A mechanical study of retting in Flax (Linum usitatissimum L.) and Hemp (Cannabis sativa L.) stems

Flax and hemp produce phloem fibres towards the periphery of their stems; individual fibre cells are cemented into discrete bundles around the xylem core by a complex matrix of pectins, hemi-celluloses and lignin. Extraction of undamaged fibres from this structure is critical in successful fibre production. Typically, the cut stems are laid on the ground in swaths for several weeks, where aerobic fungi colonize the senescing stem and hydrolytic enzymes digest the fibre-bundle matrix (dew-retting). More recently, flax stems have been chemically desiccated and the fibre-bundle matrix digested in the standing crop, prior to cutting (stand-retting). Dew-retted hemp stems were clamped horizontally into the jaws of a tensile testing machine and the work required to remove a strip of fibre-containing peel from the xylem core was measured. A similar investigation was carried out on desiccated flax stems, but samples were clamped vertically due to their finer diameters. Results show an initial increase in the work to peel associated with the dehydration of the stems, from 185–210 J m-2 in dew-retted hemp and 210–540 J m-2 in desiccated flax. In both cases this initial increase was followed by a 45% reduction in the work to peel during the following 3–4 weeks, despite relatively constant stem moisture content. This is thought to be a result of the retting process. The peel test can be used to monitor changes at the interface between the fibres and the xylem core during retting, allowing the process to be quantified and compared under different production conditions