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Hygienic Quality of Floors in Relation to Surface Texture
0960±3085/99/$10.00+0.00 Institution of Chemical Engineers Trans IChemE, Vol 77, Part C, June 1999
HYGIENIC QUALITY OF FLOORS IN RELATION TO SURFACE TEXTURE E. METTLER and B. CARPENTIER Centre National d’ Etudes VeÂteÂrinaires et Alimentaires, Laboratoire d’ Etudes et de Recherches pour l’ Alimentation Collective, France
T
he relative cleanability of seven ¯ oor materials was investigated in the laboratory. Test plates were contaminated by a one-day bio® lm of Pseudomonas ¯ uorescens containing spores of Bacillus stearothermophilus as a cleaning tracer. The test procedure involved cleaning and disinfecting operations. The classi® cation of materials was different in terms of residual spores (resulting from removal ef® ciency only) and in terms of residual bacteria (resulting from removal ef® ciency and bactericidal ef® ciency). Measurements of contamination post cleaning were also made on test plates which had been inserted in the ¯ oor of a cheese-processing site and subjected to habitual fouling-cleaning cycles for four weeks. The comparison with laboratory results showed that the classi® cation of materials obtained in the ® eld study in terms of residual contamination was close to the one obtained in the laboratory in terms of residual spores. These results indicate that the hygienic quality of ¯ oor materials in industrial conditions is linked to their cleanability, rather than to their disinfectability. A geometrical replica of each test plate surface was made using a two-step procedure: a cast of the plate was taken with a silicon-based resin; a polyurethane-based replica was then obtained from the cast. There was a close correlation between the classi® cation of materials in terms of cleanability obtained with replicas in the laboratory and the classi® cation of materials in industrial conditions. This result demonstrates that the cleanability of ¯ oor materials is linked to their surface texture. Surface roughness of materials was assessed using four standardized parameters. The results refute the criterion of mean roughness as a guide to cleanability and highlight the in¯ uence of grooves extending below the core pro® le upon hygienic quality of ¯ oors. Keywords: cleanability; ¯ oor materials; roughness; bio® lm
INTRODUCTION
MATERIALS AND METHODS Tested Materials
The relation between the hygienic quality of one material and its surface texture seems to be evident: the rougher the material surface, the lower its cleanability. However, relations between roughness and cleanability are more complex. Although the roughest materials often showed poor cleanability in comparative studies, the standardized parameter used to assess roughness Ð mainly arithmetical mean roughness Ra, or root-mean square roughness Rq Ð generally presented no correlation with the differences in cleanability1 ± 5 . Even for ¯ oor materials with totally different surface textures, neither visual assessment or arithmetical mean roughness Ra values were suf® cient to indicate the likely cleanability performance of ¯ oor materials in the laboratory6 . However, the lack of correlation between cleanability and roughness measurements could partly be due to the selection of an improper parameter. Very few published studies deal with parameters other than mean roughness1 , although numerous parameters, very different in their meaning and with no mathematical relations between them, can be used to characterize surface texture7 . The relation was studied between surface texture, characterized by four standardized parameters, and the hygienic quality of ¯ oor materials, evaluated both in the laboratory and in a cheese-processing site.
Floor materials Seven ¯ oor materials (ST1: unglazed smooth tile; RT1: unglazed rough tile; ST2: glazed smooth tile; RT2: glazed rough tile; SR1: epoxy-based smooth resin; RR1: epoxybased rough resin; SR2: polyurethane-based smooth resin) were studied, using 10 cm2 test plates. Geometrical replicas Geometrical replicas of test plate surfaces of each ¯ oor material were made using a two-step procedure (PlastiformÒ process, Rivelec, Vernouillet, France). A cast of each plate was taken by soft print molding with a twocomponent silicon-based resin (PlastiformÒ DAV). After polymerization (8 mins at room temperature), each soft print was used to make a polyurethane-based rigid counterprint (PlastiformÒ MD-PX) with a polymerization time of 1 hour at room temperature. Cleanability of Materials Fouled in the Laboratory Plates treatment A 1 cm2 zone was delimited on test plates using the 90
HYGIENIC QUALITY OF FLOORS IN RELATION TO SURFACE TEXTURE hydrophobic marker DakopenÒ (Bioblock, Illkirch, France). Plates were washed just before use according to the procedure described by Bellon-Fontaine and Cerf8 slightly modi® ed. They were washed by a 10-mins submersion under agitation in an aqueous 2% (v/v) RBS35 (Socie te de traitement chimique des surfaces, Frelinghien, France) alkaline solution (initial temperature 50°C). They were then rinsed by submersion in tap water (initial temperature 50°C) with agitation for 25 mins, followed by ® ve 1-min submersions with agitation in distilled water at ambient temperature. They were autoclaved (121°C, 20 mins). Finally, plates were submersed in sterile distilled water at ambient temperature for approximately 20 hours. Microorganisms A suspension of Pseudomonas ¯ uorescens CIP 56-90 was prepared as previously described9 . Spores of Bacillus stearothermophilus were prepared following the procedure described in standard NFT 72-2301 0 , slightly modi® ed with incubation at 58°C and pasteurization at 80°C for 10 mins. Enumerations of colony-forming units (CFU) of Ps. ¯ uorescens were performed in TSA (tryptic soy agar, Difco) and CFU of Bacillus stearothermophilus were enumerated in Shapton and Hindes agar, containing (g l- 1 in distilled water): meat trypsic peptone, 5; meat extract, 3; tryptone, 2.5; yeast extract, 1; glucose, 1; agar, 15. Enumerations were performed using a spiral plater (Spiral SystemÒ DS, Interscience, France) or by pour plating, and incubating for 24 hours respectively at 25°C and 58°C.
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bactericidal ef® ciency. The hygiene operation applied separately to each plate involved: (a) submersion in a sodium hydroxide solution (0.01 M) for 5 mins at room temperature; (b) mechanical action provided by 37 reciprocation movements of a scouring pad `for fragile surfaces’ (3M, Bernard SA, Tourcoing, France) saturated with sterile distilled water, moved on the plate surface by the Gardner washability machine1 3 (Erichsen, Rueil-Malmaison, France), adapted to the treatment of 2 cm-thick plates; (c) submersion in a sodium dichloroisocyanurate solution (200 mg of free chlorine per litre) for 5 mins at room temperature; (d) submersion in a neutralizingsolution,previouslydescribed (14), for 5 mins at 30°C. Removal of residual contamination was performed by sonication of the plate in the neutralizing solution for 4 mins at 30°C, in a 28 kHz, 2 ´ 150 W sonication bath (Delta 220, Deltasonic, Meaux, France). Enumerations were performed as described above. Each series of the cleanability test was applied to one plate of each material or to the respective geometrical replicas of their surface. Floor materials were compared in ® ve series of experiments. In each series, the difference between the residual contamination of one material and the mean value obtained with the seven materials was calculated. The ® nal result for one material was expressed by the mean value of the ® ve differences. Geometrical replicas were compared in two different series of the cleanability test, with the following modi® cations: in the second experiment, submersions into sodium hydroxide and chlorinated solution were omitted.
