The effects of different freezing methods on the quality changes of bracken fern and the root of bellflower were examined. Quality characteristics, such as the microstructure, hardness, colour, and sensory properties of these vegetables, were determined after freezing at -20°C in a freezer, at -70°C in a gas nitrogen convection chamber, and at -196°C in liquid nitrogen. Freezing produced considerable changes in the microstructure, hardness, colour, and sensory parameters of the bracken fern and bellflower root samples. The highest degree of changes in the microstructure and hardness of the vegetables were induced by freezing at -20°C, whereas the least changes were produced by freezing at -70°C. Based on the sensory characteristics, freezing at -70°C provided the highest acceptability of the vegetable samples.
In addition, the preparation of frozen lotus root was optimized. Prior to freezing, an optimal blanching condition at 100°C for 5 min was established, based on the results of microbial growth, texture, total phenolic content(TPC), and sensory evaluation. Blanched samples were then frozen under various freezing conditions such as -20°C in a freezer for 2 h, -70°C in gas nitrogen convection chamber for 7 min, and -196°C in liquid nitrogen for 20 s, and their qualities after thawing were determined. Scanning electron microscopic analysis indicated that the microstructure of the sample frozen at -70°C was similar to that of the control sample, compared with other freezing conditions such as -20 and -196°C. The antioxidant activity of frozen samples decreased compared to the control, but there was no significant(p<0.05) difference among treatments. In terms of TPC, the samples frozen at -70 and -196°C had significantly(p<0.05) higher values than that of the sample frozen at -20°C. In addition, drip loss of the sample frozen at -20°C was higher than those of other frozen samples. Therefore, these results suggest that freezing at -70°C in gas nitrogen convection chamber can be an optimal freezing method to produce high-quality frozen lotus root.