The influence of ultrasound on the structure, rheological properties, and degradation path of citrus pectin

Abstract

The effects of ultrasound on the molecular weight, structure, and rheological properties of citrus pectin were investigated. The degradation path of citrus pectin by ultrasound was also studied. The structure and rheological properties of the degradation products were identified by high performance liquid chromatography-photodiode array detector (HPLC-PAD), Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance spectroscopy (NMR), atomic force microscope (AFM), and rheometer. The results indicated that the average molecular weight of citrus pectin decreased rapidly after ultrasound treatment and reduced to one third of the initial pectin after treated for 90 min. The polydispersity reduced from 2.30 to 1.59. The degradation products had a uniform and narrow distribution of molecular weight. The main chain composition and monosaccharide constituents of citrus pectin remained unchanged after ultrasound treatment. The reduction ratio of (Gal+Ara): Rha suggested a decrease in neutral sugar side chain size of citrus pectin after ultrasonication. FT-IR and NMR results approved that the main chain of citrus pectin was not changed by ultrasound treatment. Together with the AFM results indicated that ultrasound could reduce the branched structure of citrus pectin. The viscosity of citrus pectin decreased after ultrasound treatment. Meanwhile, the ultrasound-treated citrus pectin showed predominantly viscous responses (G′ < G″) over the same frequency range.