Also in this case, root mean squares errors in leave one out cross (RMSECV-LOO) and in leave one producer out (RMSEP-LOP) validations were investigated.
The presence of some individual samples and/or variables, mainly influential for a given model, was evaluated through the calculation of Leverage [69]. Finally, the residual structure of Y-array was analyzed by inspecting the sum of squares of residuals (SSres) plot for each Y-mode.
The model was built with one latent variable (Y explained variance in cross validation, LOO, 84.38%). Analogously to the unfold-PLS case, the robustness and the predictive capability of the model was tested by Leaving One Producer Out (RMSEP-LOP) procedure. In all the six cases, the models built with one latent variable gave the best results in terms of lowest root mean squares cross-validation error. Considering the RMSEP-LOP values for each sensory parameter for the different models (data not reported), they have the same trend of the respective unfolding analysis
In order to get an overview of the variables, which mainly influence the prediction ability of the regression model, the loading matrix, corresponding to the second mode, was considered, back-transforming it into the original domain by multiplying it with the variable loading matrix of PCA decompression. In this way, it was possible to represent the loadings plot for GC-variable mode as function of the retention time. The same volatile compounds of the PLS case resulted to be relevant for predicting the sensorial parameters.
Leverage values for mode 1 and 3, ha and hc, and respective residual sum of squares (SSres), Figure 14, allow to gain combined information about the vinegar samples and/or producers which are well modeled (small SSres) and at the same time yield a unique contribution (high ha or hc).
PLEASE, INSERT FIGURE 14a, 14b and 14c HERE
In particular, from Figure 14a and 14c, 3rd, 4th and 5th producers have the largest ha and small residuals, hence contribute positively to the model, whilst the 3rd producer is influential but not well fitted. As far as the sample mode is concerned, Figure 14b and 14d, the youngest samples (number 6) is the most influential but it is not well fitted (high residuals values). On the other side, older samples (numbers 1 and 2) are not so influential in modelling the sensorial data but they are well fitted.
Summary
Both unfolded and three-dimensional regression models gave satisfactory predictions of almost all the sensorial parameters, which characterised the quality of ABTM samples. In particular, the aromatic parameters are better modelled, probably due to nature of the employed analytical data, i.e.
volatile organic compounds.
Furthermore, both models highlighted a main contribution on the prediction ability given by chemical compounds characteristic of the fermentation and the bio-oxidation processes which take place in the younger casks (samples 5 and 6), such as ethanol, ethyl acetate, and acetic acid, on the other hand by compounds which are produced during the must cooking procedure (furfural).
The relative efficiency of both unfold-PLS and N-PLS has been compared in order to understand
model. The use of N-PLS model led to a more parsimonious (1 LV) model and showed, in general, lower RMSECV values and a higher value of explained variance in prediction.
Notwithstanding, it has to be remarked that both methods allows a straightforward interpretation of the whole data set and about interrelations between sensorial and chemical parameters.
Characterisation and classification of Ligurian extra virgin olive oil
Introduction
Extra Virgin Olive Oil (EVOO) belongs to the superior olive oil category and it is solely obtained from fruit of olive tree, Olea Europera, by mechanical means.
Although the production of olive oil is concentrated in the Mediterranean countries area, the cultivation of olive trees is spreading in other many countries. The increasing consumption of EVOO is related to its peculiar properties, like seasoning of food as well as to its healthy benefits.
Nevertheless, as result of agricultural traditions, local extraction and blending practices, EVOO may be quite different in taste and quality depending on its geographical origin with consequent differences in price within the same category. In the context of developing an analytical methodology able to assess the quality and authenticity of EVOO samples [20,70,71,72], this study was focused on the characterisation of the whole aroma fraction and on the development of analytical tools able to distinguish EVOO samples coming from ‘Riviera Ligure-Riviera dei fiori’
(Northern of Italy) [73] from other Mediterranean ones. Liguria EVOO, in which cultivar
‘Taggiasca’ prevails, was designed with PDO certification and represents one of the most highly esteemed and valuable European EVOO, since it is characterised by delicate, sweet, slightly pungent and green aroma. In particular, aroma is one of the most important parameters in the estimation of the quality of an EVOO sample, hence a great number of researches has been done to evaluate inter and intra relationships between the sensory notes and the concentration of the organic compounds.
In this framework, the analysis of the volatile fraction was performed using Head Space Solid Phase Micro Extraction (HS-SPME) [67] coupled with GC-MS system for the extraction and chromatographic separation and the identification of volatile organic compounds. The obtained GC-MS signals (Total Ion Current) were processed by SIMCA analysis.
EVOO samples with certified geographical origin, object of this study, were obtained from Consortium. They belong to different olive cultivars and come from different geographical areas, namely Liguria (Northern of Italy), Apulia (Southern of Italy), Greece, Tunisia and Spain. Since the main commercial interest is to distinguish Liguria EVOOs from the rest of olive oils, the data set was split in two classes, Liguria and Not Liguria oils. SIMCA analysis was applied in order to build a classification model able to rightly classify the samples belonging to a category and correctly to reject the other ones. Furthermore, as second step, the data set was divided in three classes too, namely Liguria, Apulia and Foreign samples.
Extra virgin olive oil samples
Seventy-two EVOO samples, produced from olives of different cultivars and harvested in 2003, were analysed by means of HS-SPME/GC-MS technique. They came from five different Mediterranean countries. In particular, 22 samples were from Apulia (mainly Ogliarola and Coratina cultivars), 21 from Liguria (mainly Taggiasca cultivar), 12 from Greece (Koroneika and Athinoia cultivars), 10 from Spain (Arbequina cultivar) and 7 samples from Tunisia (Chemlali cultivar).
