Effect of water-induced changes in molecular structure and vapor pressure on the heterogeneous hygrothermal degradation of konjac glucomannan

Abstract

The heterogeneous hygrothermal degradation (HHTD) method can simply and efficiently prepare depolymerized konjac glucomannan (KGM). The degradation rate was significantly improved when the KGM powder moisture was below 20%, but basically no longer increased after the moisture was above 20%. This study systematically examined the influence of the initial moisture content of powder on the molecular structure of KGM and the vapor pressure of degradation system to elucidate the association of these changes with hygrothermal degradation. The results indicated that with the increase of powder moisture, the disorder of KGM amorphous region increased dramatically, and the ordering degree of aggregate structure decreased. Water notably enhanced the freedom of KGM molecular chain motion by reducing the intermolecular hydrogen bond interaction and expanding the free volume. Furthermore, powder moisture markedly promoted the degradation of KGM by increasing the vapor pressure and mass transfer rate of the closed degradation system. The system reached saturated vapor pressure when the moisture content was above 20%. Consequently, the synergistic effect of the water-induced changes in KGM molecular structure and system vapor pressure significantly accelerated the hygrothermal degradation rate of KGM. The promotion effect of powder moisture on degradation reached the upper limit when the moisture content was higher than 20%, thus moderately solvated KGM-20% powder can achieve an efficient hygrothermal degradation effect. This work is expected to provide theoretical bases for the industrial application of HHTD as a controllable and efficient degradation method of KGM.