan-2-one [10]6-methyl-5-hepten-2-one [14]4-methylpentan-2-one [14]1-penten-3-one [13, 14]1-octen-3-one [18, 21]4-methyl-3-penten-3-one [17]2-methyl-2-hepten-6-one [10]acetophenone [10]

† Identified by GLC-HPLC.

Tab. 3. Volatile ester identified in virgin olive oil aroma byMS.

Compound Ref

Estersbutyl acetate [12]ethyl acetate [10, 17]ethyl phenylacetate [10]ethyl propionate [10]ethyl butyrate [10, 12]ethyl octanoate [10, 12]ethyl heptanoate [11, 12]ethyl nonanoate [11]ethyl decanoate [11, 12]ethyl palmitate [11]heptyl acetate [12]hexyl acetate [12, 17]methyl acetate [14]methyl butyrate [10]methyl pentanoate [10]methyl hexanoate [10, 11]methyl heptanoate [10, 11]methyl octanoate [10, 11]methyl nonanoate [14]methyl decanoate [14]methyl oleate [11]methyl linoleate [11]1-octyl acetate [10]propyl propionate [10]butyl 2-methylbutyrate [12]ethyl 2-methylpropionate [10, 12]ethyl 2-methylbutyrate [10, 12, 20]ethyl 3-methylbutyrate [10]methyl 2-methylbutyrate [10]methyl 3-methylbutyrate [10]3-methyl-2-butenyl acetate [14]2-methyl-1-butyl acetate [10, 17]3-methyl-1-butyl acetate [10, 17]2-methyl-1butyl 2-methyl-propionate [10]2-methylbutyl propanoate [14]2-methyl-1-propyl acetate [10, 14]2-methyl-1-propyl 2-methylpropionate [10]1-propyl 2-methylpropionate [10]ethyl benzoate [10]methyl benzoate [12]methyl salicilate [11]cis-3-hexenyl acetate [10, 17]ethyl cyclohexanoate [20]

The chemical structure of most of the volatile compoundswas assigned by gas chromatography-mass spectrome-try (GC-MS) [10-21].

Tabs. 2, 3, 4 and 5 summarise carbonyl compounds, al-cohols, esters and hydrocarbons, respectively, havingbeen identified so far in virgin olive oil aromas by differentresearchers. Tab. 6 shows a compilation of other oxy-genates compounds and thiophene derivatives found tobe present among the volatile compounds of virgin oliveoil.

The number of volatile compounds detected in the aromaof an olive oil depends on the methodology adopted fortheir determination and on the quality of the virgin oliveoil.

Olive oil, obtained from healthy and rightly ripe fruits ofthe tree Olea europaea L., harvested at the right ripeness,by proper technological extraction methodologies, showsa volatile fraction mainly formed by compounds which arecommon contributors to the aroma of many fruits and veg-etables. They are produced enzymatically from polyun-saturated fatty acids through the so-called lipoxygenase(LOX) pathway [22, 23]. It is stated that in the aroma ofthese oils C6 aldehydes, C6 alcohols and their corre-sponding esters are the most abundant accumulationproducts. Moreover, reasonable amounts of C5 carbonyl

compounds, C5 alcohols and pentene dimers contributeto the virgin olive oil aroma [15].

A greater number of volatile compounds is present in thearoma of the virgin olive oils of worse categories. In thoseoils the concentrations of C6 and C5 compounds are quitelower than those detected in high quality oils or thosecompounds are even completely absent. At the sametime C7-C11 monounsaturated aldehydes [19, 24], or C6-C9 dienals [16], or C5 branched aldehydes [25] or someC8 ketones [18] become important contributors to the oilaroma, they are responsible for negative attributes (de-fects), such as rancid, winey-vinegary, fusty, muddy sedi-ment, musty. Gas chromatographic profiles of a virgin

Eur. J. Lipid Sci. Technol. 104 (2002) 639–660 Volatile compounds and sensorial analysis 641

Tab. 4. Alcohols identified in virgin olive oil aroma by MS.

Compound Ref

Alcoholsmethanol [15]ethanol [10, 17]propan-1-ol [18]butan-1-ol [12]pentan-1-ol [12, 13]hexan-1-ol [10, 12]heptan-1-ol [10, 12]octan-1-ol [10, 12]nonan-1-ol [10, 11]decan-1-ol [12]pentan-3-ol [10]octan-3-ol [18]2-penten-1-ol [13, 14]trans-2-hexen-1-ol [10, 12]cis-3-hexen-1-ol [10, 12]4-hexen-1-ol [12]1-penten-3-ol [10, 12]1-octen-3-ol [18]methylpropan-1-ol [10, 13]2-methylbutan-1-ol [10]3-methylbutan-1-ol [10, 12, 20]3-methyl-1-pentanol [12]2-phenylethanol [10, 12, 20]

Tab. 5. Hydrocarbons identified in the virgin olive oil aro-ma by MS.

Compound Ref

Hydrocarbonsn-octane [10, 11]n-nonane [12]n-decane [12]n-undecane [12]n-dodecane [12]tridecane [12]tetradecane [12]methyldecane [12]hexene [14, 17]octene [12]C8C14 [12]C11H18 [12]tridecene [12]limonene [12]α-copaene [12]α-murolene [12]α-farnesene [12]1,3-hexadien-5-yne [14]3,5-diethyl-1,5hexadiene (2 isomers) [15]3-ethyl-1,5 octadiene (2 isomers) known as pentene dimers [15]3,7-decadiene (3 isomers) known as pentene dimers [15]benzene [12]ethylbenzene [12, 17]diethylbenzene [12]trimethylbenzene [12]tetramethylbenzene [12]propylbenzene [12]isopropylbenzene [12]xilene [12]stirene [12]naftalene [11]ethylnaftalene [11]dimethylnaftalene [11]acenaftene [11]

olive oil without defects and an oil showing musty defectare depicted in Fig. 1.

3 Biogenesis of virgin olive oil volatilecompounds

Volatiles in the vegetable kingdom can be considered tobe metabolites of intracellular biogenetic pathways. Thequalitative and quantitative composition of the volatile

compounds of high quality olive oils depends closely onthe levels of enzymes involved in the pathways and ontheir activity. Genetic characteristics fix the contents ofthe different enzymes and are therefore responsible forthe qualitative composition of volatile compounds [26]. In-stead the quantitative accumulation of the differentvolatile compounds is connected with the enzyme activi-ties which, in oils of good quality, are related to the ripen-ing degree of fruits [27, 28] and on the operative condi-tions used during the oil extraction [29-33].

Other courses concerning the degradation of raw materi-al will be followed when fruits show unsanitary conditionsor are unsuitably stored before their processing.

Fig. 2 briefly summarises the main pathways involved inthe volatile production and depicts which compoundsoriginate from each of them. Nevertheless, it has to be re-membered that in addition to the volatile compounds de-riving from the mentioned pathways, also other com-pounds, especially aldehydes derived from autoxidationprocesses can contribute to the aroma of the olive oils.


Tab. 6. Miscellany of volatile compounds identified in vir-gin olive oil aroma by MS.

