Hygienic equipment design
The manufacture of many foods involves some element of batch or assembly operations or both. The equipment used for such operations is predominantly of the open type, that is, it cannot be cleaned by recirculation (clean-in-place, CIP) procedures, and must be of the highest hygienic design standards. Hygienic equipment design provides three major benefits to food manufacturers:
1. Quality - good hygienic design maintains product in the main product flow. This ensures that product is not 'held-up' within the equipment where it could deteriorate and affect product quality on rejoining the main product flow. Or, for example in flavourings manufacture, one batch could not taint a subsequent batch.
2. Safety - good hygienic design prevents the contamination of the product with substances that would adversely affect the health of the consumer. Such contamination could be microbiological (e.g. pathogens), chemical (e.g. lubricating fluids, cleaning chemicals) and physical (e.g. glass). Prevention of contamination of products with potentially allergenic agents through proper cleaning, etc., is also important.
3. Efficiency - good hygienic design reduces the time required for an item of equipment to be cleaned. This reduction of cleaning time is significant over the lifetime of the equipment such that hygienically designed equipment which is initially more expensive (compared to similarly performing poorly designed equipment), will be more cost-effective in the long term. In addition, savings in cleaning time may lead to increased production.
Texts on hygienic design include Anon. (1983a), Timperley and Timperley (1993), the European Hygienic Design Group (EHEDG 1995), Timperley (1997) and Holah (1998a). Within Europe (the EHEDG) and the USA (the 3-A Standards and the National Sanitation Foundation - NSF), a number of organisations exist to foster consensus in hygienic design and the use of these organisations' guidelines can have a quasi-legal status. It should be noted that in Europe, hygienic design guidelines tend to be more generic in nature than the more prescriptive requirements American readers may be familiar with.
In the EU, the Council Directive on the approximation of the laws of Member States relating to machinery (89/392/EEC) was published on 14 June 1989. The Directive includes a short section dealing with hygiene and design requirements which states that machinery intended for the preparation and processing of foods must be designed and constructed so as to avoid health risks and consists of seven hygiene rules that must be observed. These are concerned with materials in contact with food; surface smoothness; preference for welding or continuous bonding rather than fastenings; design for cleanability and disinfection; good surface drainage; prevention of dead spaces which cannot be cleaned and design to prevent product contamination by ancillary substances, e.g. lubricants. The Directive requires that all machinery sold within the EU shall meet these basic standards and be marked accordingly to show compliance (the 'CE' mark).
Subsequent to this Directive, a European Standard EN 1672-2 Food processing machinery - Safety and hygiene requirements - Basic concepts - Part 2; Hygiene requirements (Anon., 1997b) has recently been adopted to further clarify the hygiene rules established in 89/392/EEC. In addition to this, a number of specific standards on bakery, meat, catering, edible oils, vending and dispensing, pasta, bulk milk coolers, cereal processing and dairy equipment are in preparation. The basic hygienic design requirements as presented in EN 1672-2 can be summarised under eleven headings and are described below:
1. Construction materials. Materials used for product contact must have adequate strength over a wide temperature range and a reasonable life, and must be non-tainting, corrosion and abrasion resistant, easily cleaned and capable of being shaped. Stainless steel usually meets all these requirements and there are various grades of stainless steel which are selected for their particular properties to meet operational requirements, e.g. Type 316 which contains molybdenum is used where improved corrosion resistance is necessary.
2. Surface finish. Product contact surfaces must be finished to a degree of surface roughness that is smooth enough to enable them to be easily cleaned.
Surfaces will deteriorate with age and wear (abrasion) such that cleaning will become more difficult.
3. Joints. Permanent joints, such as those which are welded, should be smooth and continuous. Dismountable joints, such as screwed pipe couplings, must be crevice-free and provide a smooth continuous surface on the product side. Flanged joints must be located with each other and be sealed with a gasket because, although metal/metal joints can be made leak tight, they may still permit the ingress of microorganisms.
4. Fasteners. Exposed screw threads, nuts, bolts, screws and rivets must be avoided wherever possible in product contact areas. Alternative methods of fastening can be used where the washer used has a rubber compressible insert to form a bacteria-tight seal.
5. Drainage. All pipelines and equipment surfaces should be self-draining because residual liquids can lead to microbial growth or, in the case of cleaning fluids, result in contamination of product.
6. Internal angles and corners. These should be well radiused, wherever possible, to facilitate cleaning.
7. Dead spaces. As well as ensuring that there are no dead spaces in the design of equipment, care must be taken that they are not introduced during installation.
8. Bearings and shaft seals. Bearings should, wherever possible, be mounted outside the product area to avoid possible contamination of product by lubricants, unless they are edible, or possible failure of the bearings due to the ingress of the product. Shaft seals must be so designed that they can be easily cleaned and if not product-lubricated, then the lubricant must be edible. Where a bearing is within the product area, such as a foot bearing for an agitator shaft in a vessel, it is important that there is a groove completely through the bore of the bush, from top to bottom, to permit the passage of cleaning fluid.
9. Instrumentation. Instruments must be constructed from appropriate materials and if they contain a transmitting fluid, such as in a Bourdon tube pressure gauge, then the fluid must be approved for food contact. Many instruments themselves are hygienic but often they are installed unhygienically.
10. Doors, covers and panels. Doors, covers and panels should be designed so that they prevent the entry of and/or prevent the accumulation of soil. Where appropriate they should be sloped to an outside edge and should be easily removed to facilitate cleaning.
11. Controls. These should be designed to prevent the ingress of contamination and should be easily cleanable, particularly those that are repeatedly touched by food handlers to allow process operation.
The potential for well-designed and constructed equipment to be operated in a hygienic manner may be easily compromised by inadequate attention to its location and installation. Timperly (1997), when considering the accessibility of equipment, recommended that it is more effective to consider complete lines instead of individual items of equipment and recommended the following:
• There should be sufficient height to allow adequate access for inspection, cleaning and maintenance of the equipment and for the cleaning of floors.
• All parts of the equipment should be installed at a sufficient distance from walls, ceiling and adjacent equipment to allow easy access for inspection, cleaning and maintenance, especially if lifting is involved.
• Ancillary equipment, control systems and services connected to the process equipment should be located so as to allow access for maintenance and cleaning.
• Supporting framework, wall mountings and legs should be kept to a minimum. They should be constructed from tubular or box section material which should be sealed to prevent ingress of water or soil. Angle or channel section material should not be used.
• Base plates used to support and fix equipment should have smooth, continuous and sloping surfaces to aid drainage. They should be coved at the floor junction. Alternatively, ball feet should be fitted.
• Pipework and valves should be supported independently of other equipment to reduce the chance of strain and damage to equipment, pipework and joints.
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