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Romanian Journal of Food Science

Official Journal of the Romanian Association of Food Professionals

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Romanian Journal of Food Science 2011, 1(1): 3944 39

AISI 430 stainless steel behaviour at different disinfectants Mihaela BRUM 1,*, Maricica STOICA 1, Geta CRC 2 and Petru ALEXE 1 1 Biochemistry and Technologies Dept., Faculty of Food Science and Engineering, Dunarea de Jos University of Galati, 111 Domneasca St., 800201 Galati, Romania 2 Chemistry Dept., Faculty of Sciences, Dunarea de Jos University of Galati, 111 Domneasca St., 800201 Galati, Romania Received 1 October 2010; received in revised form 7 November 2010; accepted 8 November 2010

Abstract

The samples of AISI 430 stainless steel were exposed to various disinfectant solutions and then examined by scanning electron microscopy. The chemical processes that take place on stainless steel surfaces during disinfection, were put into evidence by measuring the pH of the working biocide solutions after their contact with stainless steel samples. The 430 stainless steel was chosen due to the fact that it is used for public food kitchens and catering/gastronomy industry, as well as for food processing industry. The action of three commercial disinfectants was studied: Actisept (with active chlorine as active substance), Anasept (mixture of hexamethylenediamine, polyhexamethylene biguanide and quaternary ammonium compounds) and Oxonia Active (mixture of peroxyacetic acid with hydrogen peroxide). Microscopic analysis demonstrated that disinfectants induced structure modifications of 430 stainless steel surfaces. It was concluded that the 430 stainless steel metallic surfaces are affected during the sanitization process and the damages depend on the nature of the disinfectants.

Keywords: stainless steel, disinfection, chlorine, hydrogen peroxide, quaternary ammonium compounds, food safety, pH, scanning electron microscopy.

1. Introduction

To achieve safety on regard of disease agents and to ensure shelf-life are the central concerns of hygiene of every food processing factory. Disinfection plays a major role in successful food processing and it is an essential step in preventing food contamination with pathogenic and spoilage microorganisms (Gram et al., 2007). A disinfection protocol usually ends with the elimination of the disinfectant traces by rinsing, but there are authors that consider the disinfectant application being the last step of a disinfection protocol and rinsing with water not necessary (Burfoot and Middleton, 2009). * Corresponding author: E-mail address: calin.mihaela-gl@ansvsa.ro

Furthermore, some biocide producers support the idea that as long as the remaining disinfectant in the processing food lines does not exceed the legal limits (Leveau and Bouix, 1999), it does not represent a chemical risk for the consumers health and it may reduce the general food contamination. In other words, the remaining disinfectant serves as a sentinel against microorganisms. However, the residual disinfectant can potentially lead to a significant degradation of equipments materials via corrosion that, in turn, can increase the adhering of soil (Masurovsky and Jordan, 1958; Holah and Thorpe, 1990; Leclerq-Perlat and Lalande, 1994) and affects the surface cleaning ability. The exposure time and pH are the most important factors affecting the activity and efficiency of the sanitizing agents and cleaning ability, also. Austenitic stainless steels are traditionally used for industrial applications.

Mihaela BRUM, Maricica STOICA, Geta CRC and Petru ALEXE

Romanian Journal of Food Science 2011, 1(1): 3944 40

However, they have been progressively replaced by ferritic stainless steels at lower cost, due to the absence of nickel (Sabioni et al., 2003). AISI 430, a ferritic stainless steel, is used for cutlery, kitchen sinks and catering/gastronomy industry, as well as for food processing industry (Foged et al., 2005), due to material advantages, such as: economic, aesthetic quality and low thermal expansion coefficient (ISSF, 2007). The aim of this work is to investigate the manner in which the surfaces of the AISI 430 stainless steel are affected during the sanitization process. Using scanning electron microscopy technique allows evaluating the influence of the residual disinfectants on metallic surfaces. 2. Materials and methods 2.1. Samples characterization and pre-treatment procedures

