| Pectin as an important source of dietary polysaccharides widely found in fruits and vegetables,which had the characteristics of gelling,stabilization,thickening,emulsifying and texture improvement.However,due to large molecular weight and complex structure of pectin,its actual applications in food and medicine industries are severely restricted.Ultrasound modification of pectin has become a hot topic,but the changes in solution conformations and mechanism need to be further studied.Thus,in this study,the effects of ultrasonic intensity on chain rigidity,molecular size and microstructure of commercial pectin during ultrasonic degradation were investigated by using the methods for chain conformation analyses and the theories of polymer solution.The mechanism of chain conformational transition of pectin was preliminarily discussed.On this basis,the functional properties of pectin and sonicated pectins were determined.This study will establish the theoretical basis for functional design and process control of pectin structure,and broaden the actual applications of pectin in the food industry.The main results are as follows:（1）The molecular structure and chain conformation of the original commercial pectin were analyzed by means of combined physical and chemical methods,together with theory of dilute polymer solution.The results showed that the contents of galacturonic acid（GalUA）,methoxy group and degree of esterification of commercial pectin were 71.69±1.7%,12.27±2.4%and 76.63±0.5%,respectively,indicating that the pectin is termed as a high methoxy pectin.The pectin was an acidic heteropolysaccharide mainly composed of GalUA,arabinose,rhamnose and galactose with a weight-average molecular weight（Mw）of 173.7 kDa.Moreover,the values of intrinsic viscosity（[η]）,z-average radius of gyration（Rg）,hydrodynamic radius（RH）,Mark–Houwink relation exponent（a）,conformation parameter（α）,structure characteristic parameter（ρ）of pectin were calculated to be 457.93 cm3 g-1,33.1 nm,17.5 nm,0.8205,0.6068 and 1.894,respectively,indicating that the original pectin predominantly behaved as an rigid semi-flexible chain conformation in aqueous solution.（2）In the process of ultrasonic degradation,with the increase of ultrasonic intensity and time,the molecular parameters including[η],Mw,Rg,RH and chain conformation parameters,such as a,α,andρ,of pectin decreased,while the fractal dimension increased.Specifically,the higher ultrasonic intensity led to the more obvious effect.These results revealed that the rigid semi-flexible chain conformation of pectin might turn into flexible,even flexible coils.AFM analysis showed that the network structures,which formed via inter-and/or intra-molecular entwining of the pectin chains,became disordered and partially fractured,even dissociated under the ultrasonic treatments.Moreover,the crossing points present in pectin networks were variously disrupted,and the formation of some individual strands increased,which further visually confirmed the changes in pectin chain conformation.（3）Fourier transform infrared spectroscopy and monosaccharide composition analysis showed that monosaccharide types and primary chemical structure of pectin basically remained unchanged in the process of ultrasonic degradation.Higher ultrasonic intensity and longer treatment time led to the increased GalUA content in pectin main chains,the decreased degree of methoxylation and the contents of neutral sugars in pectin side chains,and thus weakening the steric hindrances and chain rigidity of pectin chains.Meanwhile,Micro-differential scanning calorimetry and circular dichroism spectroscopy analyses showed that higher ultrasonic intensity（5.00W/mL）destroyed inter-and/or intra-molecular hydrogen bonds,leading to the destruction of molecular entanglements or network structures of pectin,and thereby enhancing the flexibility of pectin molecule.（4）With the increase of ultrasonic intensity and time,the apparent viscosity and complex viscosity of pectin reduced,which further led to the transition from pseudoplastic nonnewtonian fluid to Newtonian fluid.Moreover,dynamic viscoelastic analysis demonstrated that sonicated pectins tended to viscous-behaviors.The results of emulsifying properties revealed that the emulsions formed by sonicated pectins exhibited good emulsifying activity and stability,and showed increased tendencies as the increase of ultrasonic intensity and time.Meanwhile,the particle sizes and Zeta potentials of emulsions formed by sonicated pectins initially decreased and then increased as increasing the ultrasonic intensity and time.At sonication time of 30 min,the emulsions exhibited excellent stability.What’s more,after storage of 7 days,the emulsions still remained stable.In addition,the microscopy observation showed that the particle sizes of the emulsions formed by pectin were reduced and their distributions became uniform after ultrasonic treatment. |
