The Effect And Mechanism Of Different Processing Methods On The Multi-scale Structure Of Lentil Starch And Resistant Starch

This study focuses on the multi-scale structural characteristics of starch and resistant starch isolated from untreated lentils(ULS,UL-RS)and lentils subjected to different processing including roasting(RLS,RL-RS),germination(GLS,GL-RS),fermentation(FLS,FL-RS),conventional cooking(CLS,CL-RS),microwave cooking(MLS,ML-RS)and autoclaving(ALS,AL-RS).From the structural point of view,the reasons for high resistant starch content in lentil after processing were explored.It was found that all starch samples were categorized by size exclusion chromatography with multi-angle laser scattering into two groups,i.e.,Group A with two population of peaks appeared for samples including ULS,RLS,and GLS;and Group B with three population of peaks shown in samples including FLS,MLS,CLS and ALS.A noticeable decrease in relative crystallinity by XRD was found for all starch from the processed seeds(11.41-3.24%)compared with starch isolated from untreated seeds(15.90%),especially for Group B samples.The intensity of the absorptions related to hydrogen bonding by FT-IR(3000-3600 cm-1,and?1000 cm-1)were markedly decreased for lentil starch isolated from processed seeds compared with native starch,and,especially for treated samples of Group B,which also exhibited significantly higher degree of double helix and lower value of degree of order.Small angle X-ray scattering data manifested that the typical?9-10 nm semi-crystalline structure was increased to 15 nm for the Group B samples which were characterized with the relatively higher values of d,da,and dc.The results suggested that the densely packed crystalline structure could be shifted within the crystallites,and tended to result in small new crystal structure during the processing of lentil seeds because of starch molecules retrogradation,and such effect was particularly obvious for starch samples including FLS,CLS,MLS,and ALS.Meanwhile,the effect of different processing on the structure features of resistant starch isolated from untreated and different processed lentils was investigated.The results suggested that the semi-crystalline structure was totally destroyed,the average molecular weight of the resistant starches(RS)decreased by an order of magnitude after the enzymatic isolation process.Moreover,the relative crystallinity and degree of order of RS which was mainly composed by linear molecule chains were decreased upon processing and enzymatic isolation.The size exclusion chromatography(SEC)results showed that UL-RS,RL-RS,and GL-RS(Group A samples)exhibited higher values of Mw and Rh than FL-RS,ML-RS,AL-RS(Group C samples),and CL-RS(Group B sample).In parallel with the SEC results,other structural characteristics followed the similar trends,where Group C samples exhibited the lowest values of double helix content and crystallinity by 13C NMR and XRD,and degree of order/double helix by FT-IR.Comparatively,Group A samples exhibited the opposite trends,and displayed large amorphous aggregates on their micrographs.The results are expected to provide information for better understanding the mechanism of resistant starch formation during different processing of lentil and to lay a theoretical foundation for the future study of their structure-function relationship.