the effect of neutralizing the buffering action of any alkaline components in the brine:

2NaOH + C02 Na2C03 + CaCl2

Therefore, the pH of the brine decreases and assumes a value of about pH 6.5, which is the region where pitting incidence is highest. Furthermore, scale deposits of calcium carbonate are laid down on heat-transfer surfaces, creating the problems referred to above.

The precautions to be observed when using brine circuits are

• Ensure correct pH control and maintain in the range pH 9.5-10.

• Eliminate aeration. In particular, make certain that the brine return discharge line is below the surface in the storage tank during running and that the method of feeding brine from the tank does not cause vortexing with resultant air entrainment. Baudelot type evaporators cause aeration of the chilling liquor and should never be used on brine circuits.

• When cleaning and sterilizing the brine section of a pasteurizer, flush out all brine residues until the rinse water is free of chloride. As an added precaution, it is advisable to form a closed circuit and circulate a 1/4-1/2% caustic soda or sodium metasili-

cate solution to ensure that any brine residues are rendered alkaline.

• In plate heat exchangers and similar equipment, make sure that stainless-steel brine section components remain free of scale.

• When shutting down the plant after a cleaning run, it is advisable to leave the section full with a dilute (1/4%) caustic solution. Before startup, this should be drained and residues flushed out prior to reintroducing brine.

When operating conditions prevailing in a plant do not permit such a disciplined cleaning, operating, and shutdown procedure, only two materials can be considered for the brine section of a plate heat exchanger. These are Hastelloy C-276 or titanium. The corrosion resistance of both of these materials is such that cleaning and sanitizing of the product side of the heat exchanger can be carried out without removing the brine. Although both are more expensive than stainless steels, especially Hastelloy C-276, the flexibility of plant operation which their use permits could offset their premium price.

Alkaline Detergents

Supplied to food processing plants either as bulk shipments of separate chemicals or as carefully preformulated mixtures, the composition of alkaline detergent formulations can vary widely in accordance with individual preference or the cleaning job to be done. The detergents do, however, generally include some or all of the following compounds:

Sodium hydroxide

Sodium polyphosphate

Sodium metasilicate

Sodium carbonate

Additionally, it is not uncommon to find that a selection of sequestering agents such as EDTA and any of the many available wetting agents also may be present in the formulations.

None of these compounds are corrosive to stainless steels at the concentrations and temperatures used by the food industry for cleaning. Indeed, 316 stainless steel is unaffected by concentrations of sodium hydroxide as high as 20% at temperatures up to 160°C (320°F). They can, therefore, be used with impunity at their usual maximum concentration of 5%, even in ultra-high-temperature (UHT) operation where temperatures can rise to 140°C (284°F).

Companies have reported that some of these prefor-mulated alkaline detergents cause discoloration of the equipment.

The discoloration starts as a golden yellow, darkening to blue through mauve and eventually black. It has been established that this discoloration is caused by the EDTA sequestering agent, which complexes with traces of iron in the water. It then decomposes under certain conditions of pH and temperature to form an extremely fine film of hydrated iron oxide, the coloration being interference colors that darken as the film thickness increases. Although the film is not aesthetically pleasing, it is in no way deleterious and removing it by conventional cleaning agents is virtually impossible.

Some alkaline detergents are compounded with chlorine release agents such as sodium hypochlorite, salts of di- or trichlorocyanuric acid that form a solution containing 200-300 ppm available chlorine at their usage strength. Although the high alkalinity reduces the corro-sivity of these additives, generally speaking they should not be employed on a regular basis at temperatures exceeding 70°C (160°F).

Acidic Detergents

Alkaline detergents will not remove the inorganic salts such as milkstone and beer-stone deposits frequently found in pasteurizers. For this, an acidic detergent is required and selection must be made with regard to their interaction with the metal. As was shown earlier, sulfuric and hydrochloric acids will cause general corrosion of stainless steels. Although it could be argued that sulfuric acid can be employed under strictly controlled conditions because stainless steels, especially grade 316, have a very low corrosion rate, its use could result in a deterioration of surface finish. This, in corrosion terms, is an extremely low rate but from an aesthetic viewpoint, is undesirable.

Acids such as phosphoric, nitric, and citric, when used at any concentration likely to be employed in a plant cleaning operation, have no effect on stainless steels and can be used with impunity. Three cautionary notes are worthy of mention:

• It is always preferable to use alkaline cleaning before the acid cycle to minimize the risk of interacting the acid with any chloride salts and, therefore, minimize the formation of hydrochloric acid.

• It is inadvisable to introduce an acid into a UHT sterilizing plant when it is at full operating temperature (140°C/285°F) as part of a "clean-on-the-run" regime.

• Nitric acid, which is a strong oxidizing agent, will attack certain types of rubber used as gaskets and seals. As a general guideline, concentrations should not exceed 1% and temperature 65°C (150°F), although at lower concentrations, the upper recommended temperature is 90°C (195°F).

Another acid that is finding increasing use in the food industry for removing water scale and other acid-soluble scales is sulfamic acid. Freshly prepared solutions of up to 5% concentration are relatively innocuous to stainless steels but problems may arise when CIP systems incorporating recovery of detergents and acids are employed. Sulfamic acid will undergo hydrolysis at elevated temperatures to produce ammonium hydrogen sulfate

NH2S020H + H20 - NH4HS04

which behaves in much the same way as sulfuric acid. In situations where the use of this acid is contemplated, prolonged storage of dilute solutions at elevated temperature is inadvisable, although at room temperature the hydrolysis is at a low rate.

