Amylose Crystal Seeds: Preparation,Characterization And Their Intervention Mechanism On Long-term Retrogradation

Starch retrogradation is a common phenomenon in cooked products with high starch content,it usually includes short-term retrogradation and long-term retrogradation.Due to the complexity of the influencing factors in retrogradation and the limitation of research methods,the relationship between them has not been systematically elaborated.Clarifying the relationship between them can not only enrich the theory of retrogradation,but also provide theoretical basis for the preparation of special food for slow digestion and high-quality rice and noodle products.The essence retrogradation is the formation of starch crystal nucleus and the growth of crystal,crystal seeds and crystal nucleus are common factors in the study of retrogradation.Based on this,this article prepared amylose crystal seeds?ACS?which simulated short-term retrogradation to simplify the complex retrogradation system and intervene long-term retrogradation of starch,so that we can systematically interpret the relationship between the short-term retrogradation and long-term retrogradation.Firstly,the size and crystalline structure of ACS were prepared and characterized.Rice amylose was extracted by crystallization,temperature changing crystallization coordinated with sulfuric acid limited hydrolysis was used to prepare ACS with different crystal content.According to the different temperature of cooling environment at early stage,ACS samples were named as ACS-80?cooling at-80?,the same below?,ACS-18,ACS4,and ACS25.The results of dynamic light scattering showed that the average particle size of four crystal seeds was 240?439 nm.Through the analysis of X-ray diffraction combined with software Jade 6.5,four kinds of ACS were all B-type crystal structure and their relative crystallinity was between37.96%and 45.25%.Crystal cell of four crystal seeds were all hexagonal system structure with no significant difference in c axis.Among them,ACS4 had the longest a axis and b axis which was 10.11?,and the maximum crystal cell volume which was 1464.41?³,its crystal cell structure was the most loose.Through the analysis of small angle X-ray scattering,it was found that starch formed lamellar structure different from native starch during short-term retrogradation,the thickness of amorphous layer was obviously increased.Among four crystal seeds,the lamellar structure of ACS4 owned highest homogeneity with 4.265 nm crystal layer thickness and 7.020 nm amorphous layer thickness.ACS was introduced to intervene the long-term retrogradation process of starch,and the changes law of retrograded nucleation mode and the crystal structure of retrograded starch were studied.The Avrami equation was used to fit the retrogradation kinetics of starch,indicating that the Avrami index n of long-term retrograded starch intervened by ACS-80,ACS-18,ACS4,and ACS25 decreased from 1.869 to 1.613,1.667,1.480,and 1.689 and the retrogradation rate constant k increased from 0.024 d^-1 to 0.034,0.036,0.051,and 0.044 d^-1,respectively.The decrease of n and increase of k demonstrated that the nucleation mode under the intervention of ACS tended to promote nucleation and increase retrogradation rate.The results of X-ray diffraction showed that the relative crystallinity of retrograded starch intervened by ACS-80,ACS-18,ACS4,and ACS25 increased by 1.02%,1.50%,3.09%,and 2.13%,respectively,which indicated that the long-range order of long-term retrogradation increased.The content of retrograded starch's double helix measured by ¹³C CP/MAS NMR increased from 14.86%to highest 22.80%,the value of 1047 cm^-1/1022 cm^-1 measured by FTIR increased from 0.670 to highest 0.887,these data showed that the intervention of ACS not only increased the content of double helix,but also increased the ordered degree of double helix.Finally,the process of crystal cell parameters and lamellar transformation of long-term retrograded starch intervened by ACS were evaluated.X-ray diffraction combined with software Jade 6.5 were used to analyze the crystal cell of retrograded starch and the results showed that ACS did not result in the change of crystal cell system of retrograded starch and they were always hexagonal crystal system.But their crystal cell morphology changed,the length of a axis and b axis increased,the crystal cell volume increased from 699.84?³ to 803.53,1389.57,4437.93,and 1754.93?³,respectively.It is indicated that the crystal cell structure of retrograded starch became looser under the intervention of crystal seeds,and the influenced degree of crystal cell of retrograded starch was positively related to the crystal cell structure of crystal seeds since the crystal growth was limited by crystal nucleus.Through small angle X-ray scattering analysis,the results showed that the fractal dimension of retrograded starch was1.8?2.8,which belonged to the quality fractal.The lamellar thickness of long-term retrograded starch ranged from 14.717 to 16.365 nm,which significantly increased compared with short-term retrograded starch.At the same time,the thickness of crystalline layer of retrograded starch increased from 7.144 nm to 7.732,7.542,8.149,and 7.950 nm by the intervention of ACS-80,ACS-18,ACS4,and ACS25,respectively,and the crystalline structure homogeneity of retrograded starch increased.In summary,under the intervention of ACS which simulated short-term retrogradation,the nucleation rate in long-term retrogradation speeded up,the formation and ordered accumulation of the double-helix structure of retrograded starch was accelerated and the transition of lamellar structure from metastability to steady state was promoted,so that it could promote the long-term retrogradation of starch.