EVALUATION OF APPLE QUALITY BY REFLECTANCE SPECTROMETRY
EVALUATION OF APPLE QUALITY BY REFLECTANCE SPECTROMETRY
Doru Roiban, Elisabeth-Jeanne Popovici, Ana-Maria Kaszoni Pricop, Laura Silaghi-Dumitrescu and Rodica Grecu
“Raluca Ripan“ Institute for Research in Chemistry, 30 Fantanele, 3400-Cluj Napoca, Romania,
Abstract
Attempts were made to use the reflectance spectroscopy in order to evaluate the apple quality. The aim of the study was to illustrate the spectral differences between different apple varieties from Bistrita County.
Key words: apple, chromatic characteristics.
1. introduction
There is a continuous demand for developing alternative fruit storage techniques as well as for finding non-destructive methods for food quality control. In this context, an ample investigation was initiated in order to develop an ozone-based apple storage method. For this purpose, the quality of the as treated apples is evaluated and compared with the quality of the traditional deposited fruits [1]. In this investigation, the apple chromaticity is attempted as one of the experiment markers. Chromatic quality of apples at the beginning of the storage period was evaluated on the basis of diffuse reflection spectra.
The current investigation put in evidence the spectral differences between the three apple varieties from Bistrita County i.e. JONATHAN, GOLDEN SPUR and GOLDEN fruits.
2. EXPERIMENTAL PART
Sample preparation: Thin round disks were cut from the apple surface. The disks were slightly concave, had 32 mm diameter and 1.0 – 1.5 mm thickness. By using an original, special designed device, 2 4 uniform round disks were cut out from each apple, and then slightly pressed onto 50x50 mm glass plates. For each investigated variety, at least ten apple fruits were measured.
Colour measurement: Diffuse reflection spectra were registered with an UNICAM UV4 Spectrometer, with RSA-UC-40 integrating sphere accessory. The reflectance was registered in visible domain (380-780 nm) in comparison with a white standard (SPECTRALON; 0.3104; 0.3189; 0.3707 coordinate). The measurements were performed with Illuminant C (10C), by using CHROMA v1.0 software. By computing the diffuse reflection spectra with the sensitivity of human eye, trichromatic coordinate and CIELAB 76 colour parameters were calculated and the apple colour evaluated.
3. results and discussion
The diffuse reflection spectra of three apple varieties from Bistrita County were registered and mathematical processed in order to appreciate the fruit colours, in numerical terms. Reflectance spectra of some representative GOLDEN SPUR, GOLDEN and JONATHAN apples are depicted in figure 1. The colour parameters in CIELAB system permitted the apple placement inside the colour circle whose diameters are the blue-yellow and red-green coordinates (figure 2).
Figure 1. Diffuse reflection spectra of GOLDEN SPUR (GS), GOLDEN (G) andJONATHAN (J) apples.
Figure 2. The CIELAB colour parameters of GOLDEN SPUR, GOLDEN and JONATHAN apples
Chromatic system CIELAB 76 works with the following parameters: L-lightness, a - red-green coordinate, b - blue-yellow coordinate, c-chroma index [c = (a2+b2)1/2] and h - hue angle.
One can observe that:
GOLDEN SPUR (GS) apples present two absorption peaks at 480 nm (weak) and 676 nm (strong), respectively, and reflect well the colour in 520 640 nm domain (GS fruits show green colour); CIELAB coordinates are noticed in the green yellow quadrant, near the yellow axis.
GOLDEN (G) apples have two relatively weak absorption peaks, situated in 400-500 nm domain and 675 nm; G-fruits reflect well the light in 520650 nm domain, region which is more expanded than for GS apple (G-fruits show gold yellow colour); CIELAB coordinates are observed in the yellow-red quadrant.
JONATHAN (J) apples have a maximum of absorption at 676 nm (surprisingly in the same domain with GOLDEN SPUR apples) and have an intense absorption in 380570 nm region; J -apples strongly reflect the light in the 700780 nm domain (J-fruits show red colour); CIELAB coordinates are situated in yellow - red quadrant, closer to the red axis.
The highest lightness values are observed for GOLDEN varieties.
The reflectance spectra could be used to calculate the three x, y, z chromatic coordinates that permit the sample placement in the chromatic CIE diagram (Commission International de l’Eclairage), inside the triangle whose corners are the red, green and blue saturated colours [2,3].
The colour coordinates were calculated from the diffuse reflectance spectra of all apple samples. The variation limits of the colour coordinates in the two chromatic systems are given in table 1. The minimum and maximum values for x and y coordinates are represented on the CIE chromatic map (figure 3). The large variability of JONATHAN apples in comparison with GOLDEN SPUR and even GOLDEN apples is put in evidence.
Table 1
CIELAB coordination variation domain and three-chromatic
coordinates for the three sort of apples
Apple variety / GOLDEN SPUR / GOLDEN / JONATHANCIELAB 76 – Coordinates
coordinate red-green (a) / -9,10 2,56 / - 0,63 16,69 / - 2, 03 52,57
coordinate blue-yellow (b) / 31,74 47,04 / 34, 06 51,80 / - 3,10 39,34
chroma index (c) / 31,97 47,09 / 34,04 51,80 / 17,21 39,56
Lightness (L) / 59,88 69,85 / 58,65 71,64 / 30,11 69,44
Chromatic Coordinates
x / 0,3869 0,4116 / 0,3948 0,4521 / 0,3943 0,4618
y / 0,4126 0,4455 / 0,3859 0,4400 / 0,3110 0,3838
Figure 3. The colour of GOLDEN SPUR, GOLDEN and JONATHAN apples
in CIE coordinate map
4. CONCLUSIONS
On the basis of reflectance spectra, the apple colour can be expressed in numerical terms, fact that facilitates the evaluation of fruit quality. The trichromatic coordinate system illustrates the differences between the apple varieties whereas CIELAB system seems to be more accurate in evaluating the differences between the sorts of the same variety, i.e. to illustrate their variability. The study revealed that instrumental colorimetry could be used as a non-destructive technique to appreciate the apple quality before and during the storage period.
Acknowledgments:
The work was supported by AGRAL Research National Program , under contract 124/2002.
References:
1. Lowasz T, “Change Of Physiological Condition And Composition Of Apples During Storage”, Magyar Kemia Folyoirat, 1997, 103, 130-135;
2. Francis F.J, “Color Quality Evaluation Of Horticultural Corps, Hort. Science, 1980, 15, 58-59;
3. Varady Burgetti C, Lukacs Gy., “ Application Of Instrumental Colorimetry In The Determination Of The Optimum Picking Times Of Different Apple Varieties”, Hungarian Scientific Instruments, 1985, 60, 49-55;