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Olive Oil Sensory Analysis
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This entry is adapted from the peer-reviewed paper 10.3390/plants11030257

In inland areas of Portugal and some regions of the Mediterranean basin, olive production is based on traditional olive groves, with low intensification, local cultivars, aged plants, and centenarian trees. These plants play a key role in the ecosystem, contributing to carbon sequestration and possessing a high genetic diversity, particularly important for selecting cultivars more resistant to climatic changes. Appreciation of the value of this genetic diversity implies genetic, morphological, and physicochemical characterization of centenarian trees, which is expensive and time-consuming. Sensory evaluation is also of utmost importance. 

Côa Valley descriptive sensory profile statistical tools differentiation

1. Introduction

The olive tree is one of the most ancient cultivated crops in the Mediterranean basin. This plant is well-adapted to this region, where around 90% of the world’s production of olive oil is concentrated [1]. Apart from the economic and social importance of the olive sector, in the Mediterranean region, olive groves provide important ecosystem benefits. This is especially true of traditional orchards, with well-adapted local olive cultivars and aged plants, some of them centenarian [2][3]. The benefits generated by traditional olive groves need to be evaluated in a holistic framework related to the production of raw materials (olives, leaves, and wood) as well as other social, geographical, and environmental aspects [4]. In today’s climate change conditions, the contribution to carbon sequestration could be considered one of the most important of the ecosystem benefits provided by traditional olive groves. Due to their long life cycle, permanent fruit trees, such as centenarian olive trees, potentially sequester a high amount of atmospheric carbon accumulated in their organs, namely the trunk, branches, and roots [5]. Another ecosystem benefit provided by centenarian olive trees is their high genetic diversity, which is also particularly important for possible selection and adaptation to changes in climate [6][7]. The genetic diversity of the olive tree is vast [8][9][10], with a large number of analyzed cultivars, although only a few have a broad and worldwide distribution. However, in some areas of the Mediterranean region, the olive germplasm is under-studied. The northeast of Portugal, the second-most important region in the country with 82,767 hectares of olive groves and a production of 117,343 tons of olives in 2018 [11], is known for the high quality of its olive products. The main olive cultivars are Cobrançosa, Madural, Verdeal Transmontana, Cordovil, Santulhana, and Negrinha de Freixo [2][12], although several other minor and less-distributed cultivars exist. Nevertheless, many centenarian olive tree specimens in the region belong to unknown or unanalyzed cultivars, and suffer a high risk of disappearing. For this reason, olive tree germplasm characterization is urgent. This biodiversity analysis can take into account several factors. A survey of the morphological characteristics of olive organs (olive leaves, flowers, fruits, and endocarps) is usually carried out in complement to some aspects of plant behavior, from which genetic markers are usually used for the identification and characterization of olive cultivars [13][14][15]. In other cases, the characterization focuses on different parameters of interest, such as the yield, resistance to pests and diseases or drought conditions, adaptation to mechanical harvest, or chemical quality of the olive oil [16][17][18][19]. When it is intended to value centenarian olive tree specimens, the search for differentiated olive oils with specific and desired chemical and sensory attributes is usually taken into account. For example, the search for specimens with high amounts of antioxidants, such as phenolic compounds and tocopherols [20][21], as well as exceptional sensory properties, have been explored [2]. Recently, some studies reported that genetic effects are the main source of variation for most olive oil constituents, leading to great variability in the composition of olive oils [22][23][24]. This aspect is correlated with the sensory profile of olive oils, assessment of which is mandatory according to European Union regulations for accurately establishing oil quality [25][26]. Thus, considering the relationship between the genetic component and sensory characteristics, the use of the sensory profile of olive oils together with statistical techniques can be seen as a practical and useful tool for identifying groups of plants (i.e., centenarian trees) with similar characteristics, reducing the number of unknown specimens that must be fully characterized. 

