Using optical NIR sensor for on-line virgin olive oils characterization

https://doi.org/10.1016/j.snb.2004.11.103Get rights and content

Abstract

Near-infrared transmittance spectroscopy was applied to on-line control quality and characterization of virgin olive oils. The transmittance spectrum (log 1/T) was obtained through dispersal equipment, scanning the samples between 750 and 2500. A 1 mm optical path length flow cell and holding 120 μl was employed.

Calibration models for: acidity value (AV), bitter taste (k225) and fatty acid composition (FAME), were previously developed in the laboratory using partial least squares (PLS) regression. A total of 190 virgin olive oil samples, gathered during the usual harvesting time of three olive crop seasons, were used. The validation set gave correlation coefficients and standard error of prediction of 0.999 and 0.35, 0.936 and 0.058, 0.998 and 0.604, 0.992 and 0.674% for AV, k225, fatty acid oleic and fatty acid linoleic, respectively.

These PLS models, prior to a slope/bias correction, were used to monitor on-line values of these parameters during the processing of virgin olive oils in real olive mills. Samples of oils were obtained for chemical analysis.

Application of NIR transmittance spectroscopy on-line to virgin olive oil production line, has allowed to detect, at real-time, the changes produced in the characteristics of the oils during their production. The results indicate similarity between information both NIR and reference laboratory methods.

Introduction

The virgin olive oil is obtained from olive fruits, Olea europaea L., by extraction using only physical methods: crushing of olives in a hammer crusher, mixing of olive paste in a mixer and separation of oil using presses or centrifuges. Before storage, the oil obtained is percolated or centrifuged for clarification. Depending on the state of the fruit and on the conditions of production, the obtained oil can be classified under different categories of quality [1].

In virgin olive oil extraction process, characterization and quality control of olive oil for its classification and separation, prior to its storage, have great importance. Acidity value (AV), bitterness (k225) and oleic–linoleic relation (MP), are analytical parameters important for this [2].

The acidity value measures free fatty acids according to its hydrolytic deterioration, oils with acidity >2% cannot be consumed directly and must be refined.

k225 is a chemical index of oil bitterness. High bitterness is not well accepted by the consumer.

MP is estimated on the basis of fatty acids profile and is calculated by: summa of monounsaturated fatty acids percentage/summa of polyunsaturated fatty acid percentage. Oleic and linoleum fatty acids are the most important ones. Resistance to authorisation is directly related. High values of MP indicate oils with high stability.

Therefore, the determination of these parameters, through conventional analytical methods is carried out in a chemical laboratory. In order to carry out an effective control and classification of olive oils, at the time that they are obtained, it is necessary the instant measurement of all these parameters and, of course, without using chemical products.

Near-infrared reflectance spectroscopy has been recognised as a powerful analytical technique for rapid determination of various constituents in food [3], it is non-destructive, low-cost and provides a safe working environment. It is also a technique suitable for on-line work.

In the last year, application of NIR to oils and fats has become more popular for quality and composition studies. In olive oils, NIR is used for prediction–identification of adulteration [4], differentiation–classification of vegetable oils [5] and monitoring on-line carotenoid and chlorophyll pigments [6].

The aim of this work has been to test the feasibility of the optical NIR sensor, in transmittance method, for the monitoring of the changes in olive oils during their production in olive-mill, in order to evaluate its utilization as a potential on-line method for characterization and quality control.

Section snippets

Samples

A total of 190 virgin olive oil samples have been used for building the calibration–validation NIR models. The 64% of these samples are from ‘Picual’ cultivar, gathered during the usual harvesting time (November–March) of three olive crop seasons; the rest of the samples (36%) are from 23 different cultivars of olive oil in a unique harvesting state (December) and along the three olive crop seasons. This sampling provides a source for the incorporation in the NIR models of variability varietal,

Result and discussion

Table 1 shows the chemical characterization of oil samples used for calibration of the PLS models and their validation. The number of samples, the mean, standard deviation and range for both calibration and validation set are indicated for each parameter. The oils used in this work allowed us to obtain an adequate degree of variability for acidity value (0.12–15.14% oleic acid), bitter taste (0.06–0.66 k225), oleic fatty acid (55.04–83.58%) and linoleic fatty acid (2.26–20.25%).

Shenk and

Acknowledgements

This paper has been supported by FAGA-FEOGA: program for the improvement in the olive oil production quality (project CAO 00-L5). This is a work of Ph.D. thesis by A. Jiménez.

A. Jiménez Marquez received his PhD in Chemistry from the University of Jaén in 2003. His PhD thesis focused in application of Near Infrared Spectroscopy for on-line virgin olive oil characterization. Currently, he is researcher of from I.F.A.P.A. (Andalusia, Spain) and he works in extraction process and application of non-destructives techniques for real time control of production and quality of virgin olive oil.

References (13)

  • D. Boskou

    Olive Oil, Chemistry and Technology

    (1996)
  • M. Uceda et al.
  • B.G. Osborne et al.

    Practical NIR Spectroscopy with Applications in Food and Beverage Analysis

    (1993)
  • I.J. Wesley et al.

    Measurement of adulteration of olive oils by near infrared spectroscopy

    J. Am. Oil Chem. Soc.

    (1995)
  • E. Bertran et al.

    Near infrared spectrometry and pattern recognition as screening methods for the authentication of virgin olive oils of very close geographical origins

    J. Near Infrared Spectrosc.

    (2000)
  • A. Jiménez

    Monitoring carotenoid and chlorophyll pigment in virgin olive oil by visible–near infrared transmittance spectroscopy. Application on-line

    J. Near Infrared Spectrosc.

    (2003)
There are more references available in the full text version of this article.

Cited by (0)

A. Jiménez Marquez received his PhD in Chemistry from the University of Jaén in 2003. His PhD thesis focused in application of Near Infrared Spectroscopy for on-line virgin olive oil characterization. Currently, he is researcher of from I.F.A.P.A. (Andalusia, Spain) and he works in extraction process and application of non-destructives techniques for real time control of production and quality of virgin olive oil.

A. Molina Díaz is Analytical Chemistry professor in the Departament of Physical and Analytical Chemistry from the University of Jaén (Andalusia, Spain). He received his PhD from the University of Granada in 1982. He is the head of the research group “Analytical Chemistry from the University of Jaén” and has published over 100 papers and reports concerning different topics, especially in the field of the flow-through spectroscopic sensors, which is his main research topic.

M.I. Pascual Reguera is PhD in Chemistry from the University of Granada in 1987. Currently she is a Lecturer in Chemistry in the Department of Physical and analytical Chemistry from the University of Jaén (Andalusia, Spain). Main research interest: pesticides, olive oil, sensor flow-through optosensors applied to pharmaceutical analysis, GC-MS, HPLC-MS.

View full text