Study On Enzymatic Preparation Of Cassava Starch And Mechanism

Cassava starch was chosen as raw material to produce resistant starch(RS) by high temperature resistant alpha amylase, pululan enzyme, and enzyme combite with heat pressing method. The preparation process of the enzymatic kinetics and molecular characteristics, thermal characteristic, particle morphology and crystal type of RS after the preparation were researched, which researched by Gel permeation chromatography (GPC), X-ray diffraction, differential scanning calorimetry calorimeter (DSC) and electronic scanning electron microscopy (SEM), to reveal pregelatinization of resistant starch cassava formation mechanism. The results showed that:1. High temperature resistant alpha amylase was used to preparate RS, amylose content reached the highest22.13%and RS content reached the highest9.13%, when the enzyme dosage was30U/g starch, enzyme solution time was10mins, and the starch hydrolysis DE value was0.32. GPC results showed that long chain molecules (DPw>100) of cassava starch was down from97.98%to about20%, while new medium chain molecules (30<DPw<100) was not produced, short chain molecules (DPw<30) was up to about80%. The purification of RS molecular weight (Mw) was from6.97×103to9.72×103. The SEM results showed that:the result of the enzyme, RS particles were appeared within a fault or hollow out and the molecular particles were decreased. X-ray diffraction results showed that:the crystallization of cassava starch was typical type A, crystallinity was17.90%; The original by amylase soluted, crystal types was not changed, crystallinity was down to13.68%; The crystal type of purificated RS was C and the crystallinity was20.95%.2. Pululan enzyme hydrolysis curve of cassava starch followed level1response equation. Amylose content was23%, RS reached the highest10.43%, when the enzyme dosage was4U/g starch and enzyme solution time was4hours. GPC results showed that with the role of enzymes at pululan, long chain molecules was reduced from97.80%to about5%, while medium chain molecules was increased from1.29%to3.64%, small chain molecules was increased to about92%.This suggested that cassava starch of branches varied mainly with shorter side chain. SEM showed that with the extension of enzyme solution time, RS particles internal layer structure density increased and particles was smaller. X-ray diffraction showed that after pululan enzyme solution, the crystallization type of RS was B, crystallinity was decreased from17.90%to9.37%.3.The RS content reached highest8%, when the experiment condition was that:the moisture content was80%in the starch milk, at121℃, pressure heated30mins,4℃storaged for18hours. High water content (>60%), low temperature (4℃) and long time(within36hours) were benefited to the formation of RS. SEM showed that pressure heat treatment was benefited to the effection of hot uniform pressure, while RS particle internal damage was increased with the moisture increase from20%to80%. X-ray diffraction showed that RS crystallization type unchanged (type A), crystallinity increased from17.90%to57.89%when moisture content was20%,while the moisture content≥40%the crystallinity was reduced to zero.4.121℃pressure for30mins, and combinated with the high temperature resistant alpha amylase, which enzyme dosage was30U/g starch, enzyme solution time was10mins, the RS formation rate was highest9%, which was2%higher than the enzymatic alone, and1%higher than the pressure heat alone. Under the same conditions, combinated with pululan enzyme, which enzyme dosage was4U/g starch, enzyme solution time was4hours, the RS formation rate was highest about10%, no significant effect than use enzymatic alone. Effect on pressure heat method combined with high temperature resistant alpha amylase and pululan enzyme preparated RS was different.5. DSC results showed that:the beginning temperature of cassava starch gelatinization was52.7℃. The absorption peaks of resistant starches under different conditions to preparate were near115℃. The minus of temperature between terminate and beginning reduced. The change of enthalpy gelatinizating increased. Related analysis among X-ray diffraction peak position, the crystallinity, DSC peak temperature, the long chain molecules, medium chain molecules, short chain molecules and RS content showed that:RS content related to type B crystal structure. The more starch short chain molecules and the less long chain molecules of starch, the higher peak temperature of DSC. The higher the crystallinity, the higher the RS content. The more medium chain molecules, the easier the polycrystal formed. On the other hand, the formation of crystal would be blocked by short chain block. The formation of RS was decided by the proportion of chain molecules.