by E. R. Partington, Consultant to the Nickel Institute
Presented at the 4th European Stainless Steel Science and Market Congress, Paris, France, June 10-13, 2002. (Presented here with permission from the Association Technique de la Siderurgie Française.)
Whatever the engineering application, the process of selecting materials inevitably involves a compromise between physical and chemical properties, ease of fabrication, durability, availability and cost, but when that application is the manufacture of equipment intended for the preparation or processing of foods and beverages - at any stage 'from farm to fork' - there is an additional factor which must be taken into consideration. Both the materials with which the food will come into contact and the design of the equipment must be 'hygienic' - the materials must not contaminate the food, and the design must allow the equipment to be maintained in a clean state.
Now the European Hygienic Engineering and Design Group (EHEDG) has produced a new guideline that offers practical advice on the selection of hygienic materials and designs for food-processing equipment.
The EHEDG is a consortium of equipment manufacturers, food industries, research institutes and public health authorities, and was founded in 1989 with the aim of promoting hygiene during the processing and packing of food products. It advises CEN, the European federation of standardization institutes, on the design of safe and hygienic machinery and it has already produced an extensive series of guideline documents covering matters such as the welding and passivating of stainless steels, the hygienic design of equipment, and methods for testing whether equipment complies with these criteria.
EHEDG has brought together from throughout Europe experts in a range of materials disciplines to compile an addition to this series. The Guidance Document "Materials for Construction of Equipment in Contact with Food," to which the Nickel Development Institute is a major contributor, for the first time offers in one document practical advice to designers and fabricators to assist them in the selection of the most suitable and cost-effective material for a defined application. And with the Council of Europe's Committee of Experts introducing a Safety Evaluation Scheme to assess and authorise all materials proposed for food contact, such information will become increasingly valuable to equipment manufacturers.
This new Guideline covers a range of both metallic and non-metallic materials. Its advice on the selection of metallic materials is considered here.
The Guideline looks at both the engineering and the hygiene aspects of materials selection, but its first message is that 'engineering' and 'hygiene' should not be seen as separate entities. They will frequently inter-relate - for instance, welding will alter surface finish which will, in turn, affect both the adhesion of process soils and the cleanability of the surface. Moreover, the document is not, and cannot be, a universal specification. But what it does offer is general guidance on the materials factors which should be considered when designing such equipment and the methods that are available to test the performance of candidate materials.
So, what is the route that it recommends for selecting the most appropriate materials and fabricating them into hygienic food-processing equipment? The starting-point must be detailed definitions of both the product and the process in terms of their chemistry, stresses, times and temperatures, including those which may result from, for instance, vibration or 'water-hammer' or thermal shock, all of which are common in food-processing plant. Then the detergents and sanitising chemicals required to remove process soils and bring the plant back to an acceptable microbiological standard may introduce additional physical or chemical demands.
But it is not only these 'normal operating conditions' for which the plant and equipment must be designed. "Food Processing Machinery Basic Concepts, EN 1672-2: 1997" states that "Hazards shall be eliminated, or the associated risks reduced, by ensuring machinery is properly designed, constructed and capable of being properly installed, operated, cleaned and maintained."
Increasingly, Hazard Analysis and Critical Control Point (HACCP) analysis is being used to anticipate, evaluate and control industrial hazards. T(e basic principle of HACCP is that the operators of any commercial process should actively seek in their procedures all potential 'hazards' to health, safety and hygiene, and they should pre-empt failure by designing against them. Obviously a 'hazard' could be an infection resulting from poor design or lack of hygiene, but it could also be the complete failure of a component due to tensile stress or impact fracture, or its degradation due to wear or erosion or corrosion or fatigue.
This means that the designer and the operator are increasingly obliged to work together to understand both the materials and the operating environment better and the major purpose of the new EHEDG document is to provide practical support at this important stage of the design process.
Materials suppliers can make a major contribution. As engineers and fabricators become more skilled in anticipating the pitfalls of using the wrong materials they should begin to ask the right questions, firstly of themselves and then of their suppliers:
"What are the actual environmental conditions which we are asking these materials to meet?
"What materials and fabrication methods will do the job best?"
And, given such information, the supply industry should have a much better chance of helping them to meet those requirements first time. I have a "rogue's gallery" of unfortunate component failures from the UK drinks industry, each of which was very costly to rectify and each of which would have been avoided if a document like the new EHEDG Guideline had been available at the time. Armed with its information on how normally reliable materials may behave if subjected to conditions outside the boundaries of their capabilities, designers and constructors could have involved the materials supply experts who would have been able to help them to avoid the problems in the first place.
It took many years for brewers to realise that the aluminium aircraft alloys originally used to make light, strong kegs for their beer, despite being lined with epoxy resins to protect the aluminium from attack by the product, were corroding dangerously simply because careless handling of the kegs during transportation was breaking up the protective linings inside the kegs where no-one ever looked.
