Regulation Of Chain Length Distribution And Anti-digestion Retrogradation Structure Of Starch Via Extrusion-enzyme Synergy

Starch is the main component of carbohydrate in staple food and supplies energy for people in daily life.However,the over-processing of grains nowsdays has resulted in the production of over-refined rice,which is digested excessively in the human gastrointestinal tract.People eating them for a long time are prone to have sub-health problems such as high blood sugar and obesity,etc.Resistant starch（RS）has significant advantages in inhibiting the rise of blood sugar,promoting intestinal peristalsis and improving probiotics population status,etc.Among them,RS3is a functional starch prepared through recrystallization induced by inter-molecular hydrogen bonds and van der Waals forces after gelatinization and retrogradation.However,there are problems of RS3 such as disordered rearrangement crystal region of growth ring,poor structural controllability,and poor product taste after processing.The key of crystalline structure regulation of starch lies in the modification of the chain-length distribution（CLD）and the growth pattern of its amylopectin cluster（APC）.However,researches about fine structure-changing mechanism and structure-activity relationship of starch chains in the microscopic scale still lack evidence nowdays.Therefore,compared with the conventional high-liquid reactive system（starch chains are easily spatially dispersed in this environment）,the shearing effect of high-solid extrusion was used in this project to induce the orientation of starch chains and limit their disordered expansion.Ulteriorly,a suitable CLD will be formed and reconstruct the retrogradation system combining branching enzyme,revealing the formation and changing mechanism of crystal cluster structure induced by multi-scale shearing field created by extrusion and debranching enzyme,and further preparing novel RS3.The contents and conclusions are as follows:（1）Influence mechanism of extrusion screw shearing on the structure of compound starch with different ratios of amylose to amylopectinFixing starch moisture content of 35%,extrusion temperature zone distribution of 40-55-70-85-100℃,compound starch（amylose content:3.40～32.80%）with different ratios of amylose to amylopectin（RAA）was extruded at high（250 r/min）and low（50 r/min）screw speeds to explore the influence of retrograded crystallization structure and its APC patterns.The results showed that extrusion can effectively prepare RS3（40.77～61.52%）under the above conditions;Compared with 50 r/min,the RS3 contents were improved in a large extent（4.65～9.92%）at 250r/min;Compared with different RAA groups,starch crystallization matrices induced by amylopectin（amylose content≤4.78%）and a small amount of amylose（≥27.97%）were more conducive to obtaining high RS-modified products in extrusion environment.What’s more,controlling the moderate proportion（<2.5%）of A chains（DP 6～12）in retrograded matrix is crucial for the efficient formation of RS3;Among them,B3 chains were associated with C chains of APC skeleton after the retrogration of starch with RAA of 1:6(RAA_1:6),and a large number of B1 chains,A chains and ultrashort chains（DP<6）were connected outward in turn（non-covalent bonding,helical and steric cross-linking,etc.）,thus forming an APC lateral associative growth model.（2）Formation and changing mechanism of amylopectin cluster structure induced by multi- scale shearing fields created by extrusion and debranching enzymeGroup Control₁ of high RS3 content（RAA_w）and RAA_1:6 were selected as materials of enzymatic extrusion andα-1,6 glycosidic bond debranching enzyme with a certain concentration gradient（0～27.5 U/g）was introduced during extrusion.The synergistic reconstruction of CLD was conducted to clarify its effects on APC microcrystal aggregation,long chain distributions and its rearrangement structure.The results showed that RAA_w obtained high RS content（55.47%）at 7.5 U/g debranching enzyme concentration,while RAA_1:6 obtained high RS（59.80%and60.36%）at 7.5 and 12.5 U/g through multi-scale shearing fields created by enzymatic extrusion;There were‘indirectal protection’for maize starch granules（containing crystal nuclei of amylose）in RAA_1:6.It was speculated that debranching enzymes play a‘space-time’restricted enzymatic hydrolysis effect in high-solid micro-environment of fast extrusion;After RAA_w was processed by enzymatic extrusion,the degraded side branches of amylopectin were connected to the ends of B1,B2 and B3 chains in a‘backbone extension’manner（presumably relating to the enhanced downstream flow of enzymatic hydrolysis of starch during extrusion）,forming an APC longitudinal extension growth model.（3）‘RTD-CLD-APC’relationship analysis and regulated retrogration strategy of starch chains based on extrusion dynamicsThe tracer-pulse method（erythrosine）was used to explore the residence time distribution（RTD）of waxy starch during extrusion and enzymatic extrusion（7.5 U/g）with screw speeds of50,150 and 250 r/min.The specific mechanical energy（SME）of samples in the above two chapters was calculated.Then,the movement characteristics and mixing degrees of starch were quantitatively described based on extrusion dynamics.The results showed that mean residence time（MRT）decreased with the increase of screw speeds after extrusion and enzymatic extrusion（7.5 U/g）with screw speeds of 50～250 r/min.The order of mixing degrees and diffusion numbers were as follows:RTD₂₅₀>RTD₅₀>RTD₁₅₀.The‘RTD-CLD-APC’relationships were analyzed as follows:In APC longitudinal extension models of RAA_w,density of branch（DB）values decreased and crystallization pattern remained unchanged（A type）,Dm values decreased and an extended long-period distance(d _Lorentz)was formed.Therefore,the degree of order increased,the degree of double helix,thermostability and viscosity decreased.Among them,a typical APC longitudinal extension model was formed at 150 r/min,the mixing degree of which was the lowest（σ²:0.0204）,and the system tended to be column flow（D/u L:0.0107）;In APC lateral associative grouth models of RAA_1:6,DB values increased,and the rest of the structural and physicochemical properties were as the same as the APC longitudinal extension models.Among them,a typical APC lateral associative grouth model was formed at 50 r/min,the mixing degree of which was modertate（σ²:0.2561）,and the system tended to be arbitrary flow（D/u L:0.1508）.In conclusion,this study explored the changing regulars of APC growth models under the multi-scale regulation strategy of starch CLD jointly constructed by mechanical screws and debranching enzyme.The digestive dynamics,viscosity and its thermodynamic properties were clarified under three kinds of extrusion shearing fields so that initially revealing the relationship of‘RTD-CLD/APC growth model-structure’of starch.This will guide the regulation of novel RS3fine structure（lateral and longitudinal retrogradation models）and provide feasible synthetic strategies for preparing different building blocks or micro-domains through the combination of enzyme and extrusion.