Effect of microwave pre-dehydration on ice crystal formation in freezing process

Partial dehydration by microwave-vacuum drying has been applied to mackerels in order to reduce cell-damages caused by the formation of large ice-crystals during freezing. The samples were subjected to microwave vacuum drying at pressure of 5 kPa and temperature less than 27°C to remove small amount of the water prior to freezing. The mackerels were cooled by using the freezing chamber at temperature of -20°C and -80°C, respectively. The observations of ice crystal clearly indicated that large ice crystals were mainly formed in the intercellular spaces when the mackerel was frozen at the both cooling conditions. It is also can be understood that the formation of large ice crystals has a close relation to the cell damages. In this experiment we successfully confirmed that rapid cooling and pre-dehydration are effective in minimizing the size of ice crystal. After thawing, the pre-dehydrated samples showed a less drip loss than the un-dehydrated one. It is considered that the pre-dehydration by microwave-vacuum drying is one of promising method for the cryo-preservation of foods.