Cleanability of Materials Fouled in Industry Bio® lms formation Bio® lms were obtained using a modi® cation of the procedure of Maris1 1 . A 100-ml inoculum containing approximately 2 ´ 107 CFU of Ps. ¯ uorescens and 2 ´ 106 spores in physiological saline was deposited on test plates in the delimited zone. Plates were placed at 25°C and 95% relative humidity (RH) for 3 hours to allow adhesion to occur. Non-adherent cells were removed by rinsing with 25 ml of sterile distilled water. As culture medium, 100 ml of Columbia medium (containing, g l- 1 in distilled water: casein peptone, 12; meat peptone, 5; yeast extract, 3; meat extract, 3; corn starch, 1; sodium chloride, 5 ) were deposited in the delimited zone, before incubation at 25°C and 95% RH for 20 hours. Spores of Bacillus stearothermophilus did not germinate at this temperature. A ® nal rinse with 25 ml of sterile distilled water was used to eliminate non-bio® lm cells. With this procedure, the oneday bio® lm contained approximately 108 CFU of Ps. ¯ uorescens and 105 spores of Bacillus stearothermophilus, regardless of the material used as substratum. Cleanability test procedure The test procedure was designed to produce both no sporicidal effect and detectable levels of residual CFU’ s of Ps. ¯ uorescens. Spores of Bacillus stearothermophilus could therefore be used as a cleaning tracer, using the principle ® rst described by Galesloot et al.1 2 . Residual spores resulted from removal ef® ciency only, whilst residual vegetative cells resulted from both removal and Trans IChemE, Vol 77, Part C, June 1999
Test board The cleanability of the same materials was also studied in an industrial environnement, using techniques described by Mettler and Carpentier1 4 . Six test plates of each material were used to make a board with random positions. The board was inserted into the ¯ oor of a cheese-processing site (temperature: 20 to 30°C; relative humidity: 95%; daily processing time: 8 or 16 hours) and subjected to habitual fouling-cleaning cycles for 4 weeks. Hygiene operation The weekly cleaning involved: (a) pre-rinsing with 55°C tap water; (b) 170 bar jet washing and/or brushing using a hard plastic brush with a chlorinated alkaline solution at 60°C; (c) after 10 mins contact time, rinsing with cold tap water. The daily cleaning was limited to a rinsing with 55°C tap water. The board was removed immediately following the last daily cleaning and was shipped in refrigerated packaging for subsequent analysis in the laboratory. CFU counts Plate counts (TSA, 25°C, 48 hours) of residual contamination post cleaning were made on each plate after detachment by sonication in conditions described above. Roughness Measurements At least thirty roughness measurements were performed on each surface with cut-off wavelengths of 0.8 and 2.5 mm,
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METTLER and CARPENTER Table 1. Groups inside which surface parameters gave a similar classi® cation of tested ¯ oor materials in a previous study. Group 1
Wt* Pt
total height of the waviness pro® le total height of the primary pro® le
ISO 428715 ISO 428715
Group 2
Ra* Rq Rp Rk Rz R3z
arithmetical mean roughness root-mean square roughness levelling depth core roughness depth mean peak-to-valley height arithmetic mean third peak-to-valley height
ISO 428715 ISO 428715 DIN 476216 ISO 13565-217 DIN 476818 Daimler Benz Standard N3100719
Group 3
Rvk* Rmax Rt
reduced valley depth maximum roughness depth total height of the roughness pro® le
ISO 13565-2 DIN 476818 ISO 428715
Group 4
Rpk*
reduced peak height
ISO 13565-2 17
* parameter selected to represent the group
using a surface-tracing instrument (Perthometer S3P, equipped with a stylus of radius 5 mm and cone angle of 90°, held by a FRW 750 arm and moved by a SPVK drive unit, FeinpruÈf Perthen GmbH, GoÈttingen, Germany), following procedures described by Sander7 . Waviness (low-pass ® ltered pro® le) and roughness (high-pass ® ltered pro® le) are separated for each cut-off value. In fact, the ® lter does not `cut off’ abruptly, but has a gradual transmission curve: the cut-off value is the wavelength of a sine wave the amplitude of which is transmitted at 50%. The higher the diameter of the asperity, the lower its level of transmission. Changing the cut-off value enabled us to study different classes of asperities. Our apparatus assessed thirty surface parameters for each tracing length. Results based on trials prior to the current study, including other ¯ oor materials, showed that twelve standardized parameters could be selected to quantify surface roughness. Non-selected parameters were either unable to discriminate between the tested materials, or referred to speci® c standards. Some of the twelve parameters gave a similar classi® cation of ¯ oor materials. According to this, four groups could be distinguished and one parameter was selected to represent each group (see Table 1): Ra is the arithmetical mean value of all departures of the roughness pro® le from the mean line throughout the evaluation length; Wt is the vertical distance between two parallel lines enclosing the waviness pro® le within the evaluation length; Rpk and Rvk are parameters derived from the bearing length ratio curve (or Abbott-Firestone curve, a graphical representation of the bearing length ratio values calculated from multiple horizontal pro® le sections taken at different heights over the entire pro® le). The Rpk value depends on the height of the top portion of the surface pro® le, i.e. of the pro® le peaks exceeding the core pro® le, while the Rvk value depends on the depth of the inwardly directed portion of the surface pro® le, i.e. of grooves extending below the core pro® le. We used logarithmic transformations of the mean value of each parameter, due to the wide range of roughness represented by the tested materials. Since the surface geometry of the smooth resin SR2 was characterized by the presence of spherical holes with diameter up to 0.5 mm, six SR2 plates were analysed by visual counting of holes. Roughness pro® les obtained on this material showed that the smallest holes that were visually detected had a diameter of 0.1 mm.
RESULTS Classi® cation of Materials in the Laboratory The results of the ® ve series of the cleanability test applied to ¯ oor materials in the laboratory are given in Figure 1. The classi® cation of materials was different in terms of residual spores of Bacillus stearothermophilus (cleanability) and in terms of residual vegetative cells of Pseudomonas ¯ uorescens (both cleanability and disinfectability). In both cases, materials could be classi® ed in signi® cantly different groups.
Figure 1. Difference between the residual contamination of materials and the mean value obtained with the seven materials in terms of vegetative cells (1a) and spores (1b) after hygiene operations in the laboratory (n 5 experiments; error bars represent standard deviation; brackets represent signi® cantly different groups determined by multiple range analysis: materials with one common letter are not signi® cantly different in terms of residual contamination).
Trans IChemE, Vol 77, Part C, June 1999
HYGIENIC QUALITY OF FLOORS IN RELATION TO SURFACE TEXTURE
Figure 2. Residual contamination of materials inserted in the cheese site for 4 weeks, after hygiene operations (n 6 plates; error bars represent standard deviation; brackets represent signi® cantly different groups determined by multiple range analysis: materials with one common letter are not signi® cantly different in terms of residual contamination).
Classi® cation of Materials in Industrial Conditions The contamination of materials post hygiene operations in industrial conditions is given in Figure 2. Materials could be classi® ed in four signi® cantly different groups. The comparison with Figure 1 suggests that the classi® cation of materials obtained in the ® eld study in terms of residual contamination was close to the one obtained in the laboratory in terms of residual spores (R = 0.93, P < 0.01), even if the signi® cantly different groups were not exactly the same. On the contrary, there were marked differences between the results of the ® eld study and the classi® cation obtained in the laboratory in terms of residual vegetative cells. For instance, the relative classi® cation of ST2 and RT2 compared to ST1 was inverted. The same observation was made when comparing RT1 and SR2. Classi® cation of Geometrical Replicas in the Laboratory The validity of the counterprints as geometrical replica of the initial plate surfaces was veri® ed by comparison of the value of roughness parameters measured on both plates of each material and on their counterprints (data not shown). Figures 3 and 4 present the correlation between the residual contamination of materials in the ® eld study and the residual
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Figure 4. Correlation between the residual contamination of materials in the ® eld study and the residual contamination of geometrical replicas of materials in terms of vegetative cells after non bactericidal hygiene operations in the laboratory (n 1).
contamination of geometrical replicas of materials after respectively the test including chemical agents, in terms of residual spores, and the test without chemical agents, in terms of residual vegetative cells of Ps. ¯ uorescens. With both methods, there was a signi® cant correlation (respectively, P < 0.05 and P < 0.01) between the classi® cation of materials obtained with geometrical replicas in terms of cleanability and the classi® cation of materials in the ® eld study. Relation with Surface Roughness Parameters The relation between cleanability and surface roughness parameters was tested for six materials among the seven studied here: the polyurethane-based resin SR2 was not included in this study, for reasons discussed later. Figure 5 gives the values of the four roughness parameters, measured from at least thirty tracings with each cut-off wavelengths. The classi® cation of materials varied depending on both the surface parameter and the cut-off value. For example, materials with similar mean Ra0.8 values and close mean Ra2.5 values, such as ST2, RT2 and RR1 showed wide differences when considering Wt 0.8, Rpk0.8, Wt 2.5 and Rpk2.5. The mean Wt 2.5 values of RT2 and RT1 were the same, but the mean Wt 0.8 value of RT1 was almost twice the one of RT2. Table 2 presents the correlation coef® cient values obtained between microbial load of materials in the ® eld study and either the logarithmic value of each roughness
Table 2 . Correlation coef® cient value obtained for the linear relationship between the microbial load (log CFU cm-2) and various combinations of the logarithms of the mean values of four surface parameters, measured on six materials (n = 6) inserted for 4 weeks in the cheese-processing site. R value cut-off: 2.5 mm cut-off: 0.8 mm
Figure 3. Correlation between the residual contamination of materials in the ® eld study and the residual contamination of geometrical replicas in terms of spores after bactericidal hygiene operations in the laboratory (n 1).
