**7. Food industry and corrosion**

The corrosion processes occurs due to different factors or conditions that define the corrosive characteristics of the media in which the SS elements of the FI are exposed. To illustrate the corrosion characteristics of the FI, dairy, beverage and canned food products have been selected and are described as follows:

#### **7.1 The dairy industry**

Dairying has been an agricultural practice before the dawn of history. Milk and its derivatives are a vital part of the diet for human beings. The dairy industry (DI) collect, treat and distribute milk that is used as feedstock for the manufacturing of other products particularly cheese, butter, yogurt and creams. The corrosion control in DI plants depends on the nature of these products and the processes involved during their production. DI has a large capital investment in infrastructure, which is exposed to a mild corrosive environment. Nevertheless, a considerable effort is expended to reduce corrosion losses by selecting corrosion resistant alloys (CRA´S), in particular SS and by the implementation of preventive, corrective and preferably, predictive maintenance. At first, milk looks as a simple white liquid, but in fact, is a complex emulsion/suspension mixture of lactose, whey proteins, fat globules, minerals and vitamins among others, in aqueous solution. The fats are present as an emulsion of tiny globular particles, while casein, the most abundant protein combines

aluminum, rubber, polyester, etc., (J.T. Hola and R.H. Thorpe, 1990). Cleaning is a mechanochemical operation for sanitation and disinfection of the machinery used for the food processing, to prevent contamination of products and damage to the SS surfaces. The CIP system evolved from recirculation cleaning solutions in tubing, pipelines and equipment to an update automatic system with actuated valves, controls and timers. The easiness in which SS can be cleaned allows fast turnarounds of tankers, coolers, pumps, heat exchangers and pipes, particularly during high production cycles. The SS alloys resist corrosive attack from chemical specialties formulated for cleaning and sanitation, such as chlorine, alkalis, mixed acids, organic ammonium quaternaries, halogenated hydrocarbons and detergents. To obtain better luster finishes, soft clothes or sponges are used in order to avoid scratching highly reflective surfaces. After cleaning, the surfaces are rinsed with hot water, dried and exposed to air to rebuild the protective, passive oxide layer of SS. Mechanical conveyors constructed of SS for moving and transportation of food raw materials and processed products containers are easy to maintain clean and sanitized. Water or food steam cleaning and visual inspection can verify that all products residue has been removed and that surfaces do not become stained, corroded or abraded. Currently environmental concerns are resulting in some modifications of CIP systems to allow water

Materials scientists at the University of Birmingham, UK, made stainless steel surfaces resistant to bacteria in a project funded by the Engineering and Physical Sciences Research Council. By introducing silver or copper into the steel surface (rather than coating it on to the surface), researchers developed a technique that not only kills bacteria but the surface is very hard and resistant to wear and tear during cleaning. This type of modified steels could

The corrosion processes occurs due to different factors or conditions that define the corrosive characteristics of the media in which the SS elements of the FI are exposed. To illustrate the corrosion characteristics of the FI, dairy, beverage and canned food products

Dairying has been an agricultural practice before the dawn of history. Milk and its derivatives are a vital part of the diet for human beings. The dairy industry (DI) collect, treat and distribute milk that is used as feedstock for the manufacturing of other products particularly cheese, butter, yogurt and creams. The corrosion control in DI plants depends on the nature of these products and the processes involved during their production. DI has a large capital investment in infrastructure, which is exposed to a mild corrosive environment. Nevertheless, a considerable effort is expended to reduce corrosion losses by selecting corrosion resistant alloys (CRA´S), in particular SS and by the implementation of preventive, corrective and preferably, predictive maintenance. At first, milk looks as a simple white liquid, but in fact, is a complex emulsion/suspension mixture of lactose, whey proteins, fat globules, minerals and vitamins among others, in aqueous solution. The fats are present as an emulsion of tiny globular particles, while casein, the most abundant protein combines

reuse and to minimize waste discharges.

