Crack Propagation Mechanism of Freeze-Dried Soybean Curds during the Rehydration Process

The rehydration of a porous freeze-dried food plays a crucial role as it significantly influences the quality of the final product. In this study, we investigated the rehydration of freeze-dried soybean curd, commonly known as tofu, and elucidated the mechanism of crack propagation during the rehydration process. We rehydrated the freeze-dried tofu samples with water at four different temperatures ranging from 20 °C to 100 °C. The rehydration time, defined as the time taken to completely wet the entire surface of the sample, decreased with an increase in water temperature. We found that the cracks, formed during the production process, absorbed water significantly faster than the porous body. Interestingly, the crack expanded when immersed in high temperature water, leading to the breakage of the tofu. Environmental scanning electron microscopy revealed that tofu expands as it absorbs a significant amount of water. We concluded that the crack expansion in high-temperature water is attributed to the stress concentration caused by the difference in water rehydration rates between the cracks and porous body.