Structure of beef Longissimus dorsi muscle frozen at various temperatures: Part 2-ultrastructure of muscles frozen at -10, -22, -33, -78 and -115°C.

The changes in electron micrographs of muscles frozen at -10, -22, -33, -78 and -115°C were analyzed. The ultrastructure of muscle successively changed with decreasing freezing temperature whereas light microscopy indicated anomalous behaviour at -22°C. It appeared that, in muscles frozen at -10°C, there was no freezing of water intracellularly; in those frozen at -22°C, water was frozen intracellularly (but only in the I-band region); whereas, in muscles frozen at -33°C, water was frozen inside the fibres, both in the I- and the A-bands. In muscles frozen at -78 and -115°C, water is frozen intracellularly. These findings can be explained on the basis that, in the I-band region, the major protein is actin, which has a relatively high proportion of non-polar residues and holds water weakly, whereas the predominant protein in the A-band is myosin, which contains many polar residues and has a high water-holding capacity.