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Types of Soils:

In order to develop a cleaning plan, the nature of the soil to be cleaned must be understood. There are several classifications of soil systems and food soils are generally complex as they involve many food components which have very different chemical compositions. Overheating of carbohydrate soils is not recommended as caramelization products of the sugars and starch glues can form. Protein soils are most effectively cleaned with an alkali detergent. Mineral deposits are soluble in acid solutions, and are most commonly formed from hard water or milk.

Additionally, the age of the soil and the temperature at which it was created influence its ability to be cleaned. Burton has described the differences in milk soils derived from pasteurization and those from ultra-high temperature. Pasteurization milk soils are 50-60% protein (50% of that protein composed of BLG), while high temperature milk soils are 70% mineral and 15-20% protein. The soil characteristics are also vastly different: a soft soil as compared to a brittle soil. Knowledge of the components of a soil to be cleaned should be understood in order to develop the most effective cleaning protocol.

Food Processing Surfaces:

Food processing surfaces are composed of different materials and thus present a different variable to the equation of sanitation. It is best to use surfaces which are non-absorbent, non-corrosive, non-reactive with the product produced, and cleanable. In addition to choosing surfaces that are easier to clean, it is also important to design and select processing equipment that is easy to clean.

The surfaces used in a given food plant will be specific to their desired function. The following surfaces were found in food plants by the American Institute of Baking: stainless steel, plastics (polyethylene, ultra-high molecular weight polyethylene, polycarbonate, PVC and vinyl), rubber, glass, wood, and cloth. Similarly, stainless steel, plastic (polyethylene, polypropylene or polycarbonate), ceramic, rubber, sealed concrete, coated cast iron, and air filter material were listed as food environmental surfaces of interest in AOAC guidelines on the validation of microbiological methods.

Stainless steel is generally considered one of the best food processing surfaces due to its smooth, non-porous surface that is easily cleaned. Many types of stainless steel exist, but AISI 304 and AISI 316 are relevant to the food industry. AISI 316 is used more frequently as it can tolerate higher levels of halides (such as Cl found in salty foods and chlorinated cleaning solutions). The surface roughness should be 0.8 µm or smoother, otherwise adaptations to the cleaning protocol may be necessary to adequately clean the rougher surface.

Plastics have many uses in food production such as storage vessels, hoses, and covers. The main sanitation concern with plastics is that they can be porous and absorb portions of the food product; additionally, the plastic monomers may leach into the food. Plastics recommended for food use based on their ability to be cleaned include polypropylene, polyvinylchloride, acetal copolymer, polycarbonate, and high-density polyethylene.

Rubber’s uses in food production are mainly for seals, gaskets, and joint rings. The different properties of rubber are based on the long, repetitive molecular chains, called elastomers, that are the constituents of rubber. The recommended choices of rubber include EPDM (ethylene-propylene-terpolymers), nitrile rubber, NBR (acrylonitrile-butadiene-rubber), silicon rubber, and fluoroelastomer. The rubber choice is dictated by its desired function, as some are compatible with high temperatures and others are not oil and fat resistant.

Other materials such as ceramics, glass (plastic coated) and wood are used for specific and specialized uses.

Cleaning Mechanism:

The mechanism of cleaning is composed of four factors: time, mechanical action, concentration, and temperature. As these four factors are adjusted, it may be possible to decrease the other factors. As cleaning time is increased, generally through soaking, the other factors may be able to decrease. When mechanical action increases, the soil will be physically removed in a shorter time. Temperature can be increased throughout certain ranges and the rate of cleaning will be increased, but knowledge of the soil is necessary as high temperatures with proteins can make the soil harder to clean. The concentration of the cleaning chemical can be increased to an extent and this will decrease the amount of time spent cleaning. Additionally, higher cleaner chemical concentrations will aid in removing and suspending the soils to contribute to a more efficient rinse.

Together, these factors provide the input to accomplish cleaning. More specifically, cleaning involves breaking the cohesive forces that bind a material (soil-soil) and the adhesive forces between the soil and the surface (soil-surface). The proportion of adhesive and cohesive forces in a product is dependent on its chemical composition and soil characteristics. For example, tomato paste can generally be removed easily by overcoming the adhesive force between the surface and the soil. This enables pieces of soil to be removed because not as many cohesive forces have been broken. Soils in which the cohesive bonds mainly break will still leave residue on the surface. Protein removal from surfaces is gradual because the soil is dissolved from the surface. The adhesive forces between the soil and surface are stronger than the cohesive forces in milk soils.