Sanitizing Agents

Terminology for the process of killing pathogenic bacteria varies from country to country. In Europe, disinfection is preferred; in America, sanitizing. Regardless, the term should not be confused with sterilization, which is the process of rendering equipment free from all live food spoilage organisms including yeasts, mold, thermophilic bacteria, and most importantly, spores. Sterilization with chemicals is not considered to be feasible and the only recommended procedure involves the circulation of pressurized hot water at a temperature of not less than 140°C (285°F).

For sanitization, while hot water (or steam) is preferred, chemical sanitizers are extensively used. These include noncorrosive compounds such as quaternary ammonium salts, anionic compounds, aldehydes, amphoterics, and potentially corrosive groups of compounds that rely on the release of halogens for their efficacy. By far, the most popular sanitizer is sodium hypochlorite (chloros), and this is probably the one material that has caused more corrosion in food plants than any other cleaning agent. For a detailed explanation of the corrosion mechanism, the reader is referred to an article by Boulton and Sorenson (17) that describes a study of the corrosion of 304 and 316 stainless steels by sodium hypochlorite solutions. It is important, therefore, if corrosion is to be avoided, that the conditions under which it is used are strictly controlled. For equipment manufactured from grade 316 stainless steel, the recommended conditions are:

• Maximum concentration—150 ppm available chlorine

• Maximum temperature—room temperature that is well in excess of the minimum conditions established by Tastayre and Holley to kill Pseudomonas aeruginosa (18).

In addition, several other precautions must be observed:

• Before introducing hypochlorite, equipment should be thoroughly clean and free of scale deposits. Organic residues reduce the bactericidal efficiency of the disinfectant and offer an artificial crevice in which stagnant pools of hypochlorite can accumulate.

• It is imperative that acidic residues be removed by adequate rinsing before introducing hypochlorite solutions. Acid solutions will react with hypochlorite to release elementary chlorine, which is extremely corrosive to all stainless steels.

• The equipment must be cooled to room temperature before introducing hypochlorite. In detergent cleaning runs, equipment temperature is raised to 80-85°C (176-185°F), and unless it is cooled during the rinsing cycle, a substantial increase in temperature of the disinfectant can occur. An important point, frequently overlooked, is that a leaking steam valve can cause a rise in the temperature of equipment even though it theoretically is shut off.

• After sanitizing, the solution should be drained and the system flushed with water of an acceptable bacteriological standard. This normally is done by using a high rinse rate, preferably greater than that used in the processing run.

While these comments relate specifically to the sanitizing of plate heat exchangers, similar precautions must be taken with other creviced equipment. Examples include manually operated valves that should be slackened and the plug raised to permit flushing of the seating surface. Pipeline gaskets also should be checked frequently to make sure that they are in good condition and not excessively hardened. Otherwise they will fail to form a crevice-free seal over their entire diameter. Where it is not possible to completely remove hypochlorite residues such as in absorbent gland packing materials, hot water is preferred.

All the foregoing relate specifically to sodium hypochlorite solutions but other sanitizing agents that rely on halogen release, such as di- and trichlorocyanuric acid, should also be used under strictly controlled conditions, including such factors as pH.

Iodophors also are used for sanitizing equipment. These are solutions of iodine in nonionic detergents and contain an acid such as phosphoric to adjust the pH into the range at which they exhibit bactericidal efficacy. This group of sanitizers is employed where hot cleaning is not necessary or on lightly soiled surfaces such as milk road tankers, and farm tanks. Extreme caution should be exercised with this group for, although used at low concentrations (50 ppm), prolonged contact with stainless steel can cause pitting and crevice corrosion. Furthermore, in storage vessels that have been partially filled with iodophor solutions and allowed to stand overnight, pitting corrosion in the head space has been observed as a result of iodine vaporizing from the solution and condensing as pure crystals on the tank wall above the liquid line. Another factor is that iodine can be absorbed by some rubbers. During subsequent processing operations at elevated temperatures, the iodine is released in the form of organic iodine compounds, especially into fatty foods, which can cause an antiseptic taint. The author knows of one dairy that used an iodophor solution to sanitize a plate pasteurizer to kill an infection of a heat resistant spore-forming organism. The following day, there were over 2000 complaints of tainted milk. CIP cleaning cycles did not remove the antiseptic smell from the rubber seals, and complete replacement with new seals was the only method of resolving the problem.

Another sanitizing agent that is assuming increasing popularity, especially in the brewing industry because of its efficacy against yeasts, is peracetic acid. As such, per-acetic acid will not cause corrosion of 304 or 316 stainless steels, and the only precautionary measure to be taken is to use a good quality water containing less than 50 ppm of chloride ions for making up the solutions to their usage concentration. Because of the strongly oxidizing nature of some types of peracetic acid solutions, deterioration of some types of rubber may occur. A recent survey undertaken by the IDF for the use of peracetic acid in the dairy industry (19) found few corrosion problems reported. The general consensus of opinion was that it permitted greater flexibility in the conditions of use, compared with sodium hypochlorite, without running the risk of damage to equipment.

For comprehensive information on the cleaning of food processing equipment, albeit primarily written for the dairy industry, the reader is referred to the British Standards Institute publication BS 5305 (20).

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