Dust as a contamination source in the feed industry

Undesirable microbial contamination is the most significant factor that compromises feed product quality and safety. The number of infectious feedborne pathogens, e.g. Salmonella spp., may be only a few cells per gram or millilitre, whereas feed spoilage moulds usually need to reach populations of a million or more cells per gram to have an adverse effect on the sensory and physical attributes of the feed. Almost all ingredients containing enough bacteria may present problems for final feed without adequate process control. This was not investigated in the present case study. All mixing processes and feed material transfers result in dust formation, and dust residues accumulate on manufacturing equipment surfaces, floor drains, conveyors and even outside the factory. Dust can act as continuous culture systems, in which micro-organisms reside and may begin to multiply under beneficial conditions. The risks of dust as a microbial reservoir were investigated here. Known strains of bacteria, Escherichia coli, Salmonella agona, Enterobacter cloacae, Bacillus cereus and Staphylococcus warneri were introduced onto clean stainless-steel surfaces representing industrial manufacturing equipment surfaces. Part of the surfaces were also soiled with various amounts of powder samples of raw materials: barley flour, soy and rapeseed extracts, meat and bone meal, and ready-made compound feed, all serving as experimental dust residues. The soiled surfaces were then kept dry at room temperature. Survival of the bacteria was assessed every second day. All bacteria on clean stainless-steel surfaces died in one week, but bacteria survived for weeks on surfaces soiled with the various powders used. The decrease in viability was constant during the follow-up study. If the surfaces were wetted, all bacteria began to grow, the growth being dependent on the nature and amount of raw material or compound feed on the surface.