Effects Of Microwave Treatment On Structural Characteristics Of Rice Starch And Its Volatile Flavor Compounds

Microwave radiation is a new processing technology,which has the characteristics of good penetrability,uniform heating,rapid and less nutrition loss.It is widely used in starch food processing.In the past decades,there have been many studies on the effect of microwave radiation on starch structure,while no unified conclusion has yet been reached on the effect of microwave radiation on structural characteristics of starch.Moreover,little attention had been paid to the changes of volatile flavors in starch before and after microwave treatment.In this paper,starch was extracted from rice by alkaline extraction method.The physicochemical properties and structural characteristics of starch were firstly investigated,and then the effect of microwave treatment on the physicochemical properties and structural characteristics of rice starch was studied.Furthermore,main volatile flavor compounds of rice starch before and after microwave treatment were analyzed,in order to provide more theoretical basis and reference for the application of rice starch and its products in microwave radiation processing.The main research contents and conclusions are as follows:1.The physicochemical properties and structural characteristics of rice starch extracted by alkali extraction were analyzed.The results showed that the starch content in rice was very high,accounting for 88.46%of the dry basis.The total starch content of rice starch was 96.61%(dry basis),and the amylose content was 11.28%.The gelatinization temperature of rice starch was 86.1?,gelatinization enthalpy was 4.27 J/g;peak viscosity,valley viscosity,final viscosity,breakdown and set back were 657.0 cP,602.0 cP,986.3 cP,56.0 cP and 387.7 cP,successively.Rice starch granules showed irregular polygon with a particle size of approximately 3?8 ?m,and a specifGc surface area of 1.45 m2/g.Moreover,it had a clear polarized cross and exhibited A-type starch crystal characteristics.The weight average molecular weight MW,number average molecular weight Mn,polydispersity coefficient Mw/Mn,mean square radius of rotation Rg and degree of branching were 2.79×107 g/mol,2.58×107 g/mol,1.08,172.2 nm and 14.89%,respectively.In summary,the rice starch had highly amylopectin content,a low gelatinization enthalpy.The starch granules were small,and showed an A-type crystal structure,as having smaller the molecular weight than previous study.2.The effects of different microwave treatment conditions on the physicochemical properties of rice starch were studied.Microwave treatment conditions,including water content,temperature and time,were chosen to investigate,its effect on the physicochemical properties of rice starch,such as amylose content,particle morphology,crystallization characteristics and DSC characteristics.The results showed that microwave treatment had no significant effect on the amylose content of rice starch,but it could destroy the grain morphology,crystallization characteristics and gelatinization characteristics.In the process of microwave treatment,the water content of rice starch had little effect on the grain morphology,crystallization characteristics and gelatinization characteristics,while the temperature and time during microwave treatment had a greater effect.The higher temperature and longer time of microwave treatment could greatly influence the morphology,crystallization characteristics and DSC characteristics of starch particles of rice starch.3.The effect of microwave treatment on the fine structure of rice starch was explored.Rice starch MW1(microwave treatment conditions:moisture content 95%,temperature 60? time 1 s)and MW2(microwave treatment conditions:moisture content 95%,temperature 70?,time 10 min)were prepared under different microwave treatment conditions.The fine structure differences between the two fractions and the original starch were analyzed and compared.The results showed that the solubility,swelling,in vitro digestion and molecular weight of the rice starch MW1 with lower temperature and shorter time for microwave treatment,because the starch particles have not been damaged and the crystal structure was not changed.However,the physicochemical properties and fine structure of MW2 with higher temperature and longer time for microwave treatment was different from the original one,due to the destruction of starch particles.The crystal structure also changed from A-type crystal to amorphous state.However,no chemical bond changes of starch molecules were detected after microwave treatment.In summary,the larger the temperature and time of microwave treatment,the greater of its influence on the fine structure characteristics of starch was reached.4.Effect of microwave treatment on volatile flavor compounds of rice starch was analyzed.SPME-GC-MS was used to identify and analyze the volatile flavor compounds of rice flour,rice starch and microwave-treated rice starch.Meanwhile,flavor compounds were classified and qualitatively analyzed.The results showed that 42 volatile flavors were detected among rice flour and rice starch samples,including aldehydes,ketones,alcohols,esters and others(among which aldehydes were the most important flavors).Hexaldehyde content was the highest,followed by n-hexanol among them.When rice starch was prepared from rice flour by alkaline extraction,hexanal and n-hexanol were reduced greatly.After microwave treatment,the type of volatile flavor compounds in starch did not change a lot,as well as their relative percentage content.However,it was decreased a lot for MW2,indicating that slight microwave treatment had no significant effect on volatile compounds in starch,while the increasing of microwave treatment temperature and time resulted in serious loss of volatile compounds in starch.