Tests were performed using AISI 430 ferritic stainless steel. The stainless steel was supplied by Duramet (Bacu-Romania). The chemical analysis of the coupons was performed using the optical emission spectral analysis technique on Spectromax equipment (SPECTRO Analytical Instruments Gmb H & co. KG, Germany). All stainless steel samples were chemically cleaned and washed in distilled water before testing (Boulang-Petermann, 1997; Compre and al., 2001; Stoica et al., 2009, 2010). 2.2. Disinfectants

The disinfectants used in this study included Actisept (140 ppm active chlorine, 10 min. time action; Medicarom, UK), Anasept (0.5% in solution of mixture of hexamethylenediamine, polyhexa-methylene biguanide and quaternary ammonium compounds; Rouasan, Romania) and Oxonia Active (0.2% in solution of 5.8% peroxyacetic acid with 27.5% hydrogen peroxide, 30 min. time action; Ecolab Inc., St. Paul, MN). Fresh working solutions of each biocide were prepared by dilution in distilled water. 2.3. Experimental set-up

The pH measurement of the corrosive environ-ment was taken throughout the period of the experi-ments: initially, at 10 min, 30 min, 60 min and finally to 480 min. Prior to using the pH, the pH-meter was standardized by immersing firstly the electrode in distilled water and then adjusting the meter to pH of 7 at room temperature. The distilled water was then wiped of the electrode with a tissue paper.

Finally, the pH electrode was dipped in the medium to measure the pH. To study the disinfectants influence on the stainless steel surfaces, there were placed the coupons of stainless steels, in glass cylinders with fresh disinfectant solutions at room temperature. Each glass cylinder was covered by acrylic plate with hole. The pH measurements of the disinfectants solution was tested in time using the WTW INOLAB 720 pH-meter, to evaluate the influence of the environment pH on stainless steel surface. After disinfectants action, the modifications of stainless steel surfaces were performed by scanning electron microscope (Ishak et al., 2008; Stoica et al., 2008) using a Quanta 200 (Philips) with high magnification, in 20 fields with area of 40 m2 for each sample. 3. Results and discussion

Every production process in the food industry requires different disinfectants that vary depending on equipment and food product type. In the disinfection process, pH plays a critical role in the reduction and inactivation of fungal cells. 3.1. The pH variations of the disinfectant solutions

The pH variations of the disinfectant solutions are presented in Figure 1.

Figure 1 shows the variations of pH of different disinfectant solutions with AISI 430 stainless steel tested. In general, variations of 0.5 units of pH indicate a constant behaviour of the system studied (Queiroz et al., 2007). As a rule, for Actisept the pH of the biocide working solution was practically constant, varying less than 0.5 in time (Figure 1, series 1), but a less continuous pH increase was observed from 5.70 to 5.99 units up to 480 minutes. In the case of Anasept disinfectant, the pH of solutions was practically constant, but after 480 minutes it was observed an 0.78 units increase (Figure 1, series 2). For the Oxonia Active, the pH was practically constant, varying less than 0.5 in time (Figure 1, series 3). Modifications of pH could be explained by a balance of reactions, which means a consumption of H+ from the solution. Better pH stability on the stainless steel interface at Actisept and Oxonia Active, was observed. Instability of pH in Anasept was observed up to 480 minutes. The stability of pH was in following order: Oxonia Active > Actisept > Anasept. These results show that disinfectant solutions in generally are stable for their action time up to 480 minutes indicated by the suppliers.

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AISI 430 stainless steel behaviour at different disinfectants

Romanian Journal of Food Science 2011, 1(1): 3944 41

Figure 1. Variation of pH values against time in different disinfectant solutions:

Actisept (series 1), Anasept (series 2), Oxonia Active (series 3) with AISI 430 stainless steel.

The biocide solutions studied presented different behaviours and pH, depending on the type and the contact time. These characteristics should be considered when choosing the disinfectants useful for metallic surfaces disinfection to keep the safety food. 3.2. Aspects of surfaces morphology

Materials that present surface changes due the chemical disinfection will remain more hygienic than materials which are more easily damaged on a microscopically scale that would therefore be less cleanable. It is widely accepted in literature that disinfectant residuals increase the materials corrosion (Pisigan and Singley, 1987). Each stainless steel samples were studied by scanning electron microscopy with high magnification.

Figure 2. Scanning electron micrograph of AISI 430

stainless steel surfaces without disinfectants.