Trans IChemE, Vol 77, Part C, June 1999
log Ra log Wt log Rpk log Rvk 11 log Rvk 6 log Ra 2 log Rpk 2 log Rvk log Ra
log Wt
0.62 0.54 0.66 0.60 0.57
0.64 0.65 0.82 0.80 0.93
0.57
0.87
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METTLER and CARPENTER The correlation coef® cient was lower, but still signi® cant (P < 0.05), when using the following simpli® ed form (2). 2 log Rvk0.8
log Ra0.8
2
The correlation coef® cient with log Ra0.8 was the lowest. Figure 6 presents the correlation between the cleanability of six plates of SR2 measured by the laboratory test and the number of spherical holes visually assessed on each plate. There was a close correlation (R = 0.93, P < 0.01) between the number of holes and residual spores. DISCUSSION
Figure 5. Values of roughness parameters measured on six ¯ oor materials with the cut-off wavelengths of 2.5 and 0.8 mm. The minimum, mean and maximum values resulting from at least thirty tracings are given.
parameter or a combination of their values, measured with both wavelengths used. For each parameter or combination of parameters, the correlation was lower when the cut-off wavelength of 2.5 mm was used. When using the cut-off wavelength of 0.8 mm, the highest correlation coef® cient (P < 0.01) for a linear relationship was obtained with the combination (1), obtained by an iteration procedure using MicrosoftÒ Excel solver in a previous experiment, when measuring the cleanability of twelve materials in the laboratory (data not shown). 11 log Rvk0.8
6 log Ra0.8
log Wt0.8
2 log Rpk0.8
1
Figure 6. Correlation between the residual contamination in terms of spores and the number of spherical holes visually detected on six plates of SR2.
As expected from previous ® ndings in three other industrial situations1 4 , the industrial routine hygiene procedure used in the cheese-processing site cannot remove or inactivate all microorganisms on ¯ oors, and signi® cant differences in residual contamination between materials are observed. Field studies represent the most reliable method to compare materials, because industrial conditions are hardly simulated in the laboratory when studying open surfaces. Nevertheless, this method presents disadvantages. First, the food premises must generate suf® cient soil and must be submitted to mild hygiene procedures. Indeed, a more effective hygiene procedure would result in reducing the residual contamination of poorly cleanable materials to the level of cleanable materials. Moreover, tested materials are not fouled and cleaned independently. If the level of contamination of the surrounding surfaces is high, a cleanable material may appear strongly contaminated because fouling is partly spread over surfaces by each hygiene operation1 4 . It is therefore uncertain whether the best discrimination of materials has been reached in this ® eld study. However, a laboratory test must be validated by a ® eld study. Indeed, a comparison of materials based on our laboratory test might have led to conclusions completely different from the industrial reality, if residual vegetative bacteria had only been considered. Residual organic fouling and physiological state of microorganisms in aged industrial bio® lms are probably responsible for this difference. However, it was not necessary to use more complex fouling to simulate industrial conditions in the present case, since the classi® cation observed in the ® eld study was close to the one obtained in the laboratory in terms of residual spores. This result indicates that the relative level of residual contamination on ¯ oor materials in industrial conditions depends on the removal ef® ciency of the hygiene procedure, determined here by the soil-retaining capacity of materials, and is not linked to the bactericidal effect of the procedure. The number of signi® cantly different groups obtained with our laboratory test was not higher than in the ® eld study, although materials were fouled and cleaned independently from each other. When industrial and laboratory classi® cations are considered together, materials are classi® ed in only two groups. This result is not surprising, since surfaces are often classi® ed in only two or three signi® cantly different groups in other studies1 ,2 ,6 , whatever the method used to assess their cleanability. A maximum of four groups was reported when comparing stainless steels with eleven different surface ® nishes or nine different alloys3 . Materials tested in this study varied not only in terms of Trans IChemE, Vol 77, Part C, June 1999
HYGIENIC QUALITY OF FLOORS IN RELATION TO SURFACE TEXTURE surface texture, but also in terms of physicochemical properties, such as surface free energy or water absorption capacity (data not shown). Previous studies1 4 , including a large variety of ¯ oor materials and several industrial conditions, indicated that initial differences between unused materials in terms of surface free energy were reduced or eliminated over time, and hence could not explain differences in terms of residual contamination. Similar results were obtained with wall materials (unpublished data). Even when conditioned independently from each other in the laboratory using solutions based on hygiene chemicals and/or food products, materials exhibited the same surface free energy, in spite of initial differences1 4 . These results already suggested that the in¯ uence of physicochemical properties of materials on cleanability was negligible compared to the in¯ uence of surface texture. The counterprints technique enabled us to study the in¯ uence of surface texture of materials upon their cleanability regardless of variations in physicochemical properties. Our results clearly demonstrate that surface texture is the major parameter describing cleanability of the tested materials. However, it should be noted that ¯ oor materials containing high cement level were not included in the present study. Indeed, such materials present a high pH value when unused, with a bactericidal effect on laboratory bio® lms. These materials are nevertheless known to be highly prone to contamination in the industry, probably because of their excessive porosity2 0 . Such a material was reported to be the most contaminated in a previous 8-week long ® eld study1 4 . Our results show that the comparison of the surface texture of ¯ oor materials requires more than one parameter. Dau® n et al.1 , who used three surface parameters calculated from the roughness pro® le Ð Rq, Sm (mean spacing of pro® le irregularities), and Rt Ð to assess the surface texture of stainless steels with a range of surface ® nishes observed that Sm gave a classi® cation of surfaces very different from the one obtained with both other parameters. Since the classi® cation of ¯ oor materials varies depending on the surface texture parameter, it is of prime importance to determine which parameter or combination of parameters best enables a classi® cation of materials close to that based on their soil-retaining capacity. For years, it was hoped that some correlation could be made between cleanability and Ra, the main surface parameter recognized internationally. Although comparative studies showed that the smoothest materials were easier to clean1 ± 5 , correlation of mean roughness (Ra or Rq) values to cleanability was generally unsuccessful. For instance, Masurovsky and Jordan3 showed that some surface ® nishes could exhibit better cleanability than others with lower Rq values, for various types of materials: stainless steel, alloys, glass and plastics. The same observation was made when Ra values were used to compare stainless steel5 , sink materials2 or ¯ oor materials6 . These results clearly refute the criterion of mean roughness as a guide to cleanability. In addition, the classi® cation of materials also varies depending on the studied class of asperities, as can be seen from the modi® cation of correlation coef® cients when changing the cut-off wavelength. Our results indicate that asperities taken into account by both Wt 0.8 and parameters calculated with the cut-off of 2.5 mm did not compromise the ef® cacy of the hygiene procedure used in the present study. Similar ® ndings have been reported by Taylor and Trans IChemE, Vol 77, Part C, June 1999
