Spectral colour changes: a promising parameter for smart flax dew retting monitoring solutions

Natural fibres from plants such as flax ( Linum usitatissimum ) are gaining interest as possible green replacements of synthetic fibres to reduce textile pollution footprints. However, the extraction-facilitating process of bast fibres from the flax stem, i.e. dew retting, is still empirically evaluated, resulting in either over or under retted fibre, both of which decrease the resulting fibre quality and introduce batch variability. There is currently no in-the-field device available to help farmers to monitor and manage flax dew retting. Flax growers generally use a ‘Fried-test’ or ‘organoleptic’ evaluations to define dew retting progression. In this study, we have measured this stem surface colour change using a spectrophotometer. Spectra in 400 to 700 nm range were acquired during 3 retting periods, carried out in different retting years, involving 4 fields in total. Cluster analysis of spectra revealed four distinct stages of retting which we termed: beginning, early, middle and advanced. Moreover, by the use of a statistical pipeline including PCA and PLS-DA we established that measurements of only four selected wavelengths (480, 490, 600, and 610 nm) are sufficient to predict the retting stage with a high degree of confidence. This work paves the way for the future development of colour sensors, which could be mounted on drones. Such onboard sensors would provide farmers with a practical tool to identify the optimal moment to end retting.