MATERIALS OF CONSTRUCTION
Construction materials for open processing equipment should be as hygienic (smooth, nonabsorbent, nontoxic, easily cleanable, impervious and nonmold supporting), as chemical resistant (nondegrading and maintaining its original surface finish after sustained contact with product, process chemicals, cleaning agents), as physically durable and mechanically stable (resistant to steam, moisture, cold, the actions of cleaning and disinfecting agents, abrasion and corrosion resistant, resistant to chipping, unbreakable), and as easy to maintain (Hauser et al., 2004a; Partington et al., 2005) as possible. Table 6.1 gives an overview of the corrosion durability of the most frequently used materials in the construction of open equipment.
Use of Metals and Alloys
Nonferrous and ferrous metals and alloys are used in the construction of equipment and services for the food industry. Alloys for food contact may only contain aluminum, chromium, copper, gold, iron, magnesium, manganese, molybdenum, nickel, platinum, silicon, silver, tin, titanium, cobalt, vanadium, and carbon (EDQM, 2013). The austenitic chrome-nickel or chrome-nickel-molybdenum stainless steels are used for the construction of open equipment, as well as ancillary support systems in the food industry. Because AISI SS 304/304(L) suffers from some corrosion over a long time period, especially in the presence of chloride (e.g., salt, sodium hypochlorite), stainless steel AISI SS 316(L) is commonly used as construction material for food processing equipment. Galvanized steel should be avoided in the product contact area (the splash area included), as the zinc coating may peel off and is easily dissolved in diluted acids and bases, releasing zinc and traces of cadmium and lead. Painted steel (Fig. 6.12) should never be used in contact with food, because paints can crack or peel off. Brass, bronze, and copper quickly react with strong alkaline detergents, sodium hypochlorite, acid, and salty food, making them not suitable in the food contact zone. Aluminum is attacked by alkaline detergents, sodium hypochlorite, and acid food, but anodized aluminum is acceptable in the food contact area (Moerman & Partington, 2014).