Pitting corrosion of weld area in aluminium alloy beer keg (above).
They also failed to appreciate in this instance that welds and heat affected zones will have quite different physical structures, maybe chemical compositions and almost certainly surface finishes from the parent plate, and may be more susceptible to corrosion.
Today, S30400 stainless steel has almost universally superseded aluminium for the manufacture of beer kegs, just as it has replaced these brass components from which the same food product - beer - leached both the zinc and the lead (which the brewers did not know had been added to improve its machinability) into the product.
Brass component de-zincified by beer (above).
This stainless steel tube (photo below) was a part of the dispense mechanism inside a keg of cider. All oxygen had been excluded from the keg and replaced by carbon dioxide, and metabisulphite had been added to the cider in order to preserve it.
However, another component (photo below) encircled the tube and constantly abraded it, damaging the protective oxide film of the stainless steel tube so that the acidity of the cider eventually perforated it, allowing the gas on the outside of the tube to leak through it.
There was a simple remedy - coating with a synthetic material the ring which had been abrading the tube (photo below).
Where the flange at the top of the same cider keg tube contacted another stainless steel component, there was a very fine gap and so both parts suffered significant crevice corrosion. Again, introducing a synthetic material between the two so that the flange of the tube bedded into it and eliminated the crevice overcame the problem and enabled the operators to retain all the benefits of the strength, cleanability and long life of the stainless steel tube.
Naturally, in terms of both engineering and hygiene, some of the early stages of food preparation, such as in the orchard or the abattoir, are less demanding and involve fewer hazards than the later stages of cooking and packaging, and so low grade materials, such as painted mild steel, may be adequate for them, but after these initial stages, maintainable hygiene quickly becomes critical. This places special constraints on the design process.
According to EC Directive 98/37/EC, machinery can only be regarded as having been hygienically designed and constructed if those components which are to come into contact with the product can be cleaned before initial use and then, by applying an appropriate cleaning and disinfection programme after each use, restored to an acceptable level of cleanliness, economically and in a reasonable time. Unless it can be demonstrated that they can be cleaned by CiP (Cleaning in Place) all components which have surfaces which come into contact with food must be easily dismantled.
All surfaces, including their joints, must be smooth and must have neither ridges nor crevices which could harbour organic materials. Projections, edges and recesses should be reduced to a minimum. Components should preferably be assembled by welding or continuous bonding. The inside surfaces of pipes and vessels must have curves of a radius sufficient to allow thorough cleaning. There should be no 'back-waters' where soils can accumulate and become a problem to remove and no drain traps which can retain process or cleaning fluids. Cleanability is critical to hygiene.
Essential to cleanability is a superior surface finish. Generally, the rougher a surface is, the more readily soils will adhere to it and the more difficult it will be to clean, particularly by CiP which may not be able to apply the mechanical action which can help to remove process soils. The surface roughness of sheet or tube, usually expressed by Ra values, is relatively easy to measure where the surface is accessible, but is frequently more important inside pipes and vessels, which are less easy to inspect.
Moreover, whilst some proprietary polishing techniques produce a clean-cut surface others produce a damaged surface containing micro-cavities, tears and laps which can be difficult to clean. So the current European steel supply standard EN 10088-2: 1995 calls for "transverse Ra <0.5 µm with a clean-cut surface". However, the Ra value alone can be misleading.
For some years, an Ra value of 0.8 µm or less has been seen as acceptable for food processing plant, but cleaning efficiency is strongly influenced by the way it is performed and EHEDG Document No. 17 "Hygienic design of pumps, homogenisers and dampening devices" recognises that higher washing liquid velocities may allow surfaces with a roughness of up to Ra = 3.2 µm to be cleaned acceptably.
Electro-polishing can typically reduce the roughness by half by reducing peaks and rounding them off, and a similar effect can be achieved on stainless steels by new post-rolling brushing processes which leave a super-smooth surface that can often be cleaned without physically- or chemically-aggressive detergents. The ability to retain this advantage over a life-time of use is also valuable, and here the abrasion resistance of a material becomes important.
Surfaces subjected to wear or abrasion will, as they become less smooth, become more difficult to clean. In the processing of dry foods, the friction as they pass along pipes can cause significant abrasion of the bore. Over time this can lead not only to roughness and the resultant cleaning problems but also to very localised rises in temperature which can encourage microbial growth. If the food passing along the pipe is not dry but a fast-moving fluid containing abrasive particles, the internal surface can suffer erosion, particularly where there are any projections or features such as weld-runs to increase turbulence. Turbulence-induced pressure waves can also cause cavitation by creating bubbles and then causing them to collapse violently, damaging the surfaces of, for instance, impellers or valve-seats.
