Preparation and characterization of konjac glucomannan (KGM) and deacetylated KGM (Da‐KGM) obtained by sonication

Abstract
Background Konjac glucomannan (KGM) has been widely applied in food industry as thickening and gelation agent due to its unique colloidal properties of viscosity enhancement and gelling ability. The current study was aimed to prepare and characterize KGM and deacetylated KGM (Da-KGM) samples obtained by sonication in neutral and alkali ethanol-water solutions. Results The results showed that the deacetylation degree (DD) of Da-KGM increased exponentially with alkali concentration. Fourier transform infrared spectra (FTIR) spectra further confirmed deacetylation reaction through the dramatic decrease in the acetyl group band at 1740 cm-1. Besides, the high similarity among the tested groups in terms of X-ray diffraction (XRD) spectra implied a similar crystalline structure, while differential scanning calorimetry (DSC) curves revealed that the water binding capacity and decomposition temperature of KGM changed slightly with alkali and sonication treatment. The rheological profiles indicated that apparent viscosity (η0) of sonicated KGM samples was unchanged except for the T60 group (60 min sonication treatment). Particularly, ultrasonic treatment at high alkaline conditions (0.10 mol/L NaOH) was noted to promote deacetylation reaction, and the obtained samples showed decreased apparent viscosity and weakened the gelation process in aqueous solution. Partial correction analysis indicated that alkali rather than ultrasonic treatment resulted in the change of DD and η0 in Da-KGM. Moreover, sonication contributed to off-white color by reducing the browning caused by alkali in Da-KGM products. Conclusion Ultrasound-mediated heterogeneous deacetylation reaction is a feasible way to prepare Da-KGM samples with lightened browning and controllable DD.