Effect of enzyme‐assisted acid extraction on citrus high‐methoxyl pectin riched in RG‐I: preparation, structure characterisation and physico‐chemical properties

The properties of pectin extracted by enzyme‐assisted nitric acid (EAE‐N) and pectin extracted by enzyme‐assisted oxalic acid (EAE‐O) were analysed and compared with pectin extracted by nitric acid (AE‐N) and pectin extracted by oxalic acid (AE‐O) systematically. The yield of pectin obtained by the enzyme‐assisted acid method (23.73% ~ 25.60%) was significantly higher than that by acid extraction method (19.75% ~ 20.60%). Monosaccharide analyses demonstrated that the enzyme‐assisted acid method was able to attain more branched rhamnogalacturonan I (RG‐I) regions (36.59% ~ 42.06%) than using acid solely (31.01% ~ 34.58%). Fourier transform infrared spectroscopy (FT–IR) and nuclear magnetic resonance ( 1 H NMR) analyses indicated that there was no obvious difference in the characteristic structure of pectin. The atomic force microscopy (AFM) images revealed that EAE‐N and EAE‐O had more short side chains and cross‐link compared with AE‐N and AE‐O. Thermogravimetric analysis (TGA) shows that EAE‐N and EAE‐O have good thermal stability. All pectin samples showed a shear‐thinning fluid flow behaviour and an elastic gel‐like behaviour. The results suggested that enzyme‐assisted acid provided a viable alternative method for the extraction of pectin.