Study On The Growth Kinetics And Functional Properties Of Rice Glutelin Fibril Aggregates During Storage

In the process of rice processing,10-15%of broken rice will be produced.The development of protein resources in broken rice has great economic benefits.The main component of rice protein is rice glutelin,and it contains high content of asparagine and glutamine,which can be combined by hydrogen bond and hydrophobic interaction,resulting in poor solubility of rice glutelin.Moreover,viscoelastic gel network cannot be formed after thermal treatment,which limits its application in food processing.Previous studies of our group found that rice glutelin could self-assemble into highly ordered fibril aggregates with length of 1-5 μm and width of 20-45 nm through thermal treatment at pH 2.0 and 85℃ for 2-15 h.Furthermore,protein fibrillation can significantly improve the functional properties of proteins.It is known that some protein fibril aggregates stored at low temperatures(room temperature or 37℃)for a period of time,their structures would evolve.At present,the correlation of structural evolution and functional properties of protein fibril aggregates during storage is rarely reported.It is of great significance for the storage and application of protein fibril aggregates by studying the growth kinetics and functional properties of the rice glutelin fibril aggregates(RGFAs)during storage.This project took the rice glutelin fibril aggregates stored for different times as the research object,characterized their structural properties,explored their growth kinetics during storage,and studied the effects of storage on the functional properties of rice glutelin fibril aggregates and inhibition of wheat starch digestibility.The main research contents and conclusions are as follows:(1)The effects of storage on the molecular structure(ThT fluorescence,secondary structure,microstructure,particle size distribution,turbidity and surface hydrophobicity)of rice glutelin fibril aggregates were studied.The results showed that storage for 4 days was the inflection point of degradation of rice glutelin fibril aggregates.In the fibril samples stored for 1 day,with the extension of heating time(2-15 h),the surface hydrophobicity decreased significantly and the fluorescence intensity of ThT increased slowly with the α-helix converted into β-sheet.In addition,the particles with size distribution larger than 1000 nm disappeared,and small rice glutelin aggregates,polypeptides and short fibrils elongated to form longer and branched fibrils.Meanwhile,the turbidity value decreased.The above results indicated that heating for a long time facilitated to the formation of fibril aggregates.After storage for 4 days,the whole fluorescence intensity and turbidity value of rice glutelin fibril aggregates increased,and the surface hydrophobicity decreased significantly.The fibril samples thermal treated for 2-10 h were further elongated and grew coarser,reaching 1-4 μm in length and 20-140 nm in width.However,fibril structure broke down in the samples of 15 h fibril samples as well as the particle size distribution decreased from 93.3-627.1 nm to 71.1-122.5 nm.(2)The effects of storage on the functional properties(visual appearance,foaming property,emulsifying activity,shear viscosity,thermal stability and fibril rehydration)of rice fibril aggregates were studied.The results showed that RGFAs showed better thermal stability than native rice glutelin.The turbidity and the uniformity of RGFAs solution stored for 1 day and 4 days showed no significant difference.The 2 h-RGFAs(1 d,4 d)samples showed higher shear viscosity attributed to their larger hydrodynamic radius,whereas the 15 h-RGFAs samples fractured after 4 d storage,resulting in a significant decrease in viscosity.However,the foaming ability,foaming stability,emulsifying activity and emulsifying stability of RGFAs were improved after storage for 4 days.After freeze-freeze drying-rehydration treatment,the structure of RGFAs changed to a certain extent,resulting in a low fibril retention rate of its supernatant.It is necessary to further study the structural changes of RGFAs during freeze-freeze drying.(3)The quantity and morphology changes of rice glutelin fibril aggregates in the process of simulating in vitro gastric-pancreatic digestion were studied,and the gelatinization and in vitro digestibility of the mixture of rice glutelin fibril aggregates and wheat starch were also measured.The results showed that RGFAs stored for 1 day exhibited hydrolytic resistance to pepsin during digestion.Subsequently,the residual fibrils and peptides were easily hydrolyzed by pancreatin during pancreatic digestion.In addition,the hydrophobic interaction between RGFAs and starch or α-amylase were enhanced due to the higher hydrophobicity of RGFAs,resulting in a higher ability to inhibit starch digestion.In the RGFAs samples stored for 4 days,the hydrolysis mainly occurred in the first 60 min during the pancreatic digestion,and the 6 h-RGFAs showed higher resistance to pancreatin.Whether stored for 1 day or 4 days,2 h-RGFAs samples exhibited higher viscosity and could be adhered to the surface of starch particles,resulting the hydrolysis rate slightly lower than 6 h-RGFAs samples.At the same time,the peak viscosity and final viscosity of the mixtures of 6 h-RGFAs and starch were significantly reduced attributed to the extreme aspect ratio of fibril samples,leading to the starch gelatinization was inhibited.