Multi-spectral analyses in the VIS-NIR-SWIR for detection of ideal harvesting time and degree of ripeness

In this work we aim to develop an optical method for determining the degree of ripeness of cultivates in production and post-harvest based on spectral measurements. The method relays on a combination of measured spectral quantities and derived spectral indices that serve as a signature for the degree of ripeness. The spectral measurements combine the visible (VIS), Near-Infrared (NIR), and Short-Wave Infrared (SWIR) range and detect the most relevant sensitive molecular groups in cultivates. The measurements were carried out during the season 2023 in four apple varieties with different pigments. Subsequently, the same methodology was applied to supermarket avocados to determine their optimal degree of ripeness. The spectral features indicate distinct sensitivity bands across three different spectral ranges. Within the VIS region, three categories of molecular groups —carotenoids, anthocyanins, and chlorophylls—are identified at wavelengths of 480 nm, 550 nm, 670 nm, and 700 nm, respectively. The optical spectra were normalized by its value at 800 nm due to the consistent value throughout the season. The SWIR region exhibits six distinct sensitivity bands, categorized into C-H and O-H(H₂O) groups. The C-H overtone is identified at 1154 nm and 1770 nm, while the H₂O bands are observed at 1190 nm, 1450 nm, 1920 nm, and 2180 nm. Moreover, the findings provide a basis for developing a low-cost multi-spectral sensor that can be easily implemented in every step of the chain to reduce food losses and waste.