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

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 degrees C, 50 degrees C, 90 degrees C and in Liq.N-2) 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 degrees C to 90 degrees 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 gm. 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 degrees C and 50 C changed the color; whereas, freezing at 90 C and with Liq.N-2 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. (C) 2016 Elsevier Ltd. All rights reserved.