On the mechanism of water holding in meat: The swelling and shrinking of myofibrils

Water holding in meat has, in the past, been rather poorly understood and has not been explained at all in structural terms. A unifying hypothesis for this phenomenon is that gains or losses of water in meat are due simply to swelling or shrinking of the myofibrils caused by expansion or shrinking of the filament lattice. Myofibrils have been observed by phase contrast microscopy, and are seen to swell quickly to about twice their original volume in salt solutions resembling those used in meat processing. Such swelling is highly co-operative. Pyrophosphate reduces very substantially the sodium chloride concentration required for maximum swelling. In the absence of pyrophosphate, swelling is accompanied by extraction of the middle of the A-band; in its presence the A-band is completely extracted, beginning from its ends. We suppose that Cl− ions bind to the filaments and increase the electrostatic repulsive force between them. A crucial factor in swelling is likely to be the removal at a critical salt concentration of one or more transverse structural constraints in the myofibril (probably crossbridges, the M-line or the Z-line) allowing the filament lattice to expand. We also point out that water losses in rigor, in the PSE condition and on cooking may well result directly from shrinkage of the filament lattice.