FST3211 Engineering Materials in Food Systems Lecture Notes 2023 PDF

**FST3211 Engineering Materials in Food Systems** **1.0 Introduction** Materials of construction are usually selected based on their strength, elasticity, hardness, toughness, sensitivity to wear, corrosion and fatigue resistance, ease of fabrication, availability and cost price. However, in the c...

**FST3211 Engineering Materials in Food Systems** **1.0 Introduction** Materials of construction are usually selected based on their strength, elasticity, hardness, toughness, sensitivity to wear, corrosion and fatigue resistance, ease of fabrication, availability and cost price. However, in the construction of food processing equipment and services, the hygienic properties of materials of construction, such as sensitivity to fouling, cleanability and inertness in contact with the food produced, are as important. To select the most appropriate materials of construction for use in either the food contact, either the non-food contact area, the equipment manufacturer must have knowledge of the physical, chemical and thermal behaviour of an as large as possible range of market available materials of construction, must be familiar with their hygiene characteristics, and must have insight in the laws, regulations, standards and guidelines applicable to the materials of construction used in the design and manufacturing of his food processing equipment. 1. **General recommendations** Materials of construction for food processing equipment, process piping and utilities should be a. homogeneous, b. hygienic (smooth, nonporous, non-absorbent, nontoxic, easy cleanable, impervious and non-mould supporting), c. inert (non-reactive to oil, fat, salt, etc.; may not adulterate the food by imparting deleterious substances to it, nor affect its organoleptic characteristics), d. chemical resistant (corrosion proof; non-degrading and maintaining its original surface finish after sustained contact with product, process chemicals, cleaning agents and disinfectants), e. physically durable and mechanical stable (resistant to steam, moisture, cold, heat; resistant to impact, stress and fatigue; resistant to wear, abrasion, erosion and chipping; not prone to cracks, crevices, scratches and pits; unbreakable) and easily to maintain. The migration of heavy metals in metalware may increase when the article comes into contact with highly acid foods (e.g., fruit juices). Whether a chemical would pose health risks to the consumers depends on its toxicity and the amount of the chemical migrated into the foodstuff, which makes that the use of inert materials for food contact is of utmost importance. Product contact surfaces (all surfaces exposed to direct contact with the product as well as indirectly impacted surfaces from which splashed product, condensate, liquid or solid particles may run off, drop off, or fall into the product) should be constructed of materials that meet the highest hygienic requirements. Within the food contact areas, no chemical substances are allowed to migrate from the surfaces of these materials into the product except for technically inevitable proportions which do not harm health, odour or taste. Materials of construction for components located in the non-food contact area may be of a lower grade but must be corrosion resistant, and able to withstand all cleaning solutions normally used to clean the outside of food processing equipment. **2. 0 Materials** **2.1 Use of metals and alloys** **2.1.1 Definition** Metals are usually characterized on the basis of their chemical and physical properties in the solid state. Metals are the class of materials linked on an atomic scale by the metallic bond, being an array of positive metallic ions forming long-range crystal lattices in which valence electrons are shared commonly throughout the structure. As such, they are strong, ductile and have high thermal conductivity. Other characteristic properties are high reflectivity, and high electrical conductivity. Metallic alloys are composed of two or more metallic elements. **2.1.2 Field of application** Both non-ferrous 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, zinc, cobalt, vanadium and carbon. The machinability of these non-ferrous and ferrous materials has a large influence on the final choice for a certain construction material in a specific application. **2.1.3 Release and migration of metal compounds** Metals and alloys are used as food contact materials, mainly in processing equipment, transportation bands, knives, containers and utensils but also in foils. They play a role as a safety barrier between the food and the exterior. However, unprotected with a coating some metals can give rise to migration of metal ions into the foodstuffs, as such becoming a contaminant that contributes to and influences the total oral intake. Even though some metals are essential elements, they may either endanger human health if the total content of the metals exceeds the hygienic recommended exposure limits, or may bring about an unacceptable change in the composition of the foodstuffs, or may deteriorate their organoleptic characteristics. Moreover, release and migration of metals is closely related to metal degradation, such as corrosion and leaching of metal elements. Hence, release and migration of metals should be reduced as low as reasonably. **2.1.4 Degradation of metals and alloys** Corrosion is a phenomenon in which, in the presence of moisture and oxygen, the metal undergoes an electrochemical reaction with components of the surrounding medium. In the simple case of uniform corrosion, this reaction results in the formation of compounds of the metal (e.g. hydroxides) on the surface of the metal. The rate at which corrosion proceeds will depend in part on the composition of the aqueous medium: corrosion of iron in very pure water will be considerably slower than in water which contains, for example, acids or salts. The rate of corrosion depends also on the solubility of the formed compounds in the medium, and their rate of removal. Formed compounds may be rapidly removed in a flowing aqueous medium, increasing the corrosion rate. In a static medium, the rate of corrosion will be moderated as the ionic concentration of the surrounding medium increases. Corrosion products formed in the atmosphere are more or less adherent. More complex corrosion patterns may occur, e.g. "pitting corrosion". This occurs following attack at discrete areas on the surface of the metal that are susceptible because of, for example, surface imperfections or impurities in the metal. Pitting corrosion is generally seen as small, local, areas of corrosion. However, there may be considerably larger areas of corrosion beneath the surface that can have significant effects on the strength of the affected metal. Rust is essentially hydrated ferric oxide which usually also contains some ferrous oxide and may contain iron carbonates and/or sulfates. Similarly, verdigris on copper containing surfaces consists mainly of basic copper carbonate, but may also contain copper sulfates and chlorides. However, rust is loose and easily removed, while verdigris forms a stable patina. Further, in the assembly of food process equipment and services, the right combination of steels, alloys or metals must be used to avoid bimetallic corrosion (Fig. 1). Work in black steel and stainless steel must always be kept separate. To protect them against corrosion, stainless steel equipment components should be fully wrapped with plastic