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Holah6 , who observed that the gross topographic irregularities of ¯ oor materials were not responsible for their cleanability performances. This suggests there is a threshold value for the diameter of asperities, under which the soil is not removed by the mechanical action of the hygiene procedure. Changing the mechanical action intensity would change the threshold value. This study suggests that the hygienic quality of ¯ oors is linked to the depth of the inwardly-directed portion of the surface pro® le, rather than to the mean roughness, which includes not only valleys but also peaks. Various authors have already observed that surfaces which at ® rst glance appeared to have a readily cleanable, smooth surface but were however very dif® cult to clean, were precisely characterized by the presence of small holes when surfaces were examined in more detail using microscopy or surface pro® le analysis2 ,3 ,1 4 . This result also con® rms obvious observations: valleys, pores, crevices, cavities on any material result in mechanical entrapment of soil. The meaning of the combination (2) of surface roughness parameters appears when it is given the form (3): log Rvk0.8 Rvk0.8 /Ra0.8
3
It takes account of both the value of Rvk0 .8 and the value of the ratio Rvk0 .8 /Ra0 .8 , which depends upon the proportion of the inwardly directed portion of the surface pro® le to the mean roughness. This combination provided also the higher correlation coef® cient between surface geometry and residual contamination of materials in a second ® eld study carried out in a pastry-processing site, in conditions described elsewhere1 4 , with other ¯ oor materials (R = 0.96, P < 0.01) (data not shown). However, it should be noted that the measurement of Rvk implies an S-shaped bearing length ratio curve. For instance, the sensitivity of our apparatus is not suf® cient to enable us to obtain a bearing length ratio curve from each tracing carried out on unused stainless steel surfaces with smooth ® nishes. It is therefore uncertain whether Rvk could be used instead of Ra, the parameter currently used for geometrical speci® cations of stainless steel surfaces2 1 , although it is recognized that the information provided by Ra is not suf® cient to characterize such surfaces3 ,5 . Moreover, some surface pro® les cannot be fully described when using conventional mechanical tracing systems, since some of their surface characteristics, such as re-entrant features, do not allow the stylus to reach into them. Even an optical probe allowing non-contact measurements with a focal point diameter of only about 1 mm is not able to produce a true pro® le of the above critical surfaces7 . In this study, the surface of SR2 plates presents such characteristics. Spherical holes with diameter up to 0.5 mm result from CO2 production during polymerization of twocomponent polyurethane-based resins2 0 . Even if roughness parameters cannot be used, the signi® cant correlation between the number of holes per surface unit and cleanability shows that the hygienic quality of this material is also linked to the inwardly directed portion of the surface pro® le. It is of prime importance to note that this material, smooth at ® rst glance, presents the highest residual contamination after hygiene procedure in the ® eld study. The choice of ¯ oor materials suitable for industrial use requires the consideration of hygiene and safety issues (e.g. slip resistance)6 ,2 2 . The European Directive on `The hygiene
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METTLER and CARPENTER
of foodstuffs’ (93/43/EEC) states in Annex II that `¯ oor surfaces must be maintained in a sound condition and they must be easy to clean, and where necessary disinfect’ 2 3 . This study demonstrates that the presence of holes, crevices and all irregularities of the inwardly directed portion of the surface pro® le clearly compromise the hygienic quality of ¯ oors. These geometrical features must therefore be taken into account when choosing a ¯ oor material, together with factors that could increase their depth over time, in particular resistance to wear. CONCLUSIONS The hygienic quality of a ¯ oor material is determined by the soil-retaining capacity of its surface, which depends essentially on surface texture, but only when the depth of the inwardly directed portion of the surface pro® le is considered. The mean roughness does not re¯ ect the pro® le shape and this may be one reason for the lack of correlation with cleanability observed here and often reported in the literature. Knowledge about surface texture with respect to bacterial cleanability would provide important information, e.g. to select ¯ oor materials with both slip resistance properties and good hygienic quality. REFERENCES 1. Dau® n, G., Kerhe rve , L. and Richard, J., 1977, In¯ uence de l’ e tat de surface de l’ acier inoxydable sur son aptitude au nettoyage et sur sa sensibilite aÁ la corrosion par piquÃres, Industrie Alimentaires et Agricoles, 43±51. 2. Holah, J.T. and Thorpe, R.H., 1990, Cleanability in relation to bacterial retention on unused and abraded domestic sink materials, J Appl Bacteriol, 69: 599±608. 3. Masurovsky, E.B. and Jordan, W.K., 1958, Studies on the relative bacterial cleanability of milk-contact surfaces, J Dairy Sci, 41: 1342± 1358. 4. SchluÈssler, H.-J., 1970, Zur Reinigung fester Ober¯ aÈchen in der Lebensmittelindustrie, Milchwiss, 25: 133±145. 5. Timperley, D.A., 1984, Surface ® nish and spray cleaning of stainless steel, in Pro® tability of Food Processing, IChemE Symposium Series 84 (Institution of Chemical Engineers, UK). 6. Taylor, J.H. and Holah, J.T., 1996, A comparative evaluation with respect to the bacterial cleanability of a range of wall and ¯ oor surface materials used in the food industry, J Appl Bacteriol, 81: 262±266. 7. Sander, M., 1992, A practical guide to the assessment of surface texture, 125p. (FeinpruÈf Perthen GmbH, GoÈttingen, Germany). 8. Bellon-Fontaine, M.-N. and Cerf, O., 1990, Experimental determination of spreading pressure in solid and liquid vapor systems, J Adhes Sci Technol, 4: 475±480. 9. Leriche, V. and Carpentier, B., 1995, Viable but nonculturable Salmonella typhimurium in single- and binary- species bio® lms in response to chlorine treatment, J Food Prot, 58: 1186±1191.
10. 1988, Antiseptiques et de sinfectants utilise s aÁ l’ e tat liquide, miscibles aÁ l’ eau et neutralisables, De termination de l’ activiteÂbacteÂricide, NF T 72-230 (Association francËaise de normalisation, Paris la De fense). 11. Maris, P., 1992, Bio® lms and disinfection. Development of a microorganism carrier-surface method, Sci Aliment, 12: 721±728. 12. Galesloot, T.E., Radema L., Kooy E.G., Hup G., 1967, A sensitive method for the evaluation of cleaning processes, with a special version adapted to the study of the cleaning of tanks, J Dairy Res, 215±221. 13. 1983a, Emulsion paints for interior use; Evaluation of cleanability and of wash and scrub resistance of coatings, DIN 53 778 Part 2 (Beuth Verlag GmbH, Berlin). 14. Mettler, E. and Carpentier, B., 1998, Variations over time of microbial load and physico-chemical properties of ¯ oor materials after cleaning in food industry premises, J Food Prot, 61: 57±65. 15. 1997, Geometrical Product Speci® cations ± surface texture: pro® le method ± terms, de® nitions and surface texture parameters, ISO 4287. 16. 1989, Surface roughness ± terms and de® nitions, DIN 4762 (Beuth Verlag GmbH, Berlin). 17. 1997, Geometrical product speci® cations ± surface texture: pro® le method; surfaces having strati® ed functional properties ± Part 2: height characterization using the linear material ratio curve. 18. 1990a, Determination of roughness values of parameters Ra, Rz, Rmax, sheet 1, DIN 4768, (Beuth Verlag GmbH, Berlin). 19. 1983b, Surface parameter R3z, Works standard of Daimler-Benz N31007. 20. Carpentier, B., Leconte, A.-M. and Liot, J.P., 1997, Les reveÃtements de sols en continu utilise s dans l’ industrie alimentaire, Industries Alimentaires et Agricoles, March, 141±145. 21. Curiel, G.J., Hauser, G., Peschel, P. and Timperley, D.A., 1993, EHEDG update: hygienic equipment design criteria, Trends Food Sci Technol, 4: 225±229. 22. Liot J.-P., Carpentier B., Leconte A.-M., Fau G., Vetter F. and Saulnier H., 1998, Guide des reveÃtements de sol pour les locaux de fabrication de produits alimentaires, May, CNAMTS, Paris. 23. 1993, The hygiene of foodstuffs, Council directive 93/43/EEC, Of® cial Journal of the European Communities No. L 175/1.
ACKNOWLEDGEMENTS This research was supported by the UNIR (Ultra-propre, Nutrition, Industrie, Recherche) Association and by the Ministry of Research. Our sincere thanks to Danielle Chassaing for microbiological analysis and Anne-Marie Leconte for roughness measurements, to Marie-Pierre Que ric and Jean-Pierre Innocent for organizational help.
ADDRESS Correspondence concerning this paper should be addressed to Dr B. Carpentier, Centre National d’ Etudes Ve te rinaires et Alimentaires, Laboratoire d’ Etudes et de Recherches pour l’ Alimentation Collective, BP 332, 94709 Maisons-Alfort cedex, France. Email : [email protected] The manuscript was received 23 July 1998 and accepted for publication after revision 16 November 1998. This paper is an extended and updated version of one presented by the authors at Fouling and Cleaning in Food Processing, 6±8 April 1998, Cambridge, UK.
