Study On The Enzymatic Liquification Of Corn Starch At A High Concentration

Increasing the initial concentration of corn starch milk is one of the most effective waysto reduce starch sugar production costs. It will reduce the initial moisture content of cornstarch milk, increase the solid content of saccharification liquid and decrease the evaporatedwater. Liquification reaction system of the corn starch milk at a high concentration wasestablished firstly in the research, which initially realized the liquification of corn starch milkat a high concentration. The effects of the initial concentration on liquification wereinvestigated. The liquification of corn starch at a high concentration was optimized bymicrowave and ultrasonic pretreatment. The main research contents as follows:Firstly, the effects of initial size mixing temperature, heating rate, enzyme concentrationand incubation temperature on the liquification were studied, and the liquification system ofcorn starch milk at a high concentration was developed. The results showed that corn starchwas mixed with water at60℃for15min and stirred evenly at300r/min. The reactionconditions were that pH6.0, thermophilic α-amylase12U/g, heating to90℃at the heatingrate of1℃/min. The problems, which corn starch milk at a high concentration was stirred andmixed difficultly, were overcome at the temperature60℃. While incubating the corn starchmilk with the heating rate1℃/min, the corn starch granular was expanded and hydrolyzed byα-amylase at the same time, then the viscosity was reduced effectively during thegelatinization and liquification of corn starch milk at a high concentration.Secondly, using viscosity and DE of starch milk during liquification as the indexes, theeffects of initial concentration on liquification of corn starch milk were investigated, and itsmechanism was analyzed by NMR, DSC, LEM, XRD, SEM, etc. The results showed that theenzymatic hydrolysis rate reduced with increasing of starch milk concentration, especiallywhen the initial concentration was more than45%(w/w, dry basis), the enzymatic hydrolysisrate reduced significantly. By investigating the corn starch samples at different concentrationsincubated at90℃for1h, we found that the moisture content decreased and flow ability ofwater reduced with increasing of starch milk concentration, meanwhile swell factor of starchgranule and gelatinization enthalpy change were reduced significantly, which indicated thatstarch granule was difficult to swell and split. Further study found that, the degree of damageto starch granule structure was superior under the concentration of30%, meanwhile X-raydiffraction characteristic and polarization cross disappeared mostly, and starch paste forminto an orderly network structure after freeze drying. When the concentration increased to45%, the starch granule structure was damaged partly, molecular dispersion degree was lower,and weaker polarized phenomenon and characteristic peak of X-ray diffraction appeared;increasing of starch concentration, almost all of the corn starch granules maintained originalposition and only had varying degrees to swell, polarization cross and characteristic peak of X-ray diffraction become obvious and more granule appeared after freeze drying of starchpaste. Therefore the lower liquification rate of corn starch at a high concentration maybe beclosely related to bad flow ability of water, low gelatinization degree of starch and highcrystallinity of starch.In addition, using viscosity and DE of starch milk during gelatinization and liquificationas the indexes, the effects of microwave pretreatment on liquification of corn starch at a highconcentration were researched, and its mechanism was investigated. The results showed that,during the gelatinization and liquification of corn starch at a high concentration, the viscosityof microwave-treated starch paste was lower and the DE value was higher than the control,which could contribute to liquification of corn starch at a high concentration. The peakviscosity of corn starch milk at40%reduced by35%than the control, and high viscosityduration shortened obviously, While reacting1.0h and1.5h, the DE value increased by40.4%and24.9%than the control, respectively; the DE value of corn starch milk at45%or50%increased by25.4%or43.0%than the control after1.5h reaction, respectively. Themain reasons might be that microwave pretreatment could cause higher specific surface areain a consequence of rough and holey surface, looser granule structure, and lower crystallinityby19%and resistance to enzymatic of corn starch granule, resulting in higher rate of starchgranule degradation by α-amylase during the gelatinization and liquification. In addition,microwave pretreatment could also make some starch chains break to a certain extent.Lastly, Using viscosity and DE of starch milk during gelatinization and liquification asthe indexes, the effects of ultrasonic pretreatment on liquification of corn starch at a highconcentration were analyzed, and its mechanism was investigated. The results showed that,during the gelatinization and liquification of corn starch at a high concentration, the viscosityof ultrasonic-treated starch paste was lower and the DE value was higher than the control,which could contribute to liquification of corn starch at a high concentration. The peakviscosity of corn starch milk at40%reduced by33%than the control, and high viscosityduration shortened obviously, while reacting1.0h and1.5h, the DE value increased by50.1%and30.3%respectively; the DE value of corn starch milk at45%or50%increased by34.7%or54.2%after1.5h reaction, respectively. The ultrasonic pretreatment was superior tomicrowave pretreatment in the auxo-action of liquification of corn starch at a highconcentration. The main reasons might be that ultrasonic pretreatment could cause higherspecific surface area in a consequence of rough and hollow surface, layered structure, andlower crystallinity by25%, the crystallinity decreased more obvious than the microwavepretreatment, and resistance to enzymatic of corn starch granule, resulting in higher rate ofstarch granule degradation by enzyme during the gelatinization and liquification.