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Molecules 2007, 12(8), 1679-1719; https://doi.org/10.3390/12081679
Phenolic Molecules in Virgin Olive Oils: a Survey of Their Sensory Properties, Health Effects, Antioxidant Activity and Analytical Methods. An Overview of the Last Decade
Department of Food Science, University of Bologna. P.zza Goidanich 60, I-47023 Cesena (FC), Italy
Department of Analytical Chemistry, Faculty of Sciences, University of Granada, c/Fuentenueva s/n, E-18071 Granada, Spain
Author to whom correspondence should be addressed.
Received: 9 June 2007; in revised form: 2 August 2007 / Accepted: 2 August 2007 / Published: 6 August 2007
Among vegetable oils, virgin olive oil (VOO) has nutritional and sensory characteristics that to make it unique and a basic component of the Mediterranean diet. The importance of VOO is mainly attributed both to its high content of oleic acid a balanced contribution quantity of polyunsaturated fatty acids and its richness in phenolic compounds, which act as natural antioxidants and may contribute to the prevention of several human diseases. The polar phenolic compounds of VOO belong to different classes: phenolic acids, phenyl ethyl alcohols, hydroxy-isochromans, flavonoids, lignans and secoiridoids. This latter family of compounds is characteristic of Oleaceae plants and secoiridoids are the main compounds of the phenolic fraction. Many agronomical and technological factors can affect the presence of phenols in VOO. Its shelf life is higher than other vegetable oils, mainly due to the presence of phenolic molecules having a catechol group, such as hydroxytyrosol and its secoiridoid derivatives. Several assays have been used to establish the antioxidant activity of these isolated phenolic compounds. Typical sensory gustative properties of VOO, such as bitterness and pungency, have been attributed to secoiridoid molecules. Considering the importance of the phenolic fraction of VOO, high performance analytical methods have been developed to characterize its complex phenolic pattern. The aim of this review is to realize a survey on phenolic compounds of virgin olive oils bearing in mind their chemical-analytical, healthy and sensory aspects. In particular, starting from the basic studies, the results of researches developed in the last ten years will be focused.
Keywords:Phenols; Virgin olive oil; Sensory properties; Antioxidant activity; Analytical techniques
Phenolic molecules in virgin olive oil
Oleuropein belongs to a specific group of coumarin-like compounds, the secoiridoids, which are abundant in Oleaceae. Secoiridoids are compounds that are usually glycosidically bound and produced from the secondary metabolism of terpenes. The secoiridoids, found only in plants belonging to the family of Olearaceae that includes Olea europaea L., are characterised by the presence of elenolic acid in its glucosidic or aglyconic form, in their molecular structure. In particular, they are formed from a phenyl ethyl alcohol (hydroxytyrosol and tyrosol), elenolic acid and, eventually, a glucosidic residue. Oleuropein is an ester of hydroxytyrosol (3,4-DHPEA) and the elenolic acid (EA) glucoside (oleosidic skeleton common to the secoiridoid glucosides of Oleaceae) [1,2,3,4,5]. Secoiridoids of VOO in aglyconic forms arise from glycosides in olive fruits by hydrolysis of endogenous β-glucosidases during crushing and malaxation. These newly formed substances, having amphiphilic characteristics, are partitioned between the oily layer and the vegetation water, and are more concentrated in the latter fraction because of their polar functional groups. During storage of VOO hydrolytic mechanisms that lead to release of simple phenols, such as hydroxytyrosol and tyrosol, from complex phenols as secoiridoids may be involved [6,7,8]. The most abundant secoiridoids of VOO, identified for the first time by Montedoro et al. [1,2,3,9] and confirmed also by other authors [10,11,12,13], are the dialdehydic form of elenolic acid linked to hydroxytyrosol or tyrosol (p-HPEA) respectively termed 3,4-DHPEA-EDA and p-HPEA-EDA, and an isomer of the oleuropein aglycon (3,4-DHPEA-EA) (Table 1). In 1999 another hydroxytyrosol derivative, hydroxytyrosol acetate (3,4-DHPEA-AC) was found in virgin olive oil .
Phenolic acids are secondary aromatic plant metabolites that are widely spread throughout the plant kingdom [15,16,17]. These naturally occurring phenolic acids contains two distinguishing constitutive carbon frameworks, namely the hydroxycinnamic and hydroxybenzoic structures. Elucidation of their roles in plant life is only one of the many ongoing investigations regarding phenolic acids: one vast area of interest lies in food quality [18,19,20]. Phenolic acids have been associated with color and sensory qualities, as well as with the health-related and antioxidant properties of foods [21,22]. One impetus for analytical investigations has been the role of phenolics in the organoleptic properties (flavor, astringency, and hardness) of foods [23,24]. Additionally, the content and profile of phenolic acids, their effect on fruit maturation, prevention of enzymatic browning, and their roles as food preservatives has been evaluated . Recent interest in phenolic acids stems from their potential protective role, through ingestion of fruit and vegetables, against diseases that may be related to oxidative damage (coronary heart disease, stroke, and cancers) [26,27,28]. In particular, several phenolic acids such as gallic, protocatechuic, p-hydroxybenzoic, vanillic, caffeic, syringic, p- and o-coumaric, ferulic and cinnamic acid have been identified and quantified in VOO (in quantities lower than 1 mg of analyte kg-1 of olive oil). In this regard two research groups have extensively analyzed samples of VOO for these types of compounds [29,30,31,32]. In one of these mentioned articles, for instance, the authors found that trans-cinnamic acid, sinapinic acid, caffeic acid and 3,4-dihydroxyphenylacetic acid were present in several monovarietal VOO of the six Spanish olive cultivars analyzed ; therefore, these compounds might be potential markers of geographical origin or the olive fruit variety.
(+)-Pinoresinol is a common component of the lignan fraction of several plants such as Forsythia species  and Sesamum indicum seeds, whereas (+)-1-acetoxypinoresinol and (+)-1-hydroxy-pinoresinol and their respective glucosides have been detected in the bark of the olive tree (Olea europaea L.). According to Owen et al. , the quantity of lignans in VOO may be up to 100 mg kg-1, but as with the simple phenols and SIDs, considerable inter-oil variation exists. As suggested by Brenes et al. , the amount of lignans may be used as varietal marker, and they reported a method to authenticate VOO produced by Picual olives based on the very low content of the lignan (+)-1-acetoxypinoresinol in these oils.