Compound Ref

Acidsacetic acid [13, 20]2-methyl propanoic acid [18]3-methyl butanoic acid [18]propanoic acid [18]butanoic acid [16]hexanoic acid [16]heptanoic acid [16]

Furane derivatives2-propylfuran (two isomers) [11]2-propyl dihydrofuran [11]2-pentyl-3-methylfuran [11]ethylfuran [14]3-(4-methyl-3-pentenyl)furan [14]

Ethersmethoxybenzene (anisole) [10, 11]1,2-dimetoxybenzene (veratrole) [10]

Thiophene derivatives [11]2-isopropenylthiophene [11]2-ethyl-5-hexylthiophene [11]2,5-dithiophene [11]2-ethyl-5-hexyldihydrothiophene [11]2-octyl-5-methylthiophene [11]

Oxygenates terpeneslinalool [10]α-terpineol [10, 11]lavandulol [11]1,8-cineole [10]

Fig. 1. Gas chromatographic profiles of both a good qual-ity virgin olive oil (A) and an oil showing musty defect (B).Peaks: 1 = octane; 2 = acetone; 3 = 1-octene; 4 = ethylacetate; 5 = methanol; 6 = 2-methyl butanal; 7 = 3-methylbutanal; 8 = ethanol; 9 = ethyl 3-methyl butanoate 10 =pentan-3-one; 11 = pentene dimer; 12 = methyl 2-methylbutanoate; 13 = pentene dimer; 14 = 2-methyl propyl ac-etate; 15 = methyl 3-methyl butanoate; 16 = 1-penten-3-one; 17 = propan-1-ol; 18 = ethyl 2-methyl butanoate; 19= ethyl 3-methyl butanoate; 20 = pentene dimer; 21 =pentene dimer; 22 = pentene dimer + hexanal; 23 = 2-methyl propan-1-ol; 24 = 3-methyl butyl acetate; 25 = 2-pentenal; 26 = 1-penten-3-ol; 27 = limonene; 28 = 3-methyl butan-1-ol; 29 = trans-2-hexenal; 30 = unknown;31 = hexan-2-ol; 32 = pentan-1-ol; 33 = octan-3-one; 34 =hexyl acetate; 35 = heptan-3-ol; 36 = cis-3-hexenyl ac-etate; 37 = cis-2-penten-1-ol; 38 = hexan-1-ol; 39 = cis-3-hexen-1-ol; 40 = trans-2-hexen-1-ol; 41 = 1-octen-3-ol; 42= heptan-1-ol; 43 = acetic acid; 44 = 2-methyl propanoicacid; 45 = 3-methyl butanoic acid; i.s. = nonan-1-ol (inter-nal standard).

The different nuances of the aroma of virgin olive oils arerelated to the importance of the various pathways thatcontribute to their formation. The aroma of the oil will notbe defective when the most active pathway is the LOXcascade. On the other hand, the aroma of the oil will bedefective, if some of the volatile compounds derive fromfermentations or amino acid conversion or from enzymat-ic activities of moulds or from oxidative processes.

4 Volatile compounds analysis: Theanalytical approach to their evaluation

The volatile fraction of the virgin olive oils consists ofmany compounds differing in molecular weights andchemical nature. Their concentrations, except for trans-2-hexenal, are generally very low, they can vary widely andreach minimum levels of about a hundred ppb or less.

The analytical approach to their evaluation must solve theproblem of determining compounds at trace levels, avoidthe formation of artefacts and achieve rapid and reliablemethods for the identification and the quantitation ofchemical compounds responsible for different aromas.

Generally, several steps are requested for the quantitativedetermination and the following identification of volatiles bythe methods developed: the separation of the volatile frac-tion, its possible concentration, fractionation into the indi-vidual components and, finally, their identification. The frac-tionation of the individual components is performed by high

resolution gas chromatography (HRGC) and their identifi-cation by GC-MS. All the steps are very important [34, 35]and, therefore, they should be specified carefully and ob-served closely to obtain comparable data .

The methods reported in the literature for the collection ofvolatiles can be divided into two groups: techniques withor without an enrichment step. They were reviewed care-fully by Morales and Tsimidou in 2000 [9] and the readeris referred to these papers for deepening the argument.

Tab. 7 shows the different techniques for the quantifica-tion of the volatile compounds, briefly underlines the mainproblems of each technique, and reports referencesabout the applications to olive oils [2, 9, 10, 14, 17-21, 24-28, 30-33, 36-39, 41, 48-51].

The techniques without an enrichment step are not com-monly used because the amount of the sample is in anycase too small to achieve a good sensitivity. Furthermore,in general, high temperatures are requested which mayfavour the formation of degradation products. Moreover,applying the direct injection “memory effects” in the chro-matograph represent a further disadvantage; a staticheadspace, on the other hand, can be considered effec-tive only for highly volatile compounds [9].

Among the techniques with an enrichment step the mostpopular one is the extraction of volatiles by means of a dy-namic headspace. The volatile compounds are trappedon a suitable adsorbent and then the quantitative amount


Fig. 2. The main pathways in-volved in the production of thevolatile compounds of virginolive oil aromas.

of each individual volatile compounds is determined byHRGC after thermic desorption or elution with a solvent.In that case, the amount of volatile compounds dependson temperature, sample size, absolute quantity of the gasused for the stripping, chemical-physical characteristicsof both substances to be extracted and material em-ployed for the trapping, length as well as the diameter ofthe trap [42-48].

A special comment needs to be made on the techniqueknown as “stable isotope dilution assay” (SIDA), amethod which allows to quantify the volatile compoundsvery accurately (Tab. 7). SIDA methodology involves theadding of the deuterated compounds of all odorants to beanalysed as internal standards. Due to the structural sim-ilarity of analyte and corresponding deuterated com-pound, problems related to losses during the samplepreparation, to reactivity and chromatographic behaviourof the various analytes are overcome splendidly. Howev-er, this method requires the preparation of a number ofdeuterated compounds and, even if it is very accurate, itis not commonly used for quantifying all volatile com-

pounds of virgin olive oils. Rather they are used to identi-fy compounds considered to be the most potent contribu-tors to olive oil aroma [20, 21].

In Tab. 8 the characteristics of the different adsorbentsused for trapping volatile compounds are shown and ref-erences concerning the main applications to olive oils arereported [2-4, 9, 12, 14, 18, 19, 24, 25, 27, 30-33, 41, 42,52-54, 56-59].

5 Volatile compounds analysis: Thesensory approach to their evaluation

Volatile compounds, being responsible for most sensoryproperties of virgin olive oils, play a significant role in theevaluation of the overall oil quality and in the generationof preferences among consumers.

Several investigations were carried out to find relation-ships between volatile compounds and sensory percep-tions, but the results are not comprehensive enough todescribe all the sensations experienced during tasting


Tab. 7. Most common techniques for the quantification of olive oil aroma volatiles and their main problems.

Technique Sensitivity Problems References (olive oil)

Techniques not involving an enrichment step

Direct injection poor possible degradation products [9, 36]

Static head space poor concentrations very low, often under [17, 37-41]gas cromatographic detectability thresholds [9]

Techniques involving an enrichment step

Direct extraction with solvent impossible to be applied

Simple distillation not applied

Combined distillation- good significant and selective lacks of solute due [10]extraction to co-evaporation during the removal of con-

siderable amounts of solvent and concentrationof impurities of solvent [9]

Dynamic head space very good temperature, sample size, absolute quantity [2, 14, 18, 19, 24-28,of gas used for the stripping, chemical-physical 30-33, 37-39, 48]characteristics of both substances to be extracted and material employed for the trapping, of the length and diameter of the trap [42-47]

Supercritical fluid quite good selective against the oxygenate compounds [49, 50]extraction (SFE) with low and medium molecular weights and

also against many organic apolar compounds with low molecular weight

Solid phase microextraction quite good extraction of a lower number of compounds [51](SPME) than in both static and dynamic head-space.

