Toughness development in beef sternomandibularis muscle during cooking has been measured by a tenderometer, cleaving samples at right angles to the fibre direction. Since the muscles had been set in rigor mortis in an unshortened state, such toughening was not related to that induced by pre-rigor contraction.Two distinctly separate phases of toughening develop with increasing cooking temperature. A three-to fourfold toughening occurs in the first cooking phase between 40 and 50 "C, followed by a further doubling in the second phase between 65 and 75 "C. The first phase is associated with loss of myosin solubility, presumed to indicate denaturation in the contractile system. The second is closely associated with collagen shrinkage. This apparently induces shortening along the meat fibre, forcing out meat juice.

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SUMMARY— During the aging of fiber pieces prepared from bovine sternomandibularis muscles, a loss of adhesion occurs between adjacent myofibrils. This is evidenced by increased readiness of fiber pieces to distintegrate into individual myofibrils during a period of standard disruption. Alterations also appear within the myofibrils themselves in the regions of the Z lines, sometimes leading to the apparent dissolution of this structure. Ethylenediamine tetraacetate present in the suspensions during storage not only prevents these changes, but also preserves the refractory character of the fiber pieces. Meat aging is considered therefore to be due to disruption and possible dissolution of Z‐line material, leading to a weakening of inter‐myofibrillar linkages probably located at the junctions of adjacent Z lines, and to loss of tensile strength of the myofibrils themselves.

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