1 National Food Institute, Technical University of Denmark2 Accoat A/S
Inner surfaces of industrial process equipment for food are often coated to give the surfaces particular properties with respect to adhesion and cleanability. Existing coating materials (PTFE (Teflon®) or silicone based polymers) suffer from drawbacks when used in contact frying, because these coatings are not mechanically stable, they do not tolerate high enough temperatures (above 260⁰ C) to give the right product quality, and the surfaces wear easily calling for regular service of the equipment. The present project concerns an investigation of the possibilities of replacing the widely used non-stick PTFE coating with new surface coating solutions for contact frying processes, where the food is fried by contact with a hot surface (pan frying, stir frying). The main objective of the present work is to develop suitable, scientifically based methods for selecting and testing different surface materials for contact frying processes. The surfaces selected for this purpose cover a wide spectrum of materials that range from hydrophobic to hydrophilic materials. The different surface materials investigated include stainless steel (reference), aluminium (Al Mg 5754), PTFE (polytetrafluoroethylene), silicone, quasicrystalline alloys (Al, Fe, Cr) and ceramic coatings: zirconium oxide (ZrO2), zirconium nitride (ZrN) and titanium aluminium nitride (TiAlN) with two different levels of smoothness. In order to investigate the non-stick and cleaning properties of different surfaces, an experimental rig has been constructed which enabled a controlled fouling of different coatings on steel and aluminium substrates under realistic frying conditions. A subjective rating procedure was employed for screening different surfaces according to their non-stick properties when used for frying of a model pancake. In order to validate the subjective assessment by means of an objective method, a technique has been developed to measure the force of adhesion between the pancake and the different surfaces; a good correlation was obtained between the subjective and the objective method up to a limiting force of adhesion. Above that the pancake disintegrated by cohesive failure. Differences in the non-stick properties of different ceramic surfaces could mostly be explained by differences in the surface topography. The interfacial contact between the pancake and the frying surface was lower for a rough surface than a smooth surface; thus, a rough surface resulted in significantly less sticking than a smooth (electro-polished) surface. The relevance of using an oven to demonstrate the non-stick and cleaning properties of different surfaces for contact frying processes was also examined, and our results demonstrated that it is not realistic to test non-stick properties for contact frying processes by using a convective oven, as seems to be an established practice in industry. The different surfaces were analyzed for their cleaning properties by performing contact frying experiments with different foods, i.e. turkey meat, carrots and sweet potatoes at different temperatures with and without the use of oil; the different surfaces were cleaned by a combination of chemical and mechanical cleaning and the surfaces were subjectively rated for their cleanability. The results revealed that the cleanability of different surfaces was significantly reduced by the use of oil, especially at high temperatures. The different surfaces were re-used after each frying experiment, and after completion of the whole set of experiments they were cleaned and analyzed in scanning electron microscopy (SEM) in order to inspect their cleanability. Energy dispersive spectroscopy (EDS) was employed to elucidate the difference in elemental composition between stained and unstained spots in different surfaces that were clearly visible using SEM. In most of the surfaces, surface defects, grooves and scratches retained more carbon-containing residues confirming the significance of mechanical interlocking phenomenon on cleanability issues. Contact angle measurements were carried out with vegetable oil on different surfaces at different temperatures in order to study the relation between wettability and cleanability. The measured contact angle values gave useful information for grouping easy-clean polymer materials from the other materials; for the latter group, there is no direct relation between contact angle and cleanability, however. The study of different factors associated with wettability revealed that in addition to nature of the surface material, surface defects and surface roughness play a significant role. The wear resistance of the coatings was tested by performing abrasive wear experiments. The ceramic coatings: TiAlN and ZrN were found to show the best wear resistance properties. The experiments also revealed the poor wear resistance of stainless steel, aluminium, PTFE, silicone, zirconium oxide and quasicrystalline surfaces. The knowledge gained in this project and the methods developed to systematically test and evaluate surfaces for their non-stick and cleaning properties provide an improved basis for selecting and testing new surfaces for contact frying processes.