ABSTRACTThe effect of organic solvents on globular proteins during extraction processes has been studied in a model system of myoglobin and acetone. Acetone was added to water solutions of myoglobin and acetone‐water mixtures were added to solid samples of myoglobin. The acetone concentration range was 10–99% (v/v). The degree of irreversible transition caused by acetone in the myoglobin preparations was characterized both by differential scanning calorimetry and by water solubility. At corresponding acetone concentrations the myoglobin preparations resulting from the two different ways of adding acetone showed no significant difference in degree of irreversible transition. A constant fraction of any myoglobin preparation was resoluble in water. At medium acetone concentrations (50–70%) myoglobin underwent complete irreversible transition, whereas at low (30% and below) and high acetone concentrations (90% and above), only a fraction of the myoglobin sample underwent irreversible transition. In the low concentration range this was ascribed to the decrease in polarity of the acetone‐water medium, and in the high concentration range to the decreased availability of water, expressed as a decrease in water activity. The myoglobin samples treated with high concentrations of acetone could be divided into two groups with different susceptibility to a following heat treatment. A model for predicting the degree of irreversible transition of protein concentrates and isolates that have undergone organic solvent extraction is pr