A few years ago, Bianco et al.  investigated the presence of hydroxy-isochromans in VOO. In fact, during the malaxation step of VOO extraction, hydrolytic processes through the activity of glycosidases and esterases augment the quantity of hydroxytyrosol and carbonylic compounds, thus favouring the presence of all compounds necessary for the formation of isochroman derivatives. Two hydroxy-isochromans, formed by the reaction between hydroxytyrosol and benzaldehyde or vanillin, have been identified by HPLC-MS/MS technique and quantified in commercial VOOs.
Flavonoids are widespread secondary plant metabolites. During the past decade, an increasing number of publications on the health beneficial effects of flavonoids have appeared, such those related to cancer and coronary heart diseases [37,38,39,40]. Flavonoids are largely planar molecules and their structural variation comes in part from the pattern of modification by hydroxylation, methoxylation, prenylation, or glycosylation. Flavonoid aglycones are subdivided into flavones, flavonols, flavanones, and flavanols depending upon the presence of a carbonyl carbon at C-4, an OH group at C-3, a saturated single bond between C-2 and C-3, and a combination of no carbonyl at C-4 with an OH group at C-3, respectively. Several authors have reported that flavonoids such as luteolin and apigenin are also phenolic components of VOO [41,42,43,44,45,46]. Luteolin may originate from rutin or luteolin-7-glucoside, and apigenin from apigenin glucosides. There are also several interesting studies in which several flavonoids have been found in olive leaves and fruits [47,48,49,50].
Table 1. Phenolic compounds in virgin olive oil: compounds name, general chemical structure and molecular weight.
|Benzoic and derivatives acids|
|3-Hydroxybenzoic acid||3-OH (138)|
|p- Hydroxybenzoic acid||4-OH (138)|
|3,4-Dihydroxybenzoic acid||3,4-OH (154)|
|Gentisic acid||2,5-OH (154)|
|Vanillic acid||3-OCH3, 4-OH (168)|
|Gallic acid||3,4,5-OH (170)|
|Syringic acid||3,5-OCH3, 4-OH (198)|
|Cinnamic acids and derivatives|
|o-Coumaric acid||2-OH (164)|
|p-Coumaric acid||4-OH (164)|
|Caffeic Acid||3,4-OH (180)|
|Ferulic Acid||3-OCH3, 4-OH (194)|
|Sinapinic Acid||3,5-OCH3, 4-OH (224)|
|Phenyl ethyl alcohols|
|Tyrosol [(p-hydroxyphenyl)ethanol] or p-HPEA||4-OH (138)|
|Hydroxytyrosol [(3,4-dihydroxyphenyl)ethanol] or 3,4-DHPEA||3,4-OH (154)|
|Other phenolic acids and derivatives|
|p-Hydroxyphenylacetic acid||4-OH (152)|
|3,4-Dihydroxyphenylacetic acid||3,4-OH (168)|
|4-Hydroxy-3-methoxyphenylacetic acid||3-OCH3, 4-OH (182)|
|Dialdehydic forms of secoiridoids|
|Decarboxymethyloleuropein aglycon (3,4-DHPEA-EDA)||R1-OH (304)|
|Decarboxymethyl ligstroside aglycon (p-HPEA-EDA)||R1-H (320)|
|Oleuropein aglycon or 3,4-DHPEA-EA||R1-OH (378)|
|Ligstroside aglycon or p-HPEA-EA||R1-H (362)|
|Aldehydic form of oleuropein aglycon||R1-OH (378)|
|Aldehydic form ligstroside aglycon||R1-H (362)|
|Apigenin||R1-OH, R2-H (270)|
|Luteolin||R1-OH, R2-OH (286)|
|1-(3’-Methoxy-4’-hydroxy)phenyl-6,7-dihydroxyisochroman||R1-OH, R2-OCH3 (288)|
Why are the phenolic compounds present in virgin olive oil so important? Why is their determination so interesting and difficult?
Last year, Boskou published an interesting review  wherein the sources of natural phenolic antioxidants were discussed, and the following idea was highlighted: “Widely distributed in the plant kingdom and abundant in our diet, plant phenols are today among the most talked about classes of phytochemicals”. To answer to the question of “why are phenolic compounds so interesting?”, the author of the review summarized several issues which have been studied in depth during the last decade:
- The levels and chemical structure of antioxidant phenols in different plant foods, aromatic plants and various plant materials.
- The probable role of plant phenols in the prevention of various diseases associated with oxidative stress such as cardiovascular and neurodegenerative diseases and cancer.
- The ability of plant phenols to modulate the activity of enzymes, a biological action not yet understood.
- The ability of certain classes of plant phenols such as flavonoids (also called polyphenols) to bind to proteins. Flavonol–protein binding, such as binding to cellular receptors and transporters, involves mechanisms which are not related to their direct activity as antioxidants.
- The stabilization of edible oils, protection from formation of off-flavors and stabilization of flavours.
- The preparation of food supplements.
Focusing on phenolic compounds of virgin olive oil and bearing in mind the reasons for being so important, attention must be paid to the fact that this class of compounds has not been completely characterized due to the complexity of their chemical nature and the complexity of the matrix in which they are found. Moreover, one of the current problems for developing rapid and reproducible analysis of phenolic compounds is the absence of suitable pure standards, in particular secoiridoid molecules and lignans.
Sensory properties elicited by phenols in VOO
Virgin olive oil is a natural fruit juice obtained directly from olives without any further refining process. Its flavour is characteristic and is markedly different from those of other edible fats and oils. The combined effect of the taste, odor (directly via the nose or indirectly through the retronasal path via the mouth) and chemical responses (pungency, astringency, metallic, cooling or burning) gives rise to the sensation generally perceived as “flavor” . VOO, when extracted from fresh and healthy olive fruits (Olea europaea L.) and properly processed and stored, is characterized by an unique combination of aroma and taste that is highly appreciated [127,128]. The sensory aspect, due to the use of VOO as a seasoning on cooked and especially raw foods, has great repercussions on its acceptability. Thus, since sensory quality plays an important role in directing the preference of consumers, many attempts have been made to clarify the relationships between the sensory attributes in a VOO as perceived by assessors and its volatile and phenol profiles, which are responsible for aroma and taste, respectively .
