Cleanability of stainless steel in relation to chemical modifications due to industrial cleaning procedures used in the dairy industry

Journal of Food Engineering 23 (1994) 449-465 1994 Elsevier Science Limited Printed in Great Britain 0260-8774/94/$7.00 ELSEVIER

Cleanability of Stainless Steel in Relation to Chemical Modifications due to Industrial Cleaning Procedures Used in the Dairy Industry M.-N. Leclercq-Perlat,” I.N.R.A., Laboratoire de G&e Jules Guesde, FlewBourg

J.-P. Tissier & T. Benezech

des Pro&d& et de Technologie Alimentaires, 369 Rue B.P. 39,5965 1 Villeneuve D’Ascq cedex, France

(Received 4 January 1992; revised version received 20 April 1993; accepted 24 August 1993)

ABSTRACT The cleanability and sur$ace characterization of 08-mm thickness bright annealed finish 304 L stainless steel were assessed by standardized methods. Cleanability was previously compared to surface chemical modifications from industrial cleaning procedures commonly used in daily industries. Strips were cleaned by four different processes including rinsing phases. The reference treatment is a RBS 3.5@ sur$actant cleaning. The other cleanings were: NaOH cleaning, HNO, cleaning, and a dairy cleaning in place (including HNO, and NaOH). The industrial cleaning processes were carried out once to simulate start-up conditions and 10 times to simulate working conditions. No notable diflerences in surface texture were found between the different cleaned sur$aces studied. It was assumed that detergent and cleaning conditions used were not aggressive enough to attack stainless steel surfaces. Cleanability differences observed were due to differences in the chemical compositions used. When cleanability results and surface chemical composition results were compared, it was observed that the greater the degree of oxygen and carbon surface concentrations for the same iron sur$ace composition, the lower were the chances of soiling remaining during the test cleaning. In the same way, it was observed that the greater the degree of the iron surface concentration for the same oxygen and carbon sur$ace compositions, the greater was the change of soiling remaining during the test cleaning. *Present address: I.N.R.A.-C.B.A.I.,

L.G.P.B.A.A., 78850 Thiverval-Grignon, 449

France.

450

M-N. Leclercq-Perlat, J.-P. Tissier, T. Benezech As many diferences in sueace chemical compositions were observed between all studied su$aces, the difference in cleanability observed could be explained by an ageingphenomenon due to chemical sueace composition changes induced by cleaning detergent actions.

NOTATION Industrial cleaning procedure using first HNO, cleaning and then NaOH cleaning. Cleaning conditions (SOY, 2%, 30 min, 05 m s - ’ near the surface) Time increment (d = 5 min or 10 min) d HNO, Industrial cleaning procedure in which detergent was nitric acid without additive. Cleaning conditions (SO’C, 2%, 30 min, O-5 m s- I near the surface) Cleanability criterion (cleaning rate constant in min- ’ ) k Number of plates per modules (a= 6) Number of BSC (Bacillus stearothermophilus var. calidolactis) ;; deposed per cm2 NaOH Industrial cleaning procedure in which detergent was sodium hydroxide without additive. Cleaning conditions (80°C 2%, 30 min, O-5 m s - I near the surface) Number of cleaned plates per modules after a cleanability time Pi Of ti Industrial cleaning procedure in which detergent was R.B.S. 35@ Ref without additive. Cleaning conditions (50°C 4%, 10 min, 0.5 m s - 1 near the surface) Standard deviation s Cleanability time (min) 4 Time after which all strips of a module are clean (min) %,=n) Time after which all strips of a module are soiled (min) t(,=w CIP

‘9water fir

Water contact angle (“) used as wettability criterion Average cleanability time (min)

INTRODUCTION The importance of microscopic soiling remaining on surfaces after industrial cleaning, and the potential contamination of food, is becoming increasingly apparent in food processing. To date, only a few studies have examined the surface cleanability in relation to chemical cleaning processes for stainless steel surfaces in dairy plants.

