Effects of Different Vacuum Freeze-drying Processes on Rehydration and Quality Characteristics of Freeze-dried Rice Noodles

Dried instant rice noodles are a convenience food, and they have shown rapid development in recent years. However, there are many disadvantages of dried instant rice noodles, such as long rehydration time, high turbidity of soup, high break rate, poor taste, and disappointing textural properties. In this study, to overcome these disadvantages, freeze-dried rice noodles were prepared at freezing temperatures of −10, −20, and −40 ℃ by the vacuum freeze-drying process, which were called FDRN10, FDRN20, and FDRN40, respectively. The factors that affected the rehydration quality of freeze-dried rice noodles were investigated by analyzing the rehydration speed, microstructure, starch retrogradation, and texture characteristics at different water temperatures. The results showed that FDRN10 had the highest water content of the three freeze-dried rice noodles for the same water temperature and rehydration time. This indicated that the rehydration speed of FDRN10 was the fastest. The microstructures of the external surface and cross section were observed. FDRN10 had the loosest structure and largest aperture. The diameter of the aperture of FDRN10 was 72.58 µm, which was four times larger than that of FDRN40. This microstructure led to FDRN10 showing good water absorption. Differential scanning calorimetry showed that the enthalpy of FDRN10 was 92.98 J/g. The enthalpy of FDRN40 was the lowest (12.39 J/g). Thus, FDRN10 had the highest level of starch retrogradation among the three freeze-dried rice noodle samples, and the lowest level of starch water-absorbing swelling. Hardness analysis showed that the hardness value of FDRN10 decreased with increasing water temperature for the same rehydration time. The texture characteristics of FDRN10 were the closest to those of fresh rice noodles after rehydrating with water at 70 ℃ for 8 min. The above results indicate that the freezing temperature and rehydration temperature play important roles in the quality of freeze-dried rice noodles. This study provides an innovative process for improvement of instant-rice-noodle production.