Chemometric Analysis of Combined NIR and MIR Spectra to Characterize French Olives
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Eur. J. Lipid Sci. Technol. 2010, 112, 463–475 463 Research Article Chemometric analysis of combined NIR and MIR spectra to characterize French olives Nathalie Dupuy1, Oswin Galtier1, Yveline Le Dre´au1, Christian Pinatel2, Jacky Kister1 and Jacques Artaud1 1 CNRS UMR6263, ISM2,E´ quipe AD2EM, Groupe Syste`mes Chimiques Complexes, Case 451, Universite´ Paul Ce´zanne, Marseille, France 2 Centre Technique de l’Olivier (CTO), Maison des Agriculteurs, Aix-en-Provence, France Chemometric treatment of near-infrared (NIR) and mid-infrared (MIR) combined spectra was used firstly to predict oil and water contents in fresh olive fruit samples (n ¼ 223) and secondly to classify these samples into five principal French cultivar origins (Aglandau, Cailletier, Olivie`re, Salonenque, and Tanche). The study was carried out during four crop years (2005/2006 to 2008/2009) to take into account the seasonal variations. The comparison of the results obtained in the combined range (REP ¼ 2.6% for the water content and 3.5% for the oil content) provides an obvious advantage compared to the NIR and MIR techniques used separately. Fresh olive fruit cultivars were satisfactorily classified with the partial least squares-discriminant analysis (PLS-DA) method in the combined range. After use of the K-means clustering on the PLS-DA scores, all the samples were well classified into their five groups of origin. The use of infrared combined spectra allows a considerable improvement in estimating olive fruit quality (oil and water contents, varietal origins). Keywords: Cultivar / Oil content / Olive fruit / Partial least squares-discriminant analysis / Water content Received: September 4, 2009; accepted: January 4, 2010 DOI: 10.1002/ejlt.200900198 1 Introduction technique for evaluating the oil content. These methods are time consuming, expensive, and impractical for processing The olive tree is one of the most important crops in the large numbers of samples. Although NMR has widely Mediterranean countries. Virgin olive oil is obtained from replaced these procedures, the samples still have to be dried the olive fruit only by mechanical or physical methods which until constant mass before analysis [3], and NMR is not do not alter the oil. The quality of virgin olive oil is principally convenient and is too expensive for on-line application. a function of four parameters: varietal and geographic origin, Near-infrared (NIR) and mid-infrared (MIR) spec- olive fruit quality, and extraction process. The oil content in troscopy associated with chemometric analyses should also olive fruit was shown to have a high variability between be able to advantageously replace the traditional analytical cultivars [1]. The production of good-quality virgin olive methods to determine these parameters, because no complex oil must start with raw materials that meet well-defined preparation of sample is required and spectral acquisition is quality standards [2]. In laboratories, traditional analytical relatively fast. Moreover, it was possible to quantify various methods to determine oil and water contents are used: drying parameters using only one infrared spectrum. The determi- the sample for evaluating the water content and the Soxhlet nation of the oil and fat contents of olive pastes [4–6] or olive pomace [7–9] by NIR spectroscopy and Raman spectroscopy [8] has been reported. Recently, Leon et al. [10] have deter- Correspondence: Dr. Yveline Le Dre´au, CNRS UMR6263, ISM2,E´ quipe mined the oil and moisture contents in Arbequina, Frantoio, AD2EM, Groupe Syste`mes Chimiques Complexes, Case 451, Universite´ and Picual olive cultivars in intact frozen olive fruits with a Paul Ce´zanne, 13397 Marseille Cedex 20, France VIS/NIR analysis system in reflectance mode. Partial least E-mail: [email protected] squares (PLS) regression was used for each cultivar and for Fax: þ33-4-91289152 each harvest year. For the samples of three cultivars, the relative error of prediction (REP) for the oil and moisture Abbreviations: NIR, near-infrared; MIR, mid-infrared; PCA, principal component analyses; PLS, partial least squares; REP, relative error of contents varied according to the harvest year. So, for the years prediction; SEC, standard error of calibration; SOD, sum of distances 1996 and 1997, the estimated REP values for oil were about ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.ejlst.com 464 N. Dupuy et al. Eur. J. Lipid Sci. Technol. 