In order to study the freezing process of food materials, a freezing experiment was carried out on an onion piece. We focused on the epidermal cells of onion that were obtained from the first scaly leaves. Further, we discussed the freezing process by examining the changes in viability of these cells and the appearance of protoplasts after exposing them to a low temperature. The cells were maintained at a temperature of -34.5℃ for 0, 1, 2, 4, 8, and 15h. After thawing, the cell viability was calculated from the observations made on protoplasmic streaming for more than 160 cells, under each of these conditions. The results showed that the viability of the fresh onion cells was approximately 100% and this decreased proportionally with the cooling time until it reached almost 0% at 8h. In particular, both a rapid decrease of viability was detected after 2h of cooling, when the cells were passing through the zone of maximum ice crystal formation. This result suggested that the main cause of cell death was freezing. The appearance of cells was different, depending on cooling time. Under the condition of relatively short term cooling such as for 1 to 2h, the transform of dead protoplast was not obviously. These cells seem to be killed by mechanical damage to cell membrane by extracellular freezing; however, if the cells were cooled for more than 4h, they displayed shrinkage and broken protoplast fragments. This implies that the severe mechanical damage occurred after the shrinkage of cells due to severe extracellular freezing.