film, and eventually their inlet and outlet connections should be fitted with protective caps (Fig. 2). Immersion tests with metal coupons or specific equipment components (Fig. 3) allow to evaluate the effect of food products, detergents and disinfectants on the materials of construction used in the manufacturing of food processing equipment. Static immersion tests of the candidate materials are rapid screening test. If the plant item is to be welded, it is prudent to subject welded coupons to similar tests, as the weld metal and heat-affected zones may have different corrosion resistances from that of the unwelded material. To assess the risk of crevice corrosion, a testing procedure that involves the use of castellated washers is often used. **2.1.5 Metals** **Copper** The best-known applications of copper are vessels, traditionally used in many breweries and distilleries. Copper is largely applied in the non-product contact area, with as main application the tubes in evaporators installed in refrigerators and freezers, electrical wiring, water pipes, etc. According to recent research, copper has shown to restrict bacterial growth. Copper does not really constitute a food safety problem but it is recommended to avoid direct food contact with copper utensils, as it can cause unacceptable organoleptic effects. Moreover, copper can be quickly and severely affected by strong alkaline detergents, sodium hypochlorite, acid and salty food, making it not really suitable in the food contact zone. The rate of attack is slow enough that alkaline detergents can be used for the cleaning of copper vessels. As copper ions may leach from the copper metal, its surface roughness may increase. Oxidation of copper gives rise to the formation of toxic copper (II) oxide. **Aluminium** For food contact purposes, anodized aluminium should be used because uncoated aluminium is attacked by acid food and alkaline detergents. The use of silicates, however, prevents alkaline attack of aluminium. The use of uncoated aluminium utensils should be limited, even if the exposure to aluminium is usually not harmful. When coated, this coating must be resistant to alkaline detergents, chlorine containing bleach, and acid and salty food. Anodizing is an electrolytic passivation process used to increase the thickness of the natural oxide layer on the surface of the aluminium. Aluminium anodized coatings are resistant to many inorganic chemicals in a range between pH 4 and 8.5 but are subjected to pitting in aerated chloride solutions. An optimum resistance to corrosion is obtained if the coating is in the thickness range of 18-30 µm. The corrosion resistance of anodized coatings can be further enhanced by sealing the pores of the coating and incorporating inhibitors. However, dichromate coatings containing hexavalent Cr(VI) shall not be used for that purpose as it is toxic. Nickel Nickel intake via foodstuff does not cause hazards for the majority of consumers but a subgroup of the population (approximately 10%, mainly women) have contact allergy to nickel. In some patients with certain types of nickel dermatitis, a flare-up of eczema through ingestion of even small amounts of orally ingested nickel is observed. Nickel usually is evenly worn off, although pitting and stress-corrosion cracking may occur. Intensive aeration and high temperatures may increase the corrosion rate. Nickel may be attacked by inorganic acids such as nitric, sulfuric and phosphoric acid, but has good resistance to alkaline media and at least at not too high temperatures it is hardly attacked by organic acids, such as vinegar, lemon, and formic acid. Phosphoric acid present in some acid cleaning agents and sodium hypochlorite may leach non-alloyed nickel very easy from pure nickel, causing damage to nickel surfaces during cleaning. Where brass components are nickel-plated, damage to that coating may release physiologically unacceptable amounts of nickel in the food and the product may come in direct contact with brass. In general, pure nickel, nickel-plated steel and nickel-plated brass in the food contact area should better be avoided. Notice that the use of stainless steel and nickel alloy utensils does not elicit an allergic reaction by nickel sensitized persons. **Zinc** Zinc is easily dissolved in diluted acids and by bases, leading to the release of zinc, but also of cadmium and lead. Zinc also reacts with steam to produce zinc oxide and hydrogen gas. Zinc in the food contact zone must be avoided, especially where wet or humid acidic foodstuffs are produced. Zinc frequently contains small amounts of the toxic metals cadmium (0.01 - 0.04%) and lead as impurities. Therefore, the use of zinc, zinc alloys or zinc galvanized consumer goods with food contact is banned in some countries. **Titanium** Titanium has been suggested to be used for corrosive or delicate liquids such as dairy products, fruit juices and in the wine industry. It is practically inert, due to the phenomenon of passivation of the titanium surface by the formation of a molecular layer of titanium dioxide. This layer, which is very adherent to the metallic substrate, is scarcely removed at high temperatures even in contact with hypo-chlorites and bleach chemicals, and highly concentrated salt and acid solutions. Titanium is resistant to crevice corrosion, and impingement and pitting attack in salt water. Titanium doesn't cause health problems, as it is generally considered to be poorly absorbed upon ingestion. Titanium dioxide is also used as white pigment in paints, lacquers, enamels, coatings and plastics. Further titanium compounds (e.g., TiCl4) are also found in plastics, as they are used as catalysts in the manufacturing of certain plastics (Ziegler-Natta catalysts for synthesis of 1-alkene polymers). Recent research has demonstrated that the combined action of UV and titanium dioxide results in photocatalytic disinfection of food products, liquids and air. **Silver** Silver is used in the production of cutlery and tableware. Chemically, silver is the most reactive of the noble metals, but it does not oxidize readily; rather it "tarnishes" by combining at ordinary temperatures with sulfur-compounds or H2S (e.g., in eggs). However, migration of silver is limited. Silver may be ingested via consumption, in e.g., silver salts used as drinking water disinfectants, and as a colouring agent for decorations of confectionary and in alcoholic beverages. Silver is also used as an antimicrobial in many elastomers, plastics and within coatings of stainless steel. **Lead, cadmium and mercury** For health reasons, lead, cadmium and mercury in food contact materials should absolutely be avoided: Lead absorption may constitute a serious risk to public health. The effects of lead poisoning, which can affect nearly every bodily system, are cumulative throughout our lifetime. It is especially dangerous to infants and young children, as well as to fetuses, because it can cause slowed developments, learning or behaviour problems and lower IQ scores. Moreover, lead can increase blood pressure and cardiovascular diseases in adults. With regards to lead, the Scientific Committee for Food adopted an opinion on 19 June 1992 endorsing the provisional tolerable weekly intake of 25 μg/kg body weight proposed by the WHO in 1986. Cadmium absorption also constitutes a risk to humans, since it may induce kidney dysfunction, skeletal damage and reproductive disorders. As regards cadmium, the Scientific Committee for Food endorsed in its opinion of 2 