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HYGIENIC QUALITY OF FLOORS IN RELATION TO SURFACE TEXTURE E. METTLER and B. CARPENTIER Centre National d’ Etudes VeÂteÂrinaires et Alimentaires, Laboratoire d’ Etudes et de Recherches pour l’ Alimentation Collective, France
T
he relative cleanability of seven ¯ oor materials was investigated in the laboratory. Test plates were contaminated by a one-day bio® lm of Pseudomonas ¯ uorescens containing spores of Bacillus stearothermophilus as a cleaning tracer. The test procedure involved cleaning and disinfecting operations. The classi® cation of materials was different in terms of residual spores (resulting from removal ef® ciency only) and in terms of residual bacteria (resulting from removal ef® ciency and bactericidal ef® ciency). Measurements of contamination post cleaning were also made on test plates which had been inserted in the ¯ oor of a cheese-processing site and subjected to habitual fouling-cleaning cycles for four weeks. The comparison with laboratory results showed that the classi® cation of materials obtained in the ® eld study in terms of residual contamination was close to the one obtained in the laboratory in terms of residual spores. These results indicate that the hygienic quality of ¯ oor materials in industrial conditions is linked to their cleanability, rather than to their disinfectability. A geometrical replica of each test plate surface was made using a two-step procedure: a cast of the plate was taken with a silicon-based resin; a polyurethane-based replica was then obtained from the cast. There was a close correlation between the classi® cation of materials in terms of cleanability obtained with replicas in the laboratory and the classi® cation of materials in industrial conditions. This result demonstrates that the cleanability of ¯ oor materials is linked to their surface texture. Surface roughness of materials was assessed using four standardized parameters. The results refute the criterion of mean roughness as a guide to cleanability and highlight the in¯ uence of grooves extending below the core pro® le upon hygienic quality of ¯ oors. Keywords: cleanability; ¯ oor materials; roughness; bio® lm
INTRODUCTION
MATERIALS AND METHODS Tested Materials
The relation between the hygienic quality of one material and its surface texture seems to be evident: the rougher the material surface, the lower its cleanability. However, relations between roughness and cleanability are more complex. Although the roughest materials often showed poor cleanability in comparative studies, the standardized parameter used to assess roughness Ð mainly arithmetical mean roughness Ra, or root-mean square roughness Rq Ð generally presented no correlation with the differences in cleanability1 ± 5 . Even for ¯ oor materials with totally different surface textures, neither visual assessment or arithmetical mean roughness Ra values were suf® cient to indicate the likely cleanability performance of ¯ oor materials in the laboratory6 . However, the lack of correlation between cleanability and roughness measurements could partly be due to the selection of an improper parameter. Very few published studies deal with parameters other than mean roughness1 , although numerous parameters, very different in their meaning and with no mathematical relations between them, can be used to characterize surface texture7 . The relation was studied between surface texture, characterized by four standardized parameters, and the hygienic quality of ¯ oor materials, evaluated both in the laboratory and in a cheese-processing site.
Floor materials Seven ¯ oor materials (ST1: unglazed smooth tile; RT1: unglazed rough tile; ST2: glazed smooth tile; RT2: glazed rough tile; SR1: epoxy-based smooth resin; RR1: epoxybased rough resin; SR2: polyurethane-based smooth resin) were studied, using 10 cm2 test plates. Geometrical replicas Geometrical replicas of test plate surfaces of each ¯ oor material were made using a two-step procedure (PlastiformÒ process, Rivelec, Vernouillet, France). A cast of each plate was taken by soft print molding with a twocomponent silicon-based resin (PlastiformÒ DAV). After polymerization (8 mins at room temperature), each soft print was used to make a polyurethane-based rigid counterprint (PlastiformÒ MD-PX) with a polymerization time of 1 hour at room temperature. Cleanability of Materials Fouled in the Laboratory Plates treatment A 1 cm2 zone was delimited on test plates using the 90
HYGIENIC QUALITY OF FLOORS IN RELATION TO SURFACE TEXTURE hydrophobic marker DakopenÒ (Bioblock, Illkirch, France). Plates were washed just before use according to the procedure described by Bellon-Fontaine and Cerf8 slightly modi® ed. They were washed by a 10-mins submersion under agitation in an aqueous 2% (v/v) RBS35 (Socie te de traitement chimique des surfaces, Frelinghien, France) alkaline solution (initial temperature 50°C). They were then rinsed by submersion in tap water (initial temperature 50°C) with agitation for 25 mins, followed by ® ve 1-min submersions with agitation in distilled water at ambient temperature. They were autoclaved (121°C, 20 mins). Finally, plates were submersed in sterile distilled water at ambient temperature for approximately 20 hours. Microorganisms A suspension of Pseudomonas ¯ uorescens CIP 56-90 was prepared as previously described9 . Spores of Bacillus stearothermophilus were prepared following the procedure described in standard NFT 72-2301 0 , slightly modi® ed with incubation at 58°C and pasteurization at 80°C for 10 mins. Enumerations of colony-forming units (CFU) of Ps. ¯ uorescens were performed in TSA (tryptic soy agar, Difco) and CFU of Bacillus stearothermophilus were enumerated in Shapton and Hindes agar, containing (g l- 1 in distilled water): meat trypsic peptone, 5; meat extract, 3; tryptone, 2.5; yeast extract, 1; glucose, 1; agar, 15. Enumerations were performed using a spiral plater (Spiral SystemÒ DS, Interscience, France) or by pour plating, and incubating for 24 hours respectively at 25°C and 58°C.
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bactericidal ef® ciency. The hygiene operation applied separately to each plate involved: (a) submersion in a sodium hydroxide solution (0.01 M) for 5 mins at room temperature; (b) mechanical action provided by 37 reciprocation movements of a scouring pad `for fragile surfaces’ (3M, Bernard SA, Tourcoing, France) saturated with sterile distilled water, moved on the plate surface by the Gardner washability machine1 3 (Erichsen, Rueil-Malmaison, France), adapted to the treatment of 2 cm-thick plates; (c) submersion in a sodium dichloroisocyanurate solution (200 mg of free chlorine per litre) for 5 mins at room temperature; (d) submersion in a neutralizingsolution,previouslydescribed (14), for 5 mins at 30°C. Removal of residual contamination was performed by sonication of the plate in the neutralizing solution for 4 mins at 30°C, in a 28 kHz, 2 ´ 150 W sonication bath (Delta 220, Deltasonic, Meaux, France). Enumerations were performed as described above. Each series of the cleanability test was applied to one plate of each material or to the respective geometrical replicas of their surface. Floor materials were compared in ® ve series of experiments. In each series, the difference between the residual contamination of one material and the mean value obtained with the seven materials was calculated. The ® nal result for one material was expressed by the mean value of the ® ve differences. Geometrical replicas were compared in two different series of the cleanability test, with the following modi® cations: in the second experiment, submersions into sodium hydroxide and chlorinated solution were omitted.
