Introduction

Besides the appearance conferred to the fruit by size, color, shape and surface defects, total soluble solids (TSS), directly related with the sugar content, and titratable acidity (TA) are crucial attributes indicating the fruit internal quality (Nicolaï et al. 2007). Non-destructive optical methods based on visible/near-infrared spectroscopy (VIS/NIRS) have been evaluated for non-destructive estimation of internal starch, soluble solids content, oil contents, water content, dry-matter content, acidity, firmness, stiffness factor, and other physiological properties of a number of fruit and vegetable products indistinctly including citrus (Steuer et al. 2001; Miller and Zude-Sasse 2004; Cayuela 2008; Lu et al. 2008; Zude et al. 2008) mandarin (Kawano et al. 1993; McGlone et al. 2003); tomato (Slaughter et al. 1996); mango (Saranwong et al. 2004); kiwifruit (Osborne and Künnemeyer 1999); apple (Lammertyn et al. 1998; Park et al. 2003; Menesatti et al. 2009).

In citrus, fruit juice content, TSS and TA are the main internal quality parameters used all over the world. Packinghouse managers take representative samples of fruit to test internal quality before shipping, but controversy arises when the required minimal level of TSS is not met or when TA exceeds the highest tolerated level. Unfortunately for the same cultivars TSS and TA vary greatly in fruits harvested from the same tree and this variability depends on a large number of factors such as the length of the blooming period, which causes the setting of fruits of different age in the same tree not easily discernable at the picking time, the position of the fruits in the canopy, on the kind of inflorescences which bear the fruit. A reliable non-destructive analysis means would allow to select fruits according to their real and individual quality characteristics and to match the required internal quality standards.

The aim of this study was to evaluate the ability of VIS/NIR spectroscopy to determine TSS and TA of two thick peel mandarin cultivars. The results, elaborated through partial least squares (PLS) analysis, have been compared with standard destructive techniques to assess the respective quality characteristics and to obtain the prediction models.

Materials and Methods

The fruits were harvested on 27th November 2003 from trees grafted onto sour oranges cultivated at the CNR's experimental station (Oristano, west Sardinia). After harvest, fruit were immediately delivered at the laboratory where 400 of “Miho” satsuma and 150 of “Page” tangelo free of visible defects were chosen for the experiment. For the VIS/NIR measurements, a portable single channel spectrometer was used (Fig. 1). It performs point measurements of different emitted light quotes, in function of the probe applied: reflectance and standard color CIELAB 45/0, absorbance and interactance. The system is composed of five parts: (1) a spectrograph Hamamatsu S 3904 256Q in a special housing; a customized illumination system realized by a 20 W halogen lamp and an optical fiber bundle consisting of approximately 30 quartz glass; (2) an optical entrance with input round: 70 × 2,500 μm and diameter 0.5 mm NA = 0.22 mounted in SMA-coupling; (3) specific probes with quartz optical fiber of connection; (4) a transmission device for transmitted or absorbed light for thin solids or liquid with variable optical length; (5) a notebook equipped with specific software to acquire, calibrate, and elaborate spectral data. The Hamamatsu spectrograph has the following characteristics: grating: Flat-field, 366 l/mm (center); spectral range: 310–1,100 nm; wavelength accuracy absolute: 0.3 nm; Temperature induced drift: <0.02 nm/K; Resolution (Rayleigh criterion): DlRayleigh >10 nm; sensitivity: >1013 Counts/Ws (with 14-Bit-conversion); straylight: <0.8% with Halogen lamp and A/D converter 16 bit. For spectral acquisition, the ‘pen’ probe was used to measure the spectral reflectance response on each single fruit (spot area ≈ 10 mm2): twice in different equatorial parts. The diffuse reflectance measure is referred to the light diffuse quote that is reflected by the material and acquired by an optical quartz fibre (0.7 mm in diameter) fixed at 45° inside a circular aperture of 4 mm in diameter. The material surface due to its softness was able to include all the circular aperture avoiding any external light interference. Measurements were performed placing the probe's head perpendicularly to the fruit surface to avoid external light noise. These spectral measurements were performed in laboratory considering a white calibration (small variable in function of the external light), the instrumental integration time (light acquisition time), and subtracting the background noise (variable in function of the instrument temperature) (Fig. 1). A very low signal/noise ratio was observed in the beginning and at the end of the spectral data, affecting the accuracy measurements, so only the spectrum in the range 400–1,000 nm were take into account for the analysis.