The major differences concern volatile comp-ounds with lower molecular weight.

Stable isotope dilution assay very good synthesis of a number of deutered volatile [20, 21](SIDA) compounds

completely [9]. It should be remembered that the volatilecompounds present at higher concentrations are not al-ways the main contributors to the oil aroma [59]. The rea-son being that the thresholds – the minimum concentra-tion of a given stimulus able to give rise to a sensory re-sponse – for taste and smell sense organs depend moreon chemical factors and the stereochemical structure ofthe molecules than on their concentrations [60, 61]. An ef-fective help in estimating the importance of a flavour com-pound provides the “aroma extract dilution analysis” (AE-DA). AEDA is based upon the calculation of the ratio of a

compound concentration to its flavour threshold, the latterbeing evaluated nasally and retronasally [62]. However,the chemical determination of the volatile compounds, allof which contribute to the olive oil aroma, and of nonvolatile phenolic compounds, which are mainly responsi-ble for taste and trigeminal sensations, does not renderan account of its flavour. That is so because aroma, tasteand trigeminal sensations contribute to sensory percep-tions as well as complex interactions between these stim-uli [63, 64]. On the other hand, the recovery of volatilecompounds from virgin olive oils during their quantitative


Tab. 8. Characteristics of the adsorbents used for trapping volatile compounds.

Adsorbent Sensitivity Problems References (olive oil)

Poropak good low thermal stability and production of artefacts [52-54] [42]

Chromosorb very good good adsorption capacity of medium and high boiling [41]point compounds, the high thermal stability, poor affinity against water, easy cleaning procedures and possible recycling [9, 52]

Tenax very good good adsorption capacity of medium and high boiling [2, 4, 14, 27, 32 ,48, 56]point compounds, higher thermal stability than Chromosorb, poor affinity against water, easy cleaning procedures and possible recycling [9, 52]

Charcoal very good very strong adsorbent power against all classes of [3,12,18,19, 24,25,chemical compounds, ability of adsorbing compounds 30-33, 57-59]with low molecular weights and good affinity against water [9, 52]

Tab. 9. Recovery [%] of C5 and C6 compounds, respectively, from virgin olive oils [3, 15, 65]. The confidence limits for re-coveries at a probability level of 95% were obtained by using the standard deviation values calculated from four indepen-dent experiments.

C5 Compound Recovery C6 Compound Recovery[%] [%]

Hydrocarbonspentene dimers (C10H18) 48.0 ± 0.5

Carbonyl compounds Carbonyl compounds2-pentenal 36.8 ± 0.8 hexanal 21.6 ± 0.6

trans-2-hexenal 25.0 ± 3.0

1-penten-3-one 80.6 ± 3.0

Alcohols Alcohols

cis-2-penten-1-ol 31.0 ± 0.6 hexan-1-ol 20.2 ± 0.1

trans-2-penten-1-ol 37.6 ± 0.5 cis-2-hexen-1-ol 17.0 ± 2.0

trans-2-hexen-1-ol 18.1 ± 0.2

cis-3-hexen-1-ol 23.3 ± 0.7

trans-3-hexen-1-ol 24.0 ± 3.0

1-penten-3-ol 64.2 ± 2.0

Estershexyl acetate 11.3 ± 0.5cis-3-hexenyl acetate 11.9 ± 0.4trans-2-hexenyl acetate 10.5 ± 0.3

determination (Tab. 9) is related to the number of carbonatoms in the molecule and to the kind and position of thefunctional group [3, 65].

Therefore the sensory analysis is still the most effectivetool to evaluate quality, strength and differences amongthe stimuli elicited by the virgin olive oil during tasting andto investigate consumers’ preferences.

A common perplexity about the sensory evaluation of vir-gin olive oils is represented by the fact that each tasterjudges the organoleptic characteristics in a subjectiveway. This was true until the evaluation of the sensory at-tributes was made exclusively by one or a few very spe-cialised persons, who were able to define the quality char-acteristics and the defects of oils. Their judgement wasstill subjective, since each person has his/her own senso-ry threshold for each stimulus and may have considerableproblems in describing the personal olfactory percep-tions. The latter are based on personal previous experi-ences, and refer to an individual scale for every sensorynote. For these reasons quantitative evaluations by differ-ent tasters are not comparable.

The “quantitative descriptive analysis” (QDA) overcomes(i) the problem of the different individual thresholdsamong persons since the final judgement is the mean ofjudgements provided by a group of persons selected torepresent the totality of consumers and fully-trained toevaluate a given product organoleptically; (ii) the difficultyin defining the sensory sensations by developing a vo-cabulary that allows all tasters to describe the differentperceptions experienced during the tasting with the samewords; (iii) the use of not fixed scales by adopting a de-

fined scale to which all tasters must refer. This approachallows to compare the scores of different tasters and dif-ferent group of tasters.

Collaborative International studies, supported for manyyears by the International Olive Oil Council (IOOC), de-veloped the QDA sensory methodology for virgin oliveoils, known as COI-Panel test. The latter defines anagreed-on specific vocabulary of sensory attributes, per-forms a uniform tasting technique and eliminates all trou-bles that can compromise the sensory trial [66]. A group ofpersons, from 8 to 12, is selected in a codified way andtrained suitably to identify and measure the strength ofthe different positive and negative sensations elicited bytheir sense organs. They use a defined structured scalefor measuring the intensities of the different attributes.Since tasters are considered as measure instruments, itis absolutely essential to remove or, in any case, to min-imise all troubles that can compromise the sensory trial.With this aim possible mental or physical stress of thetasters must be attentively considered since they canmodify taste perception [67]. The physical environmentalconditions have been carefully regulated and the room forthe sensory trials must be organised with a number ofbooths where the taster can sit separately and concen-trate during the tasting. Shape and dimensions of theglass, sample volume, and oil temperature are preciselyestablished. Samples, never in a high number to avoidthe sense organ fatigue, are presented in an anonymousand random way. Tasters until August 2002 also rated theoverall grading (Fig. 3) for the characteristics of the oliveoil on a 9-point scale (9 for exceptional qualities, 1 for theworst one). The mean score was considered as a measureof the oil quality and identified its grade in relation to its


Fig. 3. Profile sheet andgrading table adoptedby European Union reg-ulations until August2002.

quality. Statistical procedures were applied to evaluatethe data provided by assessors and produced resultsthat, due to their significance levels, could be consideredas reliable as those of other methods usually adopted inscientific fields.

Since 1991 this methodology had been included in theregulations of the European Union [68] for the classifica-tion of various virgin olive oil categories (Tab. 10) and wasextensively described in the Annex II of the EU regulation(Reg. CEE n.2568/91).

