Tracking the dynamics of hemp dew retting under controlled environmental conditions

The use of natural fibers such as hemp (Cannabis sativa L.) as substitutes for nonrenewable fibers increases the life cycle performance of composite materials. The management of retting in fields as a natural pretreatment prior to fiber extraction remains challenging due to a lack of knowledge about the relative importance of environmental and biotic factors, which continually interact under field conditions. Here, we studied the dynamics of hemp retting under controlled air temperature and humidity conditions and with simulated rain. We tracked the color and infrared spectral absorbance of the stem surface, the chemical composition and microbial enzyme activities of the bast tissues, and the stem architecture over 42 days at 15 °C. Color changes on the stem surfaces were the first indicators of retting progress, with a significant decrease in the L*, a* and b* values (CIELAB) from day 14 onward. These findings were closely correlated with the surface colonization progress as revealed by scanning electron microscopy, the changes in enzymatic activities and the decohesion of the bast tissues. Additional investigations are needed to study other environmental scenarios to provide an accurate assessment of the retting process over time.