| With the corn starch and cassava starch as the research object,this paper discussed the heat-treating methods and the optimum process conditions of two kinds of starch in the condition of excess moisture content(average≥40%),and analyzed the heat-treating process by means of optical microscope and laser particle size analyzer. With the corn starch,cassava starch and the respective heat-treated samples as raw materials,the structure of the samples was characterized by means of scanning electron microscopy(SEM)and infrared spectrometer(IR),and it studied the effect of heat treatment on the physical and chemical properties which consisted of the amylose content,the solubility in cold water,chemical reactivity,enzymatic activity at room temperature,pasting properties,rheological properties of the starch paste etc.The results were as follows:(1) The measurement results of swelling factor and the sedimentation experiments showed that in the condition of excess moisture, the optimum heat-treating conditions of corn starch and cassava starch which were that the solid to liquid ratio was1:4and the heat-treating temperature were67.5℃and60℃respectively.Under the condition of optimum heat-treating process,the results of optical microscope observation showed that the morphology and surface of starch granules were changed obviously in the process of swelling;the results of granularity analysis showed that the throughout swelling process was easy to control,particles was irreversible swelling. With the increasing of heat-treating time,all of the d[0.1],d[0.5],d[0.9],the average particle diameter of the surface area and the average particle diameter of the volume increased gradually,and the tendency of d[0.9] and the volume-average particle diameter to increase is more significant;rather than the surface area decreased.(2) The results of scanning electron microscopy(SEM)analysis showed that with the increasing of heat-treating time,the starch granules occured slight swelling,the surface became rough,the number of fragments increased,some melted deformation,or even producing holes and cracks.The results of infrared spectrum analysis showed that the heat treatment did not produce new groups,but caused the width of the part of the absorption peak in the spectra.While the infrared crystallinity index of the two kinds of starch gradually reduced with the the increasing of heat-treating time,the infrared crystallinity index of corn starch,cassava starch were respectively reduced from1.117down to0.758,from1.035down to0.675.(3) The physical and chemical properties of corn starch,cassava starch and the respective heat-treated samples showed that with the increasing of heat-treating time,the amylose content of starch and the solubility in cold water increased respectively,chemical reactivity and enzymatic activity at room temperature were significantly improved.(4) The results of DSC thermal characteristics analysis showed that with the increasing of heat-treating time,the To and Tp values of corn starch and cassava starch gradually increased,the Tc,R and△H values decreased gradually.The R-value and△H of the original corn starch were20.8℃,4.34J·g-1respectively,after heat-treating2min,4min at67.5℃,they were changed to12.8℃,2.66J·g-1and9.8℃,1.78J·g-1.The R-value and AH of the original cassava starch was19.4℃,4.51J·g-1respectively,after heat-treating2min,4min at60.0℃,they were changed to18.1℃,3.46J·g-1and15.9℃,2.40J·g-1.The decreasing of R value means that the starch crystalline region was more stable and uniform,and the decreasing of the thermal enthalpy△H illustrated that the degree of particle crystallization was reduced.(5) The results of dynamic rheology showed that in the heating process,The G’and G" of the samples first increased and then decreased,the G’and G" of corn starch began to increase sharply at70℃,the G’and G" of cassava starch started to increase sharply at63℃.With the increasing of heat-treating time,the G’max and G"max of samples decreased gradually while TG’max and TG"max of the samples increased gradually;TG’max and TG"max of corn starch increased significantly while the ones of cassava starch increased to a lesser extent.In the process of cooling,the G’and G" of samples increased slowly.The results of static rheology showed that the sample paste rheology curve can be described preferably by the power law,the R2values of all rheological curve were between0.939to0.998,corn starch,cassava starch and the respective heat-treated samples were showing a typical pseudoplastic fluid(n<1),while the characteristics of the up line and down line does not coincide and the emergence of hysteresis loop showed that the starch paste had certain thixotropic. |
PDF Full Text Request