**3. Public health and hygiene**

Food can be an important source of disease-causing organisms, the investigations on risk management in food handling, diet and health (Buchanan et. al., 2001, and Oddy et. al., 1985), revealed an association between human diseases and food contaminated by flies. Food can contain beneficial bacteria, but contamination with harmful microorganisms from external reservoirs must be prevented not only by removal of food residues and disinfection but also by the modification of surfaces specially designed to avoid sites for the development of bacterial colonies, such as accumulated corrosion products or degraded protective coatings. These surfaces must not introduce toxic substances or influence flavour. Materials that fulfil these requirements are glass, earthenware, some plastics, and metals like SS and aluminium alloys. Glass is sensitive to shock and fragments are sharp, while inexpensive plastics are not heat resistant and, in thin gage, they are permeable to gases. Metals do not have these disadvantages, but they must be corrosion resistance in food environments.

The modern FI combines the application of the latest production processes, the use of cleanable corrosion resistant SS equipment and computerized information with an efficient, environmentally sound approach to meet the needs of food products consumers everywhere. Cleanability is important in relation to taste, colour, odour and contamination of edible products such as milk, processed and canned foods and alcoholic beverages. This is of particular hygienic importance in food handling. Food processing involves operations by which raw foodstuff are suitable for human consumption, it includes the basic preparation of food, preservation and packaging techniques. Many innovations have resulted in new products such as concentrated fruit juice, freeze-dried coffee and instant food.

Sterilization is one of the most important aspects of hygiene and sodium hypochlorite is a disinfectant widely used to kill microorganisms through an oxidizing reaction that yields sodium chloride as final product.

$$\text{NaCl} \blackrightarrow \text{NaCl} + \text{O}^\* \tag{2}$$

Where O\* represents oxygen species capable to kill biological material by oxidation. Both sodium chloride and sodium hypochlorite can be easily removed from surfaces used in the FI by rinsing with water, but it is not permissible to leave any residue because residual chlorides and hypochlorites can be concentrated by evaporation and induce localized pitting corrosion

#### **4. Microbiological induced corrosion in the food industry**

Microorganisms are primitive unicellular organisms capable to live in colonies at aqueous media. They are diverse and for the interest of this chapter it is appropriate to mention;

Selection of SS is an activity that requires the participation of specialized expertise in materials technology, coupled with a sound knowledge of the chemical and physical characteristics of the food products (S.H. Zhang and B. Monitz, 2006, R.M. Davidson et al., 1987). Ferritic low carbon SS UNS S44400 has the advantage to be immune to Stress Corrosion Cracking (SCC) while its resistance to localized pitting and crevice corrosion is comparable to UNS S31600 austenitic SS. The austenitic SS group is the most popular group used for the construction of infrastructure in the FI, mainly the UNS S30400 and S31600 due to their good corrosion resistance properties. The 400 series, e.g., UNS S41000, is specified for applications such as pump impellers, plungers, cutting blades, scrapes and bearings. Corrosion resistant properties of the different SS depend on the alloying elements such as chromium, nickel and molybdenum. The duplex SS is a group that contains similar amounts of ferrite and austenite in their microstructure are increasing their use in the FI in the last years. These steels have high strength and good corrosion resistance compared with the austenitic SS, but one concern is that duplex SS tend to form brittle intermetallic phases and

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In the FI, the equipment is manufactured from SS with a finely polished surface that tends to have a better corrosion resistance regarding to non-polished ones, and is easy to clean and to keep clean. A smooth surface is less susceptible to an accumulation of deposits, fouling or biofouling, which often become focal points for localized corrosion. Seamless and welded austenitic SS sanitary tubing and pipelines intended for use in the FI should have special surface finishes, as the ASTM A270 standard specifies. The finishing process results in SS

The standard designations for SS finishes are classified by mill forms. For instance, standard mechanical sheet finishes comprise unpolished or rolled finishes: No. 1; No.2D and No 2B (dull and bright respectively) and polished finishes, since called mirror finishes from No. 3 to No. 8, with a most reflective surface. No. 4 finish is a general-purpose polished finish commonly used for food equipment. In addition to the mechanically rolled and polished finishes, other types of finishes are achieved by chemical, electrochemical or thermal treatment and sometimes these techniques are applied for rehabilitation of damaged SS

SS are used extensively by the FI to ensure purity and quality of the products and cleanness and durability of the equipment. Hygiene and sanitation are basic requirements of this industry, and the metallic surfaces are cleaned, maintained or restored by manual or automated mechanical or chemical cleaning, the later referred to as ´cleaning in place´ (CIP),

The different foodstuffs have a considerable quantity of nutrients representing an ideal media for bacterial growth, which can induce corrosion processes on the metallic SS surfaces by microbiologically induced corrosion (MIC) or corrosion under acidic, salty, fatty fouling deposits or calcareous scaling from cooling water on heat transfer surfaces. The concentration of bacterial population present on a surface is the parameter to measure in order to establish the ability of such surfaces to be disinfected. Several studies have been published showing that the SS have very similar characteristics for disinfection regarding to

the service temperature must be limited to a maximum of 315 C.

with varying smoothness, brightness and light reflectivity.

**6. Cleaning and sanitation of stainless steel** 

without disassembling the equipment.

surfaces.

bacteria, fungi and algae. These microorganisms are not corrosives by themselves, but due to the impossibility to produce their nutrients by photosynthesis, they must metabolize organic content present in the media, producing other substances that are returned to the solution.