June 1995 a provisional tolerable weekly intake of 7 μg/kg body weight. Mercury may induce alterations in the normal development of the brain of infants and at higher levels may induce neurological changes in adults. EFSA adopted on 24 February 2004 an opinion related to mercury and methyl-mercury in food and endorsed the provisional tolerable weekly intake of 1,6 μg/kg body weight. Methyl-mercury is the chemical form of most concern and can make up more than 90% of the total mercury in fish and seafood. The maximum acceptable level of mercury for fish laid down in Commission regulation EC No. 1881/2006 is 0.50 mg/kg fresh weight. **2.1.6 Ferrous steels** **Cast iron** Cast iron is an alloy of iron, silicon and carbon. Typically, the concentration of carbon in cast iron is between 3 - 4% by weight. The corrosion resistance of cast iron is comparable to that of carbon steel; and sometimes even better. However, cast iron will quickly corrode on exposure to moisture and atmospheric air. To prohibit corrosion or to reduce the corrosion rate, cast iron is often painted or even galvanized. But even a thick coating of paint or zinc will not fully prohibit corrosion due to the cracks and holes within the coating. Its corrosion resistance to neutral and alkaline liquids (high pH) is fairly good, but it has poor resistance to acids (low pH) and salt water. However, cast iron can be alloyed with 13 - 16% (by weight) silicon or 15 - 35% (by weight) nickel (Ni-resist) to improve its corrosion resistance. Due to its sensitivity to corrosion and surface roughness, cast iron cannot be used in the food contact area of food processing equipment. Its use is limited to pump casings, steam piping, equipment frames, etc., away from the food contact area. **Mild steel** Mild steel (also called carbon steel) is made from iron and carbon (\< 1%) without addition of other "alloying elements". Carbon steel will quickly corrode on exposure to moisture and atmospheric air. It is also sensitive to acids, salt water and chlorine containing bleach, but fairly good resistant to neutral and alkaline liquids (high pH). Due to its corrosion sensitivity it cannot be used in the food contact area, but it is often used in the construction of valves for non-food applications. **Galvanized, nickel plated and painted mild steel** To retard its corrosion, mild steel is often galvanized (zinc plated), nickel plated or painted but, with time, these coatings get damaged and peels off (Fig. 4). Galvanized steel should be avoided in the product contact area (the splash area included), because any alkaline or acid product will rapidly attack and remove the galvanizing, so although it would prevent corrosion by water, it does nothing to protect metal against the detergents. The only permitted applications of galvanized steel are in contact with dry and non-acidic foodstuffs. Painted steel never shall be used in the neighbourhood of food because paints often contain zinc, lead, cadmium and phenolics. Moreover, paint can crack or flake, and steam and some cleaning agents rupture the physical integrity of paints. Paint that peels off can fall onto the product, creating a health risk. Paint surfaces used in non-product contact areas may crack or flake and should be repainted immediately. **Stainless steels** The main elements in all stainless steels are iron, chromium, molybdenum and nickel, with none of them being harmful to consumer health. Especially the austenitic chrome-nickel or chrome-nickel-molybdenum steels are used for the construction of equipment and services in the food industry. Stainless steel AISI SS 304(L) can be used for the construction of food processing equipment and service systems in applications with low chloride levels (up to 50 mg/L \[ppm\]), near neutral pH (between 6.5 and 8) and low temperatures (up to 25 ^0^C). Due to its sensitivity to sodium hypochlorite and salt that is usually present in food in high contents, the use of stainless steel AISI SS 304(L) should be limited to exterior equipment surfaces, motor and electrical cabinets, etc. Susceptibility to chloride attack is especially high if the water has an acidic pH, and can be further accelerated in the presence of oxidizing agents. **2.1.7 Nickel alloys** These alloys have a much higher nickel content showing higher corrosion resistance than the ferrous steels. The high nickel alloys are suitable in high salty and acidic environments at very high temperatures and in high stress applications. **2.1.8 Copper alloys** The copper alloys brass (60 - 70% copper, 30 - 40% zinc) and bronze (80 - 95% copper, 5 - 20% tin) are more prone to corrosion by alkaline and acidic detergents, salty and acid food than the ferrous steels. Brass is susceptible to de-zincification (in e.g., steam), and because cadmium and lead are co-elements to zinc, brass shall never be used in the food contact area. Although bronze was used in the production of cookware and utensils in prehistory and ancient times, its use in the food contact zone should be avoided because it quickly becomes porous in contact with acid foodstuffs, cleaning agents and steam. Notice however that bronze is widely accepted as material of construction for control valves in food gas cylinders, allowing to control gas pressure and flow without compromising the quality of the food gas. Copper alloys, such as brass and bronze, also exhibit antimicrobial activity, albeit of smaller magnitude. Electrical components in bronze or brass should be contained in enclosures. **2.1.9 Titanium alloys** Titanium alloys are stronger and more resistant to corrosion than the metal itself. Titanium is also used in certain so-called "stabilized" forms of stainless steels, which in general contain less than 1%. In medicine, titanium alloys are used in implants, and they have never indicated any local effects on tissues. However, the use in food contact materials is unknown. Studies on titanium alloys used in implants do not indicate any local effects on tissues \[7\]. **3.2 Use of Plastics** **3.2.1 Definition** Plastics are defined as shaped and hardened synthetic materials composed of long chain organic molecules called polymers, plus various additives. Additives are used to facilitate handling and processing (lubricants, mould-release agents, blowing agents, etc.), to change or improve various properties of the base polymer (heat stabilizers to cope with higher temperatures; fillers, reinforcing agents, fibers, impact modifiers to improve the durability; plasticizers to improve the flexibility; anti-statics to prevent electrical uploading; colouring agents for pigmenting; antimicrobials to prevent or retard microbiological corrosion) and to protect plastics from the effects of time and environmental conditions (flame retardants; antioxidants; UV stabilizers to absorb ultraviolet). **3.2.2 Release and migration of plastic compounds** As substances with a molecular weight above 1000 Da usually cannot be absorbed in the body the potential health risk from the polymer itself is minimal (Quotation: Reg. (EU) No 10/2011, whereas (8)). Unreacted residual monomers can be found in the polymeric material, several of which are hazardous for human health and/or the environment. Besides the residual monomers other polymerization impurities, such as oligomers and low molecular weight polymer fragments can be present in plastics. There are several ways to produce a polymer and the chosen method determines the use of solvents, suspension aids, surfactants, initiators, catalysts and other polymerization additives. The bulk process is carried out without solvents and gives the purest polymers, with only trace amounts of catalysts or initiators. In solution and dispersion polymerization techniques, organic solvents are used which may be toxic (e.g., carcinogenic) and flammable. The last traces may be very difficult to remove. Most polymerization additives, e.g. initiators, catalysts, chain transfer agents and suspension aids, are added only in small amounts (\< 2 wt.