Non-destructive analysis of anthocyanins in cherries by means of Lambert–Beer and multivariate regression based on spectroscopy and scatter correction using time-resolved analysis Manuela Zude a,⇑ , Michael Pflanz a , Lorenzo Spinelli b , Carsten Dosche c , Alessandro Torricelli d a Leibniz Institute for Agricultural Engineering Potsdam-Bornim (ATB), Dept. Horticultural Engineering, Max-Eyth-Allee 100, 14669 Potsdam-Bornim, Germany b Istituto di Fotonica e Nanotecnologie - Consiglio Nazionale per le Ricerche (IFN-CNR), Piazza Leonardo da Vinci, 32, Milan, Italy c University of Potsdam, Dept. Physical Chemistry, Karl-Liebknecht-Str. 24-25, Potsdam-Golm, Germany d Politecnico di Milano - Dipartimento di Fisica, Piazza Leonardo da Vinci, 32, Milan, Italy article info Article history: Received 19 December 2009 Received in revised form 31 August 2010 Accepted 30 September 2010 Available online 28 October 2010 Keywords: Cherry DTOF Effective pathlength Fruit maturity Lambert–Beer NIR Non-invasive Pigments PLS Ripeness Sensor fusion Spectroscopy Time-resolved spectroscopy TIRF Vis Scattering abstract In high-value sweet cherry (Prunus avium), the red coloration – determined by the anthocyanins content – is correlated with the fruit ripeness stage and market value. Non-destructive spectroscopy has been introduced in practice and may be utilized as a tool to assess the fruit pigments in the supply chain pro- cesses. From the fruit spectrum in the visible (Vis) wavelength range, the pigment contents are analyzed separately at their specific absorbance wavelengths. A drawback of the method is the need for re-calibration due to varying optical properties of the fruit tissue. In order to correct for the scattering differences, most often the spectral intensity in the visible spectrum is normalized by wavelengths in the near infrared (NIR) range, or pre-processing methods are applied in multivariate calibrations. In the present study, the influence of the fruit scattering properties on the Vis/NIR fruit spectrum were corrected by the effective pathlength in the fruit tissue obtained from time-resolved readings of the dis- tribution of time-of-flight (DTOF). Pigment analysis was carried out according to Lambert–Beer law, con- sidering fruit spectral intensities, effective pathlength, and refractive index. Results were compared to commonly applied linear color and multivariate partial least squares (PLS) regression analysis. The approaches were validated on fruits at different ripeness stages, providing variation in the scattering coefficient and refractive index exceeding the calibration sample set. In the validation, the measuring uncertainty of non-destructively analyzing fruits with Vis/NIR spectra by means of PLS or Lambert–Beer in comparison with combined application of Vis/NIR spectroscopy and DTOF measurements showed a dramatic bias reduction as well as enhanced coefficients of determination when using both, the spectral intensities and apparent information on the scattering influence by means of DTOF readings. Corrections for the refractive index did not render improved results. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Assuring healthy human nutrition and improving the economic success of farmers producing fresh fruit and vegetables are priority targets in the context of current global changes. There is general consensus that especially in agriculture new innovative technolo- gies are needed for appropriate process management. In presently developing technologies, optical methods in particular are princi- pally suitable for onsite analysis. Spectroscopy in near infrared (NIR) as well as visible (Vis) wavelength ranges can serve as a fea- sible tool for non-invasive fruit analysis due to available miniatur- ized light sources as well as accurate, robust, and inexpensive spectrophotometer modules. The compounds have been imple- mented in high-end colorimeters, handheld units for spectroscopic analyses, and sorting lines (Ozaki et al., 2006; Molto and Blasco, 2009). Based on the physics of NIR spectroscopy, a detailed review for agricultural and food products was presented by Birth and Hecht (1987) and, more recently, focusing on the initial data pro- cessing in fruit analyses, by Nicolai et al. (2007). The Vis part of the spectrum provides information on the pig- ment contents. Spectra obtained non-destructively from the fruit sample are frequently analyzed in terms of color using various col- or spaces. Since this paper uses sweet cherry as a model fruit, the literature was searched for correlations of spectral readings respec- tive to fruit quality and ripeness stage. Color readings for grading fruits are evaluated regarding the external appearance recognized by the consumer. High correlation of color values in L*a*b* color 0260-8774/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2010.09.021 ⇑ Corresponding author. E-mail address: zude@atb-potsdam.de (M. Zude). Journal of Food Engineering 103 (2011) 68–75 Contents lists available at ScienceDirect Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng