| Polyuronate is a type of food polysaccharides,which has been widely applied in the food industry.The two typical polyuronates are sodium alginate and pectin.They are applied as gelling agent,thickener,emulsifier,and stabilizer in the food industry for the sake of their naturality,nontoxicity,and biodegradability.Ionic binding and gelation of polyuronates with metal ions are the basis of these applications.This project dealt with sodium alginate and pectin.Different oligouronates were extracted from sodium alginate and pectin,and well characterized using various techniques.Their modulatory effects on the ionic binding and gelation of polyuronates with calcium were investigated.Main conclusions are as follows:1.Oligoguluronate(GB)and oligomannuronate(MB)were extracted from alginate(ALG)using selective acid hydrolysis.By controlling time and pH of hydrolysis,two GB samples with decreasing molecular weight and increasing guluronate content were obtained: GB1 with Mw=12.1 kg/mol and a guluronate content of 92%;GB2 with Mw=44.5 kg/mol and a guluronate content of 85.1%.Correspondingly,two MB samples with different molecular weights were obtained: MB1=5.4 kg/mol;MB2=52 kg/mol.Oligogalacturonate(GaB)was successfully extracted from high methyoxyl pectin(HMP)by enzyme degradation: Mw=66.2 kg/mol;degree of methoxylation = 5.2%;a galacturonate content of 81.3%.The Flory index increased from 0.585 for untreated alginate or pectin to 0.878 for the oligomers,indicating the change of conformation in solution from flexible or semi-flexible coils to rigid rods.2.The effects of addition of the oligomers on the ionic binding of sodium alginate with calcium were studied using intrinsic viscometry in dilute solutions.The addition of MB1 and MB2 did not significantly influence the binding of calcium to alginate,while in the presence of GB1 and GB2 more calcium was needed for the critical increase of relative viscosity for alginate.Higher concentration of GB1 or GB2 was more effective in suppressing the increase of relative viscosity with addition of calcium.Moreover,the effect of GB1 was stronger than that of GB2,due to a lower molecular weight and a higher content of guluronate.The addition of GaB decreased the critical concentration of calcium that was required for the increase of relative viscosity.This might be related to the crosslinking of ALG with GaB.Furthermore,the effects of the oligomers on the water holding capacity of ALG gels were compared.GB1 and GB2 could markedly improve the water holding capacity of ALG gels subjected to centrifugation,while MB1,MB2 and GaB had negligible effect.3.The mechanisms of oligoguluronate(GB)modulating the calcium-induced gelation of alginate(ALG)were investigated by rheological method.In the low calcium regime(0.25<R<0.60),the saturated storage modulus of 0.5% ALG was decreased with increasing concentration of GB,at a fixed calcium concentration of 10.24 mM.Interestingly,when R=[Ca]/[G] was fixed,the addition of GB had no effect on thegelation of ALG.Fluorescence studies showed that the fluorescence of the mixture of ALG and Rodamine 123-labbed GB increased proportionally with increasing GB concentration.Atomic force microscopic(AFM)results demonstrated that the addition of GB prevented the network formation of ALG.It was suggested that ALG and GB tended to form egg-box dimers independently,and GB competed with ALG for calcium,thus reducing its gelling ability.In the higher calcium regime(R>0.6),the addition of GB could increased the saturated storage modulus of ALG gels.AFM results showed an increased tendency of lateral aggregation of ALG in the presence of GB.It was suggested that in this calcium concentration regime GB could enhance the gelation of ALG.Based on the results above,a competitive binding mechanism was proposed for GB modulating the calcium-induced gelation of ALG.4.The modulatory effect of GB on the gelation of low methoxyl pectin(LMP)was also studied comparatively.In the lower calcium concentration regime(R<0.25),at a fixed calcium concentration of 1.8 mM or a fixed R of 0.10,the addition of GB increased the saturated storage modulus of 0.5% LMP.AFM results showed that GB tended to promote the network formation of LMP.In the intermediate calcium concentration(0.25<R<0.60),the addition of GB decreased the network-forming ability of LMP.In the higher calcium concentration regime(R>0.60),GB turned to promote the lateral aggregation of LMP.The results together suggested that GB had a promotive effect on the calcium-induced gelation of LMP in the lower and higher calcium concentration regimes,while it tended to suppress the gelation of LMP in an intermediate calcium concentration regime.The promotive effect in the lower calcium concentration regime was attributed to the crosslinking of LMP under the mediation of GB. |