2. Olive Oil Sensory Analysis as a Tool to Preserve and Valorize the Heritage of Centenarian Olive Trees

From the 150 centenarian olive trees selected in the Côa Valley region (coded t1 to t150), olive oils were only extracted from the olives collected from each of 96 trees, from which a sufficient amount of olives could be harvested. Each oil was then analyzed, having verified that all of them fulfilled the legal thresholds [25] for extra virgin olive oil classification (free acidity lower than 0.8%, peroxide value lower than 20 mEq O2/kg, extinction coefficients at 232 and 268 nm lower than 0.22 and 2.50, respectively; data not shown). All oils were also evaluated by a sensory panel, establishing a descriptive sensory profile for each one. As shown in Table 1, the panelists perceived 32 positive sensations (13 olfactory attributes and 19 gustatory attributes), although several of them were only detected in a minority number (less than 50%) of the oils evaluated (e.g., olfactory: banana, cherry, plum, rosemary, lavender, and tomato leaves; gustatory: banana, kiwi, cherry, apricot, strawberry, plum, olive leaves, rosemary, and lavender). It should be noted that the perceived sensations, as well as the intensity ranges found, are, in general, in agreement with those reported in the literature for Moroccan and Tunisian olive oils [27][28], as well as for Portuguese oils extracted from minor cultivars of centenarian olive trees [2][20]. It has been reported that olive cultivar and genetic factors influence the sensory profile of extracted olive oils [28][29][30].
Table 1. Olfactory and gustatory sensations perceived by the sensory panel in the 96 olive oils evaluated: sensation perceived, percentage of oils for which the sensation was perceived, minimum–maximum average intensity perceived, and related average (minimum–maximum) robust coefficient of variation (CVr%).
Table
Sensation Percentage of Oils with Perceived Sensation Minimum–Maximum Average Intensities Average (Minimum–Maximum) CVr%
Olfactory sensations      
Greenly fruity 100% 1.3–7.6 4.7 (0.0–17.2)
Apple 100% 3.2–7.0 3.3 (0.0–17.1)
Banana 38% 2.4–7.5 3.5 (0.0–12.2)
Tomato 98% 2.4–7.3 3.8 (0.0–15.2)
Dry fruits 100% 1.1–4.1 4.7 (0.0–17.6)
Cherry 4% 1.8–4.1 4.9 (1.9–10.7)
Plum 6% 1.8–3.7 7.0 (3.9–14.5)
Cabbage 56% 2.4–7.7 4.0 (0.0–14.2)
Fresh grass 100% 2.1–5.7 4.1 (0.0–15.1)
Rosemary 9% 2.0–5.8 3.4 (0.6–6.0)
Lavender 6% 2.1–3.8 7.0 (3.9–14.0)
Tomato leaves 44% 2.2–6.2 4.9 (0.0–15.7)
Gustatory sensations      
Sweet 100% 0.7–8.1 4.3 (0.0–14.9)
Bitter 100% 1.7–6.5 3.8 (0.0–17.9)
Pungent 100% 3.0–7.4 3.0 (0.0–11.1)
Greenly fruity 100% 1.8–7.4 3.7 (0.0–15.4)
Apple 100% 3.0–6.5 3.0 (0.0–11.6)
Banana 46% 2.5–7.6 4.1 (0.0–14.0)
Tomato 97% 1.5–7.3 3.8 (0.0–18.0)
Dry fruit 99% 1.0–5.4 4.8 (0.0–14.1)
Kiwi 8% 2.9–5.2 6.2 (0.9–18.1)
Cherry 21% 2.0–4.9 5.5 (0.0–18.5)
Apricot 6% 1.5–2.9 8.3 (2.3–17.5)
Strawberry 7% 3.0–4.4 3.3 (0.0–9.1)
Plum 17% 2.3–5.2 5.1 (2.2–12.3)
Cabbage 64% 2.6–7.2 3.2 (0.0–14.5)
Fresh grass 100% 2.2–6.6 3.9 (0.0–19.4)
Olive leaves 4% 2.0–4.1 5.5 (1.6–14.5)
Rosemary 10% 2.1–6.1 5.0 (0.6–11.4)
Lavender 8% 2.5–4.4 7.1 (1.0–10.5)
Tomato leaves 51% 1.8–6.7 4.9 (0.0–14.0)
As can be inferred from Table 1, the average CVr% for each perceived olfactory or gustatory attribute varied between 3.0% and 8.3%, with maximum values lower than 20%, which is the International Olive Council (IOC) threshold, confirming the evaluation skills of the trained panelists. The variability found in the sensory profiles of the 96 olive oils extracted from olives harvested from centenarian trees, as well as the wide range of intensities perceived by the panelists for each detected sensation, allowed the expectation that the oils, and thus the respective olive trees, could be clustered into different groups with a similar sensory pattern. The dendrogram obtained by hierarchical clustering analysis confirmed the possibility of splitting the 96 olive oils into different clusters/groups based on the dissimilarities found in the multi-dimensional sensory data established by the panelists (Figure 1). The dendrogram obtained using the sensory profiles (Figure 1) split the olive oils and, thus, the respective centenarian olive trees, into five main clusters (G1 to G5), with a Euclidean distance ranging from 0 to 25. Cluster G1 contained 30 olive oils/olive trees, G2 contained 20 olive oils, G3 contained 12 olive oils, G4 contained 21 olive oils, and G5 contained the other 13 olive oils. It should be noticed that all five clusters consisted of several subclusters, pointing out the variability in the sensory profiles of the studied olive oils.
