Study On Mechanical Activation Of Starch And Its Properties

Starch is a morphologically complex polymer substance, consisting of loose amorphousregions that are interspersed with highly regular crystalline regions, resulting from the formationof hydrogen bonds between the starch molecules. The crystalline composition consists of around25～50% of the starch granules. The scope of starch application is both enhanced and limited byits unique molecular structure. For instance, the compact arrangements of molecules in thecrystalline regions inhibit water or chemical reagents from making contact with the molecules inthe crystalline region. As a result, the gelatinization temperature is higher and chemical reactivityof starch is decreased. Meanwhile, the relative large molecular weight and the extensive networkformed by hydrogen bonds lead to high gelatinization temperature and lower fluidity. For manypurposes, the market prefers starch with less extensive crystalline regions, resulting in improvedphysico-chemical properties and increased reactivity for planned applications. Therefore, there isgreat interest in methods to modify the structure in the crystalline region, or decrease the size ofcrystalline regions. In this thesis, cassava and maize starch were mechanically activated with acustomized stirring-type ball mill. The key researchful contents are as follows:(1)The effects of mechanical activation on crystal structure, thermal properties, functionalgroups, granular morphology, size distribution and amylose content of cassava and maize starchwere investigated respectively by using granularity analysis, scanning electron microscopy,X-ray diffractometry, Fourier transform infrared spectroscopy and differential scanningcalorimetry, the amylose content were mensurated with chemical method. The mechanism ofmodification process of starch by mechanical activation was also discussed.(2)The mechanical activation effects on physicochemical properties of cassava and maizestarch were investigated by analyzing the influence of mechanical activation on cold-watersolubility, rheological characteristics, transparency and freeze-thaw stabilization. (3) Using the resistant starch (RS) content as an evaluating parameter, the mechanicalactivation effects on recrystallization of cassava and maize starch were investigated by analyzingthe influence of activation time, storage time, starch paste concentration, gelatinization andstorage temperature on the RS formation.(4) Using acetic anhydride as esterification reagent, methanesulfonic acid (MSA) as catalystand acetic acid as reaction medium, acetylated starch was synthesized from starch with differentactivation time. Moreover, using the degree of substitution (DS) as an evaluating parameter, themechanical activation effects on chemical reaction activity of starch were investigated byanalyzing the influence of mechanical activation on DS and apparent activation energy (E_a). Thestructures of acetylated starch with different DS were further characterized by using FTIR.(5) Using starch with different activation time as crude materials,α-amylase asliquefaction reagent, and dextrose equivalent (DE) as an evaluating parameter, the mechanicalactivation effects on enzymolysis reactivity of starch were investigated by analyzing theinfluence of activation time, gelatinization temperature, reaction time, reaction temperature,substrate concentration, amylase amount and pH value on DE.The following main conclusions can be drawn on the basis of the experimental results andtheoretical analysis.(1) The crystal structure of cassava and maize starch could be destroyed by mechanicalactivation, the crystallinity decreased from polycrystalline to amorphous. When subjected tomechanical activation, the starch granules shape were fractured, and agglomerating into largermore amorphous particles, congregates consisting of tiny granules, structural change fromirregular agglomerates to layered grains. The gelatinization temperature decreased withincreasing of activation time and the phase transition peak disappeared gradually. The amylosecontent increased with increasing of activation time. FTIR showed no new functional groupsproduced during the mechanical activation process.(2) The mechanical activation was evidenced to cause change of physicochemical propertiesof cassava and maize starch: cold-water solubility and transparency increased, freeze-thawstabilization reduced. All activated starch samples with pseudoplastic characteristics in accordingto the power law, starch solution in a tendency to Newton fluid. The apparent viscosity,thixotropy and shear-thinning nature of the starch pastes decreased with increasing of activation time. The cassava starch showed greater cold-water solubility, transparency and freeze-thawstabilization than maize starch under similar activation treatments, the different might beattributed to differences of size, amylose content and fragility of granules, the cassava starch aresize larger, amylose content lower and more fragile than maize starch.(3) Mechanical activation could cause degradation of starch molecules, the degree ofpolymerization (DP) and viscosity decreased and amylose content increased. As a result, mildmechanical activation treatment was proved to be in favor of the recrystallization of starchmolecule and the RS content increased evidently. RS content from activated maize starch werehigher than activated cassava starch under same experimental conditions, showed amylosecontent was one of important factors that affect yields of RS. Moreover, it was also found thatother affecting factors, such as starch paste concentration, gelatinization and storage temperaturehad a certain influence on RS formation and were closely related to activation time. XRDshowed that the structure of RS is B pattern.(4) Under the experimental conditions, the Ea of esterification from 60.95 and 66.21kJ·mol^-1 for non-activated cassava and maize starch to 48.03 and 51.92 kJ·mol^-1 (activation time0.25h), 22.98 and 25.06 kJ·mol^-1 (activation time 1.0h) for activated cassava and maize starch,respectively. The results showed that the mechanical activation pretreatment obviously enhancedenzymolysis reactivity of starch, and the dependence of esterification on reaction temperature,catalyst and acetic anhydride concentration decreased with increasing of activation time. FTIRshowed that the structural difference of between acetylated starch, native starch and activatedstarch increased with increasing of DS. The crystal structure of starch could be also destroyedduring acetylation process.(5) Mechanical activation obviously enhanced the enzymolysis reactivity of starch. Otheraffecting factors, such as gelatinization temperature, reaction temperature, substrateconcentration and amylase amount had a certain influence on enzymolysis and were closelyrelated to activation time. Moreover, the dependence of enzymolysis on these affecting factorsdecreased with increasing of activation time.