Few individuals, except for trained assessors of VOO, know that bitterness and pungency perceived by taste are positive attributes for a VOO. These two sensory characteristics are strictly connected by the quali-quantitative phenolic profile of the product. An example of the positive correlations between amount of phenols and bitter and pungent intensities is shown in Figure 2a and Figure 2b.
Some phenols mainly elicit the tasting perception of bitterness; however, other phenolic molecules can stimulate the free endings of the trigeminal nerve located in the palate and also in the gustative buds giving rise to the chemesthetic perceptions of pungency, astringency and metallic attributes. Thus, the intensity of bitterness and pungency is mainly related to the olives cultivar and the ripening stage and, as reported by many authors, are especially abundant in oils obtained from unripe fruits. For instance, Caponio et al.  showed that in Coratina and Oliarola Salentina VOO, oleuropein and its aglycon form both decrease as ripening of the olives progressed. From this data, the bitter to pungent taste would appear to be mainly ascribable to oleuropein aglycon since greater amounts of this phenolic compound are present in the Coratina oils with respect to O. Salentina oils, which are known to have a sweet taste. In order to attenuate such these taste sensations, the authors suggested the need to postpone harvesting of Coratina olives.
Figure 2. Sensory profile and phenolic content of two different VOO (HPh, high phenols oil and LPh, low phenols oil). a, sensory profiles of samples by Quantitative Descriptive Analysis (QDA); the intensity of each descriptor is evaluated on a 0-5 points scale; different perception routes: (1) orthonasal, (2) retronasal. b, single and total phenolic content of samples; A, hydroxytyrosol; B, tyrosol; C, vanillic acid; D, unknown phenolic compound with a retention time of 30.69 min; E, unknown phenolic compound with a retention time of 36.27 min; F, 3,4-DHPEA-EDA; G, (+)-pinoresinol; H, (+)-1-acetoxypinoresinol + p-HPEA-EDA; I, 3,4-DHPEA-EA; L, p-HPEA-EA.
A Quantitative Descriptive sensory Analysis (QDA) carried out by Rotondi et al.  confirmed a decreasing trend of the positive olive oil descriptors, such as bitterness and pungency, when Nostrana di Brisighella olives ripened. The highest statistically significant intensity was at the beginning of fruit skin pigmentation. The decrease in bitterness and pungency was also related to a reduction in total phenols and o-diphenols levels. In particular, a positive correlation between the secoiridoids content and bitterness and pungency was observed.
With the aim of reducing the bitterness intensity in VOOs, disfavored by many consumers when present at high intensity, some authors  developed postharvest technology based on hot-water treatments of olive fruits (cultivars Manzanilla, Picual, and Verdial) in the temperature range of 60-68°C or  with air-heating (40°C during 24, 48, and 72 h). These treatments promote a reduction in bitterness that is directly related to the time and temperature of treatment, probably due to a partial inhibition of glycosidases and esterases; in fact, these enzymes are involved in the release of secoiridoid derivatives from oleuropein during the crushing malaxation process. However, this heat treatment also affected other quality traits such as oxidative stability and color and could produce a change in the aroma profile of the VOO as well.
The standard method of analyzing the bitter taste of olive oil is by sensory analysis using a panel of tasters . However, an analytical panel is often not likely to be available, since a permanent staff of trained tasters and a highly specialized panel chief is necessary. Many consumers from extra-European countries are not accustomed to the typical high intensity of bitterness or pungency of fresh VOO and, consequently, must be blended with less bitter VOO. For this reason, methods for the evaluation of the bitterness level based on physical-chemical determinations would be very useful for the industry. Several authors have found a strong relationship between these sensory attributes and the content of phenolic compounds in the olive oils. In 1992 Gutiérrez et al.  proposed an analytical method for measurement of bitterness, based on solid phase extraction (SPE) of phenols and their spectrophotometric detection at 225 nm; this parameter termed IB or index of bitterness, was highly correlated to the sensory intensity of bitterness and is still the most widely used for its determination. Some years later, Mateos and co-workers  showed that several non-bitter phenolic compounds could also absorb at 225 nm; consequently, they asserted that this index was not appropriate for comparing bitterness of VOO obtained from blend of olive varieties characterized by very different phenolic profiles (e.g. Picual and Arbequina). Moreover, Mateos suggested that evaluation of the bitterness level of a VOO could be described by the experimental equation obtained from the regressions between intensity of bitterness and the concentrations of oleuropein aglycon using a Panel test and chromatographic analysis, respectively.
Some researchers suggest that secoiridoid derivatives of hydroxytyrosol are the main contributors to olive oil bitterness. Recently, a procedure called taste dilution analysis (TDA) was reported by Frank and co-authors to underlie the sensory threshold of bitter for oleuropein derivatives . Bitterness was assessed by preparing serial dilutions of samples in water and then tasting in order of increasing concentration until the concentration at which the diluted sample can be differentiated from water as judged in a triangle test is found. When an isomer (or isomers) of oleuropein aglycon was prepared by β-glucosidase hydrolysis of oleuropein isolated from olives and evaluated by assessors, it was found to be bitter with a threshold of 50 μmol. Using the same evaluation technique, no bitterness was observed for hydroxytyrosol or elenolic acid.
Andrewes et al.  assessed the relationship between polyphenols and olive oil pungency. p-HPEA-EDA was the key source of the burning sensation found in many olive oils. In contrast, 3,4-DHPEA-EDA, tasted at an equivalent concentration, produced very little burning sensation. This is a clear example of the different sensory properties of a secoiridoid derivative of hydroxytyrosol and tyrosol. In 2005, Beauchamp and co-authors  measured the pungent intensity of p-HPEA-EDA isolated from different VOO confirming this molecule is the principal agent in VOO responsible for throat irritation. These researchers also tested the throat-irritant properties of its synthetic form (named “oleocanthal”, with oleo- for olive,-canth- for sting, and -al for aldehyde) dissolved in non-irritating corn oil. They found an effect comparable to that of the purified compound from VOO and a dose-dependent activity.
