Development of microbial contamination in papermaking processes is a well-known problem which is enhanced by, on the one hand, the increased use of recycled fibres and, on the other hand, the closure of the water circuits. Many detrimental impacts of the microbial activity have already been reported such as biofilm formation, corrosion, catalase activity, odours generation... The work reported in this paper focused on a less expected consequence, related to modification of colloidal materials stability induced by the microbial activity and which mainly concerns recycling mills. Indeed, it was shown that pulp storage for too long time induces important physical-chemical modifications due to metabolic activity of the microbial flora. Moreover, it has been possible to demonstrate that these physical-chemical modifications are at the origin of a particular phenomenon: aged deinked pulp (DIP) acquires a strong capacity to destabilise colloidal materials even if well sterically stabilised. This aspect seems particularly detrimental when DIP and Thermo Mechanical Pulp (TMP) are mixed, as TMP brings high amount of colloids, whilst too aged DIP (due to excessive storage duration) destabilise them. This phenomenon could be at the origin of certain deposition phenomena observed in paper machines.
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