The microbiological barrier properties of food-packaging paperboards, coated with polyethylene, mineral pigment or a biodegradable polymer and of high-density paper were examined with confocal laser scanning microscopy, The results show that the spatial distribution of microscopically observable bacterial cells was uneven inside the paperboard. The concentration in the interface between the polyethylene coating and the cellulose fibers was 100-200 times higher than inside the cellulose matrix, The bacteria in the interface and the mineral coating layer grew in response to access to food and moisture, whereas no growth was observed inside the fiber web, not even after extended exposure for up to 90 days, The paper and paperboards studied contained soluble nutrients (C:N:P 54:9:1 to 309:3:1) and no measurable antimicrobial activity, The factor limiting growth and migration of bacteria inside the fiber web was most likely limited access to free water, even under conditions of extensive wetting. The studied paperboards functioned as efficient barriers against translocation of microbes, The microbes residing between the paperboard and its polymer coating facing food, was the only potential site from which microbes could leak into food. This emphasizes the need for high hygienic quality of surface-sizing chemicals, Mineral-coating pigments were a source of microbes and their application behind the PE coating facing food is contraindicated.
