Sweet cherry fruit analysis with reflectance measurements
In the experiment five cherry species (Vera, Cristalina, Germersdorfi, Noir de Mechet, and Canada Giant) were examined in order to measure the spectral differences between species. Further more, spectral alteration was examined between different health and maturity status of the fruits in the case of a specified, the Germesdorfi species. The four new indices are appropriate tools for cherry quality analysis. Thus reflectance measurements can also support more precise and automated fruit selections. The methods for the differentiation of species could also be viable at a concerned habitat; however, the climate, habitat and soil conditions strongly affect the yield quality. Concerning the fast determination of water content, WBI could be a reliable method for the assessment.
INTRODUCTION
The total sweet cherry production of the world ranges between 1.4 and 1.6 million tons. Regarding the growing area, Europe has a leading role as more than 50 per cent of sweet cherry is produced here.
The Hungarian sweet and sour cherry breeding has been going on since 1950. In the frame of this programme are 13 released and 1 candidate sweet cherry varieties, and 9 released and 2 candidate sour cherry varieties. Sweet cherry varieties in the National Variety List are the following: ‘Margit’ (1987),‘Linda’ (1988), ‘Katalin’ (1989), ‘Alex’(1997), ‘Kavics’ (1999), ‘Vera (2002), ‘Rita’(2004), ‘Petrus (2007), ‘Paulus’(2007), ‘Aida’ (2007), ‘Carmen (2007) ‘Tünde’(2008), ‘Sándor’(2008) (Apostol 2011).
The total area of new orchards planted between 1998 and 2005 with governmental support is 750 ha, out of which the intensive orchards with a plant density above 1000 trees/ha make up for only 3.3 per cent. Unfortunately, in most orchards the "semi-intensive" spacing of 7 x 5m and 6 x 4m were applied. In recent years, the cultivars planted on the largest areas were (in decreasing order): cv. ‘Katalin’, clones of cv. ‘Germersdorfi’, cvs, ‘Linda’, ‘Kordia’, ‘Bigarreau Burlat’, ‘Szomolyai fekete’, ‘Van’ and ‘Margit’. In addition to these, the following foreign cultivars were also planted by the Hungarian producers: cvs. ‘Sunburst’, ‘Stella’, ‘Regina’, ‘Valerij Cskalov’, ‘Sylvia’, ‘Sweetheart’ and ‘Krupnoplodnaja’ (Thurzó, 2008).
Based on the most recent data, the average amount of sweet cherry produced in Hungary is around 10-12 thousand tons. Therefore fast and effective method is important for sweet cherry fruit quality analyses. One of the possible methods for quality analysis is the reflectance measurement of the fruits.
MATERIAL AND METHOD
The aim of the study was to examine the applicability of reflectance measurements for sweet cherry fruit quality analyses. In the experiment five cherry species (Vera, Cristalina, Germersdorfi, Noir de Mechet, and Canada Giant) were examined in order to measure the spectral differences between species. Further more, spectral alteration was examined between different
health and maturity status of the fruits in the case of a specified, the Germesdorfi species. Out of every sweet cherry species 25 fruit samples were measured in three repetitions.
The reflectance spectra were measured by a hyperspectral (0.55nm spectral resolutions) AvaSpec 2048 spectrometer within 400-1000nm wavelength interval. The AvaSpec 2048 system consists of a spectrometer, a fiber optic and a halogen light source, and a spectral sampling box (Figure
1.).
The fiber optic has two connections; one is for the spectrometer, and one is for the light source. The light source ensures the permanent light intensity in the whole measurement range. The sampling box is insulated so the sampling is not disturbed by any external light.
Before the spectral measurement the spectrometer had to be calibrated. For the calibration white and dark reference measurement is needed. The calibration was made by a special calibration reference unit. For reflection measurements WS-2 reference tiles was used for diffuse reflection The WS-2 white reference tile is made out of a white diffuse PTFE (polytetrafluoroethylene) based material, meeting the highest demands with regard to high grade diffuse reflectance.