In 2003, Gutierrez-Rosales and co-authors  isolated the major peaks found in the phenolic profile of VOO using preparative HPLC; after dissolving in water these molecules purified were then tasted to evaluate the intensity of bitterness. It was concluded that the peaks corresponding to the 3,4-DHPEA-EDA, 3,4-DHPEA-EA and p-HPEA-EDA were those mainly responsible for the bitter taste of VOO. As previously reported, Mateos et al.  verified the better correlation between the aldehydic form of oleuropein aglycon and bitterness.
Recently some researchers  have studied the temporal perception of bitterness and pungency in monovarietal VOOs; analyses were performed by a trained sensory panel utilizing a time–intensity (TI) evaluation technique; bitterness curves had a faster rate of rising and declining than pungency curves: the curves for bitterness reached a maximum after approximately 16–20 s, whereas the maximum of the perception of pungency is registered between 26 and 29 s and is independent of the maximum intensity of the perception.
As already discussed, several authors have associated some phenols with bitterness, thus obtaining models and relationships between individual phenols separated by HPLC and bitterness intensity [135,138,140]. In these reports, bitterness was measured by a panel test or calculated from K225 values. Moreover tyrosinase- and peroxidase-based biosensors are being developed for the bitterness assessment , and are showing interesting possibilities. However, HPLC is often not available in many olive oil mill laboratories because of economic reasons, as well as specialized technical staff and biosensors. As an alternative, Beltrán and co-authors  proposed that measurement of phenol content can be used. This is a simple analytical method that involves liquid-liquid extraction and colorimetric measurement using Folin-Ciocalteau reagent . In their experimental work, the authors analyzed the relationship between phenol content and K225 for oils from four of the most important olive cultivars worldwide (Frantoio, Hojiblanca, Picual, Arbequina); 360 samples were used to develop the model. As a practical application, bitterness intensities were evaluated by sensory analysis of 25 VOO samples, and were then estimated by applying the prediction model. In order to provide an easy tool for bitterness estimation, VOO bitterness was classified by its phenol content into four categories (results expressed as mg of caffeic acid per kg of oil and intensity of bitterness between 0 and 5 values): phenol contents equal or lower than 220 mg kg-1 corresponded to non-bitter oils or oils with almost imperceptible bitterness (intensities 0–1.5); slight bitterness corresponded to 220–340 mg kg-1 (intensities 1.6–2.5); bitter oils have a phenol contents ranging from 340 to 410 mg kg-1 (intensities 2.5–2.99); and a phenol contents higher than 410 mg kg-1 corresponds to quite bitter or very bitter oils (intensities higher than 3). In general, the authors determined that the oils were classified correctly into the same bitterness categories by both methods at 92%, achieving 100% of correct classification for the lowest and highest bitterness categories.
New analytical approaches to characterization of the phenolic profile and applied studies during the last decade
In order to utilize VOO as a source of phenolic compounds, to develop complete compositional databases and to obtain more accurate data about the intake of antioxidants further chemical characterization is needed. Identification and quantitation, based traditionally on HPLC (with different detectors, such as UV, fluorescence, coulometric electrode array detection, amperometric detector) [144,145,146,147,148,149,150], GC-FID [151,152,153,154] and, more recently CE-UV, can be aided today by MS and NMR, which is a focus of the present review.
Liquid chromatography/mass spectrometry (LC-MS) has been widely accepted as the main tool in identification, structural characterization and quantitative analysis of phenolic compounds in olive oil. Using a mass spectrometer for detection offers some undoubted advantages, such as independence of a chromo- or fluorophore, lower LOD than UV in most cases , the possibility to obtain structural information and easy separation of coeluting peaks using the information about mass as a second dimension.
The sensitivity of response in MS is clearly dependent on the interface technology employed. In LC-MS analysis of phenolic compounds, atmospheric pressure ionization interfaces, i.e. APCI and electrospray ionization (ESI), are used almost exclusively today, and both positive and negative ionization are applied. In general, phenolic compounds are detected with a greater sensitivity in the negative ion mode, but the results from positive and negative ion modes are complementary, and the positive ion mode shows structurally significant fragments .
On the other hand, optimal ionization depends not only on the interface parameters, but also on the mobile phase of the liquid chromatography. As a first rule, the use of non-volatile salts in the mobile phase (common in other chromatographic methods) should be avoided, as they would interfere with the ionization source. The mobile-phase composition and its pH need also careful optimization as they may influence the ionization efficiency of the analytes.
The selection of the analyzer, apart from its accessibility, is determined by the required sensitivity and selectivity and the general objectives. LC-atmospheric pressure ionization (API)-MS typically only yields a single strong ion, which reduces its ability to make analyte accurate identifications. In the most cases, single-stage MS is used in combination with UV detection to facilitate the identification of phenolic compounds in olive oil samples with the help of standards and/or reference data. Ion Trap or QqQ provide the possibility of doing MS/MS or MSn, which can be used for structure elucidation or for additional selectivity to gain sensitivity by reducing the chemical noise . MS/MS and MSn involve two (or more) stages of mass analysis, separated by a fragmentation step. TOF MS, which is one of the most advanced MS analyzers, provides excellent mass accuracy  over a wide dynamic range if a modern detector technology is chosen. The latter, moreover, allows measurements of the correct isotopic pattern , providing important additional information for the determination of elemental composition .
Table 2 provides an overview of methodologies based on LC-tandem mass spectrometry used for the analysis of phenolic compounds in olive oil. The table does not include several publications in which the analysis of olive fruit, leaves, pulp and pomace, olive tree wood, as well as olive oil waste waters were carried out by using HPLC-MS [14,49,156,161,162,163,164,165,166,167,168]. Other important issues are the presence of phenolic metabolites of VOO in the human low density lipoprotein fraction [169,170].
