**1. Introduction**

90 Food Industrial Processes – Methods and Equipment

Yamamoto, F. & Cunha, R.L. (2007). Acid gelation of gellan: Effect of final pH and heat

Yu, H.; Sabato, S.F.; D'Aprano, G. & Lacroix, M. (2004). Effect of the addition of CMC on the

517-527, ISSN 0144-8617

(October 2004), pp. 131-135, ISSN 0969-806X

treatment conditions. *Carbohydrate Polymers*, Vol. 68, No. 3, (November 2006), pp.

aggregation behaviour of proteins. *Radiation Physics and Chemistry*, Vol. 71, No. 1-2,

Chemical disinfection of industrial facilities in food bioprocessing is a major consumer of biocides and represents an essential technological issue. Due to this requirement, the metallic surfaces of the equipments used in bioprocessing inevitably interact with electrolyte environments that are exposed to washing and disinfecting solutions with or without microorganisms through on electrochemical mechanism (Landoulsi et al., 2008; Osarolube et al., 2008). Recently the electrochemical behaviour of stainless steel surfaces have become interested to the many researchers (Hiromoto & Hanawa, 2006; Stoica et al. 2010a). However, only a few research studies were devoted to the electrochemical behaviour of stainless steels used in bioprocesses having a synergic effect on biocides and microorganism (Stoica et al., 2009; Stoica et al., 2010b). The electroanalytical techniques used in previous studies through discharge of an electric field can generate some chemical and physical processes, reversible or irreversible because of the fungi present in the environments (Shen et al., 2008 ; Yang et al., 2008) and on metallic surfaces. These processes are strongly influenced by many factors, such as: biological factors (microorganisms type; cell wall; size and shape of the cell; cells density, arrangement and cell position, fluid medium properties in medium conductivity, electric field waveforms and the number of electric pulses (Yang et al., 2008 ). The complex phenomena occurring in the electrochemical biocide-fungi-metallic surface system are studied as electrochemical interface processes occurring at the limit between molecules of aqueous solution (biocide solution) coming into contact with the metallic electrode 'live' (fungal cell membranes) and the metallic surface (AISI 304 Stainless Steel). The electron transfer in these electrochemical systems respects the general laws of charge transfer, but also presents the specific properties based on the dynamic environment in which the electron transfer occurs, at the processes of adsorption/desorption and at the surface reactions. These can occur between molecules of biocide and biological surfaces as well as between biocide penetration through these surfaces endowed with distinct architecture, composition and characteristics. The study of these complex processes requires a multidisciplinary approach regarding the metallic surfaces, fungi, biocides and electrochemical processes interface. The aim of this chapter is to systematically present the relevant aspects about the interface electrochemical processes on metallic surfaces with fungi and biocides. A relevant study on the electrochemical behaviour of the AISI 304

Electrochemical Behaviour of AISI 304 Stainless Steel

**2.1.1 Ferritic stainless steels**

**2.1.2 Martensitic stainless steels**

**2.1.3 Austenitic stainless steels**

outside, in case the insulation material gets wet.

**2.1.4 Duplex stainless steels**

2507.

2006).

for knives.

Immersed in Mixtures Consisting by Biocide and Fungal Suspensions 99

93

The ferritic Stainless Steel is a magnetic type possessing a microstructure which is primarily ferritic. There are more economical from a cost point of view, but they have limited corrosion resistance compared to the more used austenitics. Similarly the ferritics have limited toughness, formability and weldability in comparison to the austenitics. An example of ferritic grades is AISI 430 used widely for applications in cutlery, kitchen sinks and catering/gastronomy industry and enclosured equipments, where the corrosion resistance requirements are not so demanding due to material economic advantages (Carvalho et al.,

Martensitic stainless steels are similar in structure to the ferritics, but by the addition of more carbon, they can be hardened and strengthened by heat treatment in the same way to carbon steels. They are classified as a "hard" ferro-magnetic group. Their corrosion resistance is inferior to that of austenitic stainless steels, therefore they are generally used in mildly corrosive environments. The martensitic grades commonly used are AISI 403, AISI 410 and AISI 420 and they are widely used for cutting and grinding applications, especially

Austenitic stainless steel is non-magnetic (*i.e.*, has a low "permeability") and has excellent ductility, formability and toughness, even at cryogenic temperatures. Depending on the nickel content the austenitics respond to cold working by increases in strength, which can surprisingly be useful in severe forming operations, avoiding premature tearing and cracking. The most representative austenitic grades are AISI 304 and AISI 316. Most stainless steel containers, pipeworks and food contact equipments are manufactured from the most representative austenitic grades either 304 or 316 type austenitic stainless steels. They are widely used in food processing, beverage industry (Mai et al., 2006) and others: bulk storage and transportation and many other applications. For example the sugar, starch and wine industry requires equipments with good corrosion resistance and thus have adopted the AISI 304 and AISI 316 stainless steel. Beer is produce using raw materials like water, barley, hops, malt by fermentation, filtration, canning and sterilization process. Beer, wort and mashed grain is generally not corrosive to stainless steel such as 304, even though the process vessels and pipe systems during brewing operate from low temperatures up to the boiling point. In sections with temperatures above 60°C, there is a risk of chloride-induced stress corrosion cracking, often from the