Several parameters were examined based on the spectra. The colour, maturity and health status was analysed in the yellow –red (570-730) wavelength interval, where a significant sigmoid growth of reflectance appears. Besides the reflectance differences, fruit indices were calculated for a proper determination of the mentioned parameters. Since vegetation indices, which are sensitive to the chlorophyll content (Burai et al. 2009), show similar sigmoid growth in the red-NIR zone, thus the determination of fruit indices were based on the algorithms of the Normalized Difference
Vegetation Index (NDVI), the Simple Ratio Index (SRI) and the Red edge Position (REP). Differences between relative water content of the fruits were also assessed in the near infra red (NIR) zone between 900-970nm by the Water Band Index (WBI). WBI is a reflectance measurement that is sensitive to changes in canopy water status. As the water content of
vegetation canopies increase, the strength of the absorption around 970nm increases relative to that of 900 nm. WBI is defined by the following equitation: WBI=900/970 (Champagne et al. 2001). The dry material content of the fruits was also measured by drying them till the weight of
fruits became constant.
RESULTS AND DISCUSSIONS
The characteristics of the reflectance curves of each fruit species are caused by the large amount of absorption of anthocyanin content at 450-570nm wavelength intervals. On the other hand, reaching the red interval the reflectance of healthy cherry fruits are raising markedly at 700nm due to the red color (Figure 2.). Though the shapes of the spectral curves are tend to be similar, there are differences among species in intensity and the yellow –red (570-730) wavelength interval.
There are much more spectral differences between fruits regarding the ripe and health status. Healthy and ripe fruits show the highest reflectance (%) in the red and NIR spectral interval, since the amount of anthocianin is probably the highest, and there is no damage which could case oxidation of these flavonoids (Figure 3.). Due to the damages and other diseases (Monilia) brownish oxidation occurred in fruits, and the anthocyanin content is definitely decreased. Therefore the reflectance in the red interval is lower and there is no rapid sigmoid increase between yellow red intervals. Due to unripe had less anthocyanin but more carotinoids (yellowish colour), and the effect of starting oxidation processes effects in overripe fruits, low reflectance occurred in red- NIR.
Besides the reflectance values, four new fruit indices was created in order to examine the differences numerically. The Simple Ratio Yellow Index (SRYI) was calculated based on the reflectance value in yellow (Ryellow) and red (Rred) wavelength:
Normalized Red-Yellow Index (NRYI) was based on the algorithm of NDVI, and calculated as follows:
Both SRYI and NRYI values show that the rotten has the lowest and the ripe has the highest value which describes the anthocyanin content change as well as the health status of the fruits. Regarding species, the redder was the fruit of a species the higher was the indices value. Therefore these indices should be appropriate for health analysis and for differentiate species based on the redness of flavonoids.
Yellow Edge Position (YEP) was also created and calculated based on REP (Jung et al., 2006). YEP is narrow band index which shows the inflection point of the sigmoid curve. The results suggest that the place of the inflection point depends on the redness of the fruit. If the fruit is free from damages and diseases, the redder was the fruit the higher was the YEP value and probably also the anthocyanin content. Lower reflectance and slow sigmoid increase in wide wavelength interval resulted higher YEP values in the case of overripe, rotten and damaged fruits (Figure 5.). Yellow-Red Area; (YRA: the area below the sigmoid curve, between 570-730nm) was also calculated, which results less reliable data for health and maturity status.
The water content of fruits was determined by WBI, and also the dry material content was measured to evaluate the applicability of WBI in the case of fruits. Since there is good, negative correlation (R=-0.72) between WBI and dry material content WBI could be a reliable method for assessing the water content, thus the dry material content of the cherry fruits.
CONCLUSIONS
The four new indices are appropriate tools for cherry quality analysis. Thus reflectance measurements can also support more precise and automated fruit selections. The methods for the differentiation of species could also be viable at a concerned habitat; however, the climate, habitat and soil conditions strongly affect the yield quality. Concerning the fast determination of water content, WBI could be a reliable method for the assessment
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