Study On Thermal Aggregation Behavior Of Sugar Beet Pectin And Its Effect On Emulsion Properties

As a natural dietary fiber,Pectin became more and more popular because of its many physiological functions.Thermal treatment is an essential operation in the process of pectin production or processing,the quality of pectin is closely related with thermal treatment way and condition.According to the research,the functional properties of pectin under thermal treatment changed significantly,such as rheological properties,emulsion properties and solubility.However,the current research on the reaction of pectin under dry heat treatment is still at the technology level,and its dry heat reaction is not yet clear.Therefore,it is necessary to carry out in-depth research on the dry heat reaction and the molecular structure changes of pectin under thermal treatment to provide reference materials for Pectin producers and to expand the basic research on pectin.In this paper,sugar beet pectin?SBP?was selected as the research object,the dry heat reaction of SBP powder was induced at 45,60,75 and 110?.First,the aggregation behavior of SBP was characterized by its aqueous-solution properties?solubility,turbidity,particle size?,and investigate emphatically the effect of thermal treatment on side chain of SBP.The chemical composition and structural characteristics of water-insoluble molecular thermal aggregates were then studied to reveal the thermal aggregation mechanism of SBP.Finally,the effect of thermal treatment on the emulsion properties of beet pectin was investigated.The main conclusions are as follows:SBP was treated by dry heat,with the turbidity,average particle size and weight-average molecular mass of its solutionand the surface hydrophobicity index of SBP,the thermal aggregation behavior of SBP were systematically characterized.Results showed that the turbidity,average particle size and weight-average molecular mass of SBP were all increased after heating.When SBP was heated 2 h at 110?,the turbidity and average particle size increased from 0.373 to 0.651 and 472.2 to 1230 nm,respectively,which inferred the occur of thermal aggregation phenomenon.Protein was accumulated in aggregates,the aggregation degree has a positive correlation with the heating temperature and time,higher temperature is more effective to promote the aggregation,which is because protein is more sensitive to temperature changes.The hydrophobic groups of SBP were gradually exposed during thermal treatment,which increased the surface hydrophobicity index and decreased the solubility.In order to determine the variety and contents of the neutral sugar in SBP,and to research the effect of thermal treatment on side chain of SBP,a two-step hydrolysis method was proposed for the SBP.In the method,the neutral sugars in the SBP were sequentially hydrolyzed by using pectinase and trifluoroacetic acid?TFA?.First,the SBP was degraded into the fractions of smaller relative molecular mass by pectinase.Second,the neutral sugar attached to the SBP were liberated into solution by TFA.The results showed,among three commercial pectinases,Rapidase C80 was selected as the objective pectinase because of its lower endogenous monosaccharide content and better hydrolyzing activity.It is found that the optimal hydrolysis parameters of C80 are 50?,pH 5 and 15 h.Moreover,the effects of the TFA concentration,the temperature and the hydrolysis duration on the hydrolysis of the neutral sugar were further investigated.By compromising the hydrolysis rate and the degradation rate of the neutral sugar,suitable TFA concentration,temperature and hydrolysis duration were respectively determined to be 1 mol/L,110?and 4 h.SBP sequentially hydrolyzed by Rapidase C80?50?,pH 5 and 15 h??TFA?1 mol/L,110?and 4 h?,the contents of neutral sugar have no significant difference between the SBP before and after heating,which indicated that the side chain monosaccharide had not degraded during dry hear treatment.The structure characteristics of SBP thermal aggregates under dry heat were investigated.Results showed that thermal aggregates of SBP include water-soluble aggregates and insoluble aggregates.The soluble thermal aggregates are instability,its aggregation structure will depolymerize under the action of 0.1 mol/L NaNO₃,indicating that the structure of aggregates may be maintained through weak interaction forces.The insoluble aggregates were characterized by measuring its hydrophobic structures and their relative content to explore the mechanism of thermal aggregation.The results showed,compare with the SBP soluble aggregates,the amounts of neutral sugar in water-insoluble aggregates were lower,and the protein content was about 16.2%,but the molar ratio of hydrophobic amino acids was much less,which indicates that there is no direct relationship between the water insoluble property and the content of hydrophobic amino acids.Confirmed by thermogravimetry,nuclear magnetic resonance and infrared spectroscopy,the water-insoluble aggregates have slower thermal degradation rate,carboxyl and amino have dehydration condensation reaction between pectin molecule.The above results indicate that the amidation reaction of amino and carboxyl groups during dry heat is the direct cause of the formation of insoluble aggregates.,and the cross-linked sites are closely related to protein,proteins are the key components of insoluble aggregates.The formation of water-insoluble aggregates is the main reason for the change of endogenous fluorescence intensity in SBP.The effect of water soluble and insoluble aggregates on the emulsifying properties of SBP was studied.Combined with the light microscopy,the emulsifying activity and emulsion stability were characterized by measuring the particle size,particle size distribution and Zeta-potential of emulsions.Results showed that the dry thermal treatment with high temperature?75??12 or110??2 h?can improve the emulsifying activity and emulsion stability of SBP,while the dry thermal treatment with low temperature?45??has no effect.When SBP was heated 2 hours at 110?,soluble aggregates resulted in the particle size of fresh emulsion reduce from 1.51?m to 1.11?m,while the water insoluble aggregates almost have no effect on the average particle size of the emulsion.Although heat treatment leads to the formation of soluble aggregates,it has no significant effect on the Zeta-potential of the SBP emulsion,which indirectly indicates that the soluble aggregates enhance the emulsification stability by the steric effect.