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Stainless steel has been the material of choice for industrial equipment construction because of its mechanical and thermal strengths, corrosion resistance, longevity and ease of fabrication. The cleanability of stainless steel as a food contact surface has been reviewed by Milledge and Jowitt (1980) in terms of the cleanability of the various available surface finishes as compared with some other materials. Dunsmore et al. (1981) studied cleanability of some dairy soilings adhering to milking-machine surfaces and they concluded that stainless steel and glass had the same cleanability and both were more cleanable than rubber plastics. However they did not observe a variation of stainless steel surfaces with working time. Karpinsky and Bradley ( 1988) compared the microbial cleaning of six industrial stainless steel butterfly valves in relation to working conditions and industrial cleaning in-place. They determined remaining spore quantities by swabbing and they concluded that none of these valves proved to be cleanable after some working months. Wildbrett and Sauerer (1989) compared the change in cleanability with time of polymethyhnethacrylate and polypropylene with stainless steel cleanability in pilot working conditions using two types of detergents. They showed that during use, surface characteristics were changed in the case of plastics but not of stainless steel. They also showed that cleanability of the surfaces changed with working time and with the detergent used. They concluded that for dairy equipment preference should be given to stainless steel instead of polymethylmethacrylate and that polypropylene should be excluded, but they did not explain the differences in cleanability due to detergents. Holah et al. (1989) showed that there was little difference in microorganism cleanability between materials commonly used in food industry equipment, including stainless steel when new. The same authors (1990) compared the changes in the microbial cleanability of stainless steel with time to the cleanability of other materials commonly used in domestic sink manufacture. They showed that when new, all materials other than enamelled steel were equally cleanable. After an abrasive treatment which simulated a long usage time, they showed that stainless steel was more cleanable than the others. Their final conclusion was that, in the absence of organic or mineral soiling, stainless steel, owing to its resistance to damage by abrasion, was more likely to retain its hygienic properties throughout a domestic working life. Greene et al. ( 1991) examined a flowmeter used in dairy plants for sanitary conditions in relation to industrial working life (cleaning-inplace, fouling, etc.). They showed that cleaning-in-place and sanitization procedures were adequate but they did not verify the existence of surface characteristics changes. Finally, Leclercq-Perlat and LaLande ( 1994) assessed cleanability in relation to surface characteristics of various new

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materials commonly used in the dairy industry by the standard LeclercqPerlat et al. (1990) method as outlined in Section 1 below. At first, these authors verified the repeatability of their method. They showed that when surface topography was reasonably smooth on a microscopic scale, cleanability differences observed were due to differences in chemical composition. For new 304 L stainless steel, they compared the cleanability of various industrial finishes in relation to topographical aspects. They showed that surfaces with poor cleanability had many surface defects. These surfaces are likely to retain more soiling because of an increased number of attachment sites. All surface treatments that reduced the number of topographical defects increased the hygienic qualities of the surfaces. In each of the works, cleanability characterization methods were very different. Even though results of these works were similar, it is very difficult to draw a conclusion and compare the materials used over their industrial usage time. Generally, surface characteristics and industrial surface treatments like cleaning or fouling were not defined. In the majority of these works, cleanability of surfaces was observed by indirect residual soil characterization methods. For this reason, if no soiling was detected in the sample, the observed results were not reliable because it was impossible to conclude that the studied surface was really clean. Consequently, it was difficult to draw a conclusion on the effects of working treatment on material surfaces. However, direct observations of surfaces (Holah & Thorpe, 1990; Leclercq-Perlat et al., 1990; LeclerqPerlat & Lalande, 1994) offered this assurance except where any soiling particles were irreversibly adsorbed on the surfaces. In this work, the cleanability of O%nm thickness bright annealed finish 304 L stainless steel was assessed by the Leclercq-Perlat et al. ( i 990) standardized method when the surfaces were cleaned by different industrial cleaning procedures over a period of time.

