Effect Of Ultrasonic Treatment On The Gel Properties Of Waxy Rice Flour

Foods made from waxy rice flour（WRF）are more and more popular among people.The WRF with low gelatinization temperature,high peak viscosity and weak gelation ability has good application in food and is more popular in the market.Currently,some exogenous substances such as gums and carbohydrate are added to the WRF to improve the gel properties.This paper mainly explores the effect of ultrasonic treatment on the gel properties of WRF and its possible mechanismits.It will provide a theoretical basis for exploring a new and safe physical technique to improve the quality of WRF.In order to explore the effect of ultrasonic treatment on the gel properties of WRF,the orthogonal test was applied and the tanδof waxy rice flour gel reached 0.82 when the optimal parameters were 130 W of ultrasonic power,20 min of ultrasonic time,and 1:2 of solid-to-liquid ratio.Scanning electron micrograph showed that starch granules in WRF agglomerated and the starch granules in ultrasound treated waxy rice flour（UT-WRF）dispersed.The particle size of UT-WRF was significantly reduced and the color became lighter compared to that of WRF.The results of X-ray diffraction.The peak viscosity,through viscosity and final viscosity of UT-WRF were higher than those of WRF.The ultrasound treatment of WRF led to a decrease in storage modulus（G’）,hardness as well as an increase in loss modulus（G’）and adhesiveness.Therefore,the WRF showed that the characteristics of soft gel after ultrasonic treatment.The mechanism of the effect of ultrasonic treatment on the gel properties of WRF was further studied as the ultrasound caused the gels of WRF to weak.Firstly,the effect of ultrasonic treatment on the properties of starch in the WRF（containing 84.34%starch in WRF）was studied.The ultrasound treatment had no significant effect on the properties of the starch isolated from ultrasound treated waxy rice flour（S-UT-WRF）,which is the main component in WRF.There was no significant difference in the micromorphology,gelatinization properties,rheological properties and texture properties of S-UT-WRF.Secondly,the effect of ultrasonic treatment on the protein（6.95%protein in WRF）properties of WRF was studied.It was found that the effect of ultrasonic treatment on protein extracted from waxy rice flour（P-WRF）was the most significant（P<0.05）,when the ultrasonic power was 130 W,the ultrasonic time was 10 min,and the solid-liquid ratio was 1:2.The structure of the protein extracted from ultrasound treated waxy rice flour（P-UT-WRF）was loose and the holes were dense.Compared with the P-WRF,the solubility,surface hydrophobicity,endogenous fluorescence intensity,the content of free sulfhydryl group,β-sheet and random coil of the P-UT-WRF increased significantly（P<0.05）.The content ofα-helix,β-turn,disulfide and thermal stability of the P-UT-WRF decreased significantly compared to P-WRF（P<0.05）.Therefore,when the WRF was ultrasonically treated,the conformation of the protein matrix was mainly changed to effect on the gel properties of the WRF and the main component starch remains intact.The characteristics of mochi prepared from UT-WRF and WRF were studied to confirm the improvement of WRF by ultrasound.The sensory evaluation showed that the fresh mochi prepared with UT-WRF（UT-Mochi）was brighter and clearer than the fresh mochi（N-Mochi）prepared with WRF（N-Mochi）,and the score of UT-Mochi was higher.The fresh UT-Mochi had less hardness,greater elasticity,softer and a more elastic taste compared to the fresh N-Mochi.Scanning electron micrograph showed that the surface pores of fresh UT-Mochi were small and uniform in size,while the surface pores of fresh N-Mochi were larger and non-uniform in size.After the two kinds of mochi were stored at 4°C for 7 d,the surface pores were significantly smaller and the number was significantly reduced.With the prolongation of storage time,the moisture content and the T₂₂value of UT-Mochi and N-Mochi gradually decreased,while the T₂₁value gradually increased.The moisture content,T₂₁and T₂₂values of UT-Mochi were lower than those of N-Mochi at the same storage time.The results of low field NMR imaging showed that the water loss of UT-Mochi was faster compared to that of N-Mochi.The results of the freezer water content showed that the melting enthalpy of ice fusion and the freezer water content of UT-Mochi and N-Mochi gradually decreased during the storage time of 1-7 d.Under the same storage time,the melting enthalpy of ice fusion and freezable water content of UT-Mochi are smaller than those of N-Mochi.The results of texture analyzer,differential scanning calorimeter,X-ray diffractometer showed that the hardness,melting enthalpy,crystallinity of UT-Mochi and N-Mochi were increased with increasing of storage time.The hardness,melting enthalpy,crystallinity of UT-Mochi were higher than those of N-Mochi at the same storage time.According to the above results,it can be seen that the appearance,texture and sensory aspects of mochi prepared from UT-WRF have been significantly improved.Ultrasound technology has great potential for development as a green emerging technology to efficiently improve the quality of waxy rice flour.