Microstructure of protein-coated paper as affected by physico-chemical properties of coating solutions

Wheat gluten (WG), calcium caseinate (CC) and corn gluten meal (CGM), were selected as protein raw material to prepare coating solutions. Different concentrations of protein (10–20%, w/w) and deposited solution layer (50–500 μm) were used to obtain coated papers with thicknesses varying between 143(± 3) and 227(± 8) μm. As expected, the thicknesses of coated paper increased with coating weight, but, for a given coating weight, different thicknesses were observed according to the raw material used and its concentration in the coating solution. A phenomenon involving a penetration of coating solution inside paper was suspected with WG-based solutions. The microstructure of coated papers was analyzed by SEM, OM and AFM and differences in structure and morphology were observed between the three raw materials. The percentages of coating agent impregnated inside paper were also calculated from the apparent density of coated paper and the density of self-supported films prepared in the same conditions but deposited on an inert and smooth Plexiglas® support. These percentages of impregnation ranged from 4.8 to 63.3% and increased as following: CC < CGM < WG. Contact angle measurements were carried out in order to describe the behaviour of a coating solution drop just after its deposition on paper. Resulting absorption rates indicated significant differences as a function of the nature of coating agent and confirmed results obtained for the percentage of impregnation. To explain differences in the behaviour of coating solutions, it was finally concluded that not only their viscosity must be taken into account but also their affinity toward paper.