Microbiological problems in paper machine environments

Paper and board machines offer microorganisms a favourable environment, and hence their numbers may be quite high without causing significant problems for runnability or end product safety. Microbial contamination of paper machine environment may result in spoilage of raw materials, slime build-up and contamination of end product by microbes or microbial metabolites. The costs of microbial damage to the industry could be significantly reduced if the species, characteristics and life cycles of the harmful microorganisms were known in addition to the ways they spread. This literature survey compiles the information available about the species and life cycles of microorganisms causing harm in paper and board machines from raw materials to end products. The main sources of microbial contamination in the paper mill systems are described including water, cellulosic raw material, broke and papermaking chemicals as a contamination source. Paper machines provide nutrients and conditions for microorganisms to grow either sessile in shelter of biofilm on surfaces or as planctonic cells in the circulating process water. Biofilm deposits and harmful activities of their microorganisms are discussed. Contamination of end product may occur during the manufacturing process or at a later stage as recontamination resulting in microbial growth on the surface of the product. The literature survey lists the most important factors of end product contamination such as recycling, role of spore-forming bacteria, fungi and actinomycetes, migration of microorganisms from packaging materials to foods as well as offensive odours and quality problems. Microbiological methods used in studying the microbial ecology of paper machine environments have been collated. Conventional and novel methods used to reduce or to limit the amount of microbes or microbial activity, especially in circulation waters and raw materials, and to control and prevent deposit formation on paper machine are discussed.