MATERIALS

AND METHODS

Description of the method for studying cleanability A sensitive method has been developed for evaluation of procedures used to clean equipment in the dairy industry. All the steps of its assessment were described by Leclercq-Perlat et al. (1990). In this present article, only the main steps of this method and the main results of its assessment are summarized. Spores of Bacillus stearothermophilus var. calidolactis (INIU C953, coded BSC) were used as a tracer organism. The optimal growth of the

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strain occurs at 58”C, a temperature at which most airborne spoilage microorganisms and yoghurt microorganisms cannot grow. In addition, BSC spores are highly resistant to detergents and to the cleaning temperatures commonly used in dairy equipment. Indeed, in an aqueous solution, BSC spores are not affected by 0.2% (m/m) sodium hydroxide at temperatures below 75°C. Furthermore, these spores are easily incorporated into milk foods and are insensitive to drying under conventional conditions. The initial spore suspension was prepared according to AFNOR NF T72-230 (1985) method, modifying pasteurization (90°C 10 min) to ensure destruction of heat-resistant vegetative cells in the spore stocks as shown in Leclercq-Perlat et al. (1990). This spore suspension was mixed with the control organic soiling to make up the standard fouling medium. After application of O-1 ml of this fouling medium to each strip, the strips were dried for two hours at 40°C in a clean oven ventilated with sterile air to promote adhesion. The spores were assumed to be removed at the same rate as the organic soiling particles. Such an hypothesis, verified in Lalande et al. (1986), Holah and Thorpe (1990) and Leclercq-Perlat et al. (1990), was used in this paper. After testing the adhesion of some organic dairy soiling (milk; buttermilk; stirred yoghurt) Leclercq-Perlat et al. (1990) chose a commercial, whole stirred yoghurt (Danone ‘Veloute’) as the control organic soiling because of its good adhesion and easy application. This yoghurt was composed of 87.5 ( AZO*l)% water and its dry matter consisted of 93.6 ( f O-1)% organic matter and 6.4 ( f O-1)% mineral particles. Strip preparation called for some precautions. Care was taken to avoid deformation and scratching in order to preserve its surface characteristics. After testing some presoiling cleaning methods such as RBS (Mixture of surface-active agents, Traitements 35@ surfactants Chimiques de Surfaces SARL, Frelinghem, France), sulphochromic acid and acid-alkaline dairy cleaning for cleaning efficiency without surface effects, Leclercq-Perlat et al. (1990) performed the initial decontamination of the surfaces as follows: 0 Immersion in a stirred bath of 4% (m/v) RBS 35@ at 50°C for 10 min. 0 5 min rinsing in hot (50°C) running distilled water. 0 5 min rinsing in cold (20°C) running distilled water. 0 Draining and storage in an upright position, protected from dust. 0 Drying at 40°C for 2 h in a clean, ventilated oven. l Industrial cleaning procedure as described below. 0 Drying at 40°C for 2 h in a clean, ventilated oven. 0 Sterilization in an autoclave ( 130°C; 20 min).

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The strips were placed in the sterilized modules which were described by Lalande et al. (1986) and withdrawn under aseptic conditions (laminar flow closed hood). Each module contained six strips in two banks of three. The stainless steel modules were sterilized in an autoclave ( 130°C; 20 min). After each test, the modules were cleaned by immersion in a stirred RBS 35@ bath (4%; 50°C 30 min), and then rinsed and dried (100°C; 2 h). The experimental set-up is shown in Leclercq-Perlat et al. (1990). Under the hydrodynamic conditions used (Re = 2500; r= 3 Pa), rinsing did not eliminate soiling since no spores were detected in the rinses. The determination of the minimum speed required to avoid redeposition was summarized in Leclercq-Perlat et al. (1990). The average flow speed on the strips that minimized redeposition was 0.5 m s-l (i.e. Re = 5500) when the tracer concentration on each strip was below 5300 spores cme2. The effect of concentration and temperature on adhering spores was also shown in Leclercq-Perlat et al. (1990). The best standard cleaning conditions that minimized the lethal effect of the detergent (NaOH) on adherent spores were a concentration of 0.2% by weight and a temperature of 70°C. These pilot cleaning conditions were optimized in order to ensure test repeatability. In summary, this cleaning involved three steps: l

l

l

Rinsing in softened water (Th= 0) at 25°C for 10 min with an average flow rate over the strips of 0.25 m s-l (i.e. Re 2: 3000 for the module geometry). Cleaning in 0.2% (m/m) sodium hydroxide at an average temperature of 70 ( f 0.5)“C with an average flow rate over the strips of 0.5 m s- l (i.e. Re = 6000 for the module geometry). Rinsing as in the first step.

