Proposed parameters for monitoring quality of virgin olive oil (Koroneiki cv)

The overall quality of virgin olive oil (VOO) is closely related to its oxidative stability that is usually evaluated through the stability index measured by the Rancimat apparatus. Quality characteristics and also pro‐oxidant and antioxidant content for 52 Greek VOO samples (Koroneiki cv) were used to build up a model capable of predicting stability. Collinearity diagnostics, variable selection, and regression analysis were applied to the experimental data to locate the contribution of each parameter to the keeping quality of the samples. The predictive ability of the model was confirmed for a second VOO ample set of the same cultivar. It was found that except for the peroxide value, which negatively influences the stability, other important parameters were α‐tocopherol, total polar phenol and total chlorophyll content. It is concluded that the colorimetric determination of total polar phenols, the spectrometric determination of total chlorophylls and the high‐performance liquid chromatography analysis of α‐tocopherol, not presently included in the established methods of official analysis, can be used for a better evaluation of VOO quality. These parameters, which can be easily adopted as routine methods by the industry, seem to be of utmost importance for shelf life prediction and expiration dating if applied for the promotion of the most competitive products in the international olive oil market.     

Use of nitrogen to improve stability of virgin olive oil during storage

Experiments were carried out to study the possibility of improving the stability of extra virgin olive oil by using nitrogen as a conditioner gas during storage. With this aim, virgin olive oil samples, obtained from Leccino and Coratina cultivars, were stored in the dark, in closed bottles conditioned with air or nitrogen at 12–20 and 40°C. Results indicated that the FFA percentage increased over 1% only when oils were stored at 40°C. The PV and the K ₂₃₂ value (light absorbance at 232 nm) of oils increased over the limit value allowed by European Union law when the bottles were only partly filled and air was the conditioner gas. The use of nitrogen as conditioner gas helped to avoid this risk during 24 mon of storage at 12–20°C. The total phenolic content of both cultivars oils decreased during storage because their oxidation protected the oils from autoxidation. The content of total volatile compounds in oils decreased continuously during storage at 12–20°C, whereas it increased over 10 (Coratina cv.) and 15 (Leccino cv.) mon and then diminished when the storage temperature was 40°C. The same behavior, i.e., increase then decrease, was ascertained for trans-2-hexenal. The hexanal content of oils increased continuously during storage because this compound is formed by the decomposition of the 13-hydroperoxide of linoleic acid.     

High-performance liquid chromatography evaluation of phenols in virgin olive oil during extraction at laboratory and industrial scale

Phenolic compounds are of fundamental importance to the quality and nutritional properties of virgin olive oils. In this paper, the high-performance liquid chromatographic analysis of simple and complex olive oil phenols in the streams generated in the two-phase extraction system was carried out using Arbequina and Picual olives. The malaxation stage reduced the concentration of orthodiphenols in oil ca 50–70%, while the concentration of the nonorthodiphenols remained constant, particularly the recently identified lignans 1-acetoxypinoresinol and pinoresinol. Oxidation of orthodiphenols at laboratory scale was avoided by malaxing the paste under a nitrogen atmosphere. Phenolic compounds in the wash water used in the vertical centrifuge were also identified. Hydroxytyrosol, tyrosol, the dialdehydic form of elenolic acid linked to hydroxytyrosol were the most representative phenols in these waters. Hence, phenolic compounds in the wash waters came from both the aqueous and the lipid phases of the decanter oily must.     

Contribution of chemical components of cornicabra virgin olive oils to oxidative stability. A study of three successive crop seasons

Oxidation is the primary cause of virgin olive oil quality deterioration. This paper presents a correlation between oxidative stability, as determined by the Rancimat method, and some chemical components involved in the oxidation process of a set of 74 Cornicabra virgin olive oils obtained from three successive crop seasons (94/95 to 96/97). Results showed a clear influence of total polyphenols on virgin olive oil stability, with linear regression coefficients which were similar for the three seasons studied, and a much lower contribution of α-tocopherol and unsaturated fatty acids, mainly linoleic acid. A significant effect dependent on the crop season was also observed.     

Automated ultrasound‐assisted method for the determination of the oxidative stability of virgin olive oil

A fast and automated method is proposed for determining the oxidative stability of virgin olive oil by using ultrasound. The ultrasound microprobe (3 mm in diameter) was directly immersed into the olive oil sample contained in a test tube. The most influential variables in the oxidation process, namely pulse amplitude, duty cycle, irradiation time, and sample amount, were optimized. The oil absorbance at 270 nm was continuously monitored by oil recirculation through a 0.1‐mm path length flow cell connected to a fiber optic microspectrometer. This short path length allowed the direct monitoring of absorbance without needing any sample dilution. The ultrasound energy was applied during 35 min, and the resulting increase in absorbance was continuously monitored. The difference between the final and the initial absorbance at 270 nm of a set of virgin olive oil samples was closely correlated with their oxidative stability calculated by the Rancimat method (R² = 0.9915). The resulting equation enabled the prediction of the oxidative stability of virgin olive oil in a short period of time (35 min), by using a simple, inexpensive, automatic and easy‐to‐use system.     

