Effect Of Rice Protein-Starch Interaction On The Quality And Digestibility Of Rice Flour For Infants

The rice production of China has been at the leading level in the world.It is the world's leading producer of rice.Most rice in China is directly consumed as a staple food.And the processing level of related products is mostly in the primary stage.Compared with the technology of deep processing in the developed countries,our country still has a big gap.Moreover as the two-child policy openedin 2016,the number of babies born each year in China is as high as 20 million which has increased the demand for breast milk substitutes and complementary foods for infants and young children.The domestic market share of infant rice flour is dominated by the Heins,Nestleand Aptamil.Currently,the production of domestic infant rice flour has the following disadvantages:?1?serious primary processing phenomenon,?2?low technical content of products and weak innovation capability,?3?limited variety of products of varieties,?4?research on new technologies and insufficient development of new products has resulted in the waste of high quality rice resources.However,there are many problems in domestic product,such as poor reconciliation,as well as the infants and young children cannot easily digested and absorbed.Based on that,this study intends to use rice as raw material,and chooses different proportions of rice protein-starch mixed system or protein-starch thermal aggregates assembled in extrusion?glutinous rice paste?as the research object.Then investigating the effect of four different processing factors?different mix ratio,extrusion temperature,initial moisture content of the material,different rice varieties?on the paste properties and digestibility of rice cereals.The main findings are as follows:?1?As the addition of protein increased,the apparent morphology of protein-starch blends changed from a smooth and dense structure to a network structure with uniform pores.Finally,a loose and flake-like structure appears.It may be due to the rigid structure of the protein itself.In addition,the elastic modulus?G'?and of the blends were both decreased gradually.At the same time,shear thinning characteristics were exhibited,and the stability of the system was weakened,which may be attributed to the interaction between protein and starch during the wet heat treatment process.Based on the results of in vitro digestion of starch,it could be concluded that the content of rapid digestion starch?RDS?increased first and then decreased,and the content of resistant starch?RS?gradually increased.This is because as the amount of protein increased,the role of protein was changed from assisting the dispersion to the physical barrier between the starch and the enzyme,which narrowed the contact area of the starch and the enzyme.?2?As the extrusion temperature increased,the moisture content of sample and starch were decreased from 11.97%to 7.74%and from 80.9%to70.1%,respectively.The water absorption index?WAI?was decreased,while the water solubility index?WSI?was increased.It is due to the evaporation of water in the sample due to the effect of high temperature.So that the rice flour particles subjected to the shear force were degraded to generate reducing sugars and peptides,and its solubility became better.Compared with the original rice flour,the initial gelatinization temperature?T_o?of the expanded rice flour was significantly increased,while the conversion enthalpy??H?was significantly decreased.It can be attributed to the destruction of the original starch structure resulting in a change in short-range order structure of the starch.Then the ability to absorb water swelling was decreased,the temperature required for gelatinization was increased,and the thermal stability decreases.Moreover the appearance of starch crystals was changed from type A to type V because of the interaction between between amylose and natural lipids causes.The in vitro digestibility of protein in extruded rice flour was increased from 75.77%to 86.44%.The RS content was decreased significantly,while the RDS content was increased significantly.As well as the digestibility was significantly higher than that of original rice flour.?3?As the moisture content increased,the puffing degree and the WAI of puffed rice flour were gradually decreased,but the WSI was increased.It could be found from Low-field NMR analysis that the signal values of the corresponding peaks of semi-bound water and free water in the sample were lower than that of combined water,indicating that the amount of free water in rice flour products was relatively low.Compared with the original rice flour,the T_oof the extruded rice flour was significantly increased,but the?H was significantly decreased.It may be related to the destruction of the original structure of the rice flour which resulted in a decrease in the water absorption capacity and a decrease in the thermal stability.According to1047 cm^-1/1022 cm^-1,the orderly structure of starch in puffed rice flour was reduced probably for the decrease in the enthalpy??H?value.The RDS of extruded rice flour was decreased from 82.1%to 74.5%,while the RS was increased from 2.0%to5.7%.And the total digestibility of protein decreased from 86.8%to 77.1%.It may because that the mechano chemical effect degraded starch and protein and the higher moisture content lower its degree of degradation,resulting in a decrease in the digestibility of them.?4?Through the analysis of four kinds of extruded rice flour samples,it was found that there are great differences between the quality characteristics and physicochemical property.Due to the difference in the content of amylose,the extrusion rate,WSI,and WAI of extruded rice flour were different significantly?P<0.05?.The WAI of glutinous rice products was the lowest,1.56%,and WSI was the highest,72.32%,respectively.The thermodynamic characteristics indicated that the initial gelatinization temperature of early indica rice flour was the highest,61.97°C,while that of Taixiang rice was the lowest,59.12°C,respectively.It meaned that the thermal stability of the the early indica rice was the best after extrusion processing.There was insignificant?P>0.05?difference in RDS of the four expanded rice flours.And the RDS of glutinous rice products was slightly lower than that of the other three rice flours,but there was a significant?P<0.05?difference in RS of them.The RS in glutinous rice products was significantly?P<0.05?higher than the other three products.It may be due that the linear structure of amylose in the extrusion process could for mate hydrogen bonds between molecules resulting in the formation of a difficult-to-digest polymer.The protein digestibility of the early india rice sample is the highest,while that of the glutinous rice sample is the lowest.It can be conculded that the overall digestibility of the glutinous rice sample is lower than that of the other three expanded rice flours.