| Corn starch has the largest production of all varieties of starch all over the world. It has wide applications in food, textile and paper fields. However, due to the retrogradation of corn starch paste, its application effect is significantly influenced. The 1,4-α-glucan branching enzyme(GBE, EC 2.4.1.18), a member of glycosyl hydrolase family 13(GH13), catalyzes the formation of branch points in amylose and amylopectin by cleaving an α-1,4 glucosidic bond and attaching the released linear chain, through an α-1,6 glucosidic bond, to C-6 hydroxyl position of a glucan chain. In this study, corn starch was treated with the GBE from Geobacillus thermoglucosidans STB02. The effects of reaction conditions on the highly branching treatment of corn starch were investigated. The effect of highly branching treatment on the molecular structures was analyzed and the branching mechanism was also studied. Meanwhile, the effect of highly branching treatment on the starch short- and long-term retrogradation behaviors and the relationship between molecular structures and retrogradation properties were investigated. These results may provide a new method to improve starch retrogradation properties and lay the foundation for the industrial application of GBE.Firstly, the reducing sugar content of corn starch after debranching was selected as an evaluation index. The effects of all kinds of reaction conditions, including temperature, pH, amount of GBE, and time on the highly branching treatment were studied. The reducing sugar content was highest when the temperature was 50°C, the pH was 7.5, the amount of GBE was 300 U/g starch and the reaction time was 24 h, suggesting that the treatment effect was the best under these conditions.Secondly, the effect of highly branching treatment on the molecular structures were investigated. GBE treatment of corn starch could time-dependently increase the ratio of α-1,6 glucosidic bond, the weight-average molar mass of amylopectin and the content of short chains, and decrease the amylose content, the weight-average molar mass of amylose, the content of long chains, the β-amylase hydrolysis rate and the average chain length of amylopectin. After treatment for 10 h, the ratio of α-1,6 glucosidic bond was increased by 64.6%, while the amylose content, the β-amylase hydrolysis rate and the average chain length of amylopectin were decreased by 29.5%, 15.4% and 11.8%, respectively.Thirdly, the branching mechanism as catalyzed by GBE was analyzed.(1) GBE cleaves the α-1,4 glucosidic bond of amylose and attaches the released linear chain, through an α-1,6 glucosidic bond, to the amylopectin.(2) GBE cleaves the α-1,4 glucosidic bond of amylopectin and attaches the released linear chain, through an α-1,6 glucosidic bond, to the same amylopectin.(3) GBE cleaves the α-1,4 glucosidic bond of amylopectin and attaches the released linear chain, through an α-1,6 glucosidic bond, to the different amylopectin.Fourth, the effect of highly branching treatment on the starch short-term retrogradation behavior and the relationship between molecular structures and short-term retrogradation were studied. Dynamic time sweep analysis confirmed that the storage modulus(G’) decreased and the loss tangent value(tan ?) increased with increasing GBE treatment time. Pulsed nuclear magnetic resonance analysis demonstrated that the transverse relaxation times(T2) of highly branched corn starches were higher than that of the control. Brabender results suggested that the peak viscosity, breakdown value, setback value and final viscosity of highly branched corn starch were all lower than that of the control. These results indicated that the short-term retrogradation of corn starch was retarded by GBE treatment. After treatment for 10 h, the setback value of starch was reduced by 45.7%. More importantly, the setback value of starch was positively correlated with amylose content(r = 0.9948). Thus, the short-term retrogradation of corn starch was retarded may result from the decreased amylose content of corn starch.Lastly, the effect of highly branching treatment on the starch long-term retrogradation behavior and the relationship between molecular structures and long-term retrogradation were studied. Differential scanning calorimetry(DSC) results demonstrated that the retrogradation enthalpy(ΔHr) of highly branched corn starch was reduced. Fourier transform infrared spectroscopy(FTIR) analysis revealed that GBE treatment led to a decrease in hydrogen bonds within the starch. X-ray diffraction(XRD) results indicated that the relative crystallinity of starch was reduced by GBE treatment. These results suggested that the long-term retrogradation of corn starch was restrained by GBE treatment. After treatment for 10 h, the relative crystallinity of starch was reduced by 22.0%. In starch pastes aged two weeks at 4°C, the relative crystallinity showed positive correlations with the average chain length of amylopectin(r = 0.9889) and the amylose content(r = 0.9825). Therefore, the long-term retrogradation of corn starch was restrained may result from the decreased average chain length of amylopectin and amylose content of corn starch. |
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