The micrographs are presented in the Figures 25. Figure 2 presents the metallic surfaces without disinfectant treatments.

The image on the stainless steel surface without disinfectant treatments (Figure 2) shown grit lines from initial surface preparation and pitting are not observed. 3.3. Actisept effects

AISI 430 stainless steel samples were studied by scanning electron microscopy with high magnification at 10 min and 480 min, after exposure to the disinfectant solution, and the results are presented in Figure 3.

Small areas of pitting can be observed in the scanning electron micrographs of stainless steel surface after immersion in Actisept for 10 min (Figure 3a)

The image of the stainless steel after 480 min of exposure to the Actisept showing serious and numerous pitts, may be resulted from oxidation in the surface layer of the substrate (anodic dissolution). The chloride ion is not consumed, it remains in solution and attacks the stainless steel surface (Figure 3b). Thus, the layers are less protected in Actisept solution than normal passive layers. 3.4. Anasept effects

AISI 430 stainless steel samples were studied by scanning electron microscopy with high magnification, at 60 min. and 480 min., after exposure to the disinfectant solution, and results are presented in Figure 4.

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Mihaela BRUM, Maricica STOICA, Geta CRC and Petru ALEXE

Romanian Journal of Food Science 2011, 1(1): 3944 42

a b

Figure 3. Scanning electron micrographs of AISI 430 stainless steel surfaces in Actisept disinfectant at different contact time: 10 min. (a) and 480 min. (b).

a b

Figure 4. Scanning electron micrographs of AISI 430 stainless steel surfaces in Anasept disinfectant at different contact time: 60 min. (a) and 480 min. (b).

In the Figure 4a, it was not observed any

destruction of the finishing surface for the stainless steel immersed in Anasept after 60 min., while for the stainless steel immersed for 480 min. in Anasept there can be observed little dark spots on the surface, without any destruction of the finishing surface (Figure 4b). This disinfectant contains inhibitors of corrosion; therefore, the layers are protected in Anasept solution. 3.5. Oxonia Active effects

AISI 430 stainless steel surfaces were studied by scanning electron microscopy with high magnification at 30 min. and 480 min., after exposure to the biocide solution, and results are presented in Figure 5.

Figure 5 shows the scanning electron micrographs of AISI 430 stainless steel surfaces in Oxonia Active disinfectant at different contact time. A lower destruction can be observed in the scanning electron micrographs of stainless steel surface after immersion in Oxonia Active for 30 min (Figure 5a). Oxonia Active, as a strong oxidant even at very small concentrations raises the corrosion potential and decreases the repassivation potential and leads to the destruction of the passive films and thus causes pitting and crevice corrosion (Figure 5b). The observed uniform corrosion and area localized attack (cracks) could lead to corrosion-fatigue which is the result of the combined action of an alternating or cycling stresses and a corrosive environment.

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AISI 430 stainless steel behaviour at different disinfectants

Romanian Journal of Food Science 2011, 1(1): 3944 43

a b

Figure 5. Scanning electron micrographs of AISI 430 stainless steel surfaces in Oxonia Active disinfectant at different contact time: 30 min.(a) and 480 min.(b).

It causes the rupture of the autoprotective passive

film, upon which corrosion is accelerated. As a conclusion, this disinfectant produced the most remarkable surface modification compared with other tested disinfectants.

4. Conclusions The samples of the AISI 430 stainless steel were exposed to various disinfectant solutions and their behaviour was examined by scanning electron microscopy and pH measurements. The action of three commercially disinfectants: Actisept, Anasept and Oxonia Active was studied. The microscopic analysis demonstrated that disinfectants induced structure modifications of AISI 430 stainless steel surfaces, after 8 hours contact time. The surface damage morphology is mainly characterized by finishing modifications of AISI 430, more destruction for the acidic disinfectant action and less for the neutral disinfectant. It was concluded that the residual disinfectant can potentially lead to a significant degradation of equipments materials via corrosion. This characteristic could be taken into account when choosing the disinfectants useful for metallic surfaces disinfection to keep the safety food. Acknowledgments The authors thank to Dr. Alina CANTARAGIU for the scanning electron microscopy investigation.

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Abbreviations AISI American Iron and Steel Institute

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