However, a poor reproducibility of the overall gradingscores [69] was observed among different panels. Thedifferences in the evaluations of the different attributesamong the assessors of the various panels were mainlyrelated to troubles deriving from an ineffective training,due to the absence of standards which can be repro-duced because of the poor stability of the oil over time.Moreover, they were attributable to dissimilar origins, cul-ture, and food habits [70]. Therefore the InternationalOlive Oil Council promoted the revision of the organolep-tic evaluation of the virgin olive oil and a new methodolo-gy was developed.

A new profile sheet was proposed (Fig. 4) that mainly con-siders the negative attributes (e.g., fusty, musty, muddysediment, winey-vinegary, metallic and rancid notes) thatare the most commonly detectable ones in virgin olive oils[71]. Possible other defects described in the specific vo-cabulary can be named by means of designation to “oth-ers”. The profile sheet indicates only fruity, bitter and pun-gent sensations among positive notes. An unstructuredscale, 100 mm long, was chosen to overcome the prob-

lems deriving from the fact that the amplitude of all inter-vals of the old structured scale (Fig. 3) could not be con-sidered equally by tasters and to the reluctance of tastersto use the extreme portions of the structured scales [72].The intensity data, expressed as centimetres, are statisti-cally processed to calculate the median of each negativeand positive attribute. The median value of the defect per-ceived with the strongest intensity identifies the olive oilgrade, whereas the median value of the fruity attributeidentifies the extra virgin and virgin types (Tab. 11). Themethod has been included recently in EU regulation (Reg. n. 796/02) [73].

The reliability of panel assessors is measured by the ro-bust coefficient of variation that should be ≤20% for themedian of defects in extra virgin, virgin, and ordinarygrades and ≤10% in lampante. For the fruity median itshould be ≤10% in extra virgin and virgin categories.

6 Flavours: Factors affecting thecomposition of the olive oil volatilefraction

The fragrant and unique aroma of virgin olive oils of goodquality is usually described by perceptions attributable to1) the fruity sensation, the sensation reminiscent ofhealthy fresh fruit collected at the optimum of the harvest-ing time; 2) the sensations reminiscent of leaves, freshlycut grass, green fruits such as apple, banana or vegeta-bles such as artichoke or tomato etc., accompanied bymore or less intense taste notes of bitterness and pun-gency.

C6 and C5 aromatic volatile compounds are mainly re-sponsible for the green perceptions of the fragrant andunique aroma of virgin olive oils [23, 57], whereas bitter-ness and pungency have to be mainly attributed to sec-oiridoid compounds [9, 74, 75].

The perceptions indicated under point 2) are known as“green” odour notes, and characterise the flavour of oilsextracted from not completely ripe olives. They are re-garded as freshness and liveliness characteristics ofgood quality virgin olive oils by consumers.


Tab. 10. Scores (overall grading) for each category of vir-gin olive oil according to the old EU regulation.

Scores Categories

6.5 Extra-virgin olive oil

< 6.5 ≥ 5.5 Virgin olive oil

< 5.5 ≥ 3.5 Ordinary virgin olive oil

< 3.5 Lampante virgin olive oil

Tab. 11. Median values of defects and fruity aroma, and their corresponding robust coefficients of variation, in relation tovirgin olive oil categories according to the new EU regulation.

Median of defects Robust coefficient Median of fruity Robust coefficient Olive oilof variation [%] aroma of variation [%] category

0 ≤ 20 > 0 ≤ 10 Extra virgin

> 0 ≤ 2.5 ≤ 20 > 0 ≤ 10 Virgin

> 2.5 ≤ 6.0 ≤ 20 >0 Ordinary

> 6.0 ≤ 10 > 0 Lampante

Oils from unripe fruits are characterised by quite intensegreen perceptions and by very high strengths of bitter andpungent attributes. A suitable blending with oils weakerfrom a sensory point of view make them acceptable for di-rect consumption. Conversely oils obtained from ripe fruitsare lightly aromatic because of a low accumulation ofvolatile compounds that confer a typically fresh and herbalflavour, due to a reduced activity of enzymes involved in thelipoxygenase pathway [29, 58, 74-78]. They are also char-acterised by weak intensities of bitter and pungent percep-tions due to a decreasing amount of phenolic compoundsduring the ripening of fruits [29]. The whole of both fruity at-tribute and the green sensations describe the different nu-ances of the aroma of virgin olive oils.

Incorporating leaves in concentrations of about 2-3% pri-or to crushing enhances the flavour of the oils from over-

ripe fruits and improves their quality [79]. The aroma isnot distorted and tasters perceive higher intensities ofgreen fruity and bitter taste. The analysis of volatile com-pounds in fact [79] does not show newly formed sub-stances (Fig. 5), but a noticeable increase in C6 aldehy-des and C6 alcohols that, as known, are related to thegreen attributes.

The production of C6 and C5 compounds through the en-zymatic oxidation of linolenic and linoleic acids [22, 23,57] is affected by the cultivar, the degree of ripeness offruits and by their processing conditions.

The cultivar plays an essential role as the amount of theenzymes involved in the pathway is genetically character-istic. Montedoro and his group [17] already highlightedonly quantitative differences in the composition of volatile


Fig. 4. Profile sheet actual adopted byEU regulation.

fractions. Fig. 6 shows the influence of the cultivar on thepercentage of C6 aldehydes, alcohols and esters of oilsobtained from fruits of different cultivars harvested at thesame degree of ripeness and processed under the sameoperating conditions [3, 26].

Different sensory green profiles are due to the differentconcentration of these three C6 fractions of volatile com-pounds. As an example Fig. 7 shows profiles of oils fromProvenzale and Leccino fruits.

Moreover, a recent research [26] proved that, under thesame conditions, the amount of the C6 compounds com-ing only from the α-linolenic acid is practically the sameduring two harvesting years. Further, their accumulationin the oil, expressed as percentage of their total amount,is different according to the cultivars regardless of the cli-matic variables and where olives are grown. That meansthat the cultivar is a determing factor in the formation ofthe oil aroma (Fig. 8).

The amount of the different metabolites from the LOXcascade, the most important pathway for the formation ofthe olive oil aroma, changes in relation to the ripening de-gree and storage time of fruits and the operative condi-tions used during the oil extraction.


Fig. 5. Changes of the fruity intensity, of C6 aldehydesand alcohols with respect to the percentage of leavesadded to over-ripe fruits before olive crushing.

Fig. 6. Percentage of the C6 aldehydes, alcohols and es-ters of oils obtained from fruits harvested at the same de-gree of ripeness and processed in the same operatingconditions in relation to cultivars.

Fig. 7. Green note profiles of oils from fruits of two Italiancultivars, Provenzale and Leccino, harvested at the samedegree of ripeness.

Fig. 8. Percentage of the main compounds produced byenzymatic oxidation of α-linolenic acid in oils from fruits ofItalian, Spanish and Greek cultivars harvested at thesame degree of ripeness.

tant with repercussions on the activity of some enzymesinvolved in the LOX pathway [77, 78]. The comparison ofthe volatile profiles [30] of oils from the same fruits ex-tracted with the same processing diagram except for thecrushing stage, resulted in concentration changes ofsome volatile compounds. Oils extracted by means of astone mill were found to contain a higher amount of totalvolatile compounds (Fig. 10) and especially of trans-2-hexenal, hexanal and cis-3-hexen-1-ol than correspond-ing samples obtained by using a metallic crusher. Thesensory analysis applied to oils obtained by the differentcrushing methods underlines the perception of higher in-tensities of the green attributes in oils extracted with astone mill than in those obtained by metallic crushers [30].