**7. Food industry and corrosion** 

**7.1 The dairy industry** 

be used in the FI and kitchens (ASM News, 2011).

have been selected and are described as follows:

with calcium ions and remains dispersed in the liquid phase as a colloidal suspension. Fermentation processes of milk are possible due to its content of sugar in the form of lactose, which used as a nutrient by bacterial cultures naturally presents in the milk. The bacteria *Lactococcus lactis* and *Lactococcus cremoris*, convert lactose into lactic acid during souring. The minerals presents are calcium and phosphorous and the vitamins are fat soluble (A and D) and water soluble (B1, B2, B12, C and M), these last are unstable when milk is heated modifying the corrosive conditions of the media. The composition of different cow species milk is shown in Table 3.

Milk is fairly neutral in reactions, although lactic acid is present; the lactic acid content is increased by natural souring or by the artificial souring necessary for cheese and butter manufacture. This is perhaps the only constituent of milk which is responsible for metal attack. For a critical FI, such as the DI, the equipment and accessories must be constructed with a smooth, impervious, corrosion – resistant material which can be cleaned easily. Obviously the choice material that fulfills these requirements is the SS. It will be important to take care and to follow the adequate procedures in the use of SS alloys, in order to avoid affecting the odor, flavor and color of milk products.


Table 3. Composition of milk obtained from different cow breeds.

The metallic alloys to be used in the DI operations where the milk and its products will be in contact with them, must have low toxicity and physiological tolerance, corrosion resistance to milk, products and other chemical food additives and cleaners, and availability of the desirable profiles in the market at acceptable costs. The main SS equipment installed in a modern dairy plant is listed in Table 4.


Table 4. Typical stainless steel dairy equipment

blenders, filters, working tables, packaging and vending machines (J. Beddoes, 1999, H.S.

367

From a point of view of general corrosion resistance aspects, beer and their raw materials are in the pH range from 4 to 5. The S30400 SS low carbon content and stabilized variants are resistant to this pH range under cold and fully aerated conditions and are used for fermenting vessels, grist hoppers, holding tanks and the piping accessories, fabricated with welded sheet or plate. Under conditions of hot or boiling liquids the environment turns more aggressive because of the higher temperature and the depletion of the oxygen content. This inconvenient is solved by the choice of a more strongly passivating SS; S31600 is used for the mash tuns, whirlpool separators and heat exchanger plates in the wort cooler. SS30400 is also satisfactory for casings used to hold the mineral lagging used as isolating

The SCC is a common cause of corrosion failure in vessels handling hot liquids and it can occur due to external or internal sources. The external source situation appears when chloride ion is leached from the lagging by condensing water vapor and the SS surface has not an appropriate anti-chloride coating or interposed vapor sealing barriers of aluminum foil between successive layers of lagging. In the case of internal source, the chloride ions are contained in the water used for brewing or residues of disinfectants and from acids used in descaling. The chlorides become concentrated locally after thermal cycling or evaporation, which is a well known mechanism for SCC. The internal source of SCC can be controlled by deionizing the water and prohibiting the use of hypochlorite solutions as disinfectants. In the wine industry, bisulfites are widely used to prevent non desirable biochemical process. The most corrosive agent found in wines is the sulfur dioxide that forms sulfurous acid. Sulfur dioxide is usually present in wine either bound to carbonyl or unsaturated compounds and/or phenol derivatives or free, as HSO3- and SO2 (Araújo C., et. al., 2005, Ruiz-Capillas C., et. al., 2009). It is added to wine during its production as small amounts of sulphur dioxide (Preservative 220) or potassium metabisulphite (Preservative 224). Sulfur

3. Destroy the enzyme that causes enzymatic browning in juice (similar to what happens to apples when they are sliced and exposed to the air). Without SO2, wine would likely be brown or amber in color, smell oxidized (or have a sherry-like aroma), and probably

