Effect of chemical composition and cell structure on water vapor sorption behavior of parenchyma cells and fiber cells in moso bamboo (Phyllostachys edulis)

Efficient water sorption in bamboo and water transport between cells are closely related to the rapid growth of bamboo and subsequent dry processing. In this study, the water vapor adsorption behavior of parenchyma cells (PCs) and fiber cells (FCs) in moso bamboo (Phyllostachys edulis) was evaluated using a dynamic water vapor sorption (DVS) test. The effects of the chemical composition and cell structure on the adsorption mechanism were investigated with the Hailwood-Horrobin (H-H) model. The effect of heat treatment on the hygroscopicity of the PCs and FCs was determined using Fourier transform infrared (FT-IR) spectroscopy, Raman mapping, and X-ray diffraction. The results showed that the equilibrium moisture content (EMC) of the PCs was higher than that of the FCs, and the contribution ratio of the PCs and FCs to the hygroscopicity (EMC at 95% RH) of the bamboo was 12:9. The hygroscopic mechanism of the PCs was obtained from the H-H model. A high hemicellulose content provided adsorption sites (-OH) in the PCs, resulting in a high multilayer water content and EMC. The PCs structure further increased the EMC through capillary condensation due to the low cell wall density, large specific surface area, and large cell cavity size. The high EMC in the outer PCs and the low EMC in the inner FCs of the bamboo slivers formed a water pressure gradient, enhancing the trans-wall diffusion of bound water coupled with pit channels in the cell wall. The heat treatment resulted in a larger reduction in the hygroscopicity of the PCs than the FCs due to the higher degradation of CO and -OH of hemicellulose and lignin and the higher cellulose crystallinity of the PCs. The PCs possessed lower thermal stability than the FCs. This research improves our understanding of the structure-composition-function relationship between the cell structure, chemical composition, and moisture adsorption of bamboo. It also provides scientific support for developing new fiber-based composite materials and the pretreatment of high value-added PC products.