Evolution of flax cell wall ultrastructure and mechanical properties during the retting step.

•The impact of field retting time was studied on six flax fibres samples.•AFM measurements revealed a significant increase of cell wall stiffness with retting.•XRD and NMR investigations showed an increase of cellulose crystallinity with retting.•NMR evidenced a compaction of inaccessible polymers for most retted sample.•This densification can be a possible explanation of the indentation modulus increase.