%). In most cases, they do not have very hazardous properties; but they can migrate from the polymer, contributing to the overall leaching of chemical substances from the polymer material. **3.2.3 Degradation processes in plastics** Similar to metals, plastics may be prone to fatigue, erosion, spherical void expansion, creep and corrosion. These phenomena cause changes in the chemical and physical properties of a certain plastic material, while metal corrosion rather manifests as a process of surface disintegration. Degradation of plastic materials rather proceeds at the inside and is usually invisible from the outside. Water absorption as a cause of plastic failure Due to their structure, some plastics may absorb water from the surrounding liquid or atmosphere and slowly diffuse into the molecular structure where it can affect chemical and intermolecular bonds. With increasing moisture content, the stiffness of the plastic decreases and the plastic becomes soft. Where swelling of a given plastic component is prohibited, internal stress may lead to premature failure of that plastic component. On the other hand, plastics with a too low water content may become prone to embrittlement. Microbiological corrosion Some plastic material can be used by microorganisms as a source of carbon and energy, especially lower molecular weight additives such as fillers, lubricants, antioxidants, stabilizers, emulsifiers, plasticizers, colourants, etc., that are released from the plastic materials. Microbial corrosion of plastics may induce changes in the properties of plastics. They may undergo discolouration, may lose their strength, and may suffer from weight reduction because of the fact that they lose their constituents. **Stress corrosion cracking** Already at relatively low strains, triggered by stresses within plastic components, plastics may become prone to stress corrosion cracking, which may become visible as hairline cracks. Highly stressed zones within plastics may arise during the exertion of loads, and as the consequence of water retention and chemical attack. Due to these stresses, polymer chains within the plastic may become overstretched and may be torn from their entangled configuration, resulting in the rupture of chemical bonds. The damage induced cannot be reversed, and remains permanent even after stress relief. In filled and fibre reinforced plastics micro cracks mainly originate and grow at the interface of the polymer matrix and fillers, which in fibre-reinforced semi-crystalline plastics occurs at the boundary of crystallites and reinforcing fibres. **3.2.4 Hygienic requirements to be met by plastics in contact with food** Apart from their different resistance to several media and their divergent behaviour to extreme physical conditions (very high and low temperatures, UV), thermoplastics and thermosets (resins) show also differences in tensile, compression and impact strength; differences in hardness, resilience and flexibility; and divergent electrical properties. For use in the food contact area, it is important that these plastics should be odourless, nonporous, smooth and free from cracks, crevices, scratches and pits which can harbour and retain soil and/or microorganisms after cleaning. Additionally, they may not absorb product constituents and microorganisms, must have high mechanical strength (withstand the mechanical shocks that are likely to occur during normal operation; be resistant to ageing, creep, brittleness, fatigue, etc.) and good wear/abrasion resistance, shall be resistant to heat, cold flow, hydrolysis, electrostatic charging, etc. Abrasion which can occur due to the transfer of solids, slurries or pastes (e.g., tomato concentrate) can damage the surface of the material and have a significant effect on the accumulation of soils, biofilm formation and cleanability. When using a plastic material (conveyor belts, gaskets, electric cables, etc.), it is of utmost importance to secure that the material is able to withstand all temperatures from -50 0C (freezer applications) to temperatures as high 121^0^C (steam sterilization) without cracking or breaking. Moreover, the plastic material must be chemical resistant to solvents, acid, alkaline, reducing and oxidising agents, cleaning and disinfection agents, and corrosive food gases at these temperatures. The equipment manufacture should test the chemical and temperature resistance of the plastic material. Plastics also may not impart any odours and taste to the food. When food components diffuse into the plastic, they subsequently may leak back out into 'later' food, causing a loss of the perceived quality of the food, such as changes in visual appearance or organoleptic qualities (sometimes called 'tainting' of the flavour). Coolants and lubricants used for the machining of plastics may not be absorbed in the plastic material. After machining (e.g., cutting) they must be sufficiently removed, although degreasing and final cleaning of plastic components is not as easy as for metal parts. Solvents used in degreasing (halogenated organic solvents in particular) may harm plastic components or get absorbed leaving residues. **3.2.5 Thermoplastics** Thermoplastic polymers soften when heated and can be reshaped, the new shape being retained on cooling. The process can be repeated many times by alternate heating and cooling with minimal degradation of the polymer structure. Thermoplastics are largely used in the construction of food processing equipment and utilities. The physical and thermal properties as well as the chemical resistance characteristics of the most commonly used thermoplastics are given in Table 2. Commonly used thermoplastics; applications, strong and weak points: ** Polytetrafluorethylene (PTFE) or Teflon**, is inert (to all known chemicals), nontoxic, nonflammable, and has a working temperature range of -270 to 260 0C. It has an extremely low coefficient of friction, and is applied as "non-stick" coating. It is used for machine packings, seals, gaskets, insulators, tubing, vessels for aggressive chemicals, coating in cookware, conveyor belt coating, mechanical and electrical bearings, insulation for coaxial cable, fixture and motor lead wire, industrial signal and control cable, etc. Polytetrafluoroethylene (PTFE) is often considered to be a potentially attractive material, because of its high chemical resistance. However, care must be taken, because it can be porous and thus difficult to clean. ** Polyvinyl chloride (PVC)** is the most widely used plastic in industry. It appears in transparent, opaque and coloured forms. It is applied as material for drains and gutters (max. 80 0C), chilled and deionised water piping, rainwater pipes, tanks, guards, conveyor belt coating, electrical conduit and trunking, electrical cable sheathing, junction boxes, etc. PVC is tough, strong, with good resistance to chemicals, good low-temperature characteristics, but does not retain good mechanical perfor-mance above 80 0C. In a food processing plant, where there is a significant amount of