Cleanability of Materials Fouled in Industry Bio® lms formation Bio® lms were obtained using a modi® cation of the procedure of Maris1 1 . A 100-ml inoculum containing approximately 2 ´ 107 CFU of Ps. ¯ uorescens and 2 ´ 106 spores in physiological saline was deposited on test plates in the delimited zone. Plates were placed at 25°C and 95% relative humidity (RH) for 3 hours to allow adhesion to occur. Non-adherent cells were removed by rinsing with 25 ml of sterile distilled water. As culture medium, 100 ml of Columbia medium (containing, g l- 1 in distilled water: casein peptone, 12; meat peptone, 5; yeast extract, 3; meat extract, 3; corn starch, 1; sodium chloride, 5 ) were deposited in the delimited zone, before incubation at 25°C and 95% RH for 20 hours. Spores of Bacillus stearothermophilus did not germinate at this temperature. A ® nal rinse with 25 ml of sterile distilled water was used to eliminate non-bio® lm cells. With this procedure, the oneday bio® lm contained approximately 108 CFU of Ps. ¯ uorescens and 105 spores of Bacillus stearothermophilus, regardless of the material used as substratum. Cleanability test procedure The test procedure was designed to produce both no sporicidal effect and detectable levels of residual CFU’ s of Ps. ¯ uorescens. Spores of Bacillus stearothermophilus could therefore be used as a cleaning tracer, using the principle ® rst described by Galesloot et al.1 2 . Residual spores resulted from removal ef® ciency only, whilst residual vegetative cells resulted from both removal and Trans IChemE, Vol 77, Part C, June 1999
Test board The cleanability of the same materials was also studied in an industrial environnement, using techniques described by Mettler and Carpentier1 4 . Six test plates of each material were used to make a board with random positions. The board was inserted into the ¯ oor of a cheese-processing site (temperature: 20 to 30°C; relative humidity: 95%; daily processing time: 8 or 16 hours) and subjected to habitual fouling-cleaning cycles for 4 weeks. Hygiene operation The weekly cleaning involved: (a) pre-rinsing with 55°C tap water; (b) 170 bar jet washing and/or brushing using a hard plastic brush with a chlorinated alkaline solution at 60°C; (c) after 10 mins contact time, rinsing with cold tap water. The daily cleaning was limited to a rinsing with 55°C tap water. The board was removed immediately following the last daily cleaning and was shipped in refrigerated packaging for subsequent analysis in the laboratory. CFU counts Plate counts (TSA, 25°C, 48 hours) of residual contamination post cleaning were made on each plate after detachment by sonication in conditions described above. Roughness Measurements At least thirty roughness measurements were performed on each surface with cut-off wavelengths of 0.8 and 2.5 mm,
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METTLER and CARPENTER Table 1. Groups inside which surface parameters gave a similar classi® cation of tested ¯ oor materials in a previous study. Group 1
Wt* Pt
total height of the waviness pro® le total height of the primary pro® le
ISO 428715 ISO 428715
Group 2
Ra* Rq Rp Rk Rz R3z
arithmetical mean roughness root-mean square roughness levelling depth core roughness depth mean peak-to-valley height arithmetic mean third peak-to-valley height
ISO 428715 ISO 428715 DIN 476216 ISO 13565-217 DIN 476818 Daimler Benz Standard N3100719
Group 3
Rvk* Rmax Rt
reduced valley depth maximum roughness depth total height of the roughness pro® le
ISO 13565-2 DIN 476818 ISO 428715
Group 4
Rpk*
reduced peak height
ISO 13565-2 17
* parameter selected to represent the group
using a surface-tracing instrument (Perthometer S3P, equipped with a stylus of radius 5 mm and cone angle of 90°, held by a FRW 750 arm and moved by a SPVK drive unit, FeinpruÈf Perthen GmbH, GoÈttingen, Germany), following procedures described by Sander7 . Waviness (low-pass ® ltered pro® le) and roughness (high-pass ® ltered pro® le) are separated for each cut-off value. In fact, the ® lter does not `cut off’ abruptly, but has a gradual transmission curve: the cut-off value is the wavelength of a sine wave the amplitude of which is transmitted at 50%. The higher the diameter of the asperity, the lower its level of transmission. Changing the cut-off value enabled us to study different classes of asperities. Our apparatus assessed thirty surface parameters for each tracing length. Results based on trials prior to the current study, including other ¯ oor materials, showed that twelve standardized parameters could be selected to quantify surface roughness. Non-selected parameters were either unable to discriminate between the tested materials, or referred to speci® c standards. Some of the twelve parameters gave a similar classi® cation of ¯ oor materials. According to this, four groups could be distinguished and one parameter was selected to represent each group (see Table 1): Ra is the arithmetical mean value of all departures of the roughness pro® le from the mean line throughout the evaluation length; Wt is the vertical distance between two parallel lines enclosing the waviness pro® le within the evaluation length; Rpk and Rvk are parameters derived from the bearing length ratio curve (or Abbott-Firestone curve, a graphical representation of the bearing length ratio values calculated from multiple horizontal pro® le sections taken at different heights over the entire pro® le). The Rpk value depends on the height of the top portion of the surface pro® le, i.e. of the pro® le peaks exceeding the core pro® le, while the Rvk value depends on the depth of the inwardly directed portion of the surface pro® le, i.e. of grooves extending below the core pro® le. We used logarithmic transformations of the mean value of each parameter, due to the wide range of roughness represented by the tested materials. Since the surface geometry of the smooth resin SR2 was characterized by the presence of spherical holes with diameter up to 0.5 mm, six SR2 plates were analysed by visual counting of holes. Roughness pro® les obtained on this material showed that the smallest holes that were visually detected had a diameter of 0.1 mm.
RESULTS Classi® cation of Materials in the Laboratory The results of the ® ve series of the cleanability test applied to ¯ oor materials in the laboratory are given in Figure 1. The classi® cation of materials was different in terms of residual spores of Bacillus stearothermophilus (cleanability) and in terms of residual vegetative cells of Pseudomonas ¯ uorescens (both cleanability and disinfectability). In both cases, materials could be classi® ed in signi® cantly different groups.
Figure 1. Difference between the residual contamination of materials and the mean value obtained with the seven materials in terms of vegetative cells (1a) and spores (1b) after hygiene operations in the laboratory (n 5 experiments; error bars represent standard deviation; brackets represent signi® cantly different groups determined by multiple range analysis: materials with one common letter are not signi® cantly different in terms of residual contamination).
Trans IChemE, Vol 77, Part C, June 1999
HYGIENIC QUALITY OF FLOORS IN RELATION TO SURFACE TEXTURE
Figure 2. Residual contamination of materials inserted in the cheese site for 4 weeks, after hygiene operations (n 6 plates; error bars represent standard deviation; brackets represent signi® cantly different groups determined by multiple range analysis: materials with one common letter are not signi® cantly different in terms of residual contamination).
Classi® cation of Materials in Industrial Conditions The contamination of materials post hygiene operations in industrial conditions is given in Figure 2. Materials could be classi® ed in four signi® cantly different groups. The comparison with Figure 1 suggests that the classi® cation of materials obtained in the ® eld study in terms of residual contamination was close to the one obtained in the laboratory in terms of residual spores (R = 0.93, P < 0.01), even if the signi® cantly different groups were not exactly the same. On the contrary, there were marked differences between the results of the ® eld study and the classi® cation obtained in the laboratory in terms of residual vegetative cells. For instance, the relative classi® cation of ST2 and RT2 compared to ST1 was inverted. The same observation was made when comparing RT1 and SR2. Classi® cation of Geometrical Replicas in the Laboratory The validity of the counterprints as geometrical replica of the initial plate surfaces was veri® ed by comparison of the value of roughness parameters measured on both plates of each material and on their counterprints (data not shown). Figures 3 and 4 present the correlation between the residual contamination of materials in the ® eld study and the residual
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Figure 4. Correlation between the residual contamination of materials in the ® eld study and the residual contamination of geometrical replicas of materials in terms of vegetative cells after non bactericidal hygiene operations in the laboratory (n 1).
contamination of geometrical replicas of materials after respectively the test including chemical agents, in terms of residual spores, and the test without chemical agents, in terms of residual vegetative cells of Ps. ¯ uorescens. With both methods, there was a signi® cant correlation (respectively, P < 0.05 and P < 0.01) between the classi® cation of materials obtained with geometrical replicas in terms of cleanability and the classi® cation of materials in the ® eld study. Relation with Surface Roughness Parameters The relation between cleanability and surface roughness parameters was tested for six materials among the seven studied here: the polyurethane-based resin SR2 was not included in this study, for reasons discussed later. Figure 5 gives the values of the four roughness parameters, measured from at least thirty tracings with each cut-off wavelengths. The classi® cation of materials varied depending on both the surface parameter and the cut-off value. For example, materials with similar mean Ra0.8 values and close mean Ra2.5 values, such as ST2, RT2 and RR1 showed wide differences when considering Wt 0.8, Rpk0.8, Wt 2.5 and Rpk2.5. The mean Wt 2.5 values of RT2 and RT1 were the same, but the mean Wt 0.8 value of RT1 was almost twice the one of RT2. Table 2 presents the correlation coef® cient values obtained between microbial load of materials in the ® eld study and either the logarithmic value of each roughness
Table 2 . Correlation coef® cient value obtained for the linear relationship between the microbial load (log CFU cm-2) and various combinations of the logarithms of the mean values of four surface parameters, measured on six materials (n = 6) inserted for 4 weeks in the cheese-processing site. R value cut-off: 2.5 mm cut-off: 0.8 mm
Figure 3. Correlation between the residual contamination of materials in the ® eld study and the residual contamination of geometrical replicas in terms of spores after bactericidal hygiene operations in the laboratory (n 1).
