Study On The Preparation And Application Of Retrograded Sweet Potato Starch

Retrogradation of starch is the process that the amylose and amylopectin species in starch links to each other by forming covalent bond when the movements of starch moleculars slow down as temperature drops.The branched chain of starch turn to parallel arrangement,a large number of microcrystallines appear.Nowadays,retrograded starches have being used as electrode materials and their electrochemical applications have become a new hotspot in electrode related research areas.According to literature,carefully prepared corn starch has been used as cathodic materials in lithium-ion battery.However,specific preparation methods and conditions need to be figure out in order to obtain starch crystals with superior properties and meet the need of particular application.Wet heat treatment is a mild physical modification method that often used in the preparation of retrograded starch crystals.Parameters in heating process include temperature,pressure,heating time and the content of species in starch must be optimized in order to obtain retrograded starch crystal with high productivity and good quality.In this paper,retrograded sweet potato amylose and amylopectin with narrow molecular weight distributions were prepared from raw starch.A series processes include wet heat treatment,amylose digestion,organic solvent precipitation and acid neutralization were used to obtain the retrograded species.Orthogonal experiments were used to optimize the parameters of the processes.Concentrations of starch solution,incubation time,temperature and wet heat treatmen time were determined.The influences of amylose content in the mixture on the structure and physical-chemical properties of starch crystals were also studied.Finally,the possibility of using the obtained retrograded starch crystal as electrod material was explored.The experimental results showed that most of the α-1,4 glycosidic bond of amylose and amylopectin were cut off after the forth retrogradation.The main chains in them became much shorter.The maximum absorption wavelength of the two species transferred from 653 nm to 574 nm and from 643 nm to 539 nm,recpectively.The optimum condition for the preparation of retrograded crystals of amylose is: the concentration of 20 %,temperature of 80℃,incubation time of 6 h.For the mixture of amylose and amylopectin,the condition is 20 % of concentration,80 ℃,24 h for incubation.For amylopectin,is 20 % of concentration,60 ℃and 48 h of incubation.In the retrogradation process,amylose molecules played a key role of being the templates for amylopectin to precipitate on them.The crystallization process was therefore accelerated by the exsisting templates.The optimiazed crystallization condition for the mixture of amylose and amylopectin was: the ratio of amylose to amylopectin was 4 to 1,wet heat treatment time was 60 minutes.The wet heat treatment time for the pure amylose and amylopectin were 60 minutes and 45 minutes,respectively.The obtained retrograded starch crystals were tested as cathodic materials of lithium battery.Charging and discharging curves and cycle life curves were measured.It was found that the first time coulomb efficiency for pure amylose was 44.6 %,for mixture of amylose and amylopectin was 45.1 % and for pure amylopectin was 35.3 %.After 180 cycles,all the three materials showed great cycle stability.For pure amylose the specific discharge capacity was 49 mAh/g,for mixture of amylose and amylopectin the specific discharge capacity was 50 mAh/g and for pure amylopectin the specific discharge capacity was 125 mAh/g.But the discharge capacities of these materials were not high enough,nor the first time coulomb.The second discharge specific capacity droped greatly.Retrograded starch crystals should be studied systematiclly and thoroughly before they can be used as lithium-ion battery cathodic materials.