Research On The Properties And Application Of Resistant Starch Prepared By Extrusion

Choosing the staple foods with low glycemic index(GI)is the focus of diet intervention for diabetic patients.Starch is the most important carbohydrate source and main energy source for human beings.The starch in ordinary rice will be rapidly converted into sugar,which will lead to a sharp rise in blood sugar for diabetic patients after a meal.Resistant starch(RS)is slowly absorbed in the stomach and small intestine of human body,providing a feeling of fullness and maintaining the postprandial blood glucose at a relatively low state after diets.It is an ideal staple food material for diabetic patients.However,the existing rice products for diabetic patients generally suffer from poor thermal stability and low retention of effective components after washing and cooking.After thermal processing,RS fraction is difficult to retain,so RS of staple food must have high heat stability.Based on this,this article uses extrusion process to provide a one-step pathway of starch structure transformation,combined with green chemical modification(cross-linking,esterification,lipid complexation)methods to prepare a new type of starch product with high RS content.Meanwhile,the effect and mechanism of the reactive extrusion process on starch crystalline structure,molecular structure,and digestibility were analyzed,and the formation mechanism of its enzyme-resistant structure was clarified,so as to provide some ideas for the efficient preparation of recombinant rice with low GI value.Based on the extrusion method to prepare cross-linked starch(hydroxypropyl starches,phosphate starches and hydroxy propyl distarch phosphate)with low degree of substitution(DS),which could be added to food.The effects of extrusion combined with cross-linking modification on starch structure and digestibility were studied.Morphology analysis showed that the original starch granular structure was lost and replaced with a rougher and more irregular structure.Extrusion-cross-linking treatment could improve the starch molecular compactness(increased by 11.9?25.6 g/mol/nm~3).The entangled network formed by cross-linking modification within and between molecules would restrict the swelling of starch particles,hindering the attack of amylase.Dual cross-linking modification was beneficial to prevent enzymatic hydrolysis.The addition of cross-linking agents in starch extrusion process could effectively reduce the predicted glycemic index(pGI)of the extrudate.Compared with the control sample,RS content of hydroxy propyl distarch phosphate increased by 6.8%.However,the enzyme-resistant structure of cross-linked starch by extrusion has obvious structural damage,which was not an ideal object for hypoglycemic starch products.Furthermore,in order to explore the suitable hypoglycemic starch products,esterified starch(sodium starch octenyl succinate,starch acetate and acetylated distarch phosphate)with high heat stability and low DS value were prepared by extrusion.The study found that when cross-linking and esterification were used in extrusion,the physicochemical properties of the modified starch were similar with esterified starch instead of cross-linked starch,indicating that esterification was dominant during the combined esterification and cross-linking process.Esterification can reduce the degradation extent of starch structures by extrusion.Compared with the control sample,the amylose content of esterified starch was increased by 2.0%-3.5%,and the molecular weight increased by 3.6×10~6-9.5×10~6 g/mol.The molecular weight of starch modified by esterification-crosslinking was between the esterified starch and the crosslinked starch.After esterification-extrusion treatment,the X-ray diffraction patterns showed new diffraction peaks at 2?of 7.5°,13°and 20°,and the crystalline type was transformed from A-type to V-type,forming the starch-lipid complex,which were further confirmed by thermal experiments since a new endotherm at 100-115? was detected and exhibited good heat stability.However,the RS content of esterified starch was low(the highest is only 11.5%after reheating-digestion process),which was 4.7%higher than that of the control sample.Consideration of food safety,the addition amount of crosslinked/esterified agents was limited,and the modified starch was difficult to achieve the expected RS content.Based on the above research,fatty acids,the natural food component with high food safety,were added in the extrusion process to prepare starch-fatty acid complexes with heat stability and high RS content.The fatty acids(FAs)with different carbon-chain lengths(C12-C18)and degrees of unsaturation(C18:0-C18:2)were selected to prepare different fatty acid-starch complexes(FASCs).The formation mechanism of enzyme-resistant structures in FASCs by extrusion was investigated.The complexed-lipid content and degree of relative crystallinity increased with the carbon-chain length and degree of FA unsaturation.FAs with fewer carbons were more likely to generate stable complexes(e.g.,form II,melted at 100-120?),while FAs with more carbons tended to produce relatively unstable complexes(e.g.,form I,melted at 80-100?).The addition of fatty acids during starch extrusion process could form the RS fraction and reduce the pGI value.After the reheating-cooling process,a new amylose-lipid complex and an amylose-amylopectin network was formed in the unsaturated FASC samples,which restricted the penetration of enzymes into starch granules.A starch-linoleic acid complex exhibited the highest RS content(15.7%)and lowest pGI value(88.4).Starch-linoleic acid complex was selected as the raw material for the preparation of hypoglycemic starch products by extrusion process,combined with heat moisture/cycled heat moisture treatment to further improve the RS contend of the extrudate.Using the in vitro glycemic index as the response value,based on the Box-Benhnken response surface experiment design,the optimal extrusion operating parameters for preparing starch-linoleic acid complex were:the temperature of the fourth heating zone was 65?,the moisture content of raw material was 30%(w/v,starch dry basis),and the addition amount of linoleic acid was 20%(w/v,starch dry basis).Using the optimized extrusion operating parameters,combined with heat moisture/cycled heat moisture treatment to enhance the RS content.The influence of the addition of linoleic acid,extrusion,heat moisture/cycled heat moisture treatment on starch structures and digestibility were studied.The addition of linoleic acid during starch extrusion could effectively reduce the degradation extent of starch by extrusion and form starch-linoleic acid complex(form I complex),which pGI value was 17.4%lower than that of the control sample.The cycled heat moisture treatment could rearrange the gelatinized starch molecules into a loose A-type crystal structure,which could increase the relative crystallinity,but there was no significant change in digestibility.The heat moisture treatment could increase the density of the starch crystallization region by rearrangement of starch chains,and transform the form I complexes into the form II complexes with higher heat stability and RS content.The HMT-8 sample(heat moisture treatment with 8 h)exhibited the highest RS content(46.7%)and lowest pGI value(69.6).In summary,the starch-linoleic acid complex prepared by extrusion combined with heat moisture treatment exhibited excellent resistance to digestion,which could retain a high content of enzyme-resistant structure after reheating process.Based on the processing and anti-digestive advantages of the product,the starch-linoleic acid complex was used as the main raw material to simulate rice components and mixed with rice protein,inulin and konjac glucomannan to prepare the recombinant rice,and compared it with commercially rice products consumed by diabetic patients.The nutritional content,texture characteristics and cooking quality of recombinant rice were similar to those of commercially rice products.In terms of appearance,the color was yellow and the particle size of recombinant rice were larger than those of other rice samples.The sensory evaluation and cooking quality of recombinant rice were similar to those of other rice samples.After cooking,the recombinant rice exhibited a high ratio of viscosity to hardness,which could effectively retain the high heat stability and enzyme-resistant structure(form II complexes).The recombinant rice showed the highest RS content(38.1%)and lowest pGI value(76.6),indicating that it could effectively reduce starch digestion rate and pGI value.