Improvement of Mechanical Properties of Frozen Japanese Radish by Combination of Low- and High-temperature Blanching Pretreatment

In this study, LTB + HTB (low-temperature blanching followed by high-temperature blanching) pretreatment was applied to prevent excessive softening of frozen Japanese radish. It was found that heating at 60 °C for 120 min (LTB) followed by heating for 4 min in boiling water (HTB) was a better condition than only HTB because it could maintain the firmness of the sample and inactivate the enzyme. Peroxidase (POD) activity assay showed that HTB for 4 min in boiling water (100 °C) is necessary for effective blanching with or without LTB. However, HTB treatment alone led to excessive softening after freezing. Heating at 60 °C for 15, 30, and 120 min (LTB) followed by HTB were performed, and the longer the heating time at 60 °C, the higher the breaking stress. In the LTB + HTB (60 °C120 min + 100 °C4 min) frozen-thawed sample, the degree of esterification (DE) of pectin increased compared to the fresh sample. The result suggests that LTB treatment promoted the demethylation of pectin, leading to strong cell wall adhesion, and prevented the depolymerization of pectin by subsequent HTB treatment. Although the decrease in the initial elastic modulus due to cell membrane damage and ice crystal formation is inevitable, LTB + HTB pretreatment was shown to be effective in maintaining the firmness of the sample even after freeze-thawing.