Trans IChemE, Vol 77, Part C, June 1999
log Ra log Wt log Rpk log Rvk 11 log Rvk 6 log Ra 2 log Rpk 2 log Rvk log Ra
log Wt
0.62 0.54 0.66 0.60 0.57
0.64 0.65 0.82 0.80 0.93
0.57
0.87
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METTLER and CARPENTER The correlation coef® cient was lower, but still signi® cant (P < 0.05), when using the following simpli® ed form (2). 2 log Rvk0.8
log Ra0.8
2
The correlation coef® cient with log Ra0.8 was the lowest. Figure 6 presents the correlation between the cleanability of six plates of SR2 measured by the laboratory test and the number of spherical holes visually assessed on each plate. There was a close correlation (R = 0.93, P < 0.01) between the number of holes and residual spores. DISCUSSION
Figure 5. Values of roughness parameters measured on six ¯ oor materials with the cut-off wavelengths of 2.5 and 0.8 mm. The minimum, mean and maximum values resulting from at least thirty tracings are given.
parameter or a combination of their values, measured with both wavelengths used. For each parameter or combination of parameters, the correlation was lower when the cut-off wavelength of 2.5 mm was used. When using the cut-off wavelength of 0.8 mm, the highest correlation coef® cient (P < 0.01) for a linear relationship was obtained with the combination (1), obtained by an iteration procedure using MicrosoftÒ Excel solver in a previous experiment, when measuring the cleanability of twelve materials in the laboratory (data not shown). 11 log Rvk0.8
6 log Ra0.8
log Wt0.8
2 log Rpk0.8
1
Figure 6. Correlation between the residual contamination in terms of spores and the number of spherical holes visually detected on six plates of SR2.
As expected from previous ® ndings in three other industrial situations1 4 , the industrial routine hygiene procedure used in the cheese-processing site cannot remove or inactivate all microorganisms on ¯ oors, and signi® cant differences in residual contamination between materials are observed. Field studies represent the most reliable method to compare materials, because industrial conditions are hardly simulated in the laboratory when studying open surfaces. Nevertheless, this method presents disadvantages. First, the food premises must generate suf® cient soil and must be submitted to mild hygiene procedures. Indeed, a more effective hygiene procedure would result in reducing the residual contamination of poorly cleanable materials to the level of cleanable materials. Moreover, tested materials are not fouled and cleaned independently. If the level of contamination of the surrounding surfaces is high, a cleanable material may appear strongly contaminated because fouling is partly spread over surfaces by each hygiene operation1 4 . It is therefore uncertain whether the best discrimination of materials has been reached in this ® eld study. However, a laboratory test must be validated by a ® eld study. Indeed, a comparison of materials based on our laboratory test might have led to conclusions completely different from the industrial reality, if residual vegetative bacteria had only been considered. Residual organic fouling and physiological state of microorganisms in aged industrial bio® lms are probably responsible for this difference. However, it was not necessary to use more complex fouling to simulate industrial conditions in the present case, since the classi® cation observed in the ® eld study was close to the one obtained in the laboratory in terms of residual spores. This result indicates that the relative level of residual contamination on ¯ oor materials in industrial conditions depends on the removal ef® ciency of the hygiene procedure, determined here by the soil-retaining capacity of materials, and is not linked to the bactericidal effect of the procedure. The number of signi® cantly different groups obtained with our laboratory test was not higher than in the ® eld study, although materials were fouled and cleaned independently from each other. When industrial and laboratory classi® cations are considered together, materials are classi® ed in only two groups. This result is not surprising, since surfaces are often classi® ed in only two or three signi® cantly different groups in other studies1 ,2 ,6 , whatever the method used to assess their cleanability. A maximum of four groups was reported when comparing stainless steels with eleven different surface ® nishes or nine different alloys3 . Materials tested in this study varied not only in terms of Trans IChemE, Vol 77, Part C, June 1999
HYGIENIC QUALITY OF FLOORS IN RELATION TO SURFACE TEXTURE surface texture, but also in terms of physicochemical properties, such as surface free energy or water absorption capacity (data not shown). Previous studies1 4 , including a large variety of ¯ oor materials and several industrial conditions, indicated that initial differences between unused materials in terms of surface free energy were reduced or eliminated over time, and hence could not explain differences in terms of residual contamination. Similar results were obtained with wall materials (unpublished data). Even when conditioned independently from each other in the laboratory using solutions based on hygiene chemicals and/or food products, materials exhibited the same surface free energy, in spite of initial differences1 4 . These results already suggested that the in¯ uence of physicochemical properties of materials on cleanability was negligible compared to the in¯ uence of surface texture. The counterprints technique enabled us to study the in¯ uence of surface texture of materials upon their cleanability regardless of variations in physicochemical properties. Our results clearly demonstrate that surface texture is the major parameter describing cleanability of the tested materials. However, it should be noted that ¯ oor materials containing high cement level were not included in the present study. Indeed, such materials present a high pH value when unused, with a bactericidal effect on laboratory bio® lms. These materials are nevertheless known to be highly prone to contamination in the industry, probably because of their excessive porosity2 0 . Such a material was reported to be the most contaminated in a previous 8-week long ® eld study1 4 . Our results show that the comparison of the surface texture of ¯ oor materials requires more than one parameter. Dau® n et al.1 , who used three surface parameters calculated from the roughness pro® le Ð Rq, Sm (mean spacing of pro® le irregularities), and Rt Ð to assess the surface texture of stainless steels with a range of surface ® nishes observed that Sm gave a classi® cation of surfaces very different from the one obtained with both other parameters. Since the classi® cation of ¯ oor materials varies depending on the surface texture parameter, it is of prime importance to determine which parameter or combination of parameters best enables a classi® cation of materials close to that based on their soil-retaining capacity. For years, it was hoped that some correlation could be made between cleanability and Ra, the main surface parameter recognized internationally. Although comparative studies showed that the smoothest materials were easier to clean1 ± 5 , correlation of mean roughness (Ra or Rq) values to cleanability was generally unsuccessful. For instance, Masurovsky and Jordan3 showed that some surface ® nishes could exhibit better cleanability than others with lower Rq values, for various types of materials: stainless steel, alloys, glass and plastics. The same observation was made when Ra values were used to compare stainless steel5 , sink materials2 or ¯ oor materials6 . These results clearly refute the criterion of mean roughness as a guide to cleanability. In addition, the classi® cation of materials also varies depending on the studied class of asperities, as can be seen from the modi® cation of correlation coef® cients when changing the cut-off wavelength. Our results indicate that asperities taken into account by both Wt 0.8 and parameters calculated with the cut-off of 2.5 mm did not compromise the ef® cacy of the hygiene procedure used in the present study. Similar ® ndings have been reported by Taylor and Trans IChemE, Vol 77, Part C, June 1999