Evaluation of the bitter taste in virgin olive oil

An analytical method has been developed to evaluate the intensity of the bitter taste in virgin olive oil. Results from the proposed method, based on extraction of the bitter constituents of virgin olive oil with methanol/water and measurement of the absorbance at 225 nm, show a significant correlation with the intensity of bitterness that had been evaluated in a sensorial manner by a panel. The developed method, therefore, offers a real alternative to the panel test for the evaluation of this attribute.     

Antioxidant effect of natural phenols on olive oil

The total polar fraction and individual phenols present in virgin olive oil were tested for their antioxidant effect in refined olive oil. Hydroxytyrosol and caffeic acid showed protection factors greater than BHT. Protocatechuic and syringic acid were also found to have antioxidant activity. Tyrosol, p-hydroxyphenylacetic acid, o-coumaric acid, p-coumaric acid, p-hydroxybenzoic acid and vanillic acid had very little or no effect, and their contribution to the stability of the oil is negligible.     

Stability to oxidation of virgin olive oils as related to olive conditions: Study of polar compounds by chemometric methods

Polar compounds of virgin olive oils were analyzed. They influence oil flavor and aroma and improve the shelf-life of the oil. The orthodiphenolic fraction is particularly significant for oil stability because of its antioxidative activity. A relationship between the composition of the whole fraction of polar compounds and the state of health of the olives was established. For this purpose, oil samples were obtained from olives that had reached different degrees of ripeness and that had been affected by Dacus oleae infestation differently. The polar compounds were then analyzed by high-performance liquid chromatography. The data set was studied by means of chemometric methods. Partial least squares regression was used to obtain models that show a significant correlation between composition of the oil’s polar compounds and conditions of the olives sampled. In particular, compounds with antioxidative activity were directly linked with the state of health of the olives. The models obtained allow tracing of the state of health of the olives sampled through analysis of the polar fraction of virgin olive oil with a high degree of accuracy, and thus prediction of the oil’s expected shelf life.     

Evaluation of virgin olive oil bitterness by quantification of secoiridoid derivatives

The bitterness of the main compounds identified in the phenolic extract of virgin olive (Olea europaea L.) oils has been sensory-tested. The aldehydic form of oleuropein aglycone (AOA) was responsible for this attribute. Correlations between the sensory bitterness and concentrations of secoiridoid derivatives, analyzed separately or in different combinations, were obtained for olive oils from different olive varieties. The best correlation obtained corresponds to AOA content (r=0.96; P=1.83×10⁻¹⁷) in the concentration range of 0.03 to 0.5 mmol/kg. AOA concentrations ≥0.5 mmol/kg produce sensory saturation of this attribute. The correlation with AOA concentration was better than that with the absorbance of the phenolic extract at 225 nm. Therefore, the equation obtained allows the evaluation of the bitterness in virgin olive oils by HPLC analysis of the phenolic extract using detection at 280 nm.     

Hydrophilic antioxidants of virgin olive oil. Part 1: Hydrophilic phenols: A key factor for virgin olive oil quality

Virgin olive oil (VOO) consumption is increasing all over the world due to its excellent organoleptic and nutraceutical properties. These beneficial traits stand from a prominent and well‐balanced chemical composition, which is a blend of major (98% of total oil weight) and minor compounds including antioxidants. The main antioxidants are phenolic compounds, which can be divided into lipophilic and hydrophilic phenols. While lipophilic phenols such as tocopherols can be found in other vegetable oils, most hydrophilic phenols in olive oil are exclusive of the Olea europaea species endowing it with a chemotaxonomic interest. This review is focused on VOO antioxidant profile and, particularly, on hydrophilic phenols that are divided into different sub‐families such as phenolic acids and alcohols, hydroxy‐isochromans, flavonoids, secoiridoids, lignans and pigments. Analytical methods for qualitative and/or quantitative determination of these compounds are assessed. The implementation of efficient sample preparation protocols, separation techniques such as liquid chromatography, GC and capillary electrophoresis, as well as detection techniques such as ultraviolet absorption, fluorescence or MS are critical to succeed in the quality of the results. The effects of hydrophilic phenols on increasing VOO stability, its nutraceutical interest and organoleptic properties are also considered.     

