Abstract: In order to further understand the effect of polyphenols commonly present in cloudy apple juice on the structure of pectin and the stability of composite systems containing them, the three major polyphenols (phlorizin, chlorogenic acid, epicatechin) in apples were separately mixed with 5 mg/mL apple pectin in water at different concentration gradients (0.03–0.70 mmol/L) determined according to their contents in apples from different varieties and producing area and of different maturities. The structure of apple pectin were investigated by ultraviolet spectroscopy, infrared spectroscopy and scanning electron microscopy. The physicochemical properties and stability of composite systems were analyzed by rheology, thermogravimetry, particle size and turbidity retention rate. The results showed that the polyphenols interacted noncovalently with pectin mainly through hydrogen bonds, and intramolecular and intermolecular hydrogen bonds, C–H, and C=O bonds all participated in the reaction. The surface of the composite systems was smooth, showing a sheet-like structure. The composite systems had higher thermal stability, and reduced particle size and turbidity than did pectin. Chlorogenic acid at 0.08 mmol/L could increase the apparent viscosity of the system and enhance the pseudoplasticity. Phlorizin at 0.03 mmol/L could improve the turbidity retention rate of the composite system. The research results provide a theoretical reference for improving the processing technology for cloudy apple juice products.

Key words: apple polyphenols; pectin; interaction; physicochemical properties; stability