splashdown and where harsh cleaning agents are used daily to achieve sanitation standards, PVC is a better choice than PUR because it is more resistant to water and harsh cleaning chemicals. PVC is also cheaper than PUR and minimizes the risk of downtime due to cable failure. Serious drawbacks are the presence of low residual quantities of the cancerogeneous vinylchloride monomer and the extensive use of plasticizers such as di(2-ethylhexyl) phthalate (DEHP) which are believed to be pseudo-oestrogenic. Directive 78/142/EEC establishes a specific migration limit for unreacted vinyl chloride monomer in food of 0.01 mg/kg, and a maximum permitted quantity of unreacted vinyl chloride monomer of 1 mg/kg PVC. **Polyethylene (PE)** is a non-polar, semi-crystalline, translucent to opaque thermoplastic with very low density, that absorbs very little water. It provides a smooth, soft and very tough surface and waxy-like feeling, but its low strength and hardness limits its use as engineering material. Polyethylene is very chemically resistant but sensitive to halogen and oxidizing acids, as well as to oxygen in the presence of heat and UV light. Polyethylene allows maximum continuous services temperatures of about 60 - 90 0C, depending on the load. The lower temperature limit of use is about -50°C. A distinction is made between LDPE (low-density) and HDPE (high-density). LDPE is sensitive to grease and oil causing swelling and softening, or even stress cracking. The chemical resistance of HDPE is superior to that of LDPE, in particular towards oil and greases. Low-density polyethylene (LDPE) is applied as packaging material, plastic film, coating, pipes and fittings for drinking water and gases, domestic mouldings, electrical cable sheathing and insulation. High-density polyethylene (HDPE) is used to fabricate larger mouldings (transport and storage tanks), modular conveyor belts, sheet, tube, bearings, gears, etc. ** Polypropylene** is a semi-crystalline thermoplastic with high water resistance that absorbs no moisture. Polypropylene has considerably high hardness, stiffness and good creep, wear and heat resistance. The maximum temperature operation limit amounts 110 0C without exertion of load. The resistance to aqueous saline, acid and alkaline solutions, alcohol and solvent is good to very good. At higher temperatures, PP swells in contact with oils, greases and waxes. Polypropylene is used for packaging films, plastic containers, pipes and fittings for drinking water, drains, conveyor belt coating, modular conveyor belts, electrical cable sheet, etc. **Polystyrene (PS)** is an amorphous, clear transparent thermoplastic with low moisture absorption (approximately 0.05%), and available on the market in several opaque colours. In its unmodified form, polystyrene is a tough, dimensionally stable and highly resistant material but with rather poor elasticity. Polystyrene is not very useful as an engineering material because of its brittleness in unmodified forms. High-impact forms are obtained by compounding with butadiene or resins with a certain degree of elasticity, and heat-resistant forms are obtained by the use of fillers. Polystyrene also can be stabilized against ultraviolet radiation and also can be made in expanded form for thermal insulation and filler products. Polystyrene is suitable for operations at temperatures between -70 and 70 0C. Polystyrene is resistant to alkalis, lyes, diluted mineral acids, alcohols, water and aqueous salt solutions, but is prone to attack by steam, many solvents and strong oxidizing materials. Polystyrene is sensitive to stress cracking, so that any residual stress within the finished product must be avoided. Applications include food packaging material, refrigerator lining, holding tank and freezing containers, thermal and electrical insulation, etc. Applications include food packaging material, refrigerator lining, holding tank and freezing containers, thermal and electrical insulation, etc. **3.2.7 Thermosets** Thermosets are cured plastics, which means that polymer chains within thermosets are intensively crosslinked, so that they no longer can be softened at higher temperatures and re-shaped by heating. In their final state, they set to a rigid, hard, heat and solvent resistant solid. Thermosets are generally stronger and stiffer than thermoplastics, and have a high modulus of elasticity which is even maintained at high temperatures. With the addition of fillers, the properties of the thermoset resins can be further improved. Inorganic additives such as grinded rock powder, long glass fibres or lamellar mica may give thermosets a higher density, increased strength and thermal resistance, and reduced sensitivity to volume shrinkage. Thermosets are largely used in the construction of food processing equipment and utilities. The physical and thermal properties as well as the chemical resistance characteristics of the most commonly used thermosets are given in Table 3. **4.0 Use of Elastomers** **4.1 Definition** **Rubber** can be defined as a polymeric material which can be substantially deformed under stress, but rapidly recovers almost to its original stage when the stress is removed. It can be stretched repeatedly to at least twice its original length, but returns to its original length on release of the stress. The elastic properties of rubber are brought about by a combination of the chemical structure of the polymer backbone and the vulcanization (cure) process which brings about the formation of a lightly cross- linked three dimensional structure. Thermoplastic rubber is a polymer or blend of polymers that with respect to resilience and rapid recovery behaves similar to vulcanized rubber. However, to obtain these elastic properties, it does not require vulcanization or cross-linking during processing. Thermoplastic rubber can thus repeatedly be softened by heating to enable processing, and then regains its elastomeric character on cooling to room temperature. **4.2 Field of application** Rubber products are largely used in food processing equipment, such as for seals, gaskets, plate heat exchanger gaskets, hoses, conveyor belting, skirting, milk liners, butterfly valves, diaphragm pumps, feather pluckers, etc. **4.3 Release and migration of rubber components** During the processing of food, the contact of the food with the rubber present in conveyor belts, hoses, seals, gaskets, etc. becomes quite significant. The impact of a given rubber on the food with which it is in contact, is largely determined by the type of food, its temperature, the contact time and the contact area. Rubbers are chemically very complex, as they contain polymers, oligomers, residual unreacted monomers (left after the polymerization reaction) and additives. Some typical monomer remnants are styrene (left after the production of styrenic rubbers), butadiene (left after the production of poly-butadiene rubbers), toluene diisocyanate, methylene diisocyanate or hexamethylene diisocyanate (left after the production of polyurethane rubber) and acrylonitrile (left after the production of nitrile rubbers), all of which are toxic. Their concentrations can be critical to know, because their molecular weights are small, letting them migrate to the surface easily. Permissible limits of such species must be defined and followed carefully for food contact applications (in natural rubber, the concentration of free monomer is set at 1 mg/kg as a maximum). Oligomers that can exist in the system after completion of the polymerization