Air exposure time of olive pastes during the extraction process and phenolic and volatile composition of virgin olive oil

The time of exposure of olive pastes to air contact (TEOPAC) during malaxation was studied as a processing parameter that could be used to control endogenous oxidoreductases, such as polyphenoloxidase, peroxidase, and lipoxygenase, which affect virgin olive oil quality. Phenolic and volatile compounds were analyzed in the oils obtained using progressive TEOPAC at three ripening stages of olives. Multivariate statistical analysis was applied to the raw data. The phenolic concentration of virgin olive oil progressively decreased with increasing IEOPAC. On the contrary, a positive relationship was found with the concentration of several volatile compounds responsible for virgin olive oil aroma. The effect of TEOPAC, however, was strictly related to fruit ripening.     

Antioxidant activity of tocopherols and phenolic compounds of virgin olive oil

The antioxidant effects of hydrophilic phenols and tocopherols on the oxidative stability in virgin olive oils and in purified olive oil have been evaluated. Total hydrophilic phenols and the oleosidic forms of 3,4-dihydroxyphenolethanol (3,4-DHPEA) were correlated (r=0.97) with the oxidative stability of virgin olive oil. On the contrary, tocopherols showed low correlation (r=0.05). Purified olive oil with the dialdehydic form of elenolic acid linked to 3,4-DHPEA, an isomer of oleuropeine aglycon, and 3,4-DHPEA had good oxidative stability. A synergistic effect was observed in the mixture of 3,4-DHPEA and its oleosidic forms with α-tocopherol in purified olive oil by the Rancimat method at 120°C.     

Influence of ecological cultivation on virgin olive oil quality

The quality of oil extracted from ecologically cultivated olives of the Picual variety was compared with oil extracted from Picual olives cultivated using conventional methods. Olive trees were grown in a two-section plot. Fruits from each plot were harvested at various stages of ripeness, and acidity value, peroxide index, ultraviolet absorption at 232 and 270 nm, stability to oxidation, sensory analysis, fatty composition, and contents of tocopherols, phenolic compounds, and sterols were determined on oil extracted from each treatment. The results showed that the organic virgin olive oil was of a superior quality to the conventional virgin olive oil in all the quality parameters analyzed.     

High-performance liquid chromatography evaluation of phenols in olive fruit, virgin olive oil, vegetation waters, and pomace and 1D- and 2D-nuclear magnetic resonance characterization

Phenolic compounds are the most important antioxidants of virgin olive oil. This paper reports on the application of solid phase extraction (SPE) in the separation of phenolic compounds from olive fruit, olive oil, and by-products of the mechanical extraction of the oil and the complete spectroscopic characterization by nuclear magnetic resonance of demethyloleuropein and verbascoside extracted from olive fruit. SPE led to a higher recovery of phenolic compounds from olives than did liquid/liquid extraction. SPE also was used to separate phenolic compounds from pomaces and vegetation waters. Phenylacid and phenyl-alcohol concentrations in extracts obtained from SPE and liquid/liquid extraction were not significantly different (P<0.05). The recovery of the dialdehydic form of elenolic acid linked to 3,4-(dihydroxyphenyl)ethanol and an isomer of oleuropein aglycon, however, was low.     

Characterization of olive paste volatiles to predict the sensory quality of virgin olive oil

One of the main challenges that virgin olive oil producers face today is an accurate prediction of the sensory quality of the final product prior to the milling of the olives. The possibility that olive paste aroma can be used as a predictive measurement of virgin olive oil quality is studied in this paper. The study was centered on distinguishing the aroma of olive pastes that produced virgin olive oils without sensory defects from the aroma of olive pastes the virgin olive oils of which showed sensory defects. Olive pastes were analyzed by solid‐phase microextraction‐gas chromatography and a sensor system based on metal oxide sensors. Forty‐four volatile compounds were identified in olive pastes, all of them being also present in virgin olive oil. Six volatile compounds – acetic acid, octane, methyl benzene, (E)‐2‐hexenal, hexyl acetate and 3‐methyl‐1‐butanol – distinguished both kinds of pastes with only five misclassified samples. Five metal oxide sensors were able to classify the olive pastes with only two erroneous classifications.     