2010, 112, 463–475 8.5 and 16.9% and for moisture about 3.7 and 5.5%, 2.2 Reference analysis respectively. The cultivar number in the world is estimated at 2000, Of fresh olive fruits, 1.5 kg was milled in a hammer mill. and in France there are about 200 cultivars [11]. For the Then, 45 g was dried in a forced-air oven at 1048C during principal French cultivars, specific land is under cultivation 24 h; the loss of weight gave the percentage of water and outside of which production is disappointing. Some of the volatile matter of the sample. principal French olive cultivars are Aglandau, Cailletier, For oil content determination, the dried sample was trans- Olivie`re, Salonenque, and Tanche. A denomination of a ferred into a glass tube of 40 mm in diameter. The sample cultivar is recognized using different morphological descrip- was analyzed using a Time Domain NMR instrument tors (tree, leaf, fruit, and stone). However, on the basis of (Minispec Bruker) according to the AOCS [16]. these criteria, only very dedicated specialists are able to perform a strict differentiation between all the cultivated 2.3 NIR spectroscopy varieties. Currently, one major problem in the food industry is the setting of objective tools to determine the origin of raw Fresh olive fruit samples were pitted prior to measuring in a materials as well as finished products so that products can be Nicolet Antaris spectrometer interfaced to a PC and followed from the producer to the consumer. The origin and equipped with an InGaAs detector, an H2 NIR source, the authenticity of virgin olive oils have been the object of and a CaF2/germanium beam splitter. The spectrometer many studies in the past few years [12–14]. But, to our was placed in an air-conditioned room (218C). Fourier trans- knowledge, little has been done to determine the origin of form (FT)-NIR spectra were recorded by collecting the NIR raw materials with morphological characterization [15], and a energy that scatters on the surface of olive fruit, using an literature search has revealed no studies on varietal origin integrating sphere in diffuse reflectance mode. Each spec- determination of olive fruits by infrared spectroscopy. trum shown is the mean of 25 spectra obtained on 25 different The aim of this study was to show the advantages of olive fruits – from the same batch – put in an autosampler. All combined NIR and MIR spectroscopy associated with che- spectra were computed at 8 cmÀ1 resolution, with 16 scan mometric treatment for a direct and rapid test method used accumulations between 4000 and 10 000 cmÀ1 using the firstly to predict the water and oil contents of olives and software result integration 2.1 Thermo Nicolet. A back- secondly to provide variety recognition of olive fruits. The ground spectrum was collected under the same conditions results were compared with the ones obtained independently before each batch measurement. in the NIR or MIR range. 2.4 MIR spectra 2 Materials and methods Spectra of each olive sample were recorded from 4000 to 700 cmÀ1, with 4 cmÀ1 resolution and 100 scans on a Nicolet 2.1 Olive fruit samples Avatar spectrometer equipped with a DTGS detector, an Ever-Glo source, and a KBr/germanium beam splitter. The Fresh olive fruit samples were obtained from the Centre spectrometer was placed in the same air-conditioned room. Technique de l’Olivier (CTO). A total of 223 batches of Each spectrum is the mean of five spectra obtained on five 50 olive fruits were collected during four successive crops pieces of five different olive fruits – from the same batch – put (2005/2006, 2006/2007, 2007/2008, and 2008/2009) to take without preparation into a single-bound attenuated total into account the seasonal variations. reflectance (SB-ATR) cell provided with a diamond crystal. For the prediction of the water and oil contents, only olive Air was taken as reference for the background spectrum fruits collected during the first three successive crops (2005/ before each sample. Between spectra, the ATR plate was 2006, 2006/2007, and 2007/2008) were taken into account. cleaned in situ by scrubbing with ethanol solution, enabling 178 samples were used (125 for the calibration set and 53 in to dry the ATR. Cleanliness was verified by collecting a the prediction one): Aglandau (n ¼ 67), Cailletier (n ¼ 31), background spectrum and comparing to the previous back- Olivie`re (n ¼ 19), Salonenque (n ¼ 27), and Tanche ground spectrum. (n ¼ 34). For the prediction of varietal origin, 223 samples were 2.5 PLS regression used: Aglandau (n ¼ 88), Cailletier (n ¼ 34), Olivie`re (n ¼ 29), Salonenque (n ¼ 34), and Tanche (n ¼ 38), 128 PLS is a supervised analysis that is based on the relation in the calibration set and 95 in the prediction set. between the signal intensity and the characteristics of the The olive fruit samples differed in terms of cultivar origin, sample [17]. Interference and overlapping of the spectral crop year, degree of ripening, and area of growth. All the information may be overcome by using powerful multicom- fresh olive fruits were directly analyzed on receipt in the ponent analysis such as PLS regression. PLS allows a soph- laboratory. isticated statistical approach using a spectral region rather ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.ejlst.com Eur. J. Lipid Sci.