Fig. 1
figure 1

a Portable VIS-NIR spectrophotometer. b Trained operator performing measurements on mandarin fruit. c Pen probe head acquiring the signal. d Fibre light emitter

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All spectral values were expressed in terms of relative reflectance (R) (Menesatti et al. 2009). Reflectance measurements were compared to standard chemical value averaged on single fruit. All fruits were individually numbered and after taking all the measurements (readings) for non-destructive analysis were juiced and chemical analysis were performed on the juice of each single fruit in order to verify if correlations existed between non-destructive parameters and the real values of TA and TSS determined destructively. Estimation of chemical levels was performed by PLS regression model on the basis of the reflectance spectral values (Pallottino et al. 2010).The dataset was randomly separated into two subsets, one used for the model, 75% of the whole dataset, and the remaining 25% used for the independent validation test. The sample data was separated randomly into two groups: a calibration set used to develop the calibration models and the remaining samples of the population were used as prediction sets. The calibration models were also validated using full cross-validation. The x- and y-blocks were generally pre-treated using Autoscale, that centres columns to zero mean and scales to unit variance. In the case of the Page x-block was used a Savitzky–Golay smoothing and differentiation which performs a smoothing on the matrix of row vectors y. At each increment a polynomial of order is fitted to the number of points width surrounding the increment.

Results and Discussion

Table 1 shows the results and the parameters used to build the PLS model relatively to the TA and TSS estimation for the Page mandarins analyzed. The correlation coefficients (r) between observed and predicted values were equal to 0.88 for the TA (Fig. 2a) prediction and 0.85 for the TSS (Fig. 2b) for the independent test. The number of the LV used to build the model was lower for the TSS (8) then for the TA (15). The SEP coefficient of variability, calculated as the SEP divided by the average of the total observed values, was found for the Page test to be lower in the TA case in comparison with the TSS and respectively of 3.8% and 4%. Table 2 reporting the results relative to the Miho estimation shows a number of latent variables equal in both TA and TSS model prediction (15). The correlation coefficient (r) in the independent test between observed and predicted values was 0.81 for TA (Fig. 2c) prediction and 0.84 for TSS (Fig. 2d). In this case, the SEP coefficient on variability for the test resulted lower for TSS, 5.6%, with respect to TA, 8.3%. Therefore, the highest correlation coefficient (r) between observed and predicted values was identified for the TA prediction of Page.

Table 1 Results of PLS prediction relative to the titrable acidity (TA) and total soluble solids (TSS) of the Page samples
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Fig. 2
figure 2

Linear regressions between the predicted and the observed values of the: a TA, Page; b TSS, Page; c TA, Miho; d TSS, Miho

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Table 2 Results of PLS prediction relative to the titrable acidity (TA) and total soluble solids (TSS) of the Miho samples
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Internal quality features such as total soluble solids content, sugar content, juice acidity, dry-matter content and firmness are well known to be crucial attributes of the fruit (Gómez et al. 2004). The results of this study showed that a system based on a portable spectrophotometer can provide better knowledge of mandarin quality, achieving a more detailed and focused quanti-qualitative information in a shorter period of time. The autonomy of the instrument, taking into account the time needed to move from one fruit to the other, allows date acquisitions to perform on about 1,200–1,300 fruit. The power supplied by the portable batteries of the instrument and the notebook computer, guaranteed a working period of about 1.5 h. Thus, the use of the spectrophotometer, coupled with the multivariate statistical techniques used here gives the possibility to map intensively and precisely large parcel of land, obtaining a highly representative sample. Furthermore, the possibility of acquiring more detailed information, varying either in space and time, when compared with the standard chemical analysis, should prove to be a useful tool to select fruit on the base of their quality fastening an important post-harvest operation.

Most instrumental techniques to measure such properties are destructive thus, involving a huge amount of manual work. In this study, a non-destructive methods based on a high number of VIS/NIRS have been evaluated to estimate two of these important internal quality features: soluble solids content and acidity.

Conclusions

The improvement of non-destructive analysis for the determination of TSS and TA do represents an important advance in market of citrus fruit, allowing fruit quality classifying not only on the base of visual aspect, but also in relation to gustative characteristics. This could lead to changes in agricultural methods and strategies, making farmers aware of the possibility of gaining a premium price for the internal quality of the fruit and not only for their aesthetical attributes.