process can pose similar problems. Typical additives found in rubbers are plasticizers, process aids, emulsifiers, retarders, antidegradants (antioxidants, antiozonants), curing agents, cure accelerators, antistatic agents, fillers, pigments (mostly in the form of blends), reinforcing agents, resins, biocides, etc. Hence, there are a number of potential migrating agents in the system, especially because the rubber matrix is flexible with high permeabilities. Plasticizers in rubbers are usually at high concentrations, and although concentrations of antidegradants (antioxidants and antiozonants) are much lower, they are all very critical in food contact rubber products. These intentional and unintentional compounds (which are called indirect food additives by the FDA) have the potential to migrate into the food, in the case of rubber contact with the food. There is not much of published data available on the migration of chemicals from rubber into food (or food simulants) but prepared rubber samples of natural rubber, EPDM, fluorocarbon and silicone rubber cured with sulfur (and all accelerators contain sulfur) showed characteristically high levels of extracted N-nitrosamines (suspected as carcinogens), aromatic amines (e.g., phenyl-naphthylamine) and aldehydes (mainly formaldehyde). So migratory components produced during the curing of rubber can be released subsequently. Other studies also have shown the migration of metal ions (arsenic, barium and lead). Levels of nitrosamines in rubber products are restricted \< 10 µg/kg, or as extractable nitrosamines \< 1 µg/kg. **4.4 Hygienic requirements to be met by elastomers in contact with food** Elastomers must be chemically resistant to fat, cleaning agents and disinfectants; they may not show expansion and shrinking under the influence of temperature changes or chemical fluids; they must be abrasion resistant (e.g., rotary shaft seals, or seals in static applications that are subjected to abrasion from dry material product); and they must retain their surface and conformational characteristics (no loss of elasticity, no embrittlement, no rubbed-off parts, no crevices, etc.). However, elastomers can be degraded by product, cleaning agents, disinfectants, thermal and mechanical stress, much earlier than metal components, with as results: leakage of lubricants, loss of bacteria tightness, increased adherence and retention of dirt and bacteria in crevices, insufficient cleaning and problematic disinfection. Partly destroyed sealings allow ingress of liquids containing chlorides under gaskets and seals, so that a high chloride concentration may subsist between damaged sealings and adjacent metal, which favours crevice corrosion even in stainless steel. Therefore, gaskets and seals preferably should be of a removable type. The resistant characteristics of several elastomers can be found in table 4. Vulcanization (cure) of rubbers is usually complex, and a number of different reaction products (nitrosamines and nitrosatable substances) can be produced. Besides these new (unwanted) products, certain rubber cure accelerators can be left completely unreacted in the system, such as thiurams (tetramethyl thiuram disulfide and tetramethyl thiuram monosulfides), thioazoles, sulfenamides, diphenyl guanidine and dithiocarbamates. As such, both reaction products and vulcanization remnants are potential dangers for migration in food contact applications. Especially for rubbers contacting aqueous media, breakdown product migration is more common. Migration from a food contact rubber material into food (or into food simulants) is usually expressed in two ways, either as: a) Overall migration (total extractables: mass of overall migration without considering composition of the migrant), which is FDA recommended. b) Specific migration (where the composition and quantity of migrant are of interest). Only compounds on a permitted list of additives (positive list) for food-contacting rubbers should be used Typical rubbers used in the assembly of food processing equipment and services are: Natural rubber latex is obtained from rubber trees as a milky latex fluid (with small particles of rubber dispersed in an aqueous matrix). The mechanical properties of natural rubber, such as hardness and strength, can be influenced during the vulcanization with sulfur, as well as by addition of fillers and plasticizers. The tensile strength of the basic material obtained from rubber plants is already quite excellent, and the resilience of natural rubber is only surpassed by polybutadiene rubber. Natural rubber also shows very good damping behaviour and properties that makes it very suitable for dynamic applications. However, the long term service temperature only amounts 60 - 70°C. Natural rubber resists only slightly swelling in non-polar solvents, and even less in mineral oils. Also, its ozone resistance is quite poor. Natural rubber has about 1% of allergenic fraction of proteins (e.g., α-glubulins, hevein), that can cause allergic skin reactions. It is also possible to have hypersensitivity due to the chemicals added during its processing. Natural rubber found on the market is usually of the soft type, and is commonly used for car tires, gloves, spring elements, etc. ** Silicone rubber elastomers (VMQ)** combine excellent heat resistance with good resistance to hot air, ozone, UV radiation, engine and transmission oils, animal and vegetable fats. However, silicone rubber is not resistant to high-pressure steam. Silicone elastomers are suitable for operations at temperatures between -20 and 200°C (low- and high-temperature seals). Continuous short-term use at temperatures up to 230 0C is possible. However, due to its poor abrasion resistance, silicone rubber is not applicable in dynamic applications. Silicone compounds are very clean (no odour, no taste and no discolouration), making them suitable in many food applications. Silicone rubber is mainly used for seals, O-rings, gaskets, suction cups (pneumatics), conveyor belt coating and sheathing of cables. For use as food contact material in the food industry, resolution AP (99) 3 and Resolution AP (2004) 5 of the Council of Europe must be met. ** Thermoplastic polyurethane (TPU)** elastomer has high tensile strength, high tear and abrasion resistance (the best of all rubbers) and high modulus of elasticity. The flexibility of thermoplastic polyurethane elastomer remains intact over a wide temperature range from 40 up to about 80 0C under continuous use, and also the low-temperature flexibility is excellent. In the presence of water, its properties remain unchanged up to the temperature of about 60 0C. At room temperature, thermoplastic polyurethane elastomer resists attack by diluted acids and alkalis, but it is affected by these chemicals at high temperatures. Its resistance to hot water, steam and ethylene glycol is rather poor. The weathering resistance (e.g., ozone resistance) is good. **5.0 Use of Composites** Composite materials generally comprise a matrix (e.g., thermoplastic or metal) carrying added fillers to improve lubricity, strength, thermal properties, impact resistance, etc. Strength will be improved by the addition of reinforcing materials (e.g., glass or carbon fibre) which commonly will be fibrous. They may be chopped short fibres or long fibres which may be woven. Lubricity may be modified by adding, for instance, PTFE granules, laminar minerals or metal oxides. Impact resistance may be modified by adding silicones. The compatibility of any addition with the process environment must be assured. Components can react with certain wetting agents in detergents or food substances, which can be observed as porosity, increased surface roughness, absorption of flavours and taints, or by the fact that the material turns black. It has been suggested that, if the added fibres are sufficiently small, porosity is not an issue as the pore dimensions should generally be smaller than virus sizes. Woven composites could give rise to delamination problems, making that they are not used extensively at present owing to their susceptibility to break-up. However, special cases do exist, e.g. lined composite pipes. The use of carbon fibre could also help solve the problem. 