When sulfur dioxide is added to wine it is rapidly converted into bisulphite ion (HSO3-). Approximately half quickly combines with other wine constituents and is bound within complexes formed with aldehydes, ketones, phenolics, etc. The remaining sulfur dioxide/bisulfite is thus free in solution and it is this free portion that is readily available as an antioxidant. If oxygen is dissolved into a wine the sulfur dioxide levels will be reduced as they oxidize to sulfates (SO42- and HSO4-). If SO2 is greatly in excess, it can also produce a pungent aroma in white wines, considered by most to be a fault. The aroma is best described as that of a match that has just been struck. Many people have trouble smelling the sulfurous aroma of SO2, but instead perceive it as an irritation of the membranes of the nose. High SO2 can also render the palate of the wine harsh, metallic

element to conserve the heat in hot zones of these vessels.

dioxide provides three important properties in winemaking:

2. It helps to protect wine from the deleterious effects of oxygen.

1. SO2 has antiseptic qualities.

and frequently bitter.

be ruined by bacterial spoilage.

Kathak, 2002).

Electric energy is used for heating in small equipment, and for instrumentation and control of process machinery. Steam and hot water required for heating, pasteurization, sanitation and production of evaporated and dried milk is supplied by boilers. A wide variety of equipment for cooling, pasteurization, homogenization, transportation, etc, is fabricated from austenitic SS mainly USN S30400 and S31600. SS tubing and piping of different nominal diameter and wall thickness fitted with standard SS fittings are widely used for moving milk and its products throughout the plant.

The standards and specifications for design, construction, operation and sanitation of equipment for producing, handling and processing milk products have been formulated jointly by the International Association of Milk, Food and Environmental Sanitarians, the United States Public Health Service and the Dairy Industry Committee. These three associations Sanitary Standards are recognized and accepted as the official guides for the sanitary design of equipment for the DI. For the surfaces not in contact with the product the SS are preferred over other less expensive materials, nevertheless, if for economical reasons, other metals are used in the proximity to SS, precautions must be taken to avoid galvanic corrosion problems. Most of the SS are passivated and are less reactive than the majority of the engineering metals and can stimulate attack (D.E. Talbot and J.D. Talbot., 2007)

#### **7.2 The beverage industry**

A great variety of beverages includes beer and wine made by fermentation; distilled spirits requiring alcoholic fermentation and distillation, and soft drinks, carbonated and non carbonated, consisting of water flavored with sweeten natural or artificial sweetened syrup. The fabrication of beer requires barley, hops, yeast, and water as main raw materials. A series of by-products result from the processed yeast for health foods, animal feed from spent barley and the used hope is used as a fertilizer.

Beer is produced by fermentation of germinated barley and wine of the fermented juice of grape. Soft drinks are non-alcoholic beverage; they include mineral water or treated water containing sweetening agents, edible acid, flavors and sometimes fruit juices. Carbon dioxide gas gives the beverage its sparkle and tangy taste and prevents spoilage. The production of these beverages involves the use of great quantities of water in the cleaning, storage and bottling procedures. Many beverages are acidic and aggressive to carbon steel requiring corrosion –resistant SS equipment. In addition, the wet, damp and high humidity conditions contribute to plant corrosion and premature equipment failure. Application of SS will prevent the occurrence of these noxious corrosion events.

At the beginning, breweries were small places with a low production volume and their operations were carried out in wooden vessels and the first metallic components used were made form carbon steel and copper. Nowadays, many of the plants for beer, wine or sweetened beverages are large and most of the equipment is manufactured from UNS S30400 and include tanks, piping, plate coolers, pasteurizers and conveyors for bottle and can filling. Sometimes scaling occurs with chloride content in the hard, salt deposits. Remedial action involve acid treating of the water to remove dissolved carbonates and cleaning with mild acids such as phosphoric or citric to remove dissolved carbonate scaling. SS is the main material for fabrication equipment of the beverage industry infrastructure to avoid the formation corrosion products that could contaminate the beverage and affect its taste. The producing equipment include tanks, vessels, steamers pasteurizers, coolers,

Electric energy is used for heating in small equipment, and for instrumentation and control of process machinery. Steam and hot water required for heating, pasteurization, sanitation and production of evaporated and dried milk is supplied by boilers. A wide variety of equipment for cooling, pasteurization, homogenization, transportation, etc, is fabricated from austenitic SS mainly USN S30400 and S31600. SS tubing and piping of different nominal diameter and wall thickness fitted with standard SS fittings are widely used for

