T. Bénézech

Identification of surface characteristics relevant to the hygienic status of stainless steel for the food industry

Abstract The hygienic status of stainless steels used for food equipment was investigated by means of the number of residual adhering Bacillus cereus spores after a complete run of soiling and cleaning in place procedure. The 14 materials tested (304, 316 and 430 grades; pickling (2B), bright annealed (2R), and electropolished finishes) were shown to be highly hygienic with slight differences in the number of residual adhering spores. Nevertheless, they could be grouped into different classes according to their hygienic status. Grade, and to a lesser extent finish, were found to be related to the hygienic status. Among the stainless steel surface free energies, only γ + could be slightly related to the hygienic character. Moreover, surface topography by means of the R PK parameter, corresponding to the height of the top portion of the surface profile, was also shown to control the hygiene of stainless steel surfaces.

Cleaning in place: effect of local wall shear stress variation on bacterial removal from stainless steel equipment

Local wall shears stress analysis as well as cleanability experiments on di!erent pieces of equipment in various circuit arrangements were performed. Pieces of equipment used in this work, stainless steel made, were representative of production lines:straight pipes of di!erent diameters, sudden or gradual contraction or expansion pipes. Local wall shear stress values, obtained by an electrochemical method, were shown to be in9uenced by the loop arrangement. Geometry as the gradual expansion pipe had an e!ect on the upstream straight pipe where the wall shear stress values were two times lower than in other con:gurations as with a sudden expansion. In sudden geometry, the bacterial removal during cleaning was improved by the increase of the 9ow rate, corresponding to the increase of the mean local wall shear stress. However, in gradual expansion or contraction pipe, cleanability was not improved by the increase of 9ow rate. The e!ect of the mean wall shear stress on the removal has been con:rmed. Moreover, this work demonstrated a clear positive e!ect of an increase of the 9uctuation rate on the spore detachment. Low mean wall shear stress zones as in sudden expansion (around 0:15 Pa) could be cleanable because of the high 9uctuation rate (around 21%). These results underline the importance of the design and the 9ow con:guration. ? 2002 Elsevier Science Ltd. All rights reserved.

Potential occurrence of adhering living Bacillus spores in milk product processing lines

C. FAILLE, F. FONTAINE AND T. BÉNÉZECH. 2001.

Growth, morphology and surface properties of Listeria monocytogenes Scott A and LO28 under saline and acid environments

Aims: The effect of salt and acid on the growth and surface properties of two strains of Listeria monocytogenes was investigated.

Comparative evaluation of adhesion, surface properties, and surface protein composition of Listeria monocytogenes strains after cultivation at constant pH of 5 and 7

Aims:  To analyse the cellular mechanisms that influence Listeria monocytogenes adhesion onto inert surfaces under acidic growth conditions.

Cleaning-in-Place: Modelling of Cleaning Kinetics of Pipes Soiled by Bacillus Spores Assuming a Process Combining Removal and Deposition

In food industries, thorough cleaning -in-place procedures for the unheated surfaces of food processing equipment are required to ensure the safety of food products by the removal of microorganisms. The aim of this work is to model the removal kinetics of Bacillus spores during a cleaning-in-place procedure. These spores were chosen for their occurrence in dairies. The removal of Bacillus cereus spores was found to be strongly dependent on both chemical action (sodium hydroxide), which lowers the adhesion strength of spores onto hard surfaces, and hydrodynamic conditions during cleaning through wall shear stress. A simple model (first-order reaction) assuming a process combining removal and deposition during cleaning was detailed and experimentally confirmed. In addition, a significant effect on the effective removal rate constant by both the flow conditions applied during the soiling procedure and applied during cleaning was observed.

Sporulation of Bacillus spp. within biofilms: A potential source of contamination in food processing environments

Bacillus strains are often isolated from biofilms in the food industries. Previous works have demonstrated that sporulation could occur in biofilms, suggesting that biofilms would be a significant source of food contamination with spores. In this study, we investigated the properties of mono-species and mixed Bacillus biofilms and the ability of Bacillus strains to sporulate inside biofilms. Bacillus strains were able to form mono-species biofilms on stainless steel coupons, with up to 90% spores after a 48 h-incubation. These spores were highly resistant to cleaning but were easily transferred to agar, mimicking the cross-contamination of food, thereby suggesting that biofilms would be of particular concern due to a potential for Bacillus spore food contamination. This hypothesis was strengthened by the fact that Bacillus strains were able to form mixed biofilms with resident strains and that sporulation still occurred easily in these complex structures.

Influence of physicochemical properties on the hygienic status of stainless steel with various finishes

The relative hygienic status of 16 stainless steel surfaces, characterised by topography and surface free energy was investigated. B. thuringiensis spores suspended in Bechamel sauce was chosen as the test fouling suspension. Surface topography was assessed using 10 standardised roughness parameters, along with scanning electron microscope observations. The number of residual adhering spores after a fouling and cleaning in place procedure was found to be influenced by the topography of the stainless steel surface, but not by the surface free energy. Among the various roughness parameters, RA, RRR RPK and RVK were shown to be related to the hygienic status. Microscopic observations demonstrated the influence of the shape and size of surface irregularities on the level of residual soil after cleaning. This confirms that the use of only one roughness parameter, usually RA, is not sufficient in defining the hygienic status of stainless steel surfaces.

Cleanability of stainless steel surfaces soiled by bacillus thuringiensis spores under various flow conditions

Adhesion of Bacillus thuringiensis spores on stainless steel surfaces has been investigated under various fouling conditions (static and dynamic). The number of adhering spores as well as the removal of adherent spores following a standard cleaning in place (CIP) procedure were analysed. The number of adhering spores was demonstrated to depend on the fouling conditions, with a significant lower level adhering under static conditions (up to 0.6 log). Under dynamic conditions, the initial rate of contamination was found to be more than twice as high as in the turbulent conditions tested, but then appeared to slow down with time. Cleanability assessment of these surfaces revealed a weak attachment of spores adhering under static conditions as well as in turbulent conditions (removal up to 93% compared to 68% under some laminar fouling conditions). Fouling conditions should thus be taken into account as a major concern in risk assessment procedures.

A new test method for in-place cleanability of food processing equipment

A practical and quantitative method for assessing complex food equipment cleanability is described. After soiling a positive displacement pump by a composite model food made of custard and Bacillus cereus spores isolated from a food processing line, a mild cleaning-in-place procedure was carried out using basic detergents such as sodium hydroxide and nitric acid. After cleaning, surfaces potentially in contact with the contaminated food were overlaid with nutrient agar containing a tetrazolium salt. Residual contaminants appeared as small red colonies and contamination levels could be defined. A non-parametric statistical analysis was performed to compare the different areas in the pump and three cleanability levels were defined. Geometry appeared to be one of the main factors in hygiene, emphasised by the way the equipment is connected to the CIP circuit.