Key Production Technology And Frozen Storage/Freeze-Thaw Characteristics Of Quick-Frozen Raw-Dough Sticks

As a new type of industrialized product,the quick-frozen raw-dough sticks doesn’t need to be thawed and fried directly for consumption,and has the flavor and taste of traditional youtiao.However,quick-frozen raw-dough sticks are only suitable for frozen storage,and there are still many problems,such as substandard key processes,temperature fluctuations,the effect of prolonged frozen storage on the quick-frozen raw-dough sticks and cracked surface.This study explored the key production techniques for quick-frozen raw-dough sticks by critical process parameters such as mixing time,resting temperature and resting time,and the effects of frozen storage and freeze-thawing on quick-frozen raw-dough sticks further were analyzed.(1)Optimization of key production processes for quick-frozen raw-dough sticks.The level of each factor was determined by a one-way test,with specific volume of youtiao as the response value,the optimal process was obtained by response surface optimization test,with a mixing time of 15 min and a resting time of 12 h at 4°C.Under the 10°C resting temperature,the mixing time was 15 min,and the resting time was 7 h.Validation tests showed that the specific volume of youtiao at 4°C was 4.90 ± 0.07 m L/g(P > 0.05)for quick-frozen raw-dough sticks and 5.13 ± 0.10 m L/g(P > 0.05)for resting temperature at10°C,and there were no differences between the predicted values and the optimal process conditions,indicating that the optimal process conditions were reliable.(2)The samples obtained in the response surface tests in the previous chapter were fried into youtiao,and then the quality of the youtiao was comprehensively investigated.ANOVA found that the mixing time had a significant effect on the sensory properties of youtiao at4°C(P < 0.05),and at 10°C(P < 0.05)on the hardness,cohesiveness and sensory scores of youtiao;cluster analysis revealed that the youtiao made by the optimal process at 4°C resting temperature were more correlated with structure,shape and expansion,while the youtiao made by the optimal process at 10°C resting temperature were more correlated with flavor and taste;correlation analysis found a positive correlation between expansion and organizational structure(correlation coefficient 0.87)and chewiness and gumminess(correlation coefficient 0.84).Therefore,the quality of youtiao was influenced by the different processes used to make the quick-frozen raw-dough sticks.(3)The study of the water distribution and water migration of quick-frozen raw-dough sticks produced under the two optimal process conditions revealed that during the 1-60 d frozen storage period,quick-frozen raw-dough sticks produced under the same process conditions were dominated by loosely bound water,with strongly bound water gradually migrating to loosely bound water and free water.Scanning electron microscopy revealed disruption of gluten network structure and enlargement of pores during frozen storage;decreased in α-helix and increase in β-folding in protein secondary structure were observed(P < 0.05).The quality of youtiao significantly decreased in specific volume and fat content(P < 0.05),significantly increased in moisture(P < 0.05),and increased in hardness,gumminess and chewiness as the frozen storage time increasing(P < 0.05).There were no significant changes in the secondary structure of the protein in process 1(at 4°C resting temperature;P > 0.05)compared to process 2(at 10°C resting temperature;P > 0.05).Process 1 had slow moisture migration,small holes,larger specific volume and moisture,and smaller fat(P < 0.05).However,the quick-frozen raw-dough sticks made under process2 conditions had fast moisture migration,large holes,smaller specific volume and moisture,and larger fat(P < 0.05)for the youtiao made from the quick-frozen raw-dough sticks.As a result,the quality of youtiao made from quick-frozen raw-dough sticks under both processes decreased as the frozen storage time increasing,but the quality of youtiao made from quick-frozen raw-dough sticks under process 1 was relatively better,which will continue to be studied under this process.(4)Quick-frozen raw-dough sticks(4°C resting process)underwent multiple freeze-thaw treatments,which accelerated the migration of water and gradually changed the strongly bound water into loosely bound water,and their water loss rate and freezable water content increased with the number of freeze-thaws(P < 0.05),and the progressive loosening of the network structure and the increasing size of the holes were observed by scanning electron microscopy.The results of Fourier transform infrared spectroscopy showed that the protein secondary structure of the quick-frozen raw-dough sticks α-helix content decreased to 26%(P < 0.05),the β-fold content increased to 32.60%(P < 0.05),while the differences for β-turn and random curl were not significant(P > 0.05).Specific volume and fat content of youtiao decreased with increasing number of freeze-thaw cycles,while the moisture content increased(P < 0.05).In addition,the freeze-thaw treatment had a significant effect on the hardness,cohesiveness,elasticity,adhesiveness and chewiness of youtiao(P < 0.05).In conclusion,the freeze-thaw treatment had an impact on the quick-frozen raw-dough sticks,resulting in a reduction in product quality.As mentioned above,the 4°C and 10°C resting process gave a better production process,but with the extension of the frozen storage time,the quality of youtiao under the 4°C resting process was better than the 10°C resting process.During transport,freezing and thawing has a significant impact on the final quality of the quick-frozen raw-dough sticks,so the number of freeze-thaw cycles should not exceed 2.