But also the paste malaxation causes modifications ofvolatile compound compositions. The malaxation step,consisting of a low and continuous kneading of olivepastes, is essential to break up the oil/water emulsion andthus to promote the merger of the small oil dropletsformed during crushing, in particular by means of metalliccrushers, into large drops that can be easily separatedthrough mechanical systems.

Time and temperature of malaxation affect the concentra-tion of the volatile compounds and therefore the sensorycharacteristics of the resulting oils. The malaxation timemainly promotes the accumulation of alcohols and of C6

and C5 carbonyl compounds, especially of hexanal, oneof the most important contributors to the olive oil flavourbecause of its low odour threshold [83]. The increase inthe concentration of hexan-1-ol and trans-2-hexen-1-ol,the considerable decrease in the concentration of C6 es-ters and in cis-3-hexen-1-ol, accompanied by the produc-tion of very high amounts of 2-methyl butanal and 3-

Montedoro et al. [17, 29] and Solinas et al. [80], examin-ing oils from different Italian cultivars, and Olias et al. [12],examining oils from Spanish varieties, found that the con-centration of compounds responsible for the aroma,which is very high in oils from green olives, increases dur-ing olive ripeness. The latter was measured by means ofthe Jaèn index [80], based on the evaluation of thecolours of the olive skin and pulp. The amount of volatilesincreases until a maximum value is reached when fruitschange skin colour from yellow-green to purple. Beyondthis stage of ripeness, all researchers agree that theamount of the volatile fraction decreases. This behaviouris mainly affected by the changes in the concentration oftrans-2-hexenal. It follows the trend just described (Fig. 9)and has been attributed to the dry climate of the produc-tion areas of the fruits [28]. A steady decrease in the con-centration of the volatile compounds from the unripe tothe over-ripe stages, including trans-2-hexenal, wasfound by Aparicio and Morales. One exception was the oilfrom Coratina fruits that, during one of the two crop yearsof their investigation, showed an increase of the amountof trans-2-hexenal until a maximum concentration, inagreement with other researchers’ results.

However, the final volatile concentration in virgin olive oilsdepends also on technological aspects and, in particular,on the effectiveness of the grinding of pulp tissues, thetemperature reached by the olive paste during crushing,the time and the temperature of malaxation and the kindof system used for oil extraction.

Metallic crushers cause the disruption of a greater num-ber of cells containing oil than stone mills, but the temper-ature of the olive paste, because of the violence of thegrinding, rises to about 10 °C over the temperature ofpastes obtained by using a stone mill [81, 82], concomi-


Fig. 9. Changes of trans-2-hexenal in oils extracted fromfruits at different ripening degree, measured by means ofthe Jaèn index, which is based on the colour of skin andpulp of olive fruits.

Fig. 10. Influence of the crusher type on the total amountof volatile compounds of oils extracted under the sameconditions, except for the crushing stage.

methyl butanal through the activation of the amino acidconversion pathway, represent the major effects due tothe high malaxation temperature [32, 84] (Fig. 11).

A general weakening of the oil flavour is recorded bytasters, especially for walnut husk and tomato, when themalaxation temperature rises, whereas prolonged peri-ods of malaxation cause the weakening of leaf, freshly cutgrass, walnut husk, bitter and pungent sensory notes [33].Also bitter and pungent notes show the same behaviouraccording to the reduction of the phenolic compoundsduring the malaxation step as observed by some re-searchers [85].

The studies on the influence of the malaxation step on thequality of resulting oils reached the same conclusions; alow temperature (≤25 °C) and medium times (35-45 min)are the best extraction conditions to promote the forma-tion of green volatile compounds responsible for desirableperceptions. They also help to avoid high concentrationsof some compounds of which production is closely con-nected with the degradation phenomena of raw materialand inversely related to the virgin olive oil sensory quality[32, 33].

The systems adopted for extracting oil have some reper-cussion on the volatile composition. A lower content of

volatile compounds was found in oils extracted by meansof three-phases centrifugal decanters than in oils extract-ed by pressure systems. The reduction is especially evi-dent for C6 alcohols, hexan-1-ol and trans-2-hexen-1-ol,probably removed by the warm water which is used forthe dilution of olive pastes in order to facilitate their cen-trifugation [31, 86].

The introduction of new models of decanters on the mar-ket, which are able to separate the oily phase from themalaxated pastes without requiring any addition of warmwater, allowed the production of oils whose volatile com-position is more similar to that of oils extracted by pres-sure. In particular, in addition to a greater amount of phe-nolic substances, oils from dual phases decanter show agreater accumulation of C6 compounds arising from theLOX pathway [87] (Tab. 12).

7 Origins of off-flavours

When oils are of poor quality the sensory basic character-istics are considerably modified. Green, bitter and pun-gent notes are absent or very weakly perceived by tasterswho identify the presence of some unpleasant sensationsin the flavour.

Tab. 13 summarises the causes of the defects detectablein virgin olive oils. The data show that the olive preserva-tion before processing is the most important cause for themore common defects – fusty, winey-vinegary and mustyattributes – detectable in virgin olive oils.

7.1 Defects from unsuitable conditions of olivefruit preservation

The olive fruit preservation, even if carried out in ideal con-ditions (low temperature and very thin layer of olives in ful-ly-air rooms), causes a decrease in the concentrations ofvolatile compounds especially of trans-2-hexenal [88]. Theamount of the all volatile compounds, expressed as ppm ofnonan-1-ol, decreases by about 30-40% in oils obtainedfrom olives preserved for 15 d in relation to that detected inoils from fresh fruits [88]. The decrease becomes more ev-ident for longer preservation times. In parallel with the ob-


Fig. 11. Influence of malaxation timeand temperature on the amount of C6

aldehydes, alcohols and esters.

Tab. 12. Influence of the decanter type on the amount ofsome C6 compounds (ppm) arising from the lipoxygenasepathway.

Decanter type

Compound [ppm] dual-phase three-phase

1-penten-3-one 0.2 0.1

1-penten-3-ol 0.5 0.3

cis-2-penten-1-ol 0.5 0.3

trans-2-hexenal 21.1 17.0

trans-2-hexen-1-ol 1.8 0.9

cis-3-hexen-1-ol 0.6 0.4

cis-3-hexenyl acetate 1.1 0.7

hexan-1-ol 0.8 0.5

served reduced amount of the volatile fraction, the scoresof evaluation of the oil aromas show only a weakening ofthe intensity of the different attributes. Beyond 15 d the oildoes not keep the original quality, but tasters perceivesome defects at threshold level (Tab. 14).

However, as the harvesting is done in a few months, thepreservation of fruits often cannot be performed in idealconditions because of the poor size of the processingplants. The reduced areas reserved for olive storage inrelation to the fruit amount oblige to put them into jutesacks or to pile them at room temperature.

The profile of aromatic volatile compounds is significantlymodified during the olive preservation [25, 89, 90]. Com-pounds from the LOX pathway decrease significantly andrapidly in the oils from the fruits stored in piles for differenttimes. The suitable temperature conditions, the high hu-midity, and the loss from the epicarp of its ability to act asan antimicrobic barrier, due to an accelerated autolysis ofthe organic material and the fruit rotting, promote the mi-crobial colonisation of the olive tissues by all the micro-or-ganisms present in the environment [25].

