Evolution of Vis/NIR bulk optical properties of apple skin and flesh during fruit maturation

There is a large demand for the non-destructive measuring and monitoring of fruit quality to optimize the harvest time, the postharvest storage and the consumption quality. However, quality as perceived by consumers depends on many chemical and physical properties, such that the determination is not straightforward. Optical measurement techniques like Vis/NIR spectroscopy and hyperspectral imaging have proven to be useful to determine quality attributes of several fruits, but often lack accuracy and robustness. The interaction of Vis/NIR light with the fruit tissue is determined by a complex interplay of absorption related to the chemical composition and scattering related to the microstructure properties. As optical measurement techniques are based on the measurement of reflected and/or transmitted photons, more knowledge on the relation between these measurements and the sample’s absorption and scattering properties is indispensable to further improve the sensor design and prediction models for fruit quality.

Therefore, in the present work, the bulk optical properties (BOP - absorption, scattering and scattering anisotropy) of three apple cultivars (Braeburn, Kanzi and Greenstar) were studied during maturation. During a period starting 45 days before and ending 17 days after harvest, the BOP of both the skin and flesh of five apples per cultivar were characterized using a double integrating spheres setup. A supercontinuum laser in combination with a monochromator was used to illuminate the samples in the wavelength range from 500 to 1850 nm. The measured total reflectance, total transmittance and unscattered transmittance served as inputs to an Inverse Adding-Doubling algorithm to obtain the BOP.

In the studied wavelength range, the absorption coefficient (µa) showed features of anthocyanin, chlorophyll and water. During maturation, the absorption by anthocyanin in the skin (550 nm) increased towards the moment of harvest (up to 50 cm-1) for the Braeburn and Kanzi cultivars. Chlorophyll, with an absorption wavelength around 680 nm, was present in all samples. Moreover, during maturation, a downward trend was seen in the chlorophyll content, both for the apple flesh and skin. Highest concentrations of chlorophyll were found in the green cultivar Greenstar (up to 20 cm-1) and in Braeburn (up to 16 cm-1), while a lower absorption was seen in Kanzi (around 8 cm-1).
No large fluctuations were observed in the reduced scattering coefficient (µs’) and the anisotropy factor (g) during maturation. The reduced scattering coefficient showed a difference between apple skin and flesh at wavelengths below 1000 nm, especially in the Braeburn cultivar, with a µs’ of 15.3 cm-1 for the skin compared to 8.3 cm-1 for apple flesh at 600 nm. As in most biological materials, an anisotropy factor close to 1 (forward scattering) was observed.

The obtained results can be used in simulation studies to better understand the light propagation in apple tissue, which could be valuable in the development and optimization of optical sensors and data processing techniques.