The effects of pH and heating on the surface activity of muscle proteins

The effects of pH and thermal denaturation on the surface active properties of muscle proteins at the air-solvent interface were determined by the drop volume method. The surface pressures attained after 40 min, π₄₀, by myosin and actomyosin solutions heated at 45°C for 30 min were similar to those attained by the unheated protein solutions. However, the π₄₀ values were increased when the proteins were heated at 60°C prior to determining surface activity. The rate of surface tension decay was much faster following heating of myosin at 45°C compared with the unheated sample. Myosin heated at 60°C showed an initial induction period, which was followed by a very rapid rate of surface tension decay. The rate of surface tension decay for actomyosin heated at 45°C was similar to that of the unheated protein solution: however, heating at 60°C increased the rate compared to that of the control. The surface pressures attained after 40 min (π₄₀) by myosin and Factin solutions were not significantly affected by the pH of the solutions. However, π₄₀ values for both G-actin and actomyosin were greater at pH 11·0 than at pH 5·6 or 7·0. For all the proteins studied, pH greatly affected the rates of surface tension decay, the rates being slowest at pH 5·6 and fastest at 11·0.