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Holah6 , who observed that the gross topographic irregularities of ¯ oor materials were not responsible for their cleanability performances. This suggests there is a threshold value for the diameter of asperities, under which the soil is not removed by the mechanical action of the hygiene procedure. Changing the mechanical action intensity would change the threshold value. This study suggests that the hygienic quality of ¯ oors is linked to the depth of the inwardly-directed portion of the surface pro® le, rather than to the mean roughness, which includes not only valleys but also peaks. Various authors have already observed that surfaces which at ® rst glance appeared to have a readily cleanable, smooth surface but were however very dif® cult to clean, were precisely characterized by the presence of small holes when surfaces were examined in more detail using microscopy or surface pro® le analysis2 ,3 ,1 4 . This result also con® rms obvious observations: valleys, pores, crevices, cavities on any material result in mechanical entrapment of soil. The meaning of the combination (2) of surface roughness parameters appears when it is given the form (3): log Rvk0.8 Rvk0.8 /Ra0.8
3
It takes account of both the value of Rvk0 .8 and the value of the ratio Rvk0 .8 /Ra0 .8 , which depends upon the proportion of the inwardly directed portion of the surface pro® le to the mean roughness. This combination provided also the higher correlation coef® cient between surface geometry and residual contamination of materials in a second ® eld study carried out in a pastry-processing site, in conditions described elsewhere1 4 , with other ¯ oor materials (R = 0.96, P < 0.01) (data not shown). However, it should be noted that the measurement of Rvk implies an S-shaped bearing length ratio curve. For instance, the sensitivity of our apparatus is not suf® cient to enable us to obtain a bearing length ratio curve from each tracing carried out on unused stainless steel surfaces with smooth ® nishes. It is therefore uncertain whether Rvk could be used instead of Ra, the parameter currently used for geometrical speci® cations of stainless steel surfaces2 1 , although it is recognized that the information provided by Ra is not suf® cient to characterize such surfaces3 ,5 . Moreover, some surface pro® les cannot be fully described when using conventional mechanical tracing systems, since some of their surface characteristics, such as re-entrant features, do not allow the stylus to reach into them. Even an optical probe allowing non-contact measurements with a focal point diameter of only about 1 mm is not able to produce a true pro® le of the above critical surfaces7 . In this study, the surface of SR2 plates presents such characteristics. Spherical holes with diameter up to 0.5 mm result from CO2 production during polymerization of twocomponent polyurethane-based resins2 0 . Even if roughness parameters cannot be used, the signi® cant correlation between the number of holes per surface unit and cleanability shows that the hygienic quality of this material is also linked to the inwardly directed portion of the surface pro® le. It is of prime importance to note that this material, smooth at ® rst glance, presents the highest residual contamination after hygiene procedure in the ® eld study. The choice of ¯ oor materials suitable for industrial use requires the consideration of hygiene and safety issues (e.g. slip resistance)6 ,2 2 . The European Directive on `The hygiene
96
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of foodstuffs’ (93/43/EEC) states in Annex II that `¯ oor surfaces must be maintained in a sound condition and they must be easy to clean, and where necessary disinfect’ 2 3 . This study demonstrates that the presence of holes, crevices and all irregularities of the inwardly directed portion of the surface pro® le clearly compromise the hygienic quality of ¯ oors. These geometrical features must therefore be taken into account when choosing a ¯ oor material, together with factors that could increase their depth over time, in particular resistance to wear. CONCLUSIONS The hygienic quality of a ¯ oor material is determined by the soil-retaining capacity of its surface, which depends essentially on surface texture, but only when the depth of the inwardly directed portion of the surface pro® le is considered. The mean roughness does not re¯ ect the pro® le shape and this may be one reason for the lack of correlation with cleanability observed here and often reported in the literature. Knowledge about surface texture with respect to bacterial cleanability would provide important information, e.g. to select ¯ oor materials with both slip resistance properties and good hygienic quality. REFERENCES 1. Dau® n, G., Kerhe rve , L. and Richard, J., 1977, In¯ uence de l’ e tat de surface de l’ acier inoxydable sur son aptitude au nettoyage et sur sa sensibilite aÁ la corrosion par piquÃres, Industrie Alimentaires et Agricoles, 43±51. 2. Holah, J.T. and Thorpe, R.H., 1990, Cleanability in relation to bacterial retention on unused and abraded domestic sink materials, J Appl Bacteriol, 69: 599±608. 3. Masurovsky, E.B. and Jordan, W.K., 1958, Studies on the relative bacterial cleanability of milk-contact surfaces, J Dairy Sci, 41: 1342± 1358. 4. SchluÈssler, H.-J., 1970, Zur Reinigung fester Ober¯ aÈchen in der Lebensmittelindustrie, Milchwiss, 25: 133±145. 5. Timperley, D.A., 1984, Surface ® nish and spray cleaning of stainless steel, in Pro® tability of Food Processing, IChemE Symposium Series 84 (Institution of Chemical Engineers, UK). 6. Taylor, J.H. and Holah, J.T., 1996, A comparative evaluation with respect to the bacterial cleanability of a range of wall and ¯ oor surface materials used in the food industry, J Appl Bacteriol, 81: 262±266. 7. Sander, M., 1992, A practical guide to the assessment of surface texture, 125p. (FeinpruÈf Perthen GmbH, GoÈttingen, Germany). 8. Bellon-Fontaine, M.-N. and Cerf, O., 1990, Experimental determination of spreading pressure in solid and liquid vapor systems, J Adhes Sci Technol, 4: 475±480. 9. Leriche, V. and Carpentier, B., 1995, Viable but nonculturable Salmonella typhimurium in single- and binary- species bio® lms in response to chlorine treatment, J Food Prot, 58: 1186±1191.
10. 1988, Antiseptiques et de sinfectants utilise s aÁ l’ e tat liquide, miscibles aÁ l’ eau et neutralisables, De termination de l’ activiteÂbacteÂricide, NF T 72-230 (Association francËaise de normalisation, Paris la De fense). 11. Maris, P., 1992, Bio® lms and disinfection. Development of a microorganism carrier-surface method, Sci Aliment, 12: 721±728. 12. Galesloot, T.E., Radema L., Kooy E.G., Hup G., 1967, A sensitive method for the evaluation of cleaning processes, with a special version adapted to the study of the cleaning of tanks, J Dairy Res, 215±221. 13. 1983a, Emulsion paints for interior use; Evaluation of cleanability and of wash and scrub resistance of coatings, DIN 53 778 Part 2 (Beuth Verlag GmbH, Berlin). 14. Mettler, E. and Carpentier, B., 1998, Variations over time of microbial load and physico-chemical properties of ¯ oor materials after cleaning in food industry premises, J Food Prot, 61: 57±65. 15. 1997, Geometrical Product Speci® cations ± surface texture: pro® le method ± terms, de® nitions and surface texture parameters, ISO 4287. 16. 1989, Surface roughness ± terms and de® nitions, DIN 4762 (Beuth Verlag GmbH, Berlin). 17. 1997, Geometrical product speci® cations ± surface texture: pro® le method; surfaces having strati® ed functional properties ± Part 2: height characterization using the linear material ratio curve. 18. 1990a, Determination of roughness values of parameters Ra, Rz, Rmax, sheet 1, DIN 4768, (Beuth Verlag GmbH, Berlin). 19. 1983b, Surface parameter R3z, Works standard of Daimler-Benz N31007. 20. Carpentier, B., Leconte, A.-M. and Liot, J.P., 1997, Les reveÃtements de sols en continu utilise s dans l’ industrie alimentaire, Industries Alimentaires et Agricoles, March, 141±145. 21. Curiel, G.J., Hauser, G., Peschel, P. and Timperley, D.A., 1993, EHEDG update: hygienic equipment design criteria, Trends Food Sci Technol, 4: 225±229. 22. Liot J.-P., Carpentier B., Leconte A.-M., Fau G., Vetter F. and Saulnier H., 1998, Guide des reveÃtements de sol pour les locaux de fabrication de produits alimentaires, May, CNAMTS, Paris. 23. 1993, The hygiene of foodstuffs, Council directive 93/43/EEC, Of® cial Journal of the European Communities No. L 175/1.
ACKNOWLEDGEMENTS This research was supported by the UNIR (Ultra-propre, Nutrition, Industrie, Recherche) Association and by the Ministry of Research. Our sincere thanks to Danielle Chassaing for microbiological analysis and Anne-Marie Leconte for roughness measurements, to Marie-Pierre Que ric and Jean-Pierre Innocent for organizational help.
ADDRESS Correspondence concerning this paper should be addressed to Dr B. Carpentier, Centre National d’ Etudes Ve te rinaires et Alimentaires, Laboratoire d’ Etudes et de Recherches pour l’ Alimentation Collective, BP 332, 94709 Maisons-Alfort cedex, France. Email : [email protected] The manuscript was received 23 July 1998 and accepted for publication after revision 16 November 1998. This paper is an extended and updated version of one presented by the authors at Fouling and Cleaning in Food Processing, 6±8 April 1998, Cambridge, UK.
Trans IChemE, Vol 77, Part C, June 1999