The standards and specifications for design, construction, operation and sanitation of equipment for producing, handling and processing milk products have been formulated jointly by the International Association of Milk, Food and Environmental Sanitarians, the United States Public Health Service and the Dairy Industry Committee. These three associations Sanitary Standards are recognized and accepted as the official guides for the sanitary design of equipment for the DI. For the surfaces not in contact with the product the SS are preferred over other less expensive materials, nevertheless, if for economical reasons, other metals are used in the proximity to SS, precautions must be taken to avoid galvanic corrosion problems. Most of the SS are passivated and are less reactive than the majority of

the engineering metals and can stimulate attack (D.E. Talbot and J.D. Talbot., 2007)

A great variety of beverages includes beer and wine made by fermentation; distilled spirits requiring alcoholic fermentation and distillation, and soft drinks, carbonated and non carbonated, consisting of water flavored with sweeten natural or artificial sweetened syrup. The fabrication of beer requires barley, hops, yeast, and water as main raw materials. A series of by-products result from the processed yeast for health foods, animal feed from

Beer is produced by fermentation of germinated barley and wine of the fermented juice of grape. Soft drinks are non-alcoholic beverage; they include mineral water or treated water containing sweetening agents, edible acid, flavors and sometimes fruit juices. Carbon dioxide gas gives the beverage its sparkle and tangy taste and prevents spoilage. The production of these beverages involves the use of great quantities of water in the cleaning, storage and bottling procedures. Many beverages are acidic and aggressive to carbon steel requiring corrosion –resistant SS equipment. In addition, the wet, damp and high humidity conditions contribute to plant corrosion and premature equipment failure. Application of SS

At the beginning, breweries were small places with a low production volume and their operations were carried out in wooden vessels and the first metallic components used were made form carbon steel and copper. Nowadays, many of the plants for beer, wine or sweetened beverages are large and most of the equipment is manufactured from UNS S30400 and include tanks, piping, plate coolers, pasteurizers and conveyors for bottle and can filling. Sometimes scaling occurs with chloride content in the hard, salt deposits. Remedial action involve acid treating of the water to remove dissolved carbonates and cleaning with mild acids such as phosphoric or citric to remove dissolved carbonate scaling. SS is the main material for fabrication equipment of the beverage industry infrastructure to avoid the formation corrosion products that could contaminate the beverage and affect its taste. The producing equipment include tanks, vessels, steamers pasteurizers, coolers,

moving milk and its products throughout the plant.

spent barley and the used hope is used as a fertilizer.

will prevent the occurrence of these noxious corrosion events.

**7.2 The beverage industry** 

blenders, filters, working tables, packaging and vending machines (J. Beddoes, 1999, H.S. Kathak, 2002).

From a point of view of general corrosion resistance aspects, beer and their raw materials are in the pH range from 4 to 5. The S30400 SS low carbon content and stabilized variants are resistant to this pH range under cold and fully aerated conditions and are used for fermenting vessels, grist hoppers, holding tanks and the piping accessories, fabricated with welded sheet or plate. Under conditions of hot or boiling liquids the environment turns more aggressive because of the higher temperature and the depletion of the oxygen content. This inconvenient is solved by the choice of a more strongly passivating SS; S31600 is used for the mash tuns, whirlpool separators and heat exchanger plates in the wort cooler. SS30400 is also satisfactory for casings used to hold the mineral lagging used as isolating element to conserve the heat in hot zones of these vessels.

The SCC is a common cause of corrosion failure in vessels handling hot liquids and it can occur due to external or internal sources. The external source situation appears when chloride ion is leached from the lagging by condensing water vapor and the SS surface has not an appropriate anti-chloride coating or interposed vapor sealing barriers of aluminum foil between successive layers of lagging. In the case of internal source, the chloride ions are contained in the water used for brewing or residues of disinfectants and from acids used in descaling. The chlorides become concentrated locally after thermal cycling or evaporation, which is a well known mechanism for SCC. The internal source of SCC can be controlled by deionizing the water and prohibiting the use of hypochlorite solutions as disinfectants.