The spore count in the rinsing and cleaning solutions was determined by successive dilutions in a Petri dish (three per dilution). The indicatorcontaining agar of Shapton and Hindes (1963) was used. BSC spores were incubated for 24 h at 58°C. Spore growth was revealed by an indicator colour change from purple to yellow in the zone of colony development. There were no detectable effects on the yoghurt soiling when rinsing with water. In addition, more than 90% of the spores were found in the sodium hydroxide solutions. The modules were removed from the set-up under aseptic conditions (near a Bunsen burner flame) just after the second rinsing step. The surface spore counts were obtained by coating each strip with agar containing the Shapton and Hindes ( 1963) indicator. The colonies were then

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counted directly after incubation (24 h, 58°C). The parameter Nb was the initial spore concentration per strip (spore cm-‘) and S was the soiling surface area per strip (2.25 cm”). The cleaning rate was studied under the given standard cleaning conditions. The time required to clean a sample containing N,, adhering spores per strip (No = Nb*S) was used. The cleaning of microbial soiling was assumed to follow first-order kinetics characterized by a rate constant (k) which was independent of N,,. The average cleaning time (,Q required to clean a sample containing N, spores was estimated. Within the range of times for which cleaning was complete, the number of residual colonies was theoretically zero. In practice, a discrete variable was used: at least one residual colony or the complete absence of colonies. This variable was assumed to obey the Poisson distribution. In practice, the times corresponding to all six strips in a given module remaining soiled ( tp,= o) or all being cleaned ( tp,= ,J were determined. A fixed interval of 10 min was used to separate two consecutive cleaning treatments (d= 10 min). Bearing in mind the distribution law applicable to the test variable, the cleaning time resulting in cleanliness was estimated from these data using the Spearman-Karber method, which gives the following expression for the average cleaning time (pu,): P*=f p,=n----;

pp; r-O

The value pu, derived from this expression is a biased estimate of the average cleaning time. The bias depends on the time (t,,,,,) when all strips are soiled and also on the time increment separating two consecutive treatments (d), but is independent of the number of strips studied each time. Pflug and Holcomb ( 1983) consider that if tp,=o is chosen randomly and is relatively small compared to p,, then the average pupis a good approximation of the average cleaning time. The variance in average cleaning time pu, for a constant value of d and n strips is then given by the following expression:

The 95% confidence interval for p, is then given by:

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The cleaning rate constant (k) is calculated from the average cleaning time ,uupusing the following equation: k=

ln(NI,*S) + 0.58 FP

The most suitable criterion for characterization of the surface cleaning quality was the cleaning rate constant k since it is independent of the quantity of adhering soiling (A$). Therefore, k was used in order to compare the cleanability of surfaces after different industrial cleaning processes. Preparation and characterization of surfaces The cleanability of stainless steel surfaces was compared using some cleaning procedures commonly used in the dairy industry. As a reference material, 304 L stainless steel (AFNOR Z3CN1810), 0.8 mm thickness, bright annealed finish (Ugine-Aciers, B.P. 165, 59494 Lomme cedex France) was chosen. In order to ensure that the initial surface state of all strips ( 15 mm wide by 50 mm long) was the same, they were cut from the same sheet (1 mz) of industrial stainless steel. Care was taken during cutting to avoid alteration of surface characteristics through deformation or scratching. &&ace characterization After each ‘industrial’ cleaning, some surface properties were determined as proposed by Leclercq-Perlat et al. (1990). First, topographical and cleanliness characteristics (heterogeneity, adhering particles, etc.) were observed by scanning electron microscopy (SEM) with a JSH35 CF scanning electron microscope (Japan Electronic Optics Laboratory). The examination conditions of Tissier et al. (1992) were adopted when working with conducting samples (stainless steel) covered with a thin film of organic materials (non-conducting) and operated at low incident energy (2 kV). Secondly, the main chemical elements of the surface layers (analysed thickness varied between 10 and 15 run) were identified by energy dispersive X-ray analysis (EDXA). An EDXA LZ-5 probe from Link systems was used in conjunction with an SEM microscope. Spectral analysis was carried out using an 1000 multichannel analyzer from Link Systems. The operating conditions were as given in LeclercqPerlat et al. ( 1990) and Tissier et al. (1992). Thirdly, wettability was measured by doubly distilled water equilibrium angle using the sessile drop contact angle technique. This technique used to classify and compare materials according to surface energy was described by

