Abstract: Abstract: Low methoxyl pectin is widely applied as thickener, stabilizer and emulsifier in low calorie or dietetic foods. The main methods reported for the preparation of low methoxyl pectin from high methoxyl pectin have used four types of agents: acids, ammonia in alcohol or concentrated aqueous ammonia, alkali and pectin methyl esterase. The main disadvantage of the first two methods is the slowness of reaction. Alkaline de-esterification is rapid but the removal of methyl ester groups is accompanied by depolymerization of pectin chains. Enzymatic de-esterification represents an attractive alternative to chemical de-esterification, for it is without pollution, but it is time-consuming. Therefore, there is a need to explore new methods to produce pectin with high quality and efficiency. In this study, a novel method of high hydrostatic pressure combined with enzymatic treatment (pectin methyl esterase) was applied in pectin de-esterification. In order to find out the effect of enzymatic treatment assisted with high hydrostatic pressure on the properties of pectin and promote the industrial development of this technology, the physicochemical properties, molecular weight distribution and rheological characteristics of pectins de-esterified by this treatment were investigated and compared with conventional alkaline method. During the de-esterification process of enzymatic treatment assisted with high hydrostatic pressure of 200 and 300 MPa, the degree of de-esterification was reduced from 61.83% to 34.09% and 32.62% in 10 min, respectively. With further de-esterification, the reaction slowed down, while the de-esterification degree of pectin by alkaline method was reduced to 35.54% and decreased continuously when the period was prolonged to 20 min. Based on this experiment, two pairs of pectins with similar degree of de-esterification were prepared by the enzymatic treatment assisted with high hydrostatic pressure and conventional alkaline method, i.e. HHP-1 (200 MPa, 10 min; 34.09%±1.64%) & AP-1 (pH 11.0, 11 min; 31.96%±1.92%), HHP-2 (300 MPa, 15 min; 27.69%±0.51%) & AP-2 (pH 11.0, 16 min; 25.37%±0.61%). The galacturonic acid content of the pectins had no significant difference (P>0.05), while the apparent viscosity of pectins prepared by enzymatic treatment assisted with high hydrostatic pressure was higher significantly than that by alkaline method (P<0.05). The viscous flow activation energy of HHP-1 was lower than AP-1 prepared by alkaline method, indicating lower temperature dependence and better processability of HHP-1. The molecular weight distribution was determined by size exclusion chromatography. From the profile of molecular weight distribution, two peaks with the molecular weight distribution from 10 kDa to 800 kDa could be observed. The profile showed that the prepared pectins (HHP-pectins) by enzymatic treatment assisted with high hydrostatic pressure were similar to pectin untreated, but the pectins (AP-pectins) produced by alkaline method differed; the signal of Peak Ⅰ was weaker and Peak Ⅱ was stronger, and Peak Ⅱ's retention time was delayed as well, indicating that the pectin was depolymerized. The intrinsic viscosity was fitted and calculated using the Kraemer equation. The viscosity average molecular weights were calculated using the Mark-Houwink-Sakurada equation, which of the four pectins (HHP-1, HHP-2, AP-1 and AP-2) were 45 540±5 677, 47 802±3 984, 30 009±2 980 and 29 837±1 374 Da, respectively. The viscosity average molecular weight of HHP-pectins was higher than AP-pectins significantly (P<0.05). The shear stress of HHP-pectins was measured as a function of shear rate and compared with that of AP-pectins. The shear stress of HHP-pectins was much higher than AP-pectins and became strongly dependent upon the shear rate. From these results, it could be concluded that enzymatic treatment assisted with high hydrostatic pressure had no degradation action on pectin molecule and it could be taken as a highly efficient, novel and eco-friendly method for pectin de-esterification, especially for producing pectins with higher viscosity and molecular weight. Low methoxyl pectins with different degrees of esterification could be obtained by enzymatic treatment assisted with high hydrostatic pressure of 200 or 300 MPa in plant production.