Vol. 25 No. 3 May 2014 articles 6 Analysis of the influence of the skin on the near infrared absorbance spectra of potato tubers Ainara López, Carmen Jarén and Silvia Arazuri Department of Agricultural Projects and Engineering, Universidad Pública de Navarra, Campus de Arrosadia 31006, Navarra, Spain Introduction T he interfering effect of peel or skin on the acquisition of spec- tral data is a matter of investiga- tion in developing methods for non-destructive prediction of food product composition. 1 Some authors have studied the influence of the peel in the estimation of chemical components in thick-peeled fruits. 2,3 In the case of thin-peeled food products like potato (Solanum tuberosum L.), some authors pointed out that the pres- ence of the peel does not change the cor- relation between the spectral and chemical data. 1,4 However, not many studies have been developed for this type of product. Therefore, the objective of the present study is to investigate whether the peel or skin has an influence on the NIR absorbance spectra of potato tubers. In this study, among other techniques, a support vector machine (SVM) classification procedure is performed to distinguish sam- ples of peeled and unpeeled groups. SVM classification is a technique which is able to classify non-linear data by first mapping it into a high-dimensional feature space and then classifying it by the use of maximal margin hyperplanes. 5 Experimental Two commercially available varieties of potatoes were used in this study: Monalisa and Universal. Both varieties were bought in a local supermarket in Pamplona (Nav- arra, Spain) and analysed at the Post-har- vest Agricultural Products Laboratory of the Agrarian Mechatronics Research Group of the Agricultural Projects and Engineering Department of the Universidad Pública de Navarra, Spain. Two hundred and seventy (270) samples were used in this work, 120 tubers corresponding to the Monalisa vari- ety and 150 to Universal. Samples were randomly divided into two groups for train- ing (182) and test (88). Moreover, each variety was split into two groups for meas- urements on the whole and peeled tubers. Thus, 70 and 80 tubers of the variety Uni- versal were used for peeled and unpeeled measurements, respectively; for Monalisa, the numbers were 50 and 70, respectively. Data acquisition NIR spectral data were collected using a Luminar 5030 Miniature “Hand held” AOTF- NIR (Acousto-Optic Tunable Filter-Near Infra- red) Analyser (Brimrose Corporation of Amer- ica, Baltimore, Maryland) in reflectance mode. A spectral range of 1100–2300 nm with 601 points (2 nm step) was used to obtain the spectra at room temperature. Each sample was scanned at four different points and the average spectrum was used for subsequent analysis. Each spectrum was an average of 50 scans. Data were then transformed to absorbance for further analysis. Analyses First, an analysis of the two varieties was performed in order to identify undesirable scatter effects. With this purpose in mind, absorbance data of both measurements (peeled and unpeeled) of each potato vari- ety were examined to identify imperfec- tions and to compare the degree of scatter between varieties. Then, the mean absorb- ance spectrum of both peeled and unpeeled tubers of each variety was obtained to iden- tify the main differences in absorption at dif- ferent wavelengths in the NIR spectrum. Finally, and with the aim of evaluat- ing whether the differences graphically observed corresponded to the formation of statistically different groups, a classifica- tion was performed using support vector machines (SVM). Prior to SVM classifica- tion, samples were pre-treated using SNV to reduce the physical variability between the samples as a consequence of the scat- ter. 6 Data analysis was performed using Unscrambler X (version 10.0.1; Camo Soft- ware AS, Oslo, Norway). doi: 10.1255/nirn.1433 Figure 1. (a). Absorbance spectra of unpeeled potato tubers of Universal variety. (b) Absorbance spectra of peeled potato tubers of Universal variety. (c) Maximum and minimum absorbance spectra of unpeeled tubers of Universal variety. (d) Maximum and minimum absorbance spectra of unpeeled tubers of Universal variety.