Effects of freezing on microstructure and rehydration properties of freeze-dried soybean curd

• The research shows process-structure-rehydration relationships of freeze-dried tofu.
• 3-D microstructures of freeze-dried tofu were determined.
• Rehydration kinetics and solid loss are clearly controlled by freezing temperatures.
• Slow freezing gives sponge structure and modify color of dried and rehydrated tofu.
• Fast freezing enhances rehydration capacity but increases solid loss.

Ice crystallization controls the microstructure and subsequent mass transport of water in freeze-dried food matrices. This study investigated the effects of freezing on the microstructure and rehydration properties of porous, freeze-dried soybean curd. Soybean curd was prefrozen at various freezing conditions (−20 °C, −50 °C, −90 °C and in Liq.N2) prior to freeze-drying. Scanning electron microscopy showed a correlation between pore morphologies and freezing temperatures. The patterns of ice formed in the 3-D matrices were revealed using X-ray computed tomography. The decreased freezing temperature from −20 °C to −90 °C gave clearer needle ice formation in parallel with heat transfer resulting in a sponge-like structure after rehydration with a corresponding wall thickness of 2.9–21.8 μm. Conversely, the Liq.N2 freezing gave very fine ice crystals which effectively retained the fresh-soybean curd structure and color upon rehydration. Cracking increased with decreased freezing temperatures which also accelerated solid loss during rehydration. A correlation between the pore morphology (i.e. pore diameter and 1st-order rehydration kinetics) was observed. Cluster analysis revealed that freezing at −20 °C and −50 °C changed the color; whereas, freezing at −90 °C and with Liq.N2 effectively preserved the color of the rehydrated soybean curd. The results clearly demonstrated a process-structure-function relationship in freeze-drying which can be effectively utilized in the structural design of freeze-dried food materials.