Rheological Properties Of Apple High-methoxyl Pectin As Well As The Regulation Of Sucrose And Calcium Ion

This paper selected three apple high-methoxyl pectin（fast setting,medium setting and slow setting）as the research objects,the specific research contents and conclusions were as follows:（1）The research on the physical and chemical properties of apple high-methoxyl pectin:（1）The esterification degrees（DE）of three commercial apples were determined as 60.6%,66.1%and 72.4%,respectively.（2）Pectin with higher DE value was of higher weight average molecular weight（M_w）,number average molecular weight（M_n）,the particle diameters（d（v,0.5））that cumulative volume of particles reached 50%,the average length of side chains(L_sc),the molar percentage of arabinose（Ara）,the molar percentage of glucose（Glc）,etc.While the polydispersity index（PDI）,the molar percentage of rhamnose（Rha）,the molar percentage of fucose（Fuc）and linearity presented the opposite variations.（3）The intensity increment and band narrowing peak from X-ray plots as well as the structure morphology from SEM plots illustrated that the three pectins were of high crystallinity and difficult to dissolve.（4）The variations of characteristic peaks in FT-IR spectra can well present the structural differentiations of different apple high-methoxyl pectin（HMP）.（5）In addition,AFM characterization of the network structure and aggregation state of pectin aqueous sols（1.0%（w/w））showed that the structure was consecutive and different aggregates existed in the network subunits of different pectin dispersions.The agglomerates of pectin particles in the network subunits of the pectin（AP1）sol with a lower DE value are composed of curved rods（KR）,rods（SR）and spheroids（SP）.At the same time,the agglomerates are denser and bigger.The agglomerates of pectin particles in the network subunits of the pectin（AP2）sol with a moderate DE is mainly rod-shaped（SR）and spheroid（SP）,while the structure of the pectin（AP3）sol with a higher DE is mainly composed of short rods and some spheroids（SP）,and the aggregates look looser and smaller.（2）Concentration and temperature dependence of rheological properties of apple high-methoxyl pectin:（1）Under any pectin concentration and temperature conditions,shear viscosity decreased in logarithmic form with the increasing shear rate(_（50）)for all pectin dispersions.（2）On the whole,the results derived from steady shear tests demonstrated that the pectin dispersions showed their non-Newtonian pseudoplastic behaviour.（3）The frequency sweeps for pectin aqueous solutions exhibited the representative shape of macromolecular solutions,presenting a liquid-like viscoelastic behavior.（4）In detail,the shear viscosity（η）,storage modulu（G’）and loss modulu（G"）of pectin sols increased with pectin concentrations and DE.（5）η,G’and G’values all first increased and then decreased with the increase of the dissolving temperature.The maximum values ofη,G’and G’for AP1 and AP2 appeared at the dissolving temperature of 60 ^oC,while they appeared at the dissolving temperature of 40 ^oC for AP3.（6）As the ambient temperature increased,values ofη,G’and G’of the pectin systems all presented a declining trend.（7）Decreasing temperature from 60 ^oC to 25 ^oC increased the G’and G"values.（3）The effect of sucrose on the rheological properties of apple high-methoxyl pectin:All pectin systems displayed a shear-thinning nature at any tested sucrose level（45-65%,w/w）.Gelling temperature（T_g）in cooling as well as the degree of thixotropy,elastic modulus（G’）,viscous modulus（G’’）and equilibrium modulus（G_e）at ambient temperature all increased with sucrose concentration（c_s）.All above-mentioned parameters decreased with DE.This attested that sucrose favored the structure formation in pectin system,while its effect weakened in the system with a higher DE.The temperature sweep showed that a two-stage process involved in the structure formation in pectin system during cooling from 85 ^oC to 25 ^oC.The growth rate of G’in high temperature stage was lower than that in low temperature stage.（4）The effect of Ca²⁺on the rheological properties of apple high-methoxyl pectin:The zeta potential（ζ）,turbidity and fluorescent aggregation behavior increased with Ca²⁺concentration but they decreased with DE.The results of flow behavior tests showed that all HMP-Ca²⁺systems exhibited shear-thinning behavior.For the systems with Ca²⁺addtion,both shear viscosity and thixotropy all increased with the increase of Ca²⁺concentration,and decreased with DE.The results of dynamic viscoelasticity tests showed that the storage modulus（G’）for the systems with Ca²⁺addition increased with the increase of frequency and Ca²⁺concentration,but it decreased with DE.（5）The synergistic effect of sucrose and Ca²⁺on the rheological properties of apple high-methoxyl pectin:The simultaneous addition of sucrose and Ca²⁺could synergistically promote the formation of different high-methoxyl pectin systems.The results showed that sucrose addition or Ca²⁺addition contributed to the structure formation of different high-methoxyl pectin systems compared with the pure pectin sol systems.At the same time,both sucrose and Ca²⁺addition can strengthen the structure strength significantly,which reflected in the increase of viscosity and modulus.Pectin systems having fluidity exhibited their pseudoplastic behaviour and pectin systems with lower DE presented the stronger shear-thinning nature.At a certain sucrose concentration,Ca²⁺was not always conducive to the structure formation.When got to the stoichiometric saturation of free carboxyl groups,further Ca²⁺addition may hinder the structure formation,and thus reduce the gel strength.Pectin systems with lower DE were of higher gel strength.In conclusion,by adding Ca²⁺to regulate the gelation and thickening properties of HMP,it is helpful for the application of HMP in low acid or acid-free environment.The results of this study was helpful to broaden the application sope of HMP,and reduce the economic cost of Ca²⁺-related pectin gelatin products.