6.0 **Use of Coatings** **6.1 Coatings for specific purposes** Originally, coatings were developed to allow components to function reliably for long periods under extreme operating conditions without maintenance. But coatings are also used for many other reasons: Protection of equipment and structures from the aggressive environment (e.g., harsh cleaning chemicals and disinfectants). To reduce friction between two contacting surfaces. Control of fouling by using microorganism repellent surfaces or coatings that leach biocides. Hydrophilic coatings are used to increase the surface wettability, allowing condensed moisture to be quickly shed from the equipment surfaces. Such coatings may delay frost formation on the fins of evaporators within still air and blast freezers. To improve the visibility, by accentuating certain equipment components or services. To provide a pleasant appearance, or to facilitate easy detection of dirt. To change the light intensity. To contain broken glass fragments within the protective lamp envelope of a coated lamp. To modify the chemical, mechanical, thermal, electronic and optical properties of materials. **6.3 Hygienic requirements** Coatings, if used, shall be free from surface delamination, pitting, flaking, spalling, blistering and distortion when exposed to the conditions encountered in the environment of intended use and in cleaning, bactericidal treatment or sterilization. If the use of a coating is absolutely unavoidable, it should be of a contrasting colour that can easily be seen should a piece of it break off into the food, but the colourant used must also be food-compatible and must not adversely affect the properties of the coating material. **7.0 Use of Adhesives** **7.1 Definition of an adhesive** The "Association of the European Adhesive & Sealant Industry" (FEICA) defines an adhesive as a non-metallic substance capable of joining materials by surface bonding (adhesion by gluing) in a way that the final bond possesses adequate internal strength (cohesion). Organic adhesives consist basically of organic polymeric binders of high molecular weight, which determine their adhesiveness (adhesion) and internal strength (cohesion). These organic binders are either of natural (proteins, carbohydrates or resins) or synthetic origin (substances of the hydrogen-carbon type, and/ or compounds containing oxygen, nitrogen, chlorine, silicon and/or sulfur). Besides binders, an adhesive formulation also consists of one or more of the following additives: water or organic solvent carrier, plasticizers, biocides and fungicides for natural product adhesives, catalysts, emulsifiers, antioxidants, etc. These additives determine in particular the end use and processing characteristics. Inorganic adhesives are based on silicates, borates, phosphates or metal oxides. Either similar materials must be connected; either completely different materials must be connected, in e.g. glass with plastic or metal with ceramic material. Adhesives are in particular used for the bonding of plastics. Materials with a high surface energy (metals, glasses, ceramics and certain plastics such as PC or PVC) allow better adhesion than those with a low surface energy (rubber, silicone, PTFE, etc.). Materials with a high surface energy are held together by means of strong chemical bonds of the covalent, ionic or metallic type. Solids (rubber, silicone, PTFE, etc.) with low surface energy are held together essentially by physical forces (e.g., van der Waals and hydrogen bonds). **7.2 Hygienic requirements** An appropriate selection of the adhesive and the layers in between the food and the adhesive is needed. Adhesives must be nontoxic, and produced from substances that are generally recognized as safe for use in food. The constituents must be permitted for use in adhesives in contact with food by prior sanction or approval, and employed under the specific conditions of use prescribed by such sanction or approval. The recommended conditions of use should be communicated to the user within the Food Contact Status Declaration and/or via the technical documentation. To assure food safe usage of an adhesive, the container with finished adhesive must be provided with a label bearing the statement "food-safe adhesive". The adhesives must be resistant to products and process conditions such as temperature. The user also must keep in mind that adhesives used for keeping gaskets in place can cause localized corrosion of stainless steel if not used according to the supplier's specification. All bonds must be continuous and mechanically sound, which means that glued layers must be firmly bonded without any visible defects such as microscopic crevices that can form a niche for microorganisms and/or product residues and can hamper proper cleaning. Under normal conditions of use, the adhesive will remain firmly bonded to the base materials without visible separation. **7.3 Application of adhesives** In certain cases, the structure or microstructure of the materials to be joined must be changed in the area of contact (e.g., to join plastics they usually must be partially dissolved), while in other cases the surface structure may remain unchanged (e.g., to join ceramics and metals). In a first stage, to activate the surfaces that must be joined; the surface wetting must be improved by degreasing and cleaning with solvents, cleaning solution or steam. Quite often, also the roughness must be increased, which can be done either mechanically (e.g., by blasting or grinding), either chemically (e.g., by pickling or dissolving) or physically (e.g., heat treatment). Also ionization procedures such as coronary discharge between electrodes positioned in the air flow and pre-treatment with adhesion promoters have proven to be effective in the roughening of plastic surfaces. **8.0 Use of Other Materials** **8.1 Glass** Glass is transparent, and may occasionally be used as a food contact surface (e.g., light and sight glasses into vessels, and in very limited extent glass piping). For such applications, glass should be nontoxic (glasses containing lead are not allowed in the food contact area), integral (homogeneous and continuous), impervious, inert (nonabsorbent, resistant to degradation, and insoluble by process or cleaning fluids), smooth (free of cracks, crevices and pits), durable (robust, heat resistant, resistant to scratching, scoring and distortion when exposed to bioprocessing fluids). Glass shall be rated for the applicable pressure and temperature range, as well as for thermal shock. Bubbles at the glass surface are not accepted. Notice however, that the surface of glass is not completely smooth. It rather has a rough surface made of peaks and pit holes that can be filled with organic and inorganic contaminants. When these impurities react chemically with