Studies On Preparation And Crystallization Of Starch Microspheres

As a kind of natural polymer, starch is renewable, inexpensive, biodegradable andnon-toxicity, which make it become one of the most promising biodegradable materials.Due to carrying abundant hydroxyl groups, starch can be easily modified to meet differentneeds. With microporous structure, high surface area and easy accessible active hydroxylgroups, starch microspheres could adsorb free compounds effectively and prove adsorptioncapacity. So starch microspheres could be used as drug carriers, adsorbents and embeddingagents. However, there are some problems in the preparation of starch microspheres, suchas long time costing, introducing of toxic reagents and non-uniform of particle size, whichlimits its application in the area of food, medicine and so on.In this study, corn starch and β-amylase hydrolyzed amylase with different degreewere used to prepare starch microspheres by precipitating starch paste solution withethanol as the precipitant. The influence of starch suspension concentration, ethanol addingspeed, stirring rate and precipitate temperature on the particle size of starch microsphereswere investigated. Amylose with different molecular chain length were prepared byhydrolysis with β-amylase, then the effect of molecular chain length on the particle sizeand crystallization property of amylase starch microspheres was investigated. At last theinfluence of storage time, storage temperature and plasticizer on the crystallizationproperty of starch microspheres was studied. The technology of preparing starchmicrospheres with suitable size and crystallization property were explored by controllingthe conditions of ethanol precipitation, amylase hydrolysis and storage.The particle size of corn starch microspheres and amylose microspheres with differentchain length were characterized by laser granulometry measurements. The results showedthat the particle size of corn starch microspheres increased as the increasing of the starchsuspension concentration, the ethanol adding speed and the stirring rate. When theprecipitate temperature was50℃, the particle size of corn starch microspheres could beminimum. Larger size corn starch microspheres could be obtained by the reverse-formethanol precipitation. In addition, under the same preparation condition, the particle size of amylose microspheres was larger than corn starch microspheres, and the shorter themolecular chain length is the smaller amylose microspheres is.Amyloses with different molecular chain length were also prepared by hydrolysis withβ-amylase and the molecular chain length was characterized by the rotating viscometermeasurement. The kinetics of enzymatic hydrolysis of amylose by β-amylase wereinvestigated. Under the same enzymolysis conditions, the hydrolysis rate improved withthe increasing of β-amylase concentration, the hydrolysis rate decreased as the substrateconcentration decreasing, and the hydrolysis rate was the highest at pH6.0and50℃.The crystallization properties of starch microspheres were characterized by XRDanalysis. The crystal types of corn starch microspheres and amylose microspheres preparedby ethanol precipitation were V-type. The initial crystallinity of corn starch microspheresdecreased with the increasing of starch suspension concentration. Stored at the roomtemperature and33%RH for several days, the corn starch microspheres had the long-termrecrystallization but the amylose microspheres only had the short-term recrystallization.For amylose microspheres with different enzymolysis degree, when the enzymolysisdegree was higher, the molecular chain was shorter and the initial crystallinity andrecrystallization rate of amylose microspheres enhanced.To study the influence of temperature on the crystallization properties of starchmicrospheres, the corn starch microspheres and amylose microspheres with differentenzymolysis degree were stored at30℃,8℃and8/30℃for certain time respectively,and then characterized by XRD analysis. The results showed that all of the starchmicrospheres’ V-type crystal structure disappeared slowly and the A-type crystal structureformed. The recrystallization rate of starch microspheres was highest at8/30℃. In addition,changes of the crystallization properties was related to the water content of starchmicrospheres: the disappearing process of V-type crystal structure associated withincreasing of the water content of starch microspheres, and the water content decreased asthe crystallinity increasing after A-type crystal structure was forming.When amylose microspheres were soaked in the glycerin/ethanol solution andurea/ethanol solution for a certain time respectively, a small amount of glycerin and ureainfiltrate the starch microspheres and an anti-plasticization could occur, which couldperfect the crystallization structure of starch microspheres without changing thecrystallization type. In addition, the effect of anti-plasticization was enhanced as the soaking time increasing.