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Ratner et al. (1987) and summarized by Leclercq-Perlat et al. (1990). Finally, surface roughness was assessed with a surface roughness measuring instrument (Rank Taylor Hoboon Surtronic 3P) in accordance with British Standard (BS 1134; Anon, 1972). For each cleaning procedure, cleanliness and surface characteristics of six strips were studied before fouling by yoghurt and after a pilot cleaning test of cleanability for which the time was I tp,= n. Cleaning procedures For all cleaning procedures used, the first step was a RBS 35@ surfactant cleaning to ensure initial surface decontamination. This RBS 35@ cleaning was chosen as the reference treatment (trials 1 and 2). After drying (40°C 2 h), 220 strips were cleaned using three different processes including rinsing phases. They are ( 1) Nitric acid cleaning (2) Sodium hydroxide cleaning and (3) combined acid plus alkaline cleaning according to procedures below: l

l l l

Immersion in a bath of stirred (300 rds min- ’ ) detergent solution at 80°C for 30 min. These conditions were used during the final cleaning of industrial UHT sterilizers which generally preceded a stop in milk production. 10 min rinsing in hot (50°C) running distilled water. 10 min rinsing in cold (20°C) running water. Draining and storage in an upright position, protected from dust.

All the other operating steps (assembling, disassembling, mounting onto the pilot plant, pilot cleaning and retained detecting after cleaning) were performed as proposed in Leclercq-Perlat et al. ( 1990). If the detergent solution used was 2% by weight sodium hydroxide without any additive, this alkaline cleaning process was coded ‘NaOH’. If 2% by weight nitric acid without any additive was used as a detergent, this acid procedure was coded ‘HNO,‘. The third cleaning procedure used first an acid cleaning process and then an alkaline one and was coded ‘CIP’. Study of this industrial cleaning-in-place procedure was limited because Leclercq-Perlat et al. (1986) showed that whatever the composition of the milk UHT deposit, the HNO,-NaOH procedure led to a better cleaning result than the NaOH-HNO, procedure, both from the point of view of efficiency and speed. On the same group of strips each cleaning procedure was used once to simulate start-up conditions and 10 times to simulate industrial working conditions.

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The Leclercq-Perlat et al. (1990) cleanability method was used firstly on reference strips (trials 1 and 2) and secondly on the working strips (trials 3 to 14). Each cleaning procedure was repeated to verify repeatability.

RESULTS Surface characteristics of studied surfaces For each surface studied, it was observed that all initial contamination had been perfectly removed. Topographic SEM observations of the surfaces revealed some quite deep random scratches as well as shallow, uniformly distributed scratches (caused during manufacture by rolling) and some deep crevices of variable size. Moreover, their average surface roughness was equal to 0.1 1 ( f O-04) pm. No notable differences in cleanliness, topography and roughness were seen between the different cleaning procedures studied. It was assumed that the preparation of the strips was done with care and that the detergents used were not concentrated enough to corrode the surfaces and change its topography and roughness. From energy dispersive X-ray analysis, the influences of the different cleaning procedures used on the superficial composition of stainless steel were identified. In all cases studied, no notable differences in nickel or chromium surface composition was seen. Sut$aces cleaned once For these cleaning processes except the reference one, it was very difficult to draw a conclusion because the variation coefficient ( > 20%) which depended on the nature of atom analysed and on the X-ray count of each peak, was too high. This fact could be explained by surface composition heterogeneity. No notable differences in surface chemical composition were observed between reference surfaces cleaned one and ten times. Sur$aces cleaned 10 times For these cleaning processes, reproducibility of each cleaning process is quite good because no significant differences were observed between different strips analysed with the same EDXA conditions. Reproducibility of the variation coefficient ranged between 0.7 and 10%.

