| In this study, the effect of Combined Emulsifiers with Soybean Lecithins and Sucrose Esters on the survival ratio of frozen bake yeast and its develop speed & the quality of wheat flour & freezable water content and glass transition temperature, rheological properties of frozen dough & bread quality made from frozen dough were investigated.The experiment of freezing yeast in Yeast Extract Peptone Dextrose Medium(YPD) culture medium showed that the yeast with combined emulsifiers had a relatively higher survival ratio and quicker growing speed. Combined emulsifiers could diminish the damage on yeast caused by freezing. The survival ratio of yeast cultivated for 48 hours after frozen at-30℃for 5days in a culture medium with 4% combined emulsifers increased from 50% to 80% compared with the control group, and the adaptive phase shortened by 9 hours. In addition,the image taken by scanning electron microscope (SEM) indicated that there might be some interaction effects existed between the two substances.In farinograph, the results showed that the addition of combined emulsifiers could improve the quality of wheat flour. After the addition of combined emulsifiers, best stability time increased by 5.8min and 7.2min,weaken degree decreased from 48FU to 16FU and 10FU, the flour quality index upgraded from 43 to 136 and 154. Furthermore the tensile tests showed that tensile resistance of the dough was significantly enhanced by adding combined emulsifiers. The dough with emulsifiers had a higher extension energy which was propitious to process, and addition of 3% had a better effect than 2% emulsifiers.Differential scanning calorimetry(DSC) showed that Combined Emulsifiers could slow down the growing of freezable water in frozen dough, enhance the water holding capacity of protein. With the same storage time, dough with emulsifiers had a lower freezable water content than the control group. The thaw enthalpy of fresh dough declined from 22.35J/g to 15.08J/g after the addition of emulsifiers, whereas the thaw enthalpy of dough without emulsifiers increased to 29.18J/g after two months'freezing, increased by 31.14% compared with unfrozen dough. The thaw enthalpy of dough with emulsifiers increased to 16.08 J/g, increased by 6.63% compared with unfrozen dough. The glass transition temperature of dough frozen for two months was enhanced from -3.8℃to -28℃by emulsifiers, which could make the dough quality more stable.The dynamic rheological properties of frozen dough showed that at different storage time, the dough with emulsifiers had a higher storage module and average molecular weight. The dough frozen for two months with emulsifiers had a higher G'in the entire range of frequency sweeping, and it's cross point of storage module and loss module increased from 14.67Hz to 32.04Hz compared with dough without emulsifiers. The dough with emulsifiers was possessed of a bigger proportion of G'under the frequency sweeping, and the cross point moved to high frequency. The dough's average molecular weight was upgraded by emulsifiers.The experiment of bread baking showed that emulsifiers could increase the loaf volume, improve sense properties, make the bread more softer and slow down the firming rate of bread crumb. The loaf volume of fresh dough without emulsifiers, dough without emulsifiers frozen for two months and dough with emulsifiers frozen for two months were 4.65 cm3/g,3.78 cm3/g and 4.55 cm3/g, and the bread crumb with emulsifiers was much softer, more uniform. Within one week storage, the bread made from control, with 2% and 3% emulsifier's dough had 157.1g/d,114.3g/d,100.0g/d firming rate at 4℃,121.4g/d,74.3g/d,60.0g/d at 20℃.
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