Comparative study on volatile compounds from Tunisian and Sicilian monovarietal virgin olive oils
Introduction
Virgin olive oil is a valuable vegetable oil, which is often used without any preliminary refining process (Olias, Perez, Rios, & Sanz, 1993). It is extracted from fresh and healthy olive fruits (Olea europeae L.) by mechanical or other physical methods (washing, decantation, centrifugation or filtration). These technological procedures, if correctly adopted, are able to preserve the volatile and non-volatile compounds which are mainly responsible for fragrant and delicate flavour of virgin olive oil which is highly prized by consumers. The distinctive aroma of virgin olive oil is attributed to a large number of chemical compounds of different chemical classes, aldehydes, alcohols, esters, hydrocarbons, ketones, furans and, probably, other as yet unidentified volatile compounds (Kalua et al., 2007, Kiritsakis et al., 1998, Vichi et al., 2003a). The volatile compounds of virgin olive oil do not contribute to its whole aroma with the same importance and the high concentration volatile compounds do not necessarily serve as the major contributors of odour. Several chemical factors such as volatility, hydrophobic character, type and position of functional groups seem to be more related to the odour intensity of a volatile compound than its concentration. Therefore, the influence of different volatile compounds must be evaluated not only on the basis of their concentration, but also on the basis of their odour thresholds, that play a key-role. Indeed, the compound’s odour threshold value is its minimum concentration able to give rise to an olfactory response. As an example, hexanal seems to contribute more to green odour than E-2-hexenal because of its lower odour threshold (75–300 vs. 420–1125 μg kg−1) (Angerosa, 2002, Aparicio and Luna, 2002, Kalua et al., 2007, Morales et al., 2005, Reiners and Grosch, 1998).
From the biochemical point of view, the volatiles found in virgin olive oil are mainly produced in plant organs by the oxidation of fatty acids though intracellular biogenic pathways (Angerosa and Basti, 2001, Kalua et al., 2007). Some of these volatiles are present in the intact tissue of the fruit and others are formed during disruption of cell structure during virgin olive oil production due to the enzymatic reactions in the presence of oxygen. It is generally agreed that endogenous plant enzymes, through the lipoxygenase pathway (LOX) are responsible for the positive aroma perceptions in olive oil, whereas chemical oxidation and exogenous enzymes, usually from microbial activity, are associated with sensory defects (Angerosa and Basti, 2001, Kalua et al., 2007, Morales et al., 1995, Morales et al., 1997). The major volatile compounds responsible for odour notes of virgin olive oils are the C6 and the C5 volatile compounds coming from primary or secondary LOX pathway, respectively (Angerosa, 2002, Aparicio and Morales, 1998, Kiritsakis et al., 1998). Moreover, the presence of minor volatile compounds may provide useful quality markers and lead to an improved understanding of the formation or degradation of the major volatile compounds (Kalua et al., 2007). It is accepted that the volatile profile of virgin olive oil depends on the level and the activity of enzymes involved in the various pathways (Angerosa, 2002, Angerosa and Basti, 2001, Angerosa et al., 1999). The enzymatic levels are genetically determined (Campeol, Flamini, Chericoni, Catalano, & Cremonini, 2001) whereas, other factors influenced their activities. In fact, cultivar, geographic region, fruit maturity, processing methods and parameters influence the volatile composition of olive oil (Angerosa et al., 1998, Aparicio and Morales, 1998, Ben Temime et al., 2006, Haddada et al., 2007).
A study of Italian, Spanish and Moroccan extra virgin olive oils (Reiners & Grosch, 1998) confirmed the richness of C6 volatile compounds in Italian oils but showed that they were poor in fruity esters. Ben Temime et al. (2006) and Haddada et al. (2007) reported that the headspaces of Tunisian virgin olive oils were particularly rich in C6 aldehydes (hexanal, (E)-2-hexenal and (Z)-3-hexenal).
