Spectral Measurements on ‘Elstar’ Apples during Fruit Development on the Tree

Monitoring of apple fruit development on the tree is necessary for determination of optimum harvest date. The current method is to pick several times during fruit development fruit samples and to analyse them in a destructive way. Sufficient experience is necessary to correctly interpret the results of analyses. Appreciable errors can occur, if an apple cultivar shows high variability in fruit maturation. However, reduction of errors by taking enlarged samples is not viable. More reliable results could be expected if non-destructive continuous monitoring of the same fruits during fruit development on tree would be possible. Measurements of fruit ripening on the tree were carried out by using portable equipment with a miniaturised spectrophotometric module. By means of a specific glass-fibre probe, the partial light transmittance through fruit was detected in wavelength range from 500 to 1000 nm. Tests were performed on ‘Elstar’ apples in a commercial orchard during pre-harvest and harvest period 2003. From the spectral data, different indices were derived to characterise the fruit pigment change and maturity progress in comparison with conventional laboratory analyses for fruit maturity analysis. The portable spectrophotometer worked reliably and indicated sensitively changes of the spectral signature of apple fruit during the measuring period. Spectral indices as well as laboratory data demonstrated very high variance of maturity progress between individual ‘Elstar’ fruits. The spectral indices of each single fruit changed continuously over the time. The red-edge as well as a normalised chlorophyll index exhibited moderate decline, while a normalised anthocyanin index increased sigmoidally. The results obtained with these indices showed the potential of non-destructive fruit maturity monitoring on the tree.