Effects of freezing processes on the structure and flavor of frozen tofu.

Background: Frozen tofu with a porous and elastic structure and a unique taste is formed by steaming, soaking and freezing fresh tofu. At present, no unified standard exists for the manufacture of frozen tofu. Hence, this study aimed to investigate the effects of conventional freezing processes (fresh tofu was frozen at -10 °C or -20 °C for 8 days and then ripened at -4 °C for 2 days after steaming and soaking) on tofu by examining the changes in the structure and volatile substances of frozen tofu.

Results: During the 10-day storage, the color change was more significant in the water-soaked groups than in the salt-soaked groups, especially after steaming treatment. During frozen storage for 4-8 days, the pH decreased 10.9% in the water-soaked groups and 3.8% in the salt-soaked groups. The protein denaturation was more obvious in the salt-soaked groups especially in synergistic steaming, resulting in a 48.2% and 32.0% increase in carbonyl content. In addition, ten volatiles were considered as flavor markers in frozen tofu, of which ethyl formate was a key special substance produced after tofu freezing.

Conclusion: Steaming was very important for the oxidation of tofu protein, it reconstructed the protein structure and redistributed water molecules and salt ions, which made the reformed peptide chain more stable. Furthermore, water soaking was more conducive to lipid oxidation and increased the content of the main flavoring substances compared with salt soaking. © 2025 Society of Chemical Industry.