Adhesion of food emulsions to packaging and equipment surfaces

The adhesion of food emulsions to food contact surfaces is a problem of utm ost importance in the recycling of packages and cleaning of industrial equi pment. Bulk adhesion was measured experimentally by weighing the mass of fo od emulsion remaining on solid surfaces after contact (i.e., amount adhered or adhesion amount), a matter which is of industrial concern. Surfaces of different hydrophilicity have been tested: polytetrafluoroethylene (PTFE), low-density polyethylene (LDPE), poly(ethylene terephthalate) (PET), stainl ess steel and glass. Their polar free adhesion energy in contact with water (Wa(water)(P)) varied in the range 0.8 to 7 mJ m(-2). The observed decrease in the bulk adhesion amount of oil-in-water (o/w) emu lsions, when stabilized by whey protein isolates and soybean lecithin, with decreasing Wa(water)(P) may be explained by increasing hydrophilization of the outside of adsorbed protein layers formed on solid substrates. This ph enomenon would be due to conformational rearrangements of the macromolecule s by hiding their hydrophobic moieties in contact with hydrophobic substrat es, leading to a consequent decrease in adhesion forces between emulsion dr oplets and the substrates. A correlation was established between adhesion measurements and solid surfa ce tension, gamma s or its electron-donor component from the van Oss model, gamma s(-). Results could be interpreted on the basis of physicochemical m echanics which relates rheological and adhesive properties of emulsions to microscopic adhesion forces acting between liquid droplets and surfaces. Th e relative importance on adhesion amount of surface thermodynamical propert ies and emulsion rheology was demonstrated and several hypotheses for bulk adhesion mechanisms are forwarded. (C) 1999 Elsevier Science B.V. All right s reserved.