Stainless steel's inherent hardness maintains the smoothness which enables it to resist the adhesion of soils and biofilms and renders it very easy to clean and sanitise. Indeed, stainless steel has been proved in clinical tests to be significantly more hygienic than other food-contact surfaces. Claims have, of course, been made that other (non-ferritic) materials offer strong antibacterial properties by effectively poisoning virulent pathogens, so having to depend less upon the smoothness and abrasion resistance of their surfaces, but stainless steel remains hygienic without imparting any of its constituents either to pathogens or to the food being prepared.
Stainless steel's inherently smooth surface has an additional, but less obvious, advantage. When used for vertical surfaces in kitchen areas (photo below) it forms a most effective barrier up which vermin cannot climb, significantly restricting their freedom to spread throughout a building. For this reason, stainless steel is increasingly being used to face the vertical surfaces of vehicle loading-bays in factories as these are favourite entry-points for mice.
Smooth stainless steel surfaces (above) can resist the spread of vermin.
Whilst there is no one grade of stainless steel which meets every demand of the food-processing industry, the stainless steel 'family' provides such a range of properties that almost every individual need can be met. The demands of strength, corrosion resistance (particularly to strong cleaning agents), formability and weldability, etc., will frequently indicate stainless steels of the austenitic 18%Cr - 10%Ni AISI 304 type as these will, in the majority of environments meet all these requirements very well.
But chloride levels may be high in supply waters, in heating, cooling, or process fluids, in detergents and sanitising agents (some of which are based on sodium hypochlorite or organic chlorine donors) and in the brines widely used as refrigerants. And, of course, the food product itself very often also contains sodium chloride. In this case, one of the 17%Cr - 12%Ni - 2.5%Mo AISI 316 types (see Table 1 below) may be specified. Given tensile stresses and chlorides and a processing temperature above about 60� C, there may be a risk of stress corrosion cracking, to which the solution may be a duplex or high alloy grade such as 23%Cr - 4%Ni or 22%Cr - 5%Ni, or super-stainless steels such as those containing 6% Mo, or even special alloys with nickel contents of up to 33%.
Stress corrosion cracking may not be restricted to food-contact surfaces - it has been experienced where the chloride-containing insulation around hot pipes became wet from dripping condensation in a steamy cooking area, resulting in stress corrosion cracking of the external surfaces of the pipe-work. For their greater hardness and therefore wear-resistance the 1.4021-grade martensitic steels, whilst less corrosion-resistant, are commonly employed for knife blades or cutters. But for fish preparation knives, where both hardness and better corrosion resistance are required, grade 1.4122 may be appropriate.
Stainless steels which contain sulphur, lead or selenium to improve their machinability may offer less corrosion-resistance than the 304 and 316 types (and may require special welding techniques) and so should only be used with due reference to the chemistry of the food, the production processes and the detergents which will be used to clean the plant if those elements are not to be leached into the food or into the plant to contaminate a later batch of food. So important is this inertness that there is a proposal under consideration by the EC to require food contact components to carry batch numbers and to require total traceability.
It is already a requirement of EC Directive 89/109/EEC that food-contact materials do not transfer their constituents to the food in quantities which could endanger human health, or result in an unacceptable change in its nature or substance or quality, or cause a deterioration in the smell or the taste of food. And the Council of Europe (CoE) has now produced guidelines concerning the formal safety evaluation of materials to be used in contact with food. This scheme requires that all materials proposed for contact with foods be submitted for evaluation of such risks, although to date there has been no formal evaluation of stainless steel products used in food contact applications.
However, it is well-established that no flavours or discoloration are imparted to foods and beverages in contact with stainless steels, and numerous studies of the uptake of metals by foods give rise to no concern for health due to excessive intakes of nickel or chromium from the stainless steels. Indeed, no significant difference in migration has been noted between these steels and glass.
So the experienced selection of the correct grade of material, coupled with good design and fabrication, make all these potential engineering or hygiene 'hazards' avoidable and it is the purpose of the new EHEDG Guideline to increase designers' and fabricators' appreciation of just how much expertise is available from the supply industries on how materials will behave under a range of operating conditions.
The Nickel Institute welcomes its contribution and feels that the spread of such information is greatly to be encouraged and its availability publicised as widely as possible - equipment manufacturers who draw upon these resources will be better equipped to avoid technical pitfalls and to produce efficient and hygienic plant with the excellent reliability and lifetime economics for which stainless steels are renowned.
But even with stainless steel's impressive portfolio of engineering and hygiene attributes, there do remain significant markets not yet converted to its use. It is interesting that 98% of the chocolate processing and packaging plant operated world-wide is still constructed from painted mild steel. Despite their supremacy as the materials of construction for food-preparation equipment, there is still potential for even wider use of stainless steels.
Eric Partington is a U.K.-based consultant to the Nickel Institute. To submit comments or questions about this article, please contact us.