The stages of cleaning and soil removal have been investigated and described as following:

  • Possible bulk reaction between components of the chemical and the bulk fluid
  • Transport of chemical to the surface, affected by temperature, concentration and flow.
  • Transport into the deposit: penetration of chemicals into the deposit is dependent on its structure. Surface active agents can increase penetration due to wetting.
  • Reaction between the deposit and cleaning chemical include melting, mechanical break-up, wetting, swelling, desorption, emulsification, hydrolyzation, saponification and dispersion.
  • Transport to the interface: reaction products diffuse out of the deposit.
  • Transport to the bulk: concentration gradients and hydrodynamic conditions allow the transport of the reaction products into the bulk.

Cleaning for allergen removal is focused on proteins. The cleaning mechanism for proteins and allergens is similar to the general scheme of cleaning described above, but specific measures are taken in response to the soil characteristics, namely protein. The stages of protein cleaning have been described as follows: 1) swelling stage, the native protein reacts to form an open protein matrix; 2) uniform stage, the rate of cleaning is constant and the deposits are removed through surface shear and diffusion; 3) decay stage, protein matrix breaks down into a non-uniform layer and the deposits are removed through shear stress and mass transport.

The swelling initially begins as the soil is rinsed with water and the protein absorbs the water. Furthermore, alkaline solutions containing hydroxyl anions react with the protein and subsequently the protein swells, dissolves and is suspended. Sometimes chlorine is also added in the form of a chlorinated alkaline detergent which additionally helps to break down the proteins and minimize mineral deposits. An acidic cleaner is not initially used for proteins because it will precipitate the protein and adhere it to the surface, making it much more difficult to clean and remove.

Detergents and Cleaning Solutions:

Detergents and cleaning solutions are a minor component (5%) in the cost of cleaning, but impart a large impact on the efficacy of cleaning. An ideal cleaning agent is able to dissolve readily in water, rinse freely, be compatible with other components, penetrate soils, emulsify fats, suspend precipitates, hydrolyze proteins, and comply with regulations. The choice of cleaner will be based on the properties of the soil to be cleaned.

Water is of course a main component of all cleaners. The other components of cleaning solutions can be divided into two categories, physically active ingredients and chemically active ingredients. The physically active ingredients alter the physical characteristics of the soil such as solubility or colloidal stability, while the chemically active ingredients modify soil components to make them more soluble.

Surfactants are a physically active ingredient. The hydrogen bonds in water are disrupted by the polar heads of surfactants. This action decreases the surface tension of the water droplet and enables it to moisten a greater surface area, thus penetrating more soils and surfaces, and increasing the cleaning action.

The chemically active ingredients include alkaline solutions, acidic solutions, and water conditioners. The alkaline options include sodium or potassium hydroxide, and sodium, potassium or ammonium salts of phosphate, silicates or carbonates. The alkaline detergents aid in protein dissolution. Sodium hydroxide alone is hard to rinse from surfaces, but the addition of wetting agents may help. Acidic components aid in the dissolution of mineral deposits and in the food industry are generally used in periodic cleans. Water conditioners include sequestering and chelating agents which assist in the prevention of mineral deposit accumulation. This prevention occurs through the formation of soluble complexes with Ca and Mg which also helps regulate water hardness.

Wet cleaning methods  

Wet cleaning methods include clean in place, clean out of place, foam or gel cleaning and manual or hand cleaning. CIP allows the equipment to stay assembled while a normal stepwise cleaning process occurs including rinsing, caustic wash, rinsing, acid wash, and a final rinse. When using a COP method, the equipment is disassembled and placed into a cleaning vat capable of heating and recirculation. When using a foam or gel cleaning method, the solution is directly sprayed onto the soiled surface. Manual cleaning involves disassembling equipment and then physically brushing and cleaning the equipment. Aspects of wet cleaning programs for allergen removal have been studied on lab-scale, pilot plant and industrial levels.