High-resolution spectroscopic techniques, and particularly NMR spectroscopy, are finding interesting applications in the analysis of complex mixtures of various food extracts that contain phenols.
During the past decade proton nuclear magnetic resonance spectroscopy (NMR) has been successfully used in olive oil analysis [171,172]. Currently available high-resolution spectroscopic techniques, coupled with the facilities of computerized mathematical or other treatment of data have found interesting applications in the field of agricultural and food science without the necessity for a separative technique coupled with NMR, as commented by Gerothanassis . Additionally the usefulness of 1H NMR spectroscopy has been increasingly recognized for its non-invasiveness, rapidity, and sensitivity for a wide range of compounds in a single measurement. However, difficulties may arise in relation to the information obtained from spectra of multicomponent mixtures such as olive oil. Strong signal overlap, dynamic range problems, diversity of intensities due to various concentrations of the food constituents, and the inherent lack of scalar coupling information between different moieties lead to ambiguous or incomplete assignments, thus hindering detection even with the use of multidimensional NMR . One possible approach to these problems involves the combination of the advantages of NMR spectroscopy with those of chromatography. Coupled techniques such as LC-NMR or LC-NMR/MS may provide information on overall composition and enable the identification of individual phenols in complex matrices. Moreover, on-line solid phase extraction (SPE) in LC-NMR for peak storage after the liquid chromatography separation prior to NMR analysis or similar techniques have been recently applied.
Table 2. Summary of separation of phenolic compounds in the polar fraction of VOO using HPLC-MS methods.
|Time of analysis||Mobile phase||Stationary phase||Type of elution||Extraction System||Detection System||Observations||References|
|120 min||A: H2O|
|Spherisorb ODS 25 cm x 4.6 mm i.d.; 10µm||Gradient||Combination between LLE  and SPE||UV in HPLC.|
|Separation of the polar fraction of VOO in two parts. Antioxidant activity assessment|||
|93 min||A: H2O + CH3COOH 0.5%|
|Spherisorb ODS 2, 25 cm x 4.6 mm i.d.||Gradient||LLE with methanol/water (80:20 v/v)||UV; MS (ESI) in positive ion mode||Flavonoids such as luteolin and apigenin were detected as phenolic components of VOO|||
|25 min||H2O:CH3CN (82:18 v/v) + CH3COOH 0.02%||Nucleosil ODS, 25 cm x 2.1 mm or 25 cm x 1.1 mm i.d. 5 µm||Isocratic||LLE with buffer; SPE with phenyl cartridges (acidification)||UV, fluorescence, MS, MS/MS|
HPLC-APCI (negative ion mode)
|60 min approx.||A: H2O + HCOOH 0.045%|
B: MeOH + HCOOH 0.045%
|Nucleosil ODS, 25 cm x 2.1 mm i.d. 5 µm||Gradient||-Phenolic acids as Cartoni |
-HYTY and TY:
3 g oil across cartridge phenylic
|MS; MS/MS||Olives and VOOs.|
MS/MS using Multiple Reaction Monitoring (MRM) (high specificity and sensitivity in MS spectra)
|HPLC method and conditions of Cortesi et al. ||C18 column (RP) Alltech 25 cm x 4.6 mm i.d.||Gradient||LLE: Montedoro et al. |
using butylated hydroxytoluene (BHT)
|MS; MS/MS||Analysis of oleuropein aglycon by APCI-MS. Phenolic compound profile|||
|HPLC method of Romani et al. ||Lichrosorb RP18, 25 cm x 4.6 mm i.d. 5 µm||Gradient||LLE with EtOH/water (70:30 v/v), the water was acidified with formic acid (pH 2.5)||DAD; MSD||HPLC analysis of phenolic acids, secoiridoids and flavonoids|||
|60 min||A: H2O + CH3COOH 2mM|
B: MeOH + CH3COOH 2mM
|Nucleosil ODS, 25 cm x 2.1 mm i.d. 5 µm||Gradient||LLE with methanol/water (80:20 v/v), acidification and passed through a C18 cartridge||MS and MS/MS (API/MS in negative ion mode)||Identification of a new class of phenolic compounds in olive oils: hydroxy-isochromans|||
|HPLC method and conditions of Brenes et al. ||UV; electrochemical, fluorescence, MS.||Use of a lignan (1-acetoxypinoresinol) to authenticate Picual VOOs. Use of GC too.|||
|50 min or 70 min||A: H2O + H3PO4 0.5%|
B: MeOH/ MeCN (50:50 v/v)
|Lichrospher 100 RP18, 25 cm x 4.0 mm i.d. 5 µm||Gradient||SPE (diol-bound phase)||UV,|
HPLC-MS in ESI(positive ion mode)
|Dialdehydic and aldehydic forms of oleuropein aglycon and ligstroside aglycon|||
|65 min||A: H2O + CH3COOH 2%|
B: MeOH/ MeCN (50:50 v/v)
|C18 Luna column, 25 cm x 3.0 mm i.d. 5 µm||Gradient||Comparative study of 5 extraction methods (LLE and SPE)||UV, DAD; MS||HPLC and CE methods. (HYTY, TY, oleuropein, ligstroside aglycon and decarboxymethyl oleuropein aglycon)|||
|65 min||A: H2O + CH3COOH 2%|
|Phenomenex Luna (phenyl-hexyl)phase; 25 cm x 4.6 mm i.d. 5 µm||Isocratic||LLE with methanol/water (80:20 v/v) Montedoro et al. ||UV; MS (ESI in negative ion mode)||Isolation of individual polyphenols to study sensory properties|||
|60 min approx.||A: H2O + HCOOH 0.09%|
B: MeOH + HCOOH 0.09%
|Nucleosil ODS, 25 cm x 2.1 mm i.d. 5 µm||Gradient||Separation of phenolic compounds in two fractions after C18 cartridge.|
Group A: 12 g oil
Group B. 3 g oil
|UV; fluorescence; MS; MS/MS||Improve of extraction system of .|
Determination of isomer of dihydroxy- and dimethoxybenzoic acids. Comparison among LOD in HPLC-UV, HPLC-FL and HPLC-MS/MS
|75 min||A: H2O + CH3COOH 0.5%|
|C18 Luna column, 25 cm x 3.0 mm i.d. 5 µm||Gradient||LLE with methanol/water (60:40 v/v)||DAD; MS (ESI in negative ion mode)||Effect of olive ripening degree on the oxidative stability and organoleptic properties of olive oil|||
|HPLC method of Rotondi et al. .||Gradient||LLE with methanol/water from olive oil. SLE from olive fruits||DAD; MS (ESI in positive and negative ion mode)||HPLC and CE analysis.|
3 simple phenols, a secoiridoid derivative and 2 lignans
|60 min||A: H2O + 0.2% acetic acid|
|Lichrospher 100, 12.5 cm x 4.0 mm i.d. 5 µm||Gradient||LLE with methanol (500 mg of oil)||Refractive index detector; MS||TY, Vanillic acid,|
Lut and Apig.Squalene (with Refractive Index detector).
