Simple optical criterion of the ripeness level of strawberries

Abstract

Reflection spectra from strawberries of four botanical varieties at seven different ripeness levels were investigated in the visible range of 400–700 nm. Reflection spectra of unripe berries exhibit two maxima at wavelengths of 550 and 700 nm and a characteristic dip between them in the range of chlorophyl absorption at 670–680 nm. In the course of ripening, a significant transformation of the first maximum occurs up until its complete disappearance from the spectra of overripe berries, along with its shift to the long wavelength range of the visible spectrum. A gradual increase in the reflection coefficient at the wavelength of 670 nm and smoothening of the spectral curve of ripe and overripe berries are observed at the same time. We established that the ratio of reflection coefficients at the wavelengths of 670 and 630 nm, R670/R630, can serve as a universal ripeness criterion, including the final ripening stages.