The instrumental signals were arranged in bi-dimensional matrix with as many rows as samples (72) and as many columns as GC-points (1680) recorded during data acquisition (retention time, Rt: 72 min).
Before data analysis, the first 4.5 minutes and the last 7 ones of the signals were cut because there were no peaks at all, giving a GC vector of 1404 points for each sample.
In Figure 15, the average of the EVOO chromatograms for each of the five different geographical proveniences was reported.
PLEASE, INSERT FIGURE 15 HERE
In general, extracted volatile fraction includes a large number of hydrocarbons, aldehydes, alcohols, ketones, esters and other minor compounds. It is possible to observe that the average Liguria signal reports the lowest intensities of the different chromatographic peaks, followed by the Spanish one, perfectly in agreement with their much more delicate, sweet and slightly astringent flavour with respect to the other ones. In particular, the chromatographic peaks, which seem to characterise the volatile fraction of Liguria olive oil, are trans-2-hexenal (Rt: 34-34.5 min), and hexanal (Rt: 28-29 min), both C6 linear unsaturated aldehydes characteristics of high quality virgin olive oils.
As far as classification analysis is concerned, firstly, the 72 samples were split into two categories, Liguria and not Liguria (NL) samples, aiming to find a predictive classification rule to discriminate Liguria samples. Then, the samples were divided into three categories: Liguria, Apulia and Foreign (Greece, Spain and Tunisia) for investigating which category could mainly ‘overlapping’ with Liguria one.
The different data sets were separately and randomly split into training (for building a calibration model) and test sets (for validating it) as schematised in Table 11.
PLEASE, INSERT TABLE 11 HERE
The data matrices were separately mean centered and a model was built for each class (Liguria and Not Liguria and Liguria, Apulia, Foreign). The number of principal components (PCs) was chosen according to the best compromise among the minimum root mean square error in cross validation (RMSECV), sensibility and specificity of each model.
As first step, SIMCA was used to perform the classification among Liguria and Not Liguria classes.
Twelve PCs were chosen for Not-Liguria class model, while three PCs for the Liguria one. The SIMCA results reported in Table 12 highlight an excellent sensibility, meaning a right classification of both training and test sets, as well as an excellent specificity for Liguria samples.
PLEASE, INSERT TABLE 12 HERE
As concern Not Liguria class, almost all training set (except three samples) is well modelled.
However, considering the test set samples, the sensibility of the model fairly decreases (68%). Eight Liguria samples fall inside the confidence limits of the model giving a very low specificity value (43%).
Afterwards, in the second step, as previously explained, samples were divided in three categories, Liguria, Apulia and Foreign (Greece, Spain and Tunisia). Internal and external cross validation were done following the same procedure of the previous case (random group and test set samples).
Three PCs were retained for both Liguria and Apulia classes, while seven PCs gave the best results for the Foreign model. As far as Liguria class model is concerned, the results were unchanged with respect to the previous case. Indeed, all the training and test samples were in the confidence limits of the model (sensibility of 100%) and all the external samples, belonging to Apulia and Foreign classes, were rightly rejected (specificity of 100%). Furthermore, an improvement was evident in the performance of Apulia and Foreign classes, as well. In particular, Apulia class model was totally able to model both training and test set, showing a sensibility of 100%. Six Foreign samples and no
Liguria ones fall inside the Apulia model confidence limits, giving a relatively high specificity (83%).
Finally, as regards Foreign class, all the training set was inside the confidence limits and only two samples, belonging to test set, were outside (sensibility: 83%). At the same time, the model became more specific with respect to the previous step, since only four Liguria samples were not rejected by Foreign class model (specificity: 91%).
Summary
The aim to discriminate Liguria class from the other ones was successfully accomplished, showing as the use of HS-SPME/GC-MS technique coupled with chemometrics analysis can be an useful tool for the solution of such challenging classification problem in the control of food quality research.
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Tables
Table 1. Summary of all the variables used for the selection of the representative farms for the two investigated areas.
Table 2. Scheme of available concentrated must samples, listed on the basis of their geographical origin and variety.
Concentrated musts
Geographic origin white red extra-red rosé
Emilia Romagna 17 15 14
Apulia 3 5 1
Argentina 3
Spain 7 2
Total 67
Table 3. SIMCA model sensitivity and specificity values for both training and test sets for Emilia-Romagna class, based on NIR signals.
Class LVs Explained variance (%)
SENSITIVITY (%)
SPECIFICITY (%)
Emilia Romagna 10 99.76 Training set 100 --
Test set 53 62
Table 4. SIMCA model sensitivity and specificity values for both training and test sets for Emilia-Romagna class after WPTER variable selection, based on NIR signals
Class LVs Explained variance
(%) SENSITIVITY
(%) SPECIFICITY
(%)
Emilia Romagna Training set 100 --
Test set 71 72
Table 5. PLS-DA model sensitivity and specificity values for both training and test sets for Emilia-Romagna class, based on NIR signals.
Class LVs Explained variance
(%) SENSITIVITY
(%) SPECIFICITY
(%)
Emilia Romagna 10 99.52 Training set 93 93
Test set 60 71
Table 6. SIMCA model sensitivity and specificity values for both training and test sets for Emilia-Romagna class, based on MIR signals.
Class LVs Explained variance (%)
SENSITIVITY (%)
SPECIFICITY (%)
Emilia Romagna 8 99.49 Training set 100 --
Test set 70 72
Table 7. PLS-DA model sensitivity and specificity values for both training and test sets for
Table 7. PLS-DA model sensitivity and specificity values for both training and test sets for