Underlying mechanisms behind adhesion of fermented milk to packaging surfaces

Fermented milk is commonly eaten during breakfast or as a snack between meals either in one-portion cups or from a bowl with a spoon. Approximately 5-10% of the fermented milk remains in the packages upon pouring, which during the last years, has received attention in public media as newspapers and television. This is a problem, not only from an economical point of view, but also from an ecological and unethical perspective. In this study, we have investigated the influence of rheology, surface characteristics, and storage time; on product adhesion, and this study is a continuation of an earlier study published in this journal. The physical properties of the surface as roughness and morphology were determined by Scanning Electron Microscopy, Interferometry, contact angle measurements, and Fourier Transform Infrared/Attenuated Total Reflectance Spectroscopy. After incubation, the adhered amount was determined by gravimetrical studies. The results have shown that there are two mechanisms over different time scales that predominate. Initially (seconds-minutes) the excess product is drained of the surfaces until the gravity is opposed by the rheological strength such as the yield stress and the apparent viscosity. The second mechanism occurs on a longer time scale (minutes-hours) and it includes dewatering of the product residue at the surface. Concluded in this work is that the rheological properties of fermented milk is of major importance and that the problem with fermented milk remaining in the packages could be regarded as a flow and dewatering problem rather than an adsorption problem. (C) 2014 Elsevier Ltd. All rights reserved. (Less)