Influence of seasonal conditions on the composition and quality parameters of monovarietal virgin olive oils

The aim of this work was to determine the effect of the climatological conditions of the olive crop season on the composition of monovarietal virgin olive oils obtained from the Arbequina cultivar with special emphasis on the phenolic fraction, its percent distribution, and related oil quality parameters such as oxidative stability and bitter index. The main differences were due to freeze injuries caused by low temperatures in December 2001. The levels of chlorophylls and carotenoids in olive oil or pulp from frost-damaged olive trees were lower as a consequence of faster ripening. The olive oil extracted from frost-damaged olive pulp had lower contents of secoiridoid and especially lower levels of 3,4-DHPEA-EDA (the dialdehydic form of elenolic acid linked to hydroxytyrosol). In the following crop seasons, a significant increase in phenolic compounds, especially in secoiridoid derivatives such as 3,4-DHPEA-EDA, was observed. This increase may be due to the fact that olive trees that suffered frost damage in December 2001 were more sensitive to stress caused by the water deficit during summer in the subsequent crop seasons, which is usual in this olive-growing region. Moreover, important correlation coefficients were observed between the main secoiridoid derivative compound (3,4-DHPEA-EDA) and oxidative stability and the bitter index.     

Low molecular weight polypeptides in virgin and refined olive oils

Twenty-eight virgin olive oils—from different regions of Spain and prepared from olive drupes of different varieties—and six refined olive oils were analyzed to determine the presence of proteins in these oils. All oils studied showed the presence of proteins in the range of 7–51 μ/100 g of oil. There were no significant differences in protein content in oils from different varieties or between virgin or refined oils. In addition, all oils exhibited analogous amino acid patterns, suggesting a similarity among protein fractions obtained from different oils. A polypeptide with an apparent M.W. of 4600 Da was common to the isolated protein fractions. These results suggest that this polypeptide is a previously unknown minor component in olive oils. No clear influence of this component on oil stability was observed when oil stabilities were estimated as a function of phenol, tocopherol, phosphorus, and protein contents of the oils.     

Evolution of phenolic compounds in virgin olive oil during storage

Phenolic compounds are of fundamental importance to the shelf life of virgin olive oils because of their antioxidative properties. In this paper, the evolution of simple and complex olive oil phenols during 18 mon of storage is studied by high-performance liquid chromatography (HPLC) analysis. The olive oils under examination were from various olive cultivars, harvested in two sectors in the same region at different stages of ripeness. The findings indicate that it is not the variety but rather the ripeness of the olives and the soil and climate that influence the phenol composition of virgin olive oil. In addition, a positive correlation was found between the age of the oils and the tyrosol to total phenols ratio. Lastly, gas chromatography-mass spectrometry analysis confirmed that the unidentified peaks detected by HPLC were of a phenolic nature.     

Polyphenol oxidase from eggplant reduces the content of phenols and oxidative stability of olive oil

Activity of the polyphenol oxidase (PPO) from eggplant fruit (Solanum melongena L.) on phenolic compounds of an extra virgin olive oil (EVOO) was studied. In standardized reaction solutions, the eggplant PPO, isolated in the laboratory, depleted completely chlorogenic and caffeic acids, oleuropein, and verbascoside, while the levels of hydroxytyrosol reduced by half. Conversely, no activity of the PPO was observed on the gallic and protocatechuic acids nor on mono‐phenols, such as tyrosol and the p‐coumaric, o‐coumaric, and ferulic acids. PPO activity on phenols extracted from eggplant fruit and EVOO confirmed the enzyme substrate specificity and caused a significant decrease in the measure of total phenols and o‐diphenols. Similarly, PPO crude extract caused a significant decrease of polyphenols directly in the EVOO. Moreover, maximum degradation of EVOO polyphenols was observed when olive oil was homogenized with eggplant fruit pulp to form a cream‐like purée. In fact, immediately after the preparation, total phenols and o‐diphenols of the olive oil recovered from the eggplant‐oil purée were decreased by ～80% and 100% compared to those of the initial EVOO. As a consequence, the oxidative stability of the recovered oil was ～60% lower than that of the initial EVOO. In conclusion, in the preparation of vegetable preserves, a residual activity of phenol oxidase may adversely affect the quality and shelf life of the extra virgin olive oil used as covering.     

Changes in the main components and quality indices of virgin olive oil during oxidation

This work reports on changes in the major and minor components of virgin olive oil during oxidation, details modifications found in the standardized quality indices, and analyzes the most significant relationships between the components of the oil and its oxidative stability. During the induction period or slow phase of oxidation, polyphenols, tocopherols, and pigments undergo the most important alterations. Other compounds, such as FA or volatiles, suffer significant modifications only during the rapid or exponential phase of oxidation when the natural antioxidant systems fall to minimal values. Among the quality indices, PV and the specific extinction coefficients K ₂₃₂ and K ₂₇₀ increase markedly from the earliest stages of oxidation, whereas titratable acidity does not change appreciably during the induction period. The evolution of the different compounds and parameters analyzed suggests that the tocopherol and orthodiphenol contents are the best indices to determine the average life of the oils.