| Starch is a renewable resource with a wide range of sources,but the problem of prone to retrogradation of native starch limits its industrial application.Maltogenicα-amylase modification is one of the effective methods to inhibit starch retrogradation.Enzymatic modified starch granule retains the gelatinization ability of starch,and avoids the problems of high viscosity and retrogradation of the gelatinized system,and can carry out high-concentration starch modification,so it has unique advantages compared with modified gelatinized starch.At present,the studies on enzymatic modification of starch granules mostly focus on the improvement of the physicochemical properties of starch,and there are few studies on the effect of modification on starch structure.And there are few studies on the relationship between the degree of modification and the physicochemical properties of starch because the lack of quantitative method of the degree of modification.In this research,wheat starch was used as research object,and modified with maltogenicα-amylase under non-gelatinized condition to study the effect of enzymatic modification on starch structure and retrogradation properties.The correlation between degree of modification and retrogradation properties was analyzed.The factors affecting the degree of modification of starch granules by enzymes were explored.The main research contents are as follows:(1)Preparation of modified wheat starch granules with maltogenicα-amylase and study on structure changes of modified starch.The wheat starch granules were modified by maltogenicα-amylase at 55℃,and the structure of the modified starch was characterized.Scanning electron microscopy(SEM)showed that the modified starch granules had surface pitting.Polarizing light microscopy(PLM)showed that the polarized crosses of modified starch granules were not completely destroyed;X-ray diffraction(X-RD)showed that the modified starch granules remained A-type crystalline pattern,and the relative crystallinity was higher than that of the control group;Small angle X-ray scattering(SAXS)showed that the long period of starch granules and the thickness of crystal lamellae decreased and the thickness of amorphous lamellae increased after modification.The molecular structure of modified starch showed that the enzymatic modification did not cause significant changes in amylose content in wheat starch,but shortened the molecular chain length of amylose,and the chain length distribution of amylopectin changed significantly,the content of medium and long chains(DP9-30)decreased,and the content of short chains(DP<9)increased.(2)Retrogradation properties of modified wheat starch granules.The swelling power test showed that the swelling power of the modified starch granules decreased;rapid viscosity analyzer(RVA)showed a decrease in peak viscosity,trough viscosity,final viscosity and setback value during gelatinization of the modified starch.The setback value of wheat starch treated with maltogenicα-amylase at the dosage of 15.0 U/g starch(dry basis)was 88.91%lower than that of the control group.Differential scanning calorimetry(DSC)showed that the gelatinization onset temperature,peak temperature and conclusion temperature of modified starch were higher than those of native starch,the range of gelatinization temperature was narrowed,and the gelatinization enthalpy decreased.The retrogradation enthalpy decreased significantly.The 28-day retrogradation enthalpy of wheat starch treated with maltogenicα-amylase at the dosage of 15.0 U/g starch(dry basis)was 97.35%lower than that of the control group.(3)Study on the relationship between the degree of hydrolysis and retrogradation properties of modified wheat starch.The degree of hydrolysis was used as the measure of the degree of modification of wheat starch granules by maltogenicα-amylase,and the correlation between the degree of starch modification and retrogradation was analyzed.It was found that the degree of hydrolysis and the setback value in the RVA curves and the retrogradation enthalpy measured by DSC showed a linear negative correlation(the correlation coefficient R~2 are above0.9).Therefore,the retrogradation properties of modified starch can be regulated by adjusting the degree of hydrolysis during the modification process.(4)Study on the influencing factors of the enzymatic modification on starch granules.The wheat A and B starch granules were separated by natural sedimentation method and modified by maltogenicα-amylase.The factors affecting the enzymatic modification of starch granules were explored from the perspective of the difference in the structure of the two kinds of starch granules.The results of the degree of hydrolysis showed that under the same conditions,the degree of hydrolysis of wheat B starch was higher and the degree of modification was greater,so the short-term and long-term retrogradation of B starch after enzymatic modification were more inhibited.The particle size distribution results showed that the volume-average diameter of B starch granules was 5.43μm,which was smaller than that of A starch granules(18.98μm),and its specific surface area was larger and there were more enzyme adsorption sites.SEM showed that the surface of B starch granules was rougher;PLM showed that the polarized cross of B starch granules was weaker than that of A starch,and the double helix order in the crystalline region of starch granules was lower;X-RD showed that the relative crystallinity of B starch granules was 23.85%,which was smaller than that of A starch(27.76%);SAXS showed that the mass fractal dimension(D_m)of B starch granule was 2.08,which was smaller than that of A starch(2.44),and the granule structure of B starch was looser.The correlations between the degree of hydrolysis and retrogradation properties of wheat A and B starch were analyzed respectively,and it was found that they all showed a linear negative correlation(R~2>0.9),which further confirmed the correlation between the degree of starch modification and retrogradation properties. |
PDF Full Text Request