THE INFLUENCE OF WORK SOFTENING ON THE VISCOELASTIC PROPERTIES OF BUTTER AND MARGARINE

Creep compliance-time studies at low stresses give more information about the structural changes in work softened butter and margarine than the Cone Penetrometer. They show that margarine undergoes a greater structural change than butter. Margarine loses most of its instantaneous elasticity in an unrecoverable manner. However, most of the Newtonian viscosity loss is recovered. Butter loses much less of its instantaneous elasticity, and most of what is lost is recovered during aging, but less of the Newtonian viscosity which is lost is recovered. Retardation spectra derived from creep compliance-time curves for both butter and margarine before work softening show a peak at ∼10⁴ sec. In contrast to margarine, butter does not lose this peak when work-softened. For margarine, the peak slowly redevelops during aging and occurs at ∼10² sec. Past interpretation of Cone Penetrometer data has been based on the simplified concept of primary irreversible bonds and secondary reversible bonds, and the changes in their relative proportions during work softening. The present approach avoids the allocation of bond strengths into these arbitrary categories, and is based on the realization that butter and margarine contain a wide range of bond strengths.