Quantitation in 7 samples.
|45 min||A: H2O + 0.2% acetic acid|
|Inertsil ODS-3, 15 cm x 4.6 mm i.d. 5 µm||Gradient||LLE with methanol/water (80:20, v:v) (45 g of oil)||UV; ESI-MS||Antioxidant activity of olive pulp and olive of Arbeq. cv|||
|40 min||A: H2O + 0.2% acetic acid|
|C18 Luna column, 15 cm x 2.0 mm i.d. 5 µm||Gradient||Diol cartridge (3 g of oil)||UV (DAD); MS; MS/MS (QqQ)||Quantification of 23 compounds in 3 olive oils.|
Possible models of derived secoiridoids (nine basic models of Lig and Ol aglycons found in bibliography)
|70 min||A: H2O + 0.5% acetic acid|
|C18 Luna column, 25 cm x 4.6 mm i.d. 5 µm||Gradient||LLE with methanol/water (60:40, v:v) (60 g of oil)||UV (DAD); MS||Isolation of several phenolic compounds and study of their antioxidants properties (DPPH, OSI and electrochemical method)|||
|50 min||A: H2O + HCOOH (pH 3.2)|
|C18 Luna column, 25 cm x 3.0 mm i.d. 5 µm||Gradient||LLE with ethanol/water (7:3, v:v) (25 ml of oil)||UV (DAD); ESI-MS||Evaluation of lignans free and linked HYTY and TY in VOO. TLC to determine the presence of lignans.|||
|30 min||A: H2O + 0.1% acetic acid|
|RP C18 2.1 x 100 mm, 3.5 mm particle size; XTerra MS||Gradient||SPE-Diol (60 g of oil) diluted 1:10||ESI-TOF (TOF)||Determination of all the well-known phenolic compounds of oil and more than 25 “new” compounds|||
One of the pioneers in this field was Montedoro  who identified four new phenolic compounds in olive oil in 1993. This paper reported the NMR, UV and IR characterization of the compounds under study, and finally, concluded that the newly identified compounds were an isomer of oleuropein aglycon, the dialdehydic from of elenolic acid linked to hydroxytyrosol, and the dialdehydic from of elenolic acid linked to tyrosol. The results obtained by Limiroli  and Bariboldi  were useful in contributing to a more in-depth understanding of the secoiridoid fraction of VOO. Following these results, several authors have used NMR to analyze phenolic compounds in olive oils, which summarize the reports which include methodologies combining HPLC and NMR (Table 3).
Table 3. Summary of separation of phenolic compounds in the polar fraction of VOO using HPLC-NMR (as coupled techniques or by NMR as off- line technique after HPLC).
|Time of analysis||Mobile phase||Stationary phase||Type of elution||Extraction System||Detection System||Observations||References|
|45 min||A: H2O + CH3COOH 2% (pH 3.1)|
|Erbasil C18, 15 cm x 4.6 mm i.d.||Gradient||LLE with methanol/water||UV; NMR and IR||Spectroscopic characterization of secoiridoid derivatives|||
|60 min||A: H2O + CH3COOH 0.2%|
|Spherisorb ODS 2, 25 cm x 4.6 mm i.d.||Gradient||Same as Montedoro et al. ||Photodiode array; MS; NMR||Simple phenols, flavonoids, secoiridoids|||
|HPLC method of Montedoro et al. ||Column RP18 Latex; 25 cm x 4.0 mm i.d. 5 μm||Gradient||LLE with methanol (500 g of olive oil)||UV; MS (ESI) in negative and positive ion mode; NMR||Identification of lignans as major components in polar fraction of olive oil.|
Preparative thin-layer chromatography (PLC).
|HPLC method of Montedoro et al. [1,2,3]||Column RP18 Latex; 25 cm x 4.0 mm i.d. 5 μm||Gradient||LLE with absolute methanol and methanol/water (80:20 v/v)||UV; MS (ESI) in negative and positive ion mode; NMR||Use of TLC, GC, GC-MS|
Study of antioxidant/anticancer capacity
|50 min||A: H2O + CH3COOH 3%|
B: MeCN:MeOH (50:50 v/v)
|Lichrosphere 100 RP18, 25 cm x 4.0 mm i.d. 5 μm||Gradient||Comparative studies of LLE and SPE using diol-phase cartridges; unwanted substances washed out with hexane and hexane/ethyl acetate (90:10, v/v)||UV; DAD|
NMR (for ligstroside aglycon)
|Phenols, flavones and lignans.|
Colorimetric determination of o-diphenols.
