Pseudomonas sp. biofilm development on fresh-cut food equipment surfaces - a growth curve - fitting approach to building a comprehensive tool for studying surface contamination dynamics

The capacity of two primary growth models to describe Pseudomonas fluorescens and Pseudomonas grimontii biofilms' development was assessed. The Baranyi and the 'Logistic with Breaking Delay' models were applied for biofilms grown in various laboratory and pilot-plant devices, including pipes or mock-ups mimicking vegetables washing tanks in the fresh-cut food industry. An initial short transitional period not described by the growth models was observed during which cells rapidly attached to pilot-plant devices' surfaces. The following observed surface contamination growth patterns were consistent with both growth models. However, only the Baranyi model was relevant to the occurrence of wide variability and/or growth curves with no lag or stationary phases. Both surface design and hydrodynamics in pilot-plant devices strongly influenced biofilm growth curves. Based on fitted parameters, it was possible to differentiate between areas and relate these to design parameters such as sharp corners, welds or specific hydrodynamics as 2D, 3D or near-static flow conditions. 15-25 h lag phases reflecting equilibrium between cell attachment and release were observed for wall shear stresses exceeding 0.5 Pa under dynamic flow conditions. Consequently, flow pattern design improvements by increasing the shear rate would greatly reduce food cross-contamination risk, as cleaning operations could start before any significant biofilm developments.