| Rice cake is a traditional rice-made food in China.In the traditional production of rice cake,pounding is the most distinctive processing step,and it is also the key step in the quality formation.In the modern production process,screw extrusion has replaced the traditional manual pounding.Although the production efficiency is improved,the taste of rice cake is reduced.Using mechanical pounding to imitate the traditional manual pounding,studying the quality formation of rice cake in the pounding process,comparing the quality difference between pounding process and extrusion process will help to understand the impact of different processing processes on the quality of rice cake and improve the sensory quality.Therefore,firstly,this paper used response surface design to optimize the pounding process of rice cake.Then using texture analysis,scanning electron microscopy,size exclusion chromatography,confocal laser scanning microscopy and other methods to measure the texture,microstructure,starch and protein properties of extruded rice cake and pounded rice cake produced under different pounding times.On one hand,it showed the changing process of the characteristics of rice cakes during the pounding process,on the other hand,it compared the quality differences of the rice cakes produced by the pounding and extrusion processes,which comprehensively reflected the influence of processing technology on the quality of the rice cakes.The main research conclusions were shown as follows:1.The single factor experiment was used to investigate the effects of water addition,pounding height and pounding times number on the texture index,cooking quality and sensory evaluation of rice cakes.The pounding height had the largest effect on cooking loss,the pounding times number had the largest effect on water absorption,the water added had the largest effect on texture index,and both the water added and pounding times number had a greater effect on sensory evaluation.The optimal pounding processing parameters for rice cake were determined through the design of the response surface center combination experiment: the amount of water added was72%,the pounding height was 0.30 m,and the pounding times was 900 times.2.Extruded rice cake and pounded rice cake had different texture characteristics.Although the two types of rice cakes had similar elasticity,the hardness of extruded rice cake was significantly higher than that of pounded rice cake.Scanning electron microscopy results showed the pores formed in the extruded rice cake had smaller pore sizes and uneven distribution,while the pores formed in the pounded rice cake had larger pore sizes and uniform distribution.3.Processing changed the physico-chemical structure of starch.?-amylase and pullulanase had different enzymatic hydrolysis in different rice cakes,which indicated that different processing methods and processing degrees had resulted in different arrangements of starch molecular structure.The effects of processing technology on the molecular structure of starch were investigated by size exclusion chromatography and the apparent amylose content changes.SEC results showed that pounding and extrusion promoted the cross-linking and entanglement of amylopectin chains with other branched or amylose chains,and formed uniform starch network is in the rice cake.The entanglement of starch molecular chains caused by the higher pounding times was more significant.The entanglement between the molecular chains may provide better elasticity for pounded rice cake.The apparent amylose content increased with the extrusion process and the number of pounding times,which indicating that the amylopectin degradation had occurred to varying degrees during processing.The effects of processing technology on the thermal and gelatinization properties of starch were investigated by DSC and RVA.DSC data showed that all rice cakes had a high level of gelatinization and the rice cake produced a small amount of starch retrogradation during the cooling process,but no change in starch-lipid complex was observed.RVA data showed that processing promoted the gelatinization of starch,and the starch degradation and entanglement caused by processing delayed starch retrogradation.4.Processing changed the physico-chemical structure of protein.The effect of processing technology on the microstructure of proteins was investigated by laser confocal microscopy,it was found that processing reduced the size of protein aggregation and the distribution was more uniformly.The aggregation state of protein aggregates was investigated by measuring the solubility of the protein in different buffer solutions,and measuring the sulfhydryl content and average particle size.Solubility results showed that the solubility of proteins in different buffers is increased by processing,indicating that protein aggregates undergo degradation during processing.The increase in the total sulfhydryl content of the protein indicated that the disulfide bonds of some protein aggregates were broken.The particle size data showed a decrease in the diameter of the soluble protein and the aggregates were destroyed.The effects of processing technology on protein structure were investigated by SDS-PAGE,zeta potential and fourier infrared spectroscopy.SDS-PAGE results showed that processing did not cause degradation of protein subunits.Zeta potential data showed that processing exposed more positively charged amino acid residues to the surface of protein molecules.Infrared spectroscopy results showed that pounding increased the content of ?-sheet structure,which may improve elasticity.Overall,the changes in protein properties caused by extrusion were similar to the rice cake pounded by 500 times. |
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