According to the temperature and the degree of humidityreached in the pile, one genus of epiphytic microflora candevelop better than another, thus it is possible that theproduction of different metabolites is responsible for dif-ferent defects. The preferential growth of yeasts givesrise to the formation of ethanol and ethyl acetate. As aconsequence, the winey defect appears when their con-centrations are higher than those corresponding to theirsensory thresholds. The possible presence of Acetobac-ter is responsible for the vinegary defect because it pro-motes the production of acetic acid [25].

Generally, Enterobacteriaceae, Clostridia and Pseudo-monas meet with the better conditions for their growth.The accumulation of their metabolites, represented bybranched aldehydes, branched alcohols and their corre-sponding acids [91, 92], is promoted by the contact timeand the suitable temperature. In a few days, they over-


Tab. 13. Causes of main defects perceived in virgin olive oil and description of the resulting flavour.

Cause Defect Flavour description

Bad sanitary conditions – grubby typical of oils obtained from olives which have suffered a Dacusof fruits oleae infestation

Wrong harvesting – ground picked typical of oils obtained from ground-picked olives, spontaneouslyprocedure olives fallen from trees and remained on the ground for several days

Time and conditions of – fusty typical of oils obtained from olives stored in piles which sufferedfruit storage degradative phenomena

– winey typical of oils obtained from olives stored in piles which suffered somefermentation

– musty typical of oils obtained from olives stored in piles which suffered themore or less considerable fungal invasion

Unsuitable extraction – earth typical of oils obtained from fruits collected with earth or bespattered technology with mud and processed without washing

– heated typical of oils obtained when too long times or too high temperaturesare adopted in the malaxation step

– metallic typical of oils extracted with both new processing plants and/or usedthe first time during the crop year

Unsuitable oil – rancid typical of oils strongly oxidizedstorage conditions – muddy sediment typical of oils stored for a long time on their sediment

– cucumber typical of oils stored for a long time during the hermetic bottling

Tab. 14. Scores, total amount of volatile compoundsexpressed as ppm of nonan-1-ol, their % loss in oilsobtained from fruits stocked for different times withrespect to the volatile fraction of the oil coming from freshfruits.

Days Panel test Total Loss score of volatile of volatile

compounds compounds [ppm] [%]

0 7.8 1400 0

6 7.9 1070 24

13 7.1 780 44

21 6.4 650 54

27 6.0 240 83

step the threshold concentrations for the perception offusty defect. In particular a significant correlation (R = -0.919) was found between the intensity of the fusty de-fect and 2-methyl butan-1-ol + 3-methyl butan-1-ol/1 +√trans-2-hexenal. Hereby 1 is a mathematical artefact toavoid that the function shows values to infinity which donot have any physical meaning for trans-2-hexenal valuesnear to zero [90].

When the storage time is prolonged for several days, thehumidity and temperature conditions encourage the de-velopment of moulds, whose pectolytic activity acceler-ates the complete rotting of fruits. An investigation carriedout to isolate and identify micro-organisms present on theolive skin proved that the most moulds belong to Penicilli-um and Aspergillus species [93]. The enzymes of mouldsinterfere with those of olive fruit in the LOX pathway [94].So that there is, parallel to the growth of moulds, a steadydecrease in C6 compounds and, at the same time, an in-crease in C8 compounds which are common metabolitesof the LOX pathway of moulds. In addition, as expected[95, 96] in parallel to the fungal growth great amounts ofpropan-1-ol, 2-methyl propan-1-ol, 3-methyl butan-1-oland their corresponding acids and esters are produced.The musty defect average intensity was positively related(R = 0.9332) with the percentage of 1-octen-3-ol in rela-tion to the total amount of C8 compounds [18] (Fig. 12).

The storage temperature plays an essential role in deter-mining how long the preservation time can last withoutgiving rise to the appearance of off-odours. Kiritsakis andco-workers [97] suggest that a storage temperature ofabout 5 °C in air reduces the fungal growth considerably,so that olives can be preserved for at least 30 days. Theresulting oil, sensory tested, was found to be still of goodquality. The same authors state that storage temperaturesnear 0 °C are detrimental because of the destruction ofthe natural antioxidants of olive fruits due to chilling injury.

7.2 Defects from the oil preservation

Other more common defects of virgin olive oils, such asmuddy sediment, rancid, cucumber, originate during oilpreservation.

The oxidation is an inevitable process that starts after thevirgin olive oil has been extracted and leads to a deterio-ration that always becomes more serious during oil stor-age. Initially lipids are radically oxidised to hydroperox-ides, which are odourless and tasteless [98] and do notaccount for sensory changes. However, they are suscep-tible to further oxidation or decomposition into products ofsecondary reactions, which, conversely, are responsiblefor typical unpleasant sensory characteristics, identifiedon their whole as rancid attribute.

Decomposition occurs through a homolytic cleavage ofthe hydroperoxide group with production of various com-pounds, including aldehydes, ketones, acids, alcohols,hydrocarbons, lactones, furans and esters [99]. Light,temperature, metals, pigments, unsaturated fatty acidcomposition, quantity and kind of natural antioxidants, aswell as the amount of sterols promote the free radicalmechanism of the autoxidation process in a different way[9, 99].

During the oil preservation the original volatile composi-tion, mainly formed by compounds deriving from the LOXpathway responsible for pleasant properties, changes.The concentrations of C6 aldehydes, especially that oftrans-2-hexenal, and C6 alcohols undergo a drastic re-duction, whereas those of new compounds, C5-C11 satu-rated and unsaturated aldehydes, increase gradually [24,100]. The most advanced oxidation stages are charac-terised by the complete disappearance of compoundsarising from the LOX cascade and by very high concen-trations of the mentioned aldehydes [24]. They contributemainly to undesirable aromas, because of their low odourthresholds [101]. Other contributors are represented byunsaturated hydrocarbons, furans and ketones. Unsatu-rated aldehydes and ketones can be further oxidised pro-ducing new off-flavour compounds, whose presence ac-counts for the different nuances of the unpleasant aromasdescribed by tasters as rancid, painty, fishy, etc. [102].

Snyder et al. [103] observed that the volatile fraction ofoxidised oils consisted mainly of saturated carbonyl com-pounds. Hexanal is always present in the flavour of goodquality oils since it is formed through the LOX cascade.But its amount, because of the strict specificity displayedby some enzymes involved in its production [77], is ratherlow as compared with that of trans-2-hexenal, whose for-mation is promoted. However, Morales and his group[104] could not consider the considerable increase in theconcentration of hexanal in oxidised oils, especially in


Fig. 12. Relationship between musty defect intensity andpercent of 1-octen-3-ol with respect to total amount of C8

compounds.

thermoxidixed oils, as an adequate marker for the begin-ning of oxidation. The reason being that a significant pos-itive correlation (R = 0.96) was proved between hexanaland the overall acceptability of potential and habitual con-sumers of virgin olive oils [105]. Therefore they proposedto monitor the oxidation stages of a virgin olive oil duringan accelerated thermoxidation process through the deter-mination of the concentration of nonanal. The latter whichis absent or present only in traces in fresh virgin olive oilsof good quality shows a positive relationship with the oxi-dation (R = 0.89). The ratio hexanal/nonanal was found tobe an appropriate way to detect the beginning of oxidationand its evolution, even if the hexanal is present in the orig-inal flavour [104].