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Reference and lo-times-NaOH (tests 5 and 6) cleaning processes induced more surface carbon and oxygen than the HNO, cleaning processes alone (tests 9 and 10) or with NaOH (tests 13 and 14). The iron concentration of the cleaned reference strips increased near the surface, although it remained roughly constant for the HNO, and CIP processes. It was lower for the NaOH cleaning procedures. The carbon concentration near the surface was higher for the NaOHcleaned strips. No notable differences in carbon surface concentration were observed between HNO,-cleaned strips, either alone or with NaOH. The oxygen surface concentration is strongly dependent on the cleaning treatment. NaOH-cleaned surfaces give higher oxygen surface concentrations and reference cleaned strips contained a little less surface oxygen than HNO, cleanings (alone or with NaOH). Use of HNO, (alone or with NaOH) markedly reduced the oxygen surface concentration. In conclusion, it was observed that acid cleanings (alone or with NaOH) lowered the surface oxide layer. Moreover, NaOH cleanings (alone or CIP) seemed to be less efficient with regard to carbon and oxygen surface adsorption. It was assumed that C and 0 surface concentration increases were due to carbonation of the NaOH-cleaned surfaces which took place during industrial NaOH cleaning. Yet Na was never detected. Significant differences in wettability between the given cleaning procedures were observed, primarily due to modifications in the surface chemistry of the stainless steel. It was assumed that the detergents used were not concentrated enough (2% by weight) to modify the surface topography and, consequently, to change the average surface energy of the material. For a reference cleaned surface (tests 1 and 2), a similar wettability ( ewater= 45( f 3”)) was found. For surfaces cleaned only once, except for the reference strips, it was very difficult to draw a conclusion because the standard deviation (8”) on wettability (8,,,,, = 45”) was too high. This fact could be explained by contact angle hysteresis due to surface chemistry heterogeneity. The reference surfaces and those cleaned 10 times were significantly different in wettability. The 1O-timesNa-OH-cleaned surfaces ( Owater= 51( + 2)) were slightly more hydrophobic than reference ones (8,,,,, = 45( + 2”)). The surface cleaned 10 times with HNO, (8,,,,, = 40( f 2”)) was slightly more hydrophilic than the other cleaned surfaces. For the surfaces cleaned 10 times with CIP, wettability ( t9,,,,, = 46( f 3”)) was estimated to be between those of the 10 times NaOH and HNO, cleaned surfaces; that is to say, of the same order of magnitude as the reference cleaned surfaces.

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Cleanability results The most suitable criterion for characterization of the cleanability of materials was the cleaning rate constant (k) due to its independence of the adherent soiling quantity. k was determined in order to compare the influences of different industrial cleaning procedures on the cleanability of 304 L stainless steel (0.8 mm thickness bright annealed finish). These results are summarized in Table 1. For all cleaning procedures, no notable difference in cleanability criterion (k) was found between the two identical tests. Indeed, this value had the same order of magnitude for both of the tests. On the other hand, it seemed that surface chemical modifications had an important influence on surface cleanability for the same topography. No notable differences in cleanability (k= 0*55( z?zO-44) min-‘) were seen between the reference surfaces cleaned once and 10 times. Moreover these strips were the most cleanable. For all cleaning procedures, except the reference ones, surfaces cleaned once were less difficult to clean than those cleaned 10 times. Indeed, the cleanability criterion (kltime= O-12( k 0.01) rnin- ’ and klorimes = 0*08( + 0.01) min- ‘) for the HNO,-cleaned strips was the same order of magnitude as that for the CIP-cleaned surfaces.

