A comparison of thermodynamic approaches to predict the adhesion of dairy microorganisms to solid substrata

Four different thermodynamic approaches were compared on their usefulness to predict correctly the adhesion of two fouling microogranisms from dairy processing to various solid substrata. The surface free energies of the interacting surfaces were derived from measured contact angles according to: 1. The equation of state; 2. The geometric-mean equation using dispersion and polar components neglecting spreading pressures; 3. The geometric-mean equation using dispersion and polar components while accounting for spreading pressures; and 4. The Lifshitz-van der Waals/Acid-Base approach. All approaches yielded similar surface free energies for the low energy surfaces. Application of approach 1 with different liquids did not give consistent values for the high surface free energy substrata. The dispersion or Lifshiftz-van der Waals components were nearly equal for approaches 2, 3, and 4; however, the polar or acid-base components differed greatly according to the approach followed. Approaches 1 and 2 correctly predicted that adhesion should occur, although the trend with respect to the various solid substrata was opposite the one experimentally observed, as was also the trend predicted by approach 4. Only approach 3 correctly predicted the observed bacterial adhesion with respect to the various solid substrata. In approach 3 and 4, adhesion was frequently found, despite a positive free energy of adhesion. This was attributed to either possible local attractive electrostatic interactions, inadequate weighing of surface free energy components in the calculation of free energies of adhesion, or to additional forces arising from structured interfacial water.