In contrast, the increasing tenderness as pH falls below 6.0 has been attributed to enhanced acidic protease activity. Watanabe et al. [160] also suggested non-enzymatic causes of the increased meat toughness at pH values in the range 5.8–6.3. The authors recognized that reduced sarcomere length is an important cause of increased toughness in meat, and it appears that sarcomeres increase in length as the ultimate pH decreases below 6.2. Alternatively, tend- erisation has been attributed to the direct effect of calcium ions on myofibrillar proteins, without involvement of enzymatic proteolysis, and this process has been shown to be pH dependent. The toughness of meat immediately post- mortem is the same whether the meat is of low, medium or high pH, when the aging is inhibited by ZnCl2. Following limited aging, Watanabe et al. [160] found a curvilinear relationship between pH and shear force values, with a toughness peak at pH 6.07. However, the authors con- cluded that continued aging eventually achieves the same degree of tenderness at all pH values.