An approach to non-destructive apple fruit chlorophyll determination

Development on the tree of 'Elstar', 'Jonagold' apples (CW 30-CW 45), and 'Idared', and 'Golden Delicious' apples (CW 36-CW 41) was monitored using a recently available, accurate and inexpensive spectroscopic technique. With this technique a spectrum can be obtained at 256 discrete wavelengths in the range from 300 to 1100 nm within 400 ms. In practice the spectral response to fruit temperature in the range 4 (storage conditions) to 30 degrees C (orchard) has to be taken into account. However, no temperature influence was found in the wavelength range of 600-750 nm, while a high response appeared in the range of 800-1100 nm. Spectral recordings in the visible wavelength range using reflectance or transmittance mode were tested to obtain information on the fruit pigment content, which is related to fruit development. Spectral transmittance recordings were correlated with development of 'Jonagold' (R2 = 0.89) and 'Elstar' (R2 = 0.93) fruit, while reflectance recordings provided less relevant information. However, the spectral-optical response to fruit development is determined by changes in fruit pigment contents. Estimation of chlorophyll content was achieved with R2 = 0.57 ('Elstar'), 0.82 ('Jonagold'), 0.65 ('Idared') and 0.80 ('Golden Delicious') by linear regression of chlorophyll red-edge on extracted chlorophyll content, even if the green ground colour was visually covered by the red blush colour. The correlation coefficient was improved up to R2 = 0.88 ('Elstar'), 0.98 ('Jonagold'), 0.90 ('Idared'), and 0.87 ('Golden Delicious') by multivariate analysis of absorbance values. Fruit ripeness was predicted by fruit firmness, SSC, starch degradation, the chlorophyll red-edge and multivariate predicted chlorophyll content. The prediction of time-related fruit development using the Streif index resulted in a validation correlation of 9 and 7 days for 'Elstar' and 'Jonagold', respectively, but was slightly improved by inclusion of the chlorophyll analysis to 8 and 6 days.