On the other hand, another secondary aldehyde, trans-2-heptenal, was considered to be a compound that coulddescribe the different stages of the autoxidation processas it is absent in extra virgin olive oils. Solinas and co-workers [19, 24] found that the aldehydes of which con-centrations increase considerably in the dynamic head-space of the oils oxidised are trans-2-pentenal, hexanaland trans-2-heptenal. The authors suggested to usetrans-2-heptenal as marker of oxidation rather than trans-2-pentenal and hexanal, since these two compounds arealready present in the headspace of extra virgin olive oilsand some troubles could occur in the detection of trans-2-pentenal. Trans-2-heptenal shows a positive relationshipwith the rancidity perception. It was evaluated that thesensory threshold for this attribute corresponds to a con-centration of 1.5 ppm of trans-2-heptenal in the virginolive oil [24].

During an accelerated oxidation, e.g. during frying, thereis obviously an immediate great increase in volatile com-pounds [106], especially in carbonyl compounds [19]. Thecomparison of the volatile composition of virgin olive oilwith those of seed oils proves that the unsaturation levelis connected with the amounts of carbonyl compoundswhich are formed during frying and that the virgin olive oilshows the highest aroma stability [19]. During long-termoil preservation in bottles or, especially, in tins, where theoil is hermetically stored, it is possible to perceive an un-pleasant sensation reminiscent of cucumber, attributed tothe production of 2,6-nonadienal [71].

However, another type of sensory defect can appear dur-ing oil preservation. Virgin olive oil is often preserved andconsumed without filtering in the production countries, sothat it keeps vegetation water and also sugars, proteinsand enzymes. After a few months of preservation a layerof sediment is formed at the bottom of the container inwhich virgin olive oil is stored. Under suitable temperatureconditions the sediment can ferment and give rise to theproduction of unpleasant compounds responsible for the

typical muddy sediment defect. The micro-organisms in-volved have been poorly studied so far. But due to thelarge number of butyrates and 2-ethyl butyrates present itseems likely that the micro-organisms belong to theClostridium genus which originates butyric fermentation.

7.3 Defects from unsuitable harvestingmethods

Olive harvesting is an important operation that contributesto a good oil quality. In several areas of olive production,fruits are picked by hand from the tree; however, as this isvery expensive, harvest mechanisation often becomesessential for a profitable olive culture. Oils of good qualitywill be obtained from fruits collected manually or mechan-ically, if olives are addressed without delay to the extrac-tion process [107]. Unfortunately, when olive trees arehigher than 4-5 meters and are formed by trunks too bigto allow mechanical harvesting, the fruits are left till theyare over-ripe and fall to the ground spontaneously. Thegathering of olives from the ground, suitably prepared, isrepeatedly made with brushes and aspirators until theend of the spring [107]. The increase in contact time be-tween olives and ground causes an increase in the con-tent of volatile alcohols and carbonyl compounds with un-pleasant aroma and the appearance of a typical defect,which can be described by mouldy and earthy tastes atthe same time [108]. Its intensity seems to be related tothe benzaldehyde concentration (R = -0.967).

7.4 Defects related to technological aspects

Olive harvesting occurs in a time of the year in which theweather is not always suitable for this operation. Thus af-ter several rainy days, olives may be bespattered withmud or soiled with earth. A washing operation is alwaysrecommended by technologists, but not always per-formed. Especially when olives are very ripe washing isdispensed with, since it could cause the removal of somepulp tissue and therefore oil losses spring [109]. Whenolives are processed without washing, the resulting oilsare characterised by a defective aroma reminiscent of wetearth. A possible relationship between volatile com-pounds and this sensory defect has not been studied yet.

Another defect related to technological aspects is ametallic attribute, typical of oils obtained by using newprocessing plants to extract oil or by old plants used thefirst time during the crop year. Under the mentioned con-ditions free fatty acids contained in the oils solubilise thethin layer of iron oxide that covers the plant surfaces(made of stainless steel) during the period in which theplant is not used, imparting the metallic taste to the re-sulting oil. The amount of iron in oils obtained by the firstextraction processes is 4-5 folds higher than the mean


content of following extractions [110] (Tab. 15). Obviouslythe kind of crushers affect the amount of iron in the result-ing oils significantly with repercussions on their stability tooxidation [110]. It is recommended to preserve oils com-ing from the first extraction separately.

Too long times or too high temperatures adopted in themalaxation step can induce the appearance of heated de-fect. It is related to the production of very high amounts of2-methyl butanal and 3-methyl butanal through the acti-vation of the amino acid conversion pathway and to poor-ly accumulation of their corresponding alcohols [33].

The separation of oily must from the olive pastes is per-formed with centrifugation and pressing methods. Cen-trifugation methods are more wide-spread but some tradi-tional plants still adopt the pressing method using filteringdiaphragms. Nowadays the latter are generally made ofplastic fibres and do not give any particular odour to theolive oil. However, if the pressing plant does not work in acontinuous way residues of pulp and of vegetable waterremaining on the filtering diaphragms can undergo fer-mentations and/or degradation phenomena which giverise to the defect named pressing mats [107].

7.5 Defects from damaged fruits

An essential aspect to obtain high quality oils is the quali-ty conditions of fruits, since the oil quality cannot be at-tained by processing raw material damaged by some in-festation. The most common olive pest is represented byDacus oleae G., now named Bactrocera oleae, that at-tacks fruits from early summer to harvest time. The dam-age to the fruit increases with the development stages ofthe larva. The most serious damage occurs when the lar-val development is complete and the olive fly pierces thefruit skin in a characteristic way. Changes in the volatilecomposition and also in the phenolic compounds of re-sulting oils are considerable and affect the organolepticproperties and their stability negatively [111, 112]. Theconcentrations of carbonyl compounds and alcohols in-crease notably in parallel to the broadening of infestation.The ratio hexanal/total alcohols was found to be correlat-ed well (R = 0.990) with the extent of infestation [112].

8 Relationships between volatilecompounds and the sensorycharacteristics of virgin olive oils

Searching for relationships between volatile compoundsand the sensory properties of virgin olive oils represents aproblem difficult to solve.

Once it was assumed that the concentrations of the differ-ent volatile compounds may be related to some sensorynotes. In some cases this presupposition proved true: asan example the amount of trans-2-heptenal was found tobe related to the degree of oil rancidity, the concentrationsof ethanol and ethyl acetate, greater than their corre-sponding sensory thresholds, are related to the winey at-tribute and the benzaldehyde content to the simultaneouspresence of mouldy and earthy defects. In other casessome ratios were found to be correlated with other attrib-utes: the ratio hexanal/nonanal with the rancidity, the per-centage of 1-octen-3-ol with respect to the total amount ofC8 compounds with the mouldy sensory note, 2-methylbutan-1-ol + 3-methyl butan-1-ol/1 + √trans-2-hexenalwith the fusty defect and the ratio hexanal/total alcoholswith the importance of Dacus oleae infestion.