|60 min||A: H2O + CH3COOH 0.5%|
B: MeOH/ MeCN (50:50 v/v)
|Lichrospher 100 RP18, 25 cm x 4.0 mm i.d. 5 μm||Gradient||Comparative study of LLE and SPE (diol and C18-phase)||Photodiode array detector; MS, NMR.||Simple phenols, secoiridoids and lignans|||
|30 min||A: H2O + 0.1% trifluoroacetic (TFA-d)|
B: MeCN+ 0.1% (TFA-d)
|Phenomenex RP-C18, 25 cm x 4.6 mm i.d. 5 μm||Gradient||LLE with methanol/water (80:20 v/v) (50 g of oil)||LC-SPE-NMR system||Complete characterization of 27 phenolic compounds in olive oil. 7 compounds not detected in the past|||
Recently, Christophoridou et al.  reported the first application of the hyphenated LC-SPE-NMR technique using postcolumn solid-phase extraction to direct identification of new phenolic compounds in the polar fraction of VOO. The addition of a post-column SPE system to replace of the loop system of the LC-NMR, resulted in higher sensitivity (significant increase of the signal to noise [S/N] ratio); in fact, S/N improvements by up to a factor of 4 could be demonstrated with this new technology . The spectra recorded were one dimensional (1D) 1H-NMR and two dimensional (2D) NMR. The presence of phenols was confirmed from the respective LC-SPE-NMR spectra, which were assigned on the basis of existing 1H-NMR databases and with total correlation spectroscopy (TOCSY). The most interesting findings of this study were the verification of the presence of the lignan syringaresinol, the presence of two stereochemical isomers of the aldehydic form of oleuropein and the detection of homovanillyl alcohol.
As commented above the researches that studied olive mill waste, brines olive drupes, tissues of olive cultivars, alperujo, olives, olive leaves were not included in Table 3 [49,161,194,195,196,197,198,199]. Servili et al.  in a HPLC investigation of the phenols present in olive fruit, VOO, vegetation waters and pomace, and subsequently by 1D- and 2D-NMR achieved the complete spectroscopic characterization of demethyloleuropein and verbasoside extracted from olive fruit.
There are also several interesting reports describing the analysis of olive oil by ionspray ionization tandem mass spectrometry (IS-MS/MS) and ESI-MS/MS with NMR, without the use of a previous separative technique [201,202,203]. For the purposes of this review it is important to include a recent publication by Christophoridou et al. , where the authors demonstrate the potential of 31P-NMR spectroscopy to detect and quantify a large number of phenols in VOO extracts. This novel analytical method is based on derivatization of the hydroxyl and carboxyl groups of phenolic compounds with 2-chloro-4,4,5,5 tetramethyldioxaphospholane and the identification of the phosphitylated compounds on the basis of the 31P chemical shifts.
Even if the characterization and quantification of phenolic compounds have been successfully carried out by GC and HPLC, the use of faster analytical techniques and screening tools, allowing a rapid screening of phenolic compounds from VOOs, is strongly recommended. Although compared with GC or HPLC, CE is a relatively new technique in food analysis. A large variety of foods have already analyzed by this technique, as CE can represent a good compromise between analysis time and satisfactory characterization for same classes of phenolic compounds in VOO.
CE is characterized by high separation efficiency, small sample and electrolyte consumption, and the separation requires only several minutes. This last characteristic is the main advantage versus chromatographic methods, which makes CE useful for routine analysis as well as for controlling and monitoring processes in a number of industrial fields [205,206,207,208,209,210,211,212,213]. Moreover, CE is relatively well suited to analysis of samples with complex matrices, like VOO.
CE technique can be coupled with different detectors (UV, FIL, electrochemical detectors, MS…). To date, for the analysis of phenolic compounds in VOO, there are several papers reporting the use of CE with ultraviolet detection; it is possible to study results obtained by using CE-MS in only two papers (Table 4).
Along these lines, the use of CE as an analytical separation technique coupled with mass spectrometry as a detection method can provide important advantages in the analysis of phenolic compounds of olive oil because of the combination of the high separation capabilities of CE and the power of MS for identification and confirmation method.
Using mass spectrometric detection, differences in optical detection must be considered. First, the separation electrolyte has to be volatile, reducing the choice of buffering system primarily to ammonia, acetate, or formate. While there are reports documenting nonvolatile buffers from UV-CE, only low buffer concentrations can be used and thus lower sensitivity must be accepted. Generally, nonaqueous solvents are well-suited for hyphenation with MS and add another parameter to modify selectivity.
As commented before for HPLC coupled with MS, CE can also be coupled with different MS analyzers (i.e., with quadrupole, ion trap, time-of-flight, etc.) and use several ionization methods (APCI, ESI, MALDI). ESI is one of the most versatile ionization methods and is the natural method of choice for the detection of ions separated by capillary zone electrophoresis. Regarding the analyzers, ion trap (IT) and TOF systems are the two analyzers more common in the lab of food analysis , although single-quadrupole MS is still often used as an easy and affordable detector.
Of particular interest is the coupling of CZE to ESI-TOF-MS. This coupling combines the abovementioned benefits of CZE separation with the high selectivity due to mass accuracy of 5 ppm, which opens the possibility of determining elemental compositions. The analysis of the true isotopic pattern by ESI-TOF-MS has recently been shown to provide an additional analytical dimension for identification .
During the last decade, concerning phenolic compounds present in VOO, it is possible to find reports in which applicative work is carried out, as well as other where a new analytical method is developed. Herein, the publications including CE-UV and CE-MS are summarized (see Table 4 and Table 5).
Table 4. Summary of optimized conditions of capillary electrophoresis methods where VOO samples are analyzed. V, voltage; T, temperature, i. d., internal diameter of capillary; Lef, effective length of capillary; [Buffer]; buffer concentration.
