| Natural starch cannot meet the needs of food and non-food industry.People often modify natural starch to extent its application.Extrusion is a commonly used physical modification method.Our group developed the improved extrusion cooking technology?IECT?in the research of nutritional texturized rice,and found that the texture of nutritional texturized rice was similar to nature rice.In addition,IECT could improve the freeze-thaw stability of rice starch.However,the effects of IECT on the structure and properties?gelatinization,retrogradation,digestion and rheological properties?of rice starch are still unknown,which limit the use of IECT in starchy food.The aim of the study is comparing the difference of IECT on the gelatinization,retrogration,digestion and rheological properties of rice starch,characterizing the effect of IECT on the different level of rice starch structure,establishing the relationship between structure and physicochemical properties.In addition,the study will also prepare the slowly digestible starch and instant brown rice products through IECT,which could expand the application of IECT in starchy food.The main results of the research are as follows:In this paper,pre-gelatinized rice starch was prepared by IECT?IPS?with different moisture content?30-70%?.When the moisture content of the raw material was more than 60%,the starch was almost completely gelatinized.IPS has higher WAI and WSI at low temperature?30°C?,lower breakdown value?740 cP?and setback value?1440cP?compared with the nature rice starch,indicating that IECT could improve the stability of starch paste and reduce the degree of short-term retrogradation,the higher the moisture content,the effect was more obvious.The change of starch paste property was mainly caused by the degradation of starch molecules.The molecular size of the nature starch was 39.2 nm and which of IECT modified rice starch was 20.7 nm.The molecular degradation caused by IECT mainly occurred in amylopectin molecules.The high screw speed?50 Hz?and high temperature?130°C?of IECT resulted in most of the degradation of rice starch?molecular size was reduced to 55%of nature starch?,and the degradation occurred in amylopectin instead of amylose molecules under 60%moisture content.This was consistent with the results of the preparation of IPS.Degraded small molecules could inhibit the short-term retrogradation of rice starch by delaying the decrease of water mobility and the increase of storage modulus,but accelerate the long-term aging of amylopectin.The greater the starch degradation by IECT,the more short-term retrogradation was inhibited and long-term retrogradation accelerated.In addition,molecular dynamics simulation of amylopectin retrogradation results were consistent with long-term retrogradation results of modified starch,indicating that molecular simulation technology is an effective method in studying the long-term retrogradation of starch at the molecular level.By comparing the digestion,gelatinization and rheological properties of nature rice starch and different molecular size of IECT modified starch?ERS?,it was found that digestion rate of the ERS with the smallest molecular size was significantly higher than that of native starch which was up to 3 times.And the digestion rate increased as the decreased molecular size of ERS.The content of rapidly digestible starch of ERS increased 3 times,the slowly digestible starch decreased 58%,and the resistant starch decreased 87%.As the molecular size decreased,the WSI of ERS increased when ERS gelatinized at high temperatures,but the WAI decreased.Both native rice starch and ERS exhibited weak gel and shear thinning properties,and belonged to pseudoplastic fluid.The critical strain value,storage modulus and apparent viscosity and thixotropy of ERS were decreased,and the degree of reduction has positive correlation to the degree of degradation of starch molecules.IECT accelerated the rate of formation of the starch gel network structure and improved the shear recovery capacity of the starch gel which was 136%of native starch.The digestibility of rice starch,potato starch and pea starch were changed by IECT combining with retrogradation.The retrogradation would decrease the digestibility of starch,and the digestion rate of retrograded modified starch gel was slower than which of retrograded native starch gel,and decreased to 76%,83%and 72%of retrograded rice starch,potato starch and pea starch,respectively.The retrograded modified pea starch gel has the lowest digestion rate which was 3.02×10-2/min,and the digestion rate of the modified potato starch was 4.61×10-2/min,and the modified rice starch has the fastest digestion rate which was 6.05×10-2/min.Moreover,the increased storage modulus and hardness of modified starch were less than which of native starch during retrogradation which were 5-13×103 Pa and 227-719 g respectively.It indicated the harm of retrogradation on the quality of the modified starch was less than native starch.IECT combining with retrogradation is an effective method which can reduce the digestion rate of starch.In addition,the study of starch-based food retrogradation properties requires a comprehensive consideration the changes of starch properties and structural.The instant brown rice products were prepared by IECT with different particle size?103,28.3 and 15.3?m?which were processing by ultrafine pulverization.The contents of protein and fat of instant products were reduced by 6-38%and 19-49%,respectively.IECT and ultrafine pulverization promoted the conversion of insoluble dietary fiber into soluble dietary fiber,increased total dietary fiber content to 1.47 times of native brown rice,and reduced particle size of dietary fiber which has the minimum size with16.5?m in the products,but when the raw material particle size was 15.3?m,the particle size of dietary fiber would not change after IECT treatment.Although IECT would increase the content of phenolic acid in instant brown rice products by 16-30%,it reduced the in vitro antioxidant activity.In addition,instant brown rice products were more stable than brown rice during storage.Although the small particle size of the raw material was helpful for the improvement of certain nutrients,it also has some negative effects.The smaller the particle size of the raw material,the color of the product was darker,accompanying a burnt smell,which would affect people's acceptance of the product.Therefore,in the future production,it is necessary to comprehensively consider various factors and select a suitable raw material particle size to prepare brown rice products.In summary,IECT could change the gelatinization,retrogradation,digestion and rheological properties of rice starch by changing the different level of rice starch structure,making it suitable for different use.The changes of these properties were mainly due to degradation of amylopectin.IECT combining with retrogradation could slow down the digestion of starch.In addition,IECT could improve the eating quality and nutritional properties of brown rice. |
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