Plants 11 00257 g001 550
Figure 1. Dendrogram with the identification of five clusters/groups (G1, G2, G3, G4 and G5), for a Euclidean distance from 0 to 25, based on the dissimilarities of the sensory profiles of oils obtained from centenarian olive trees grown in the Côa Valley region.
To further understand the sensory patterns of each of the abovementioned five clusters/groups of olive oils/olive trees, boxplots and one-way ANOVA were used to compare the olfactory (Figure 2) or gustatory (Figure 3 and Figure 4) sensations perceived among the five established groups of oils by the sensory panel. From those figures, it can be inferred that oils clustered in G1 showed high olfactory and gustatory intensities of greenly fruity sensations, with intense notes of tomato and cabbage olfactory–gustatory sensations in addition to high olfactory intensities of tomato leaves, possessing high bitter and pungent sensations and low sweetness. Oils belonging to cluster G2 were distinguished from the previous ones mainly due to the higher olfactory–gustatory intensity of banana, and the perceived gustatory fruit notes of cherry, apricot, plum, and tomato leaves attributes rather similar to the trends of the oils from G1. In contrast, oils from cluster G3 showed a lower olfactory–gustatory intensity of greenly fruity sensations, with lower bitterness and pungency, and a markedly higher sweetness; the perceived rosemary and lavender olfactory–gustatory intensities were probably responsible for the unique sensory pattern of these oils. Olive oils grouped within cluster G4 also possessed lower olfactory–gustatory intensities of greenly fruity as well as tomato sensations, showing a lower bitterness and pungency with high sweetness; the near absence of fruit and herbaceous notes was most likely responsible for their unique sensory fingerprint. Finally, oils from cluster G5, which from an overall sensory pattern were quite similar to those from clusters G1 and G2, can be distinguished from the other oils due to the slightly higher intensity of olfactory–gustatory sensations of fresh grass and lower olfactory–gustatory intensities of cabbage sensations. Interestingly, fresh grass sensations have been described as a characteristic of olive oils extracted from olives from olive tree cultivars grown in northeast Portugal [2]. These findings clearly pointed out that the sensory profiles of olive oils from centenarian olive trees, together with hierarchical clustering analysis, may be used as a practical fingerprint approach to identify centenarian trees that would produce oils with unique sensory patterns, contributing to an appreciation of their value and protection of their exceptional and intrinsic genetic diversity.
Plants 11 00257 g002 550
Figure 2. Sensory profile boxplots (olfactory sensations) found in olive oils extracted from olives from centenarian olive trees in the Côa Valley region. Different lowercase letters signify statistically significant differences at a significance level of 5% (one-way ANOVA followed by Tukey’s multi-comparison test).
Plants 11 00257 g003 550
Figure 3. Sensory profile boxplots (bitter, pungent, and sweet intensity) found in olive oils extracted from olives from centenarian olive trees in the Côa Valley region. Different lowercase letters signify statistically significant differences at a significance level of 5% (one-way ANOVA followed by Tukey’s multi-comparison test).
Plants 11 00257 g004 550
Figure 4. Sensory profile boxplots (gustatory sensations) found in olive oils extracted from olives from centenarian olive trees in the Côa Valley region. Different lowercase letters signify statistically significant differences at a 5% significance level (one-way ANOVA followed by Tukey’s multiple comparison test).

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