Study On The Structural Characterizations Of A Pectin Extracted From Premna Microphylla Turcz And Its Cold Gelation With Whey Protein Concentrate

Premna microphylla Turcz is a deciduous shrub belonging to Premna,Verbenaceae species.Premna microphylla Turcz leaves contain high content of pectin,which can be utilized to prepare gel-like food.Whey protein concentrate(WPC)is obtained from whey by a series of purification and concentration processes,such as ultrafiltration,double filtration and concentration,to remove lactose.It can provide many nutritional advantages for food products.The aim of this study was to study the chemical structure of the ammonium oxalate soluble pectin(AOP)in Premna microphylla Turcz and its cold gelation with whey protein concentrate(WPC).The mechanism of the cold gelation between AOP and WPC is thus proposed.The main results are as follows:1.AOP was extracted by ammonium oxlate extraction method,and the yield of AOP was 17.63%.The protein and galacturonic acid contents of AOP were detected by Coomassie brilliant blue method and m-hydroxyldiphenyl method,respectively.The results showed that AOP contained 0.29%protein and 73.5%galacturonic acid.The result of titration experiment showed that the degree of esterification(DE)of AOP was 13.69%,indicating that the AOP extracted in this study was a low-methoxy pectin(LMP).In addition,HPAEC of AOP demonstrated that the neutral sugars of AOP were mainly composed of rhamnose,arabinose,galactose and glucose.The values of GalA/(Rha+Ara+Gal),Rha/GalA,(Ara+Gal)/Rha and HG/RG-I of AOP were 7.88,0.02,4.52 and 6.51 respectively.The results above indicated that AOP was a linear low-methoxy pectin rich in homogalacturonan with low branching degree of RG-I.The results of SEC-MALLS showed that molecular weight and polydispersity of AOP were 128.60 kg/mol and 1.56,reflecting the high homogeneity of AOP.The apparent viscosity study exhibited that the higher concentration of AOP solution showed shear-thinning flowing characteristics and was a pseudoplastic fluid.This was due to the local network and weak network structure formed in the AOP solution.High temperature and high pH would lead to the destruction of the weak network structure in the AOP solution,thus reducing the apparent viscosity of AOP solution.2.Gelation study showed that AOP and WPC could cold gelation through intermolecular interactions without preheating and acidification.In addition,the cold gelation between AOP and WPC could occur at low content of total biopolymers.Zeta-potential analysis,gel hardness test and dynamic rheological study showed that there was an optimal complex ratio(AOP:WPC=1:5),which greatly strengthened the degree of interaction between AOP and WPC to form a gel with the highest strength and the best solid-like properties.When the complex ratio was higher than the optimal complex ratio,the binding between AOP and WPC was weakened due to the charge neutralization effect and the increase of electrostatic repulsion between molecules,which made the gel strength and solid-like property of the composite gel decrease.3.AOP and WPC can form gels in a wide range of pH.Turbidity curves,FT-IR analysis,CLSM and ITC tests showed that when the pH was around the pH? of the AOP-WPC complex(pH 4.5),the gelation between AOP and WPC was an enthalpy driven process caused by electrostatic forces.With the increase of pH,the electrostatic interaction between AOP and WPC gradually weakened,while the hydrophobic interaction constantly increased.When the pH was higher than the pHc of the complex(pH 7.5),the complexation of AOP and WPC was an entropy driven process caused by hydrophobic forces.It can be inferred that the structure of composite gel formed by AOP and WPC was mainly based on AOP,and WPC played the role of crosslinking agent through electrostatic and hydrophobic interaction in the process of cold gelation.