Roder, et. al studied cleaning of equipment after processing hazelnut cookie dough on a pilot and industrial scale. Their cleaning methods were based on manual cleaning with subsequent additions of hot water rinses and dish detergent. After these cleaning cycles were completed, cookie dough without hazelnut was processed on the equipment and samples at different equipment sites such as the spiral kneader, rotary molder and wire-cutting machine were sampled and tested with a hazelnut ELISA. When manual cleaning alone was used, higher amounts of hazelnut were found at the wire- cutter and thus product push-through would not be an effective method at this site. They did find that the excess hot water rinses were effective in reducing hazelnut residue in subsequent non-hazelnut cookies. The addition of detergent was not found to additionally decrease hazelnut residues. It is important to identify an appropriate cleaning procedure, although wet cleaning of commercial baking equipment is not generally recommended.

Dry cleaning methods

Dry cleaning methods include vacuuming, sweeping, scraping, wiping with cloths or brushes, and compressed air. There is not a lot of research currently about dry-cleaning methods and their implications for allergen removal. A study by Jackson et al investigated Sani-Wipes sanitizing wipes and vacuuming for allergen removal. Slurries of peanut flour, NFDM, whole egg powder, soy flour, soy milk, and soy-based infant formula were applied to urethane, stainless steel, and Teflon and then baked for one hour at 80°C. The surfaces were then vacuumed or wiped with the sanitizing wipes and swabbed for testing with ELISA, ATP, and total protein. Positive results of allergenic resides were found after vacuuming; this method may not be effective for allergen removal. The sanitizing wipes were found to clean the surface effectively.

When using vacuuming or compressed air, it is important to consider potential cross-contact of airborne allergen particles to other processing lines during cleaning. It has been found that brushing and compressed air can move dry powders a significant distance.

Validation and Verification of Cleaning Methods:

Validation and verification of cleaning methods for allergen removal are a complex and unique situation with many variables due to the allergen, food matrix, processing equipment, cleaning methods, and detection methods available. Validation is defined as the “process of assuring that a defined cleaning procedure is able to effectively and reproducibly remove the allergenic food from the specific food processing line or equipment”. Verification is the process of “demonstrating that validated cleaning protocols have been properly performed once the commercial manufacture of a product begins”.

Acceptance Limits

When validating and verifying cleaning methods, an acceptable measure of cleanliness or acceptance limits of the allergen must be used to determine when the equipment or food product is effectively cleaned. Sometimes the level of cleanliness is determined by the sensitivity level of the detection method. The detection limits of analytical methods are not necessarily practical or justifiable levels for cleaning in the context of a food allergic response. Recently, advances in determining threshold levels for various allergenic foods have been made. The threshold values give information about the amount of food that provokes an allergic reaction; the information is gathered through double-blind challenge studies of allergic populations. These threshold values or reference doses will enable informed decisions throughout risk-assessment after cleaning and could be applied to decisions about precautionary allergen labeling as well.

Insights into applications of cleaning threshold levels can be obtained from the pharmaceutical industry, which has employed these methods for some time. It is recommended that all cleaning limits of active pharmaceutical ingredients be “practical, achievable and justifiable” and based on toxicity data and acceptable daily intakes.

Sampling by Swabbing

Developing an effective sampling plan can be a statistical exercise similar to finding the needle in a haystack. It is generally best to start with “problem areas” such as gaskets, corners, and hard to reach places that may not have been fully cleaned. The physical act of swabbing these selected locations should be approached with a methodical manner to decrease subjective differences between different users (116). The swab should be absorbent and have minimal particulates, while also able to release swabbed residue into the extraction solution. It is recommended that the swab head be moist but not saturated prior to testing. The method of swabbing is not always specified, but a cross- hatch procedure that covers the area in two different directions is recommended. An AOAC guideline for microbiological methods recommends swabbing a 1” x 1” area, while some lateral-flow device kits recommend swabbing a larger area. When developing and validating a swabbing protocol, it is important to determine the % recovery or swabbing efficiency of the swab. A study by Schlegel et al. investigated direct sampling methods of peanut solutions from a stainless-steel surface (80). Using a flat zapped-head foam swab, they found that the first swab had a recovery of 68%, and by swabbing the same area with a second swab, the total recovery increased to 93% of peanut proteins applied to the surface.

Detection Methods for Cleaning Assessment

Common detection methods for food allergens have been previously described. Options such as visual cleanliness, lateral flow devices, ELISA, ATP, and general protein swabs have also been used to detect allergens after cleaning by swabbing food processing surfaces or testing final product. When choosing a detection method, it is important to consider if it can detect allergen residues at the level of cleanliness that is desired. Interferences of residual cleaning solution or product matrix should be considered.

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