TABLE 1 Infhtences of Different Cleaning Procedures on the Cleanability of 08-mm Bright Annealed 304 L Stainless Steel

Test Cleaning no. process

: 3 4 5 6 7 8 9 10 11 12 13 14

Ref Ref NaOH NaOH NaOH NaOH HNO, HNO, HNO, HNO, CIP CIP CIP CIP

Number of cycle 1 10 1 1 10 10 1 1 10 10 1 1 10 10

d ti t2 Nb (Spores cm-l) (min) (min) (min) 1960 1960 2400 2400 2670 2660 3560 3550 1380 1380 1020 1020 1840 1840

( f 180) ( f 180) ( 31230) ( + 230) ( + 200) ( k 230) ( f 230) ( f 200) ( f 50) ( f 60) ( f 50)

( Ik 50) ( f 50) ( + 50)

5 5 5 5 10 10 10 10 10 10 10 10 10 10

0 0 35 35 60 60 50 50 50 50 30 30 50 50

40 40 80 80 130 130 130 130 160 160 100 100 160 160

pfs

(min)

Thickness

kks (min - ‘)

16 ( + 1) 0.56 (+004) 17(kl) 0*53(+0*04) 67(f2)0-137(+@004) 66 ( f 2) 0.135 ( + 0.004) 100(&3)0.093(f@004) 101 ( f 3) 0.092 ( *O-004) 85(+3)0~113(+0~004) 86(f:3)0.111(+0.004) 112 ( * 3) 0.077 ( f 0.004) 110(+3)@078(+0~004) 67(f3)0.112(f@004) 68(~3)0~110(f0~004) 110(~3)0~081(+~004) 1 1 1 ( f 3) 0.082 ( f 0.004)

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In the two cases, NaOH-cleaned surfaces seemed more cleanable than HNO,- or CIP-cleaned strips using k as a cleanability criterion to estimate the cleaning of a microscopical adherent soiling. The surfaces cleaned with HNO, (or CIP) and NaOH were significantly more difficult to clean than the reference strips. Indeed, the surface cleanability for strips cleaned with HNO, (or CIP) was above five times less for one cleaning cycle and about seven times less for 10 cleaning cycles than the reference ones. Likewise, NaOH-cleaned surfaces were about four times less cleanable than reference cleaned plates in the case of one cleaning and six times less cleanable in the case of 10 cleanings. DISCUSSION

AND CONCLUSIONS

In this method, it is assumed that spore tracer was eliminated at the same rate as yoghurt soiling. For this, SEM observations and EDXA surface chemical analysis (analysis thickness = 10 nm) were made on some strips which were cleaned in a pilot plant for a time of tp,= n. In this way, it was verified that their surfaces were perfectly cleaned without adherent soiling particles or adherent dead microorganisms and that they had the same composition as the initial clean strips. For all the cleaning procedure surfaces which were tested at cleaning times lower than t,,,=., the presence of microorganisms was systematically observed as well as that of adherent yoghurt particles. The qualitative reality of the assumption of the Leclercq-Perlat et al. (1990) cleanability method was therefore verified in this manner. Nevertheless, the absence of irreversible soiling particles adsorbed into the first external layers of the surfaces ( I a few A) could not be verified. Brassard (1990) verified the reversibility of microscopic protein soiling adsorption for a new 304 L stainless steel surface by X-ray photoelectron spectroscopy (XPS). It was shown, in this way, that there was no adherent matter diffusing into the external layer of 304 L stainless steel. Wildbrett and Sauerer (1989) showed the absence of irreversible adsorption and diffusion for the stainless steel studied. In our conditions and for all the surfaces cleaned by industrial cleaning procedures studied, it was assumed that there was no irreversible surface adsorption or diffusion of yoghurt soiling into the superficial layers. Material texture and topograpy strongly influence surface cleanability. Soiling in pits and crevices would not receive the same shear forces as soiling attached to the smooth surfaces. Those surfaces that are more pitted and creviced, on a microscopical scale, would therefore be less cleanable (Holah & Thorpe, 1990; Freeman et al., 1990; Leclercq-Perlat &

