Optical measurement techniques for the ripeness determination of Braeburn apples

摘要

The manual picking of fruits is a labor intensive operation and hence one of the major costs in fruit production. As a solution, the mechanical harvest of fruits has recently been investigated extensively. However, the mechanical harvest brings forward several challenges such as automatic fruit ripeness detection. So far, apple harvest has relied on the subjective ripeness decision of the picker which can lead to postharvest disorders or highly heterogeneous products. An automated mechanical harvester has to be able to solve this problem by assessing if apples to be harvested are ripe or not in a fast, reliable and non-destructive way. In this sense, optical methods are promising in several fruit and food applications. Therefore, in this study, three optical techniques have been evaluated with respect to their potential for non-destructive evaluation of apple ripeness in the orchard. The experiments were conducted on the bicolored Braeburn apple variety picked weekly until commercial harvest. The first technique investigated is VIS/NIR spectroscopy in which light with wavelengths ranging from 380 nm to 1690 nm is sent onto the apple sample. After interaction with the apple, the light is dispersed in its different wavelengths (post-dispersive), and captured on the diode array spectrophotometer to give a spectrum per measurement. The second method is the Multiplex® apparatus which uses light-emitting diodes (LEDs) with different excitations (UV, blue, green and red) and silicon photodiodes to capture the fluorescent light after interaction with the apple. From these fluorescence measurements it calculates several physiology-related parameters like flavonol content, anthocyanin content, chlorophyll content, blue-green fluorescence and other fluorescence ratios. The last method is Hyperspectral Laser Scatter Imaging (HLSI) which is a contactless technique that projects a small beam of monochrome light in the 550-1000 nm range onto the apple to generate a glow spot caused by diffuse reflectance. A CCD camera is used to capture an image of this spot per wavelength (pre-dispersive). From these scatter images information on scattering and absorption properties of the apple is derived. Destructive measurements were performed on the apples as a reference for ripeness. The apple firmness, refraction value and starch degradation were combined in the Streif Index parameter. Braeburn apples are considered to be ripe enough for picking if the Streif index is below 0.2. Using partial least squares (PLS) analysis, the different output parameters of the optical measurement techniques were correlated with the Streif index to give an indication of fruit ripeness. All techniques show good correlations in this cultivar with R2 ranging from 0.83 to 0.96. The potential of the different techniques to be used in a robotic harvesting system mainly relies on the cost, system dimensions and measurement speed.