Fruits obtained from different cultivars grown under the same environmental conditions produce oils with different volatile compounds, as does fruit of the same cultivar grown in different geographic regions (Angerosa et al., 1999, Baccouri et al., 2007, Ben Temime et al., 2006, Haddada et al., 2007). Moreover, both the processing and the storage of the fruit and the oil contribute greatly to the flavour and overall quality of olive oil (Angerosa, 2002, Venkateshwarlu et al., 2004). Analysis of the aroma volatile compounds has been used to evaluate the degree of ripeness of the olive fruit (Aparicio & Morales, 1998). Indeed, an understanding of the stages at which volatile compounds are formed can be used to control the volatile composition of olive oil, allowing the production and consumption of better quality oils. Thus, the aroma compounds increased with the degree of fruit maturity to a certain point (Aparicio and Morales, 1998, Kiritsakis et al., 1998, Ranalli et al., 1998). Selection of premium olive fruit at optimum ripeness and optimum processing conditions are factors that can be used to control the process of volatile compound formation. Apart from the condition of the fruit at harvest, differences in post-harvest handling of the fruit and the oil lead to different volatile profiles. Extraction methods and conditions, in particular, the malaxation time and temperature, produce olive oils with different flavours (Angerosa et al., 1998, Ranalli et al., 2001). A prolonged storage of either the fruit or the oil produces volatile compounds that are responsible for off-flavours (Kiritsakis et al., 1998). The absence of the C6 aldehydes, alcohols and esters from the lipoxygenase pathway and the presence of many saturated and unsaturated aldehydes from chemical oxidation, including hexanal, characterize this kind of off-flavour of virgin olive oil known by assessors as rancid defect (Angerosa, 2000, Ha et al., 2004). Furthermore, recent researchers reported that the volatile profile of Cornicabra virgin olive oil was influenced by the irrigation and the maturity stage of the olive. The levels of major volatile components decreased in the course of ripening but were higher in irrigated olive oils (Gomez-Rico, Salvador, La Greca, & Fregapane, 2006).
A very recent paper (Tena, Lazzez, Aparicio-Ruiz, & Garcia-Gonzalez, 2007) is focused on the characterization of several Tunisian oils at four stages of ripeness by means of volatile compounds analyzed by SPME–GC and metal oxide sensors (MOSs); authors highlighted the importance of these informations on a future traceability of Tunisian virgin olive oils produced in costal and inland farms of different geographical areas. Nevertheless, up to now, there is no detailed information available on the influence of irrigation on the volatile composition of Tunisian VOO. Concerning this, a correct evaluation of the effects of sustainable irrigation conditions on Chétoui VOO aroma is a very important aspect, especially for this variety cultivated in the Tunisian north, where the water resources allowed the use of the irrigation treatments, while, this agronomic practice is not foreseeable for the Chemlali trees cultivated in central and southern areas of the country, due to very limited water resources available in these regions.
To analyze volatile fraction of virgin olive oil, solid-phase microextraction (SPME) has been introduced as a pre-concentration method prior to GC analysis in alternative to the dynamic headspace. SPME is an analytical technique where the analytes are either adsorbed or absorbed onto the fibre from the sample matrix and subsequently desorbed into an analytical instrument. SPME is simple, rapid, cheap and solvent-less. This technique has gained popularity in the analysis of volatile compounds since its development in the early 1990s (Arthur & Pawliszyn, 1990).
This paper intends to first characterize and compare the volatile profiles of different monovarietal oils largely produced in the north, the centre and the south regions of Tunisia (cvv. Chétoui and Chemlali) and in Sicily (Nocellara del Belice, Biancolilla and Cerasuola); secondly, determine the effects of the olive ripening degree on the volatile composition of the studied virgin olive oils; finally, evidence the influence of the irrigation regime on the formation of volatiles of Chétoui virgin olive oil. Concerning the last purpose, in this paper the behaviour of only cv. Chétoui has been studied so the results obtained should be considered as preliminary. A study in depth on higher number of samples will be indispensable to confirm the trend of volatile compounds observed.
Section snippets
Oil samples
Monovarietal virgin olive oils, properly obtained from healthy fruits without any kind of infection or physical damage, were collected in triplicate, at three different stages; the beginning of the harvest period when fruits were green colored (unripe) in the middle of the harvesting (medium ripe) and at the end when fruits were colored in black both on the skin and on the pulp (over-ripe) (FLAIR, 1991). Five varieties were studied: (Chétoui and Chemlali, Tunisia; Nocellara del Belice,
Results and discussion
The characteristic and the unique flavour of virgin olive oil, in particular, its green and fruity attributes depend on many volatile compounds (Angerosa, 2000, Angerosa, 2002). Concentration and odour threshold of volatile compounds, whether major or minor, are crucial to virgin olive oil quality. Consequently, the identification and the quantification of the compounds causing the flavour or off-flavour is considered the key for quality control. The analysis of volatile fractions from oils
Conclusions
The application of SPME to the analysis of virgin olive oil headspace allowed the detection of significant differences in the proportions of volatile compounds from Tunisian and Sicilian monovarietal virgin olive oils tested in their original cultivation area in relation to olive ripening stages and some agronomic practices such as irrigation.
As Tunisian and Sicilian oils were tested in their original growing area, additional works will be addressed in these local cultivars obtained in
Acknowledgement
This work was supported by the Tunisian Ministry of High Education, Scientific Research and Technology. Part of this study was carried out at the Dipartimento di Scienze degli Alimenti of Università di Bologna at Campus of Food Science, Cesena, Italy.
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