|Instrumental Variables||Experimental Variables|
|References||λd [nm]||V [kV]||T [ºC]||i. d. [μm]||Lef [cm]||tinj [s]||Type of Buffer||[Buffer] [mM]||pH||Organic modifiers and other variables|
|||200||27||30||50||40||3 s (0.5 p.s.i)||Sodium Tetraborate||45||9.6||-|
|||CZE method of Bendini et al.|
|||CZE method of Bendini et al.|
|||210||25||25||75||50||8 s (0.5 p.s.i)||Sodium Tetraborate||25||9.6||-|
|||200||18||25||50||36||2 s (1.5 p.s.i)||Sodium Tetraborate||40||9.2||-|
|||210||-25||25||75||50||8 s (0.5 p.s.i)||Sodium Tetraborate||50||9.6||20% 2-propanol|
|||214/250||25||25||75||100||8 s (0.5 p.s.i)||Sodium Tetraborate||30||9.3|
|||214/MS (ESI-IT)||25||25||50||100||10 s (0.5 p.s.i)||NH4OAc||60||9.5||5% 2-propanol|
Sheath liquid (60:40 v/v 2-propanol/ water and 0.1% v/v of TEA at a flow rate of 0.28 mL/h)
|||200/240/280/330||28||22||50||40||8 s (0.5 p.s.i)||Sodium Tetraborate||45||9.3|
|||CZE method of Carrasco-Pancorbo |
|||MS (ESI-TOF)||30||25||50||85||10 s (50 mBar)||Ammonium hydrogen carbonate||25||9.0||Sheath liquid (2-propanol/ water 50:50 v/v at a flow rate of 4 μL/min)|
Table 5. Summary of extraction systems used and compounds detected in VOO samples with the application of each method. HYTY, hydroxytyrosol; TY, tyrosol; DHPE, 2,3-dihydroxyphenylethanol; VA, vanillic acid; DOA, decarboxymethyloleuropein aglycon (3,4-DHPEA-EDA); Ac Pin, (+)-1-acetoxypinoresinol; Lig Agl, ligstroside aglycone; Ol Agl, oleuropein algycone; EA, elenolic acid
|References||Initial quantity of oil Final quantity of solvent (MeOH/H2O (50:50 v/v) ) in the extraction process (kind of extraction)||Detected compounds in olive oil||Other relevant aspects|
|||2 g → 1 mL (LLE )||HYTY, TY, unidentified secoiridoids compounds||1st paper where CE is used for the analysis of phenolic compounds from oils|
|||2 g → 1 mL (LLE )||HYTY, TY, DHPE, unidentified oleuropein aglycone derivatives|
|||2 g → 0.5 mL (LLE , as modified in )||HYTY, TY, VA, DOA, Ac Pin|
|||60 g → 0.5 mL (LLE )||13 phenolic acids + taxifolin (flavanonol||Potent extraction system which permits detection of small quantities of phenolic acids|
|||10 g → non specified (Combination of LLE-SPE )||5 phenolic acids|
|||60 g → 0.5 mL (LLE )||13 phenolic acids + taxifolin (flavanonol||Co-electroosmotic CE|
|||60 g → 2 mL (SPE-Diol )||TY, Pin, Ac Pin, DOA, Lig Agl, HYTY, Ol Agl, EA||Use of standards obtained by semipreparative-HPLC|
|||60 g → 2 mL (SPE-Diol )||11 phenols (simple phenols, lignans, complex phenols and EA)||1st paper in which CZE-ESI-IT MS is used for the analysis of phenolic compounds from oils|
|||60 g → 2 mL (SPE-Diol )||26 compounds belonging to all the different families of phenolic compounds present in olive oil||26 compounds in less than 10 min. 1st paper in which flavonoids are detected by CE, and 1st “multicomponent” method for the determination of olive oil phenols|
|||60 g → 2 mL (SPE-Diol )||Applicative work using a previously method ||Interesting from a quantitative and applicative point of view|
|||60 g → 2 mL (SPE-Diol ) and diluted 1:10||All the “well-known” phenolic compounds and 28 other analytes||1st paper in which CZE-ESI-TOF MS is used for the analysis of phenolic compounds from oils. TOF permits the “identification” of new compounds in the oil’s profile|
Concluding remarks and future outlook
The amount of phenolic compounds is a very important parameter when evaluating the quality of VOOs. Phenols are closely related with both the resistance of the oil to oxidation and the typical bitter and pungent tastes. Furthermore, some studies have shown that the amount of phenols, particularly those with a catecholic structure, together with a favorable monounsaturated to polyunsaturated fatty acid ratio, is related to several healthy attributes. These different aspects make VOO a very valuable and appreciated dietary lipidic condiment, and add importance to the determination of its phenolic compounds, both qualitative and quantitatively. The most commonly methods used for phenolic determination in VOO are based on GC and HPLC, and more recently on CE, coupled with different detector systems (UV, FLD, amperometric or coulometric). If the literature regarding phenolic compounds of VOO is analyzed in detail, it is evident that this class of compounds has not been completely studied, because of the complexity of their chemical nature and the complexity of the matrix in which they are found. During the last ten years, MS and NMR have become indispensable to study the quali-quantitative profiles of phenols and their oxidative forms, and detectors with the power to identify compounds and provide the analyst with information about the molecular structure are essential.
Apart from the interest on knowing in composition of the polar fraction of VOO, the determination of these compounds also helps to understand their health benefits that include reduction of risk factors of coronary heart disease, prevention of several varieties of cancer and modification of immune and inflammatory responses. It is also of interest to distinguish what phenolic molecules are responsible for bitterness, pungency, astringency and metallic sensations and to evaluate the antioxidant activities of the polar fraction.
Although excellent progress has already been made, it is expected that the use of different methodologies of potent techniques coupled with rapid, reliable and sophisticated detectors will become more common in the near future; there are still many “unknown” compounds present in the polar fraction of olive oil and it is very important to carry out collaborative studies to join the efforts of the scientific community.
3,4-dihydroxyphenyl-ethanol or hydroxytyrosol
3,4-dihydroxyphenyl-ethanol acetate or hydroxytyrosol acetate
3,4-dihydroxyphenyl-ethanol linked to elenolic acid
3,4-dihydroxyphenyl-ethanol linked to dialdehydic form of elenolic acid
electrospray ionization mass spectrometry
Flame Ionization Detector
High Performance Liquid Chromatography
HPLC-atmospheric pressure chemical ionization mass spectrometry
ion-trap mass spectrometry
limit of detection
tandem mass spectrometry
multiple-stage mass spectrometry
nuclear magnetic resonance
Oxidative Stability Instrument
p-hydroxyphenyl-ethanol or tyrosol
p-hydroxyphenyl-ethanol linked to elenolic acid
p-hydroxyphenyl-ethanol linked to dialdehydic form of elenolic acid
virgin olive oil
time of flight mass spectrometry
Total Peak Area Ratio
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