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

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), without disassembling the equipment.

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

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

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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

**COW: FAT % PROTEIN % LACTOSE % ASH % TOTAL SOLIDS %** 

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

Centrifugal pumps Tubular and plate heat exchangers

Cream separators Vessels for cleaning solutions

Centrifuges Piping and tubing equipment fittings

Brown Swiss 4.0 3.6 5.0 0.7 13.3 Ayrshire 4.1 3.6 4.7 0.7 13.1 Cebu 4.9 3.9 5.1 0.8 14.7 Jersey 5.5 3.9 4.9 0.7 15.0 Holstein 3.5 3.1 4.9 0.7 12.2 Guernsey 5.0 3.8 4.9 0.7 14.4

milk is shown in Table 3.

affecting the odor, flavor and color of milk products.

modern dairy plant is listed in Table 4.

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

Milking machines Homogenizers Vacuum pumps Pasteurizers

Milk coolers Milk dryers

Clarifiers Conveyors

Table 4. Typical stainless steel dairy equipment

Agitators and Mixers Vacuum evaporators

Bulk milk storage tanks Spray milk dryers

Bulk milk tankers CIP spray devices

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 reuse and to minimize waste discharges.

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