In the wine industry, bisulfites are widely used to prevent non desirable biochemical process. The most corrosive agent found in wines is the sulfur dioxide that forms sulfurous acid. Sulfur dioxide is usually present in wine either bound to carbonyl or unsaturated compounds and/or phenol derivatives or free, as HSO3- and SO2 (Araújo C., et. al., 2005, Ruiz-Capillas C., et. al., 2009). It is added to wine during its production as small amounts of sulphur dioxide (Preservative 220) or potassium metabisulphite (Preservative 224). Sulfur dioxide provides three important properties in winemaking:


When sulfur dioxide is added to wine it is rapidly converted into bisulphite ion (HSO3-). Approximately half quickly combines with other wine constituents and is bound within complexes formed with aldehydes, ketones, phenolics, etc. The remaining sulfur dioxide/bisulfite is thus free in solution and it is this free portion that is readily available as an antioxidant. If oxygen is dissolved into a wine the sulfur dioxide levels will be reduced as they oxidize to sulfates (SO42- and HSO4-). If SO2 is greatly in excess, it can also produce a pungent aroma in white wines, considered by most to be a fault. The aroma is best described as that of a match that has just been struck. Many people have trouble smelling the sulfurous aroma of SO2, but instead perceive it as an irritation of the membranes of the nose. High SO2 can also render the palate of the wine harsh, metallic and frequently bitter.

Corrosion Network. SS for food processing industry is reported by the International

369

The Nickel Development Institute (www.nidi.org) presents info on the use of SS and conducts workshops on related subjects. The SS Appeal (SSA), which groups SS equipment suppliers, launched a website (www.stainlessappeal.com) to promote the application of SS. The International Association of Milk, Food and Environmental Sanitarians (www.iamfes.org) provides food safety professionals worldwide with a forum for exchange of information on sanitation practices. Many producers of SS and fabricators of SS processing equipment are listed in the Stainless Steel World Buyer's Guide published in the

The National Sanitation Foundation (NSF) in the USA is best known for its role in the development of standards for equipment, primarily in the food service area. The NSF's materials and finishes guide refers to three zones: the food zone, the splash zone and the non-food zone. The NSF stipulates that the food zone "surface materials shall be smooth, corrosion resistant, non toxic, stable and non absorbent under use conditions. They shall not impart odor, color or taste, nor contribute to the adulteration of the food. Exposed surfaces

Corrosion control in the FI involves three different methods to reduce corrosion: design

The variety of structures: tanks, heat exchangers, cooling systems, hygiene control systems, conveyors, cans, fillers, etc., used in the FI, should be designed to provide functional

 It is necessary to avoid risks by underdesign or excessive costs due to overdesign considerations. Knowledge, technical information, and creative engineering principles must be used in an intelligent way to avoid corrosion. There are three important aspects in general to remember when designing a system for demanding service (P. Roberge, 2008,

Cathodic protection and corrosion inhibitors are also employed to protect some installations for water supply, cooling system and underground tanks and pipelines. Normally they are applied as a dual anti-corrosion system together with coatings, linings

Corrosion resistance is the main property to be considered in the choice of materials for such plant, but the final selection must be a compromise between technological and economic factors. It is sometimes more economical to use high-priced SS that will provide long and trouble-free service than to use a lower priced material that may require frequent

Although the materials selection by trial and error frequently works, it does not always lead to an optimal or innovative decision. The evolution of computer and software has

Association of Food Industry Suppliers (www.iafis.org).

considerations, materials selection and protective coatings.

SS World Journal, the Netherlands.

shall be easily cleaned".

**8.1 Design considerations** 

Avery et al., 1992):

**8.2 Materials selection** 

maintenance or replacement.

or paints.

qualities along to all the process steps.

1. Design for complete and free drainage

2. Eliminate or seal weld service 3. Make it easy to inspect.

In addition citric, tartaric, tannic, acetic and malic acids are also found in wines. SS is corrosion resistant to all of them in the concentration ranges in which they usually are found in wine fabrication steps (B.J. Connolly, 1971 in ASM).