But the simple concentrations of volatile compounds can-not take into account the complex interactions occurringin the olfactory system or between taste and smell, or in-teractions between colour and taste and smell or betweenoral trigeminal irritations and taste and smell [59, 60, 84].On the other hand, the relationships do not always resultfrom the interactions between a single odour note and asingle volatile compound, but sometimes from the con-nections between a single attribute and the sum of thevolatile compounds [113] or from the perceptual fusionand blending of taste and odour sensations which couldgive rise to new qualities [59].

In addition the volatile compounds of the virgin olive oil donot contribute to its whole aroma with the same impor-tance. Chemical factors such as volatility and the hy-drophobic character, size, shape, conformational struc-ture, type, and position of functional groups seem to bemore related to the odour intensity of a volatile compoundthan its concentration [63, 64, 114]. The different nuances


Tab. 15. Amount of iron, chrome and nickel (ppb) found in oils processed in new and old plants for the first time during thecrop year.

Fe† [ppb] Cr† [ppb] Ni † [ppb]

Crusher A B A B A B

Pression (stone mill) 2868.1 556.5 19.8 24.2 12.9 15.2Centrifugation (fixed hammers) 2059.6 237.7 35.5 19.5 26.2 15.8

† A = first 1-2 extractions, B = following extractions.

of green odour and the degree of pleasantness seem tobe affected by cis/trans isomerism and by the position ofthe double bond in volatile compounds [115]. Thereforethe influence of different volatile compounds must beevaluated not only on the basis of their concentration, butalso on the basis of their sensory thresholds, whose de-termination is also affected by individual differences insensitivity observed in human normal subjects [116, 117].

The independent odour quality of the different volatilecompounds that contribute to form the various virgin oliveoil aromas were investigated intensively by means of GC-sniffing techniques [12, 14, 23], whereas the contributionof each compound was evaluated by means of the ratiobetween its concentration and its odour threshold, knownas odour activity value [65].

The calculation of the odour activity values proved to bevery useful to identify the compounds that are the maincontributors to the oil aroma. As an example, cis-3-hexe-nal seems to contribute more to green odour than trans-2-hexenal because of its lower odour threshold [20]. Amongthe contributors to high quality olive oil the most importantones, besides cis-3-hexenal, are cis-3-hexen-1-ol andhexanal, for their low odour thresholds, and trans-2-hexe-nal [21].

The essential role played by hexanal in the formation ofmost of the green attributes is confirmed by results of arecent research [75] that studied possible correlations be-tween fruity, leaf, almond, banana, freshly cut grass, wal-nut husk, wild flowers, tomato, bitter, pungent, and sweetattributes, and the concentrations of compounds derivingfrom the LOX pathway and the total amount of the sec-

oiridoid compounds by means of a Linear RegressionAnalysis (LRA). Tab. 16 summarises which compoundsare correlated mainly positively and negatively, respec-tively, to the mentioned attributes.

Statistical procedures were applied to the concentrationvalues of the volatile compounds and to the sensory at-tributes included in the IOOC profile sheet and also to thefigures representing the overall sensory evaluation of vir-gin olive oils to highlight possible relationships.

The application of an artificial neural network (ANN), us-ing the back-propagation algorithm, to the concentrationsof all the compounds present in the head spaces of 204virgin olive oils extracted from fruits of various cultivars,differing in their quality, ripeness, sanitary state and geo-graphical origin, allowed to predict the panel test scoresgiven by tasters according to IOOC panel test methodolo-gy with a high degree of accuracy [118].

Servili and co-workers [119] applied Principal ComponentAnalyses (PCA) and Partial Least Square (PLS) for relat-ing sensory and instrumental data. PLS provided goodpredictions of headspace data of some attributes de-scribed by means of free-choice profiling-quantitative de-scriptive analysis.

The inter-intra-relationships between the sensory attribut-es produced by the whole virgin olive oil matrix and itsvolatile compounds were highlighted using multidimen-sional scaling technique [2].

A sensory wheel, a technique already used to explore therelationships between sensory attributes [120, 121], was


Tab. 16. The main compounds positively and negatively correlated to green attributes.

Sensory note R2 Volatile compounds positively related Volatile compounds negatively related

Bitter 0.80 1-penten-3-one, polyphenols hexanal, cis-3-hexen-1-ol

Pungent 0.80 1-penten-3-one, polyphenols hexanal, trans-2-hexenal

Sweet 0.72 hexanal trans-2-hexenal, trans-2-pentenal

Fruity 0.66 cis-2-penten-1-ol trans-2-hexen-1-ol,trans-2-pentenal, 1-penten-3-one

Leaf 0.65 1-penten-3-one, polyphenols hexanal

Freshly cut grass 0.57 trans-2-hexenal hexanal

Almond 0.62 cis-2-penten-1-ol trans-2-hexenal, 1-penten-3-ol,cis-3-hexen-1-ol, polyphenols

Banana 0.60 hexanal, cis-3-hexenyl acetate trans-2-pentenal, trans-2-hexenal, cis-2-penten-1-ol

Walnut husk 0.57 cis-3-hexenyl acetate, hexan-1-ol, cis-3-hexen-1-ol

Wild flowers 0.56 trans-2-hexen-1-ol hexyl acetate, hexanal

Tomato 0.51 hexan-1-ol trans-2-hexen-1-ol, hexanal,1-penten-3-one 1-penten-3-ol

constructed by Aparicio and Morales [122] by means ofstatistical procedures. Mean values of the intensities ofthe most important sensory notes, after detection and re-moval of outliers and validation, were analysed by PCA.Each attribute was plotted on the plane identified by thetwo principal components. A circle with a radius of 1.0 wasdrawn at co-ordinates (0,0) onto this plane. Sensorywheel sectors were computed applying the circular stan-dard deviation [122]. This statistical sensory wheel clus-ters many sensory attributes having similar semantic de-scription into seven principal sectors: fruity, ripe fruit, ripeolives, undesirable, green, bitter-pungent and sweet.Some attributes fall outside the mentioned sectors andthey form so-called miscellanies.

Once the sensory wheel was built, the relationship be-tween sensory data and the volatile compound concen-trations were determined by attributing sensory wheel co-ordinates to each volatile compound. They were the twoprincipal components of PCA. Thus each volatile com-pound was projected onto sensory wheel [4].

The position of volatiles and sensory attributes on thesensory wheel establishes their information content. Themajor information is provided by volatile compoundswhich are situated close to the perimeter of the circle.Generally the place of each volatile compound in the sec-tors of the sensory wheel has been appropriate and cor-responded to the perception produced by the pure com-pound previously tested by HRGC-sniffing technique. Asexpected due to the basis of their sensory properties, cis-3-hexen-1-ol, cis-3-hexenal and cis-3-hexenyl acetateare included in the green sectors, whereas hexanal isplaced in the sweet one. Several volatile compounds, forexample trans-2-hexen-1-ol, hexan-1-ol, ethyl acetate,generally present only in low concentration in oils of goodquality, belong to the undesirable sector. It can be as-sumed that a low number of compounds falling in the bit-ter-pungent sector are responsible for these sensory at-tributes.

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[Received: September 10, 2001; accepted: July 29, 2002]


review - uclm · 2011-02-23 · review 1 introduction virgin olive oils, being mechanically...

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