Effects Of Protein On The Physicochemical Properties Of Indica Rice Flour During Rice Aging

Indica rice is the conventional strategic grain reserve in China and the raw material for traditional rice-based food,such as rice noodles,fermented rice cakes,and so on.Generally,storage of freshly harvested rice grains for a period of time is essential to obtain rice-based products with good qualities.Protein is the second most abundant nutrient in rice after starch,which is closely related to the quality of rice products.Therefore,in this study,four indica rice varieties,namely Zhongzao 35,BB02,TM130 and 87 Zao,were used as raw materials to explore the physicochemical properties changes of indica rice after storage at 36℃ for 0-90 day,and to analyze the changes in protein composition and structure during aging as well as its impact on rice physicochemical properties.These will provide a theoretical basis for the deep processing of grain and improve the stability of rice quality.The major findings and results are summarized as follows:The effects of ageing on the physicochemical properties of indica rice flour showed that the peak viscosity,tough viscosity,and breakdown viscosity of indica rice were both increased first and then decreased with the extension of storage time,while the setback,pasting temperature and final viscosity were increased.The solubility of aged rice was increased,while the swelling power was decreased.After storage,the elastic modulus and viscosity modulus were increased,indicating an enhanced gel network structure.In addition,the loss tangent values(Tan δ)of all samples were less than 1,exhibiting a solid-like behavior and weak gel properties.The correlation analysis between pasting characteristic parameters showed that the peak viscosity was the most sensitive index of pasting properties during aging,and could be used to predict the changes of other pasting characteristic indexes.It was found that the content of crude protein,prolamin and glutenin had no significant changes,but the content of albumin,globulin and sulfhydryl,and the activities of α-amylase,starch branching enzyme,and starch debranching enzyme were decreased.The coordination among α-amylase,starch branching enzyme,and starch debranching enzyme changed the distribution of amylopectin chain length,increased the proportion of short chains(A chains)or super long chains(B3 chains)in different varieties,and affected the pasting and rheological properties of rice flour.The infrared spectra of protein showed that no new peak was shown during aging,that is,the groups of molecular were not changed.However,the relative proportion of protein secondary structure was changed,and protein tended to be unstable.The results of SDS-PAGE electrophoresis showed that no protein subunit band was appeared or lost,but the intensities of the subunit bands were changed.The band strength of the subunits with molecular weights of 13,20,and 56 kDa was increased,while the band intensity of the subunits with molecular weights of 31 and 33 kDa was decreased.Pearson correlation analysis showed protein composition and structure highly correlated with the physicochemical properties of indica rice flour.Compared with untreated samples,the deproteinized rice flour showed higher peak viscosity,breakdown viscosity,solubility,and swelling power,but lower pasting temperature,indicating that endogenous proteins significantly affected the pasting and hydration properties of indica rice flour.The results of scanning electron microscopy and optical microscopy showed that protein wrapped starch to form composite particles,resulting in uneven particle size and irregular shape.The Fourier transform infrared spectra showed that the characteristic absorption peaks of the protein amide II band(1600-1500 cm-1)located at 1535 cm-1 was disappeared in deproteinized samples.The absorption peaks of the starch intramolecular and intermolecular O-H stretching vibration at about 3400 cm-1 with a blue shift phenomenon were not as sharp as those of the untreated indica rice flour,indicating that the strength of hydrogen bond was weakened and the hydrogen bond interaction between protein and starch molecules existed.In addition,the ratio of the crystalline region to non-crystalline regions was decreased,that is,the short-range order of rice starch was decreased,demonstrating that the presence of endogenous proteins enhanced the short-range order of starch molecules.