The nutrients needed include a source of energy such as glucose that supplies carbon and minerals containing phosphorous, sulfur and nitrogen for the cell structure of bacteria. The growth, development activity and death of bacteria are also influenced by the pH and concentration of some ions, which establish the best conditions. Bacteria are diverse and can live in extreme conditions such as high concentration of chlorides (halophilic), higher temperatures (thermophilic) or to realize different chemical transformations; reduce sulfates, reduce sulfides, use hydrogen or reduce nitrates, etc. Bacteria can participate in corrosion processes in different ways: changing the environment by replacing a substance by another, covering partially the metallic surface with their biofilms creating local corrosion cells or inducing the corrosion process by depolarization of hydrogen at the metal surface.

#### **5. Stainless steel in the food industry**

The corrosion resistance of SS is due to the presence on its surface of a protective, passive oxide film which is stable, tightly adherent and very thin: about 50 Å. When broken, the film regenerates itself by exposure to air or moisture. A broad summary of the utilization of various categories of SS in the FI is given in Table 2.


Table 2. Stainless steel for use in the food industry

bacteria, fungi and algae. These microorganisms are not corrosives by themselves, but due to the impossibility to produce their nutrients by photosynthesis, they must metabolize organic content present in the media, producing other substances that are returned to the solution. The nutrients needed include a source of energy such as glucose that supplies carbon and minerals containing phosphorous, sulfur and nitrogen for the cell structure of bacteria. The growth, development activity and death of bacteria are also influenced by the pH and concentration of some ions, which establish the best conditions. Bacteria are diverse and can live in extreme conditions such as high concentration of chlorides (halophilic), higher temperatures (thermophilic) or to realize different chemical transformations; reduce sulfates, reduce sulfides, use hydrogen or reduce nitrates, etc. Bacteria can participate in corrosion processes in different ways: changing the environment by replacing a substance by another, covering partially the metallic surface with their biofilms creating local corrosion cells or inducing the corrosion process by depolarization of hydrogen at the metal

The corrosion resistance of SS is due to the presence on its surface of a protective, passive oxide film which is stable, tightly adherent and very thin: about 50 Å. When broken, the film regenerates itself by exposure to air or moisture. A broad summary of the utilization of

**Characteristics/Uses Cr Ni Mo Cmax**

blades, valve trim, freezer blades.

gates.

cladding over steel.

purposes

equipment.

equipment.

of the dairy industry

**S41000** 11.5-13.5 --- --- 0.15 Hardenable by heat treatment. Turbine

**S41600** 12-14 --- --- 0.15 Easy machinable. Valve stems, plugs and

**S42000** 12-14 --- --- 0.35-0.45 Hardenable by heat treatment. Cutlery,

**S44000** 16-18 --- --- 0.6 Very hard. Pumps, plungers, gears, bearings. **S43000** 16-18 --- --- 0.2 Good corrosion resistance. Structural

**S30200** 17-19 8 --- --- Good corrosion resistance. General purpose. **S30400** 18-20 8-12 --- 0.08 Good corrosion resistance. General dairy

**S31600** 16-18 10-14 2-3 0.10 Superior corrosion resistance. The workhorse

**S31000** 24-26 19-22 --- 0.25 Heat resistance. High temperature

surface.

**SS UNS \*** 

**5. Stainless steel in the food industry** 

various categories of SS in the FI is given in Table 2.

**number Chemical composition % w/w** 

**S43100** 15-17 1.5-2.5 0.16-0.17

**Austenitic chromium-nickel steels** 

**\*UNS: Unified Numbering System** 

Table 2. Stainless steel for use in the food industry

**Martensitic and Ferritic chromium steels** 

Selection of SS is an activity that requires the participation of specialized expertise in materials technology, coupled with a sound knowledge of the chemical and physical characteristics of the food products (S.H. Zhang and B. Monitz, 2006, R.M. Davidson et al., 1987). Ferritic low carbon SS UNS S44400 has the advantage to be immune to Stress Corrosion Cracking (SCC) while its resistance to localized pitting and crevice corrosion is comparable to UNS S31600 austenitic SS. The austenitic SS group is the most popular group used for the construction of infrastructure in the FI, mainly the UNS S30400 and S31600 due to their good corrosion resistance properties. The 400 series, e.g., UNS S41000, is specified for applications such as pump impellers, plungers, cutting blades, scrapes and bearings. Corrosion resistant properties of the different SS depend on the alloying elements such as chromium, nickel and molybdenum. The duplex SS is a group that contains similar amounts of ferrite and austenite in their microstructure are increasing their use in the FI in the last years. These steels have high strength and good corrosion resistance compared with the austenitic SS, but one concern is that duplex SS tend to form brittle intermetallic phases and the service temperature must be limited to a maximum of 315 C.

In the FI, the equipment is manufactured from SS with a finely polished surface that tends to have a better corrosion resistance regarding to non-polished ones, and is easy to clean and to keep clean. A smooth surface is less susceptible to an accumulation of deposits, fouling or biofouling, which often become focal points for localized corrosion. Seamless and welded austenitic SS sanitary tubing and pipelines intended for use in the FI should have special surface finishes, as the ASTM A270 standard specifies. The finishing process results in SS with varying smoothness, brightness and light reflectivity.

The standard designations for SS finishes are classified by mill forms. For instance, standard mechanical sheet finishes comprise unpolished or rolled finishes: No. 1; No.2D and No 2B (dull and bright respectively) and polished finishes, since called mirror finishes from No. 3 to No. 8, with a most reflective surface. No. 4 finish is a general-purpose polished finish commonly used for food equipment. In addition to the mechanically rolled and polished finishes, other types of finishes are achieved by chemical, electrochemical or thermal treatment and sometimes these techniques are applied for rehabilitation of damaged SS surfaces.