the glass, the glass easily may become stained and discoloured. Glass also may become prone to hydrolytic and chemical attack by certain alkali and acid solutions, which even can make the glass surface much rougher. Resistance to water and/or acid/alkaline solutions varies from excellent to poor depending on the composition of the glass. It is important to choose the right type of glass. In most cases, the use of glass is not recommended because it is also brittle, may break and cannot endure thermal cycling. Replacement of glass by transparent alternatives like Perspex (poly methyl methacrylate) or polycarbonate is recommended. **8.2 Ceramics** Properties Ceramics are produced by the fusion and hardening of mineral substances. Fired at high temperatures, they become pressure, temperature, abrasion, high-temperature corrosion and erosion-corrosion resistant. They may reduce friction and wear; but are brittle (they rather break than bend) and weaker in tension. However, by adding small amounts of long organic polymers, less brittle and less prone to fracture, more flexible organo-ceramics can be obtained. In general, ceramic materials are also very resistant to acids and sufficiently resistant against lye. Application Ceramics are more and more employed in the food industries due to their resistance to extreme operating and cleaning conditions. They are used in the coating of other stable materials, in the production of ceramic membranes and in the construction of processing equipment for very sensitive products. **Hygienic requirements** To meet all applicable hygiene criteria, ceramic materials must be/have: Inert. Nonporous and non-absorbent. To prohibit any porosity, ceramic pieces should be fired to their full maturity so that ceramic particles melt together enough to form a waterproof surface. When not correctly fired, ceramic pieces may remain porous, enough to let fluids penetrate the surface and soak into the ceramic material. Smooth, unbroken surfaces, entirely free of crazing (small hairline cracks) and blemishes, including the lip of recipients. Crazing may compromise the strength of the ceramic body, and may allow bacteria to hide in the hairline cracks. Even low doses of bacteria can become a large culture in the food. Moreover, ceramic bodies that absorb liquids via the cracks become less durable and will retain heat more if subjected to heating. Cracks also incline to release toxic materials from the ceramic body much easier. Resistant to scratching, scoring and distortion when exposed to the conditions encountered in the environment of intended use (harsh cleaning chemicals and disinfectants, sterilization). Nontoxic, free of leachable lead, cadmium and other potentially toxic heavy metals. Compounds of nickel, chrome, cobalt, antimony and manganese (most often their metal oxides) are often used, and they are known to cause health problems, especially when used in large quantities. Other metals that could be problematic are barium and lithium, which commonly are found in their carbonate form and may leach into (especially acid) foods or liquids. Any barium ingested can lead to severe abdominal pain, nausea, diarrhea, vomiting, muscle weakness, muscle cramps and heart problems, and in a worse case difficult breathing, high blood pressure, tachycardia and possible death. Lithium carbonate is fairly safe but some people take lithium carbonate for mental problems. Good thermal shock resistance must allow the ceramic piece to withstand sudden temperature changes without cracking. The ceramic material shall have good acid and alkali resistance. **8.3 Wood** As it is inexpensive and durable, wood (hard marple, ash, basswood, beech, birch, butternut, cherry, oak and American black walnut) has been a traditional material for many applications in the food industry: ice cream sticks, cutting boards, vegetable and fruit boxes, pallets, etc. In the American regulations, hard maple or an equivalently hard, close-grained wood may be used for cutting boards, cutting blocks, butcher's blocs, bakers tables and utensils such as rolling pins, chopsticks, etc. Because of the high acidity and salt content of brines in some products, which can cause severe corrosion problems even for expensive metallic construction materials such as stainless steel, fermentation tanks and storage containers in wood are still accepted in the food industry for wine, pickles and olives. However today, the usage of wood in the food industry remains under debate. The reason for the negative attitude towards wood seems to be caused by the food legislation and the interpretation thereof in Europe and elsewhere. Wood is out of grace because of hygienic and mechanical strength problems: risk of splinters, porosity of wood (promotes the absorption of blood, fat and moisture), difficulties to keep it smooth and free of cracks, difficulties to keep it clean and hygienic due to the lack of cleaning and/or sanitation methods, etc. Moreover, strong and oxidizing acids and diluted alkalis may attack wood. To avoid pest infestations and the growth of moulds with concomitant production of mycotoxins, wood in contact with food is also often treated with pesticides and fungicides. Control for the presence of residual levels of these fungicides and pesticides in the food in contact with the wood should be performed. In conclusion, the use of wood is not really recommended. On some exceptions, wood is certainly not allowed within the product contact area, and should not be exposed to the outside. It must be permanently and tightly sealed off from the product zone. **8.4 Insulation** Thermal insulation must be adequate to maintain the heat or cold within given equipment. Non-chloride-releasing insulation material should be used, that does not absorb and retain water. Styrofoam, foam glass or another rigid foam are better choices over fibrous materials. The problem with fibreglass batting is that it has already proven to be an excellent harbourage of dust, insects and rodents. The insulation should not be exposed to the outside but must be permanently and tightly sealed off from the product zone. It is highly recommended to install fully welded, vapour tight, plastic, aluminium or stainless steel cladding of appropriate thickness that resists tear and abrasion. The exterior of the insulation protection should be smooth, and installed in a correct way to avoid dust traps, for example, joints should face downwards. It should be impossible to walk on the insulation during maintenance. Pipe cladding should be applied in both dry and wet areas. Asbestos shall never be used as insulation material as it may cause lung and peritoneum mesothelioma. **8.5 Nanomaterials** The use of nanomaterials in the food industry may present potential risks, requiring the need for risk assessments to identify and quantify these risks. Some nanoparticles have been found to exhibit negative effects on tissues such as inflammation, oxidative stress and signs of early tumour formation. **8.6 Antimicrobial compounds** The European Hygienic Engineering & Design Group clearly states that materials which have been modified with antimicrobial chemicals may not be considered as a substitute for hygienic design or cleaning/disinfection practices. Microorganisms may build up resistance against such chemicals over a period of time, and antimicrobial chemicals may gradually leach out of the surface material with time. Antimicrobial nanoparticles may create adverse health effects as shown in several scientific papers.