The duplex stainless steels have a balanced or mixed structure of austenite and ferrite and as a result have characteristics of both "basic" types. Just like the ferritics, they are ferromagnetic with a good formability and weldability as the austenitics. In adition, the duplex stainless steels have the general corrosion resistance similar to or better than that of AISI 304 and 316 (Tavares et al., 2010). Examples of duplex grades are AISI 2304, AISI 2205, AISI

Stainless Steel in mixtures consisting by biocide and fungal suspensions is presented. The results show that there is a synergic effect between the active substances from the disinfectant and fungal suspensions and the applied electric potential during tests, thus this effect can be taken into consideration in the food bioprocessing safety.

#### **2. Theoretical aspects regarding the stainless steel, some factors that influence the hygiene of food contact surfaces and metallic surface corrosion**

A variety of materials are used in the construction and fabrication of different food equipments. Various metals as well as non metals (*e.g.* plastics, rubber) are used as materials depending on the applications and because of the high demands imposed on the corrosion resistance and the hygiene the food processing industry. These materials vary in their properties regarding their workability, compatibility with type of food product, processing conditions and sanitary design features, depending the applications. The stainless steel is the obvious choice of better material for equipments used on food contact surfaces (Holah & Thorpe, 1990; Saikhwan et al., 2006). This choice is due to the corrosion resistance of stainless steels coupled with their strength and durability, their ability to be readily cleaned and sterilized without deterioration using a wide range of cleaning/sterilizing systems, and the fact that they impart neither color nor flavor to foodstuffs and beverages.

#### **2.1 Stainless steel categories**

There are more than 70 standard types of stainless steel and many special alloys, fabricated by multiple processing steps which modify their properties. These steels are produced in the rough form (AISI - American Iron and Steel Institute - types). Generally, stainless steels are mainly iron based with 12% to 30% chromium, up to 22% nickel and minor amounts of carbon, copper, molybdenum, selenium and titanium. The AISI designation for these materials is well known with the number series 300 referring to austenitic stainless steels and the 400 series covering the ferritic and martensitic stainless steels. These stainless steels can be classified into distinct categories including ferritic, martensitic, austenitic and duplex stainless steels (SS) (Figure 1).

Fig. 1. The stainless steels categories

### **2.1.1 Ferritic stainless steels**

98 Food Industrial Processes – Methods and Equipment

Stainless Steel in mixtures consisting by biocide and fungal suspensions is presented. The results show that there is a synergic effect between the active substances from the disinfectant and fungal suspensions and the applied electric potential during tests, thus this

A variety of materials are used in the construction and fabrication of different food equipments. Various metals as well as non metals (*e.g.* plastics, rubber) are used as materials depending on the applications and because of the high demands imposed on the corrosion resistance and the hygiene the food processing industry. These materials vary in their properties regarding their workability, compatibility with type of food product, processing conditions and sanitary design features, depending the applications. The stainless steel is the obvious choice of better material for equipments used on food contact surfaces (Holah & Thorpe, 1990; Saikhwan et al., 2006). This choice is due to the corrosion resistance of stainless steels coupled with their strength and durability, their ability to be readily cleaned and sterilized without deterioration using a wide range of cleaning/sterilizing systems, and

There are more than 70 standard types of stainless steel and many special alloys, fabricated by multiple processing steps which modify their properties. These steels are produced in the rough form (AISI - American Iron and Steel Institute - types). Generally, stainless steels are mainly iron based with 12% to 30% chromium, up to 22% nickel and minor amounts of carbon, copper, molybdenum, selenium and titanium. The AISI designation for these materials is well known with the number series 300 referring to austenitic stainless steels and the 400 series covering the ferritic and martensitic stainless steels. These stainless steels can be classified into distinct categories including ferritic, martensitic, austenitic and duplex

**2. Theoretical aspects regarding the stainless steel, some factors that influence the hygiene of food contact surfaces and metallic surface**

the fact that they impart neither color nor flavor to foodstuffs and beverages.

effect can be taken into consideration in the food bioprocessing safety.

**corrosion**

92

**2.1 Stainless steel categories**

stainless steels (SS) (Figure 1).

Fig. 1. The stainless steels categories

The ferritic Stainless Steel is a magnetic type possessing a microstructure which is primarily ferritic. There are more economical from a cost point of view, but they have limited corrosion resistance compared to the more used austenitics. Similarly the ferritics have limited toughness, formability and weldability in comparison to the austenitics. An example of ferritic grades is AISI 430 used widely for applications in cutlery, kitchen sinks and catering/gastronomy industry and enclosured equipments, where the corrosion resistance requirements are not so demanding due to material economic advantages (Carvalho et al., 2006).