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Lalande, 1994). For this reason, all cleaned surfaces were characterized by SEM topography and average roughness. No notable differences in surface texture were found between the different cleaned surfaces studies. It was assumed that the detergent types and cleaning conditions used were not concentrated enough to attack 304 L stainless steel surfaces and change the topography and/or roughness of surfaces. Using the Leclercq-Perlat et al. (1990) method for testing the cleanability of yoghurt soiling adhering to OS-mm thickness bright annealed finish 304 L stainless steel surfaces, some surfaces cleaned by procedures commonly used in the dairy industry were classified in ascending order or cleanability in relation to their chemical nature: HNO, ( 10 cycles) = CIP ( 10 cycles) < NaOH ( 10 cycles) < HNO, ( 1 cycle) = CIP ( 1 cycle) < NaOH ( 1 cycle) 4 reference In terms of the surface nature, soiling attachment will depend on the number of attachment sites available (related to surface area and topography) for soiling surface chemical interactions (Gibbons & Denton, 1981). As there were significant differences in cleanability between the cleaned surfaces studied which had the same topography (smooth at microscopical level), it could therefore be suggested that soiling attachment was not similar between these cleaned surfaces of the 304 L stainless steel in terms of soiling/surface chemical boundings. On the contrary, for yoghurt soiling adhesion, Leclercq-Perlat and Lalande (1994) observed that there was a notable difference in cleanability between the materials studied. This fact was explained by a great difference in nature of chemical interactions between bacteria/surface bonding and yoghurt/surface reactions. Indeed, for the stainless steel, it was shown that if carbon and oxygen concentrations of cleaned surfaces were classified by EDXA for the same iron surface concentration from the lowest (HNO, or CIP) to the highest (NaOH), the same ascending order of cleaned surface cleanability was obtained. This result suggests that the greater the degree of oxygen and carbon surface concentration, the lower the chance of soiling remaining during pilot cleaning. Likewise, if the iron surface concentration of stainless steel was classified by EDXA for the same carbon and oxygen surface concentrations from the lowest (NaOH or HNO,) to the highest (reference), the same ascending order of cleaned surface cleanability was obtained. This result would suggest that the greater the degree of iron surface concentration, the greater the chance of soil elimination during cleaning.

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For 304 L stainless steel, the differences in cleanability of various cleaned surfaces could be explained by some changes in iron, carbon and oxygen chemical surface compositions which induced some variations in attachment site numbers and/or soiling/surface boundings. When cleanability of surfaces cleaned 10 times with RIB 35@ was compared with that of surfaces cleaned 10 times with HNO, or NaOH, a great change in cleanability ( X 0.14 and X 0.20 respectively) was observed. All chemical cleaning of materials which changed the chemical composition of the material surfaces had a notable influence on cleanabilty properties. In conclusion, materials that resist surface chemical changes due to irreversible chemical adsorption will remain more hygienic than materials which are more easily ‘damaged’. Likewise, detergents which do not change surface composition will remain more hygienic than those detergents which are more ‘aggressive’ for the same efficiency on the soiling. Moreover, Wildbrett and Sauerer (1989) showed the small effect of any ageing phenomenon on stainless steel surfaces after a long industrial use. Holah and Thorpe (1990) also showed this phenomenon but they attributed it to abrasion of surfaces during working. A notable difference in cleanability was observed between surfaces cleaned only once and those cleaned 10 times, except for reference cleaned strips. As many differences in surface chemical compositions between all surfaces studied were observed, the difference in cleanability observed could be explained by an ageing phenomenon due to chemical surface composition changes induced by cleaning detergent action during the industrial use. For each material surface, cleanability was determined by the Leclercq-Perlat et al. (1990) method which allows comparison of the relative cleanability of some stainless steel surfaces cleaned by industrial procedures in relation to surface chemical composition modifications. For each surface and soiling, this method was specific for the food product-tracer combination used. Indeed, the cleanability criteria depend on the material and its surface characteristics but also on the type and nature of soiling (specific adhesion of constituents). Consequently, all treatments commonly used in the food industry which induced some surface characteristic changes also induced some cleanability ones. Stainless steel with a smooth finish on a microscopic scale is likely to retain its hygienic properties throughout an industrial working life because of its resistance to corrosion and mechanical damages. What is more, for this material, the efficient detergent which does not change the chemical surface composition after a long industrial use is more likely to be retained as the best.

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M.-N. Leclercq-Perlat, J.-P. Ttksier,T. Benezech

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