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Hygienic design of equipment for open processing

The risk of contamination of food products during open processing with relevant microorganisms increases with the opportunity to grow in poorly designed equipment as well as with their concentration in the environment. This means that, supplementary to requirements for hygienically designed equipment, the environmental conditions are an important factor. In addition, the type and level of product contamination and the stage of the manufacturing process must be taken into consideration. Open processes include very different types of equipment, e.g. machines for dairy products, alcoholic and non-alcoholic drinks, ice-cream products, sweet-oil, nutrient fat, coffee products, sugar, cereals, vegetables, fruits, bakery products, meat, and fish. In relation to the stage of processing and manufacturing different levels of hygienic requirements can be demanded when handling raw products, for example, which have to be heat treated in comparison to microbial unstable ready-made products and consumer goods. In principle, the design of equipment and the environmental conditions must not allow any increase in concentration of relevant microorganisms.

The subgroup Design Principles of the European Hygienic Equipment Design Group (EHEDG) produced in previous papers guidelines on “Hygienic Design Principles” (1), “Hygienic Welding” (3), and “Hygienic Design of Equipment for Closed Processing” (2). The guidelines have been approved by EHEDG.


The current paper deals with principal hygienic requirements for equipment used in open processing. It describes methods of construction and fabrication giving examples of how the principal design criteria can be met in open process equipment. For machines for special products and special manufacturing processes more stringent requirements may be necessary which have to be defined specifically. If requirements can not be met, cleanability must be demonstrated by tests. Environmental conditions are not taken into consideration in this paper. Requirements on this subject which can also be applied to open processes have been published.

Food contact equipment

Food contact areas include all surfaces that are directly exposed to the product and al1 indirect surfaces from which splashed product, condensate, liquid or dust may drain, drop or be drawn into the product. This means that for hygienic design of product contact areas of equipment for open processing the area above the open product surface must also be taken into consideration.


Many countries have directives for materials in contact with foodstuffs. It should be ensured that the use of specific materials is in accordance with legislation. Al1 materials must be non toxic, mechanically stable, inert and resistant to the product and to al1 cleaning and antimicrobial agents at the full range of concentrations, operating pressures and temperatures. The control of Wear must be prescribed.
Stainless steel is frequently the logical choice for metallic applications and, in consequence, the mostly used material in the food industry. The correct selection and application of speciîic types mainly depends on the corrosive properties of products, disinfecting and cleaning agents (above all, chloride containing fluids can lead to pitting corrosion or stress corrosion cracking) as well as on welding requirements. Adequate AISI, DIN and AC1 types for cast products should be used. Specific recommended types are published2.
Aluminium is not sufficiently corrosion resistant and should generally be avoided for food contact. If nickel or chromium plated equipment is used the plating must be manufactured reliably and its integrity checked. It must be ascertained that under conditions of use the plating cannot flake or otherwise contaminate the product. Chemically plated materials should be preferred over electroplating because of higher durability and more compact and dense surface layers. In addition, applications of plated materials must be chosen referring to their corrosion resistance in relation to the properties of products and cleaning and disinfecting agents. Plastic materials are used to protect tools and implements from metal to metal contact (e.g. for shear edges of cutters), as guides and covers, or for hoses because of their plasticity and corrosion resistance2. It must be noted that some plastics are porous and can absorb product constituents and harbour microorganisms. Special attention must be paid to this effect by careful cleaning and periodical inspection.
Rubber materials and other elastomers are commonly used for gaskets, seals, scrapers, etc. Excessive mechanical or thermal compression or deformation causes damage to such components adversely affecting cleanability z.
Wood is appropriate only in a limited number of cases, for example when it plays a favourable role for relative humidity regulation andor microbiological ecology (e.g. cheese ripening, the production of wine, vinegar, etc.) or when its mechanical properties cannot be obtained with other available materials (e.g. butcher’s blocks). Wooden surfaces must be cleaned effectively and disinfected because they can retain microorganisms which can subsequently grow in the presence of product nutrients. Splinters can result in foreign body contamination.


Al1 surfaces in contact with foodstuffs must be easily cleanable. Therefore, surfaces must be smooth, continuous and free from cracks, crevices, scratches and pits which can harbour and retain soil andor microorganisms after cleaning. The recommended surface finish of stainless steel should be equal to or less than Ra = 0.8 pm . It should be rnaintained during the usable life of the equipment.

It is preferred to use permanent joints against disrnountable ones to reduce hygienic risks by projections, protrusions, edges, recesses, metal to metal contact and crevices of sealing gaskets. Permanent joints of equipment should preferably be welded. There are several types of common defects arising in welded joints (eg misalignment, cracking, porosity, inclusions) which can act as a source of microbiological problems. Principal requirements on the welding process and on welding seams are published3. The product contact surface of welds must be smooth (ground flush with the surrounding surface). To avoid crevices, by metal to metal contact, the welded seams must not be intermittent but continuous. Overlapped welded joints should not be used by reason of retaining soil and forming areas at the overlap edge which are difficult to clean. If overlapping is unavoidable reliable draining and cleaning conditions of shadow areas must be taken into consideration. The welded seams should be ground flush and smooth. In the case of thick sheets the edge of the upper plate must be sloped. If necessary they must be ground as shown in Fig. 1.

Fig. I: Welded joints. (a) Overlapped sheets with intermittently welded seams create crevices and metal to metal contact areas between the seams; (6) improved design of overlapped sheets must have continuous welds and sloped edges for easy cleaning; (c) correct design is characterized by smooth continuously welded sheets.

Welding in sharp corners of equipment must be avoided (Fig. 2). Radiussed corners and welding searns in the plain area are recommended for hygienic design.

Fig. 2: Welded joints in corners. (a), (b) Weided seams in corners create un-cleanable areas; (c) radiussed corners and correetly welded seams in the plain area avoid any hygiene risk.

If adhesives are used for permanent joints they must be compatible with materials, products and cleaning/disinfecting agents with which they are in contact. Al1 bonds shall be continuous and mechanically sound so that the adhesives do not separate from the base materials to which they are bonded.

Dismountable joints (e.g. of plates or appendages) fixed by fasteners (e.g. screws or bolts) must only be used if dismantling is unavoidable. Ends of metal to metal contact surfaces of the overlapped edges must be carefully sealed considering defined compression of the seals. This cannot be achieved by threads of screws because of tolerances (Fig. 3a) but by bolts or pins (Fig. 3b). The design of grooves for seals has to consider space for expansion to avoid extension of seals into the product area during heating (Fig. 3c). There shall be no exposed screw threads on product contact surfaces (Fig. 3a). Appropriate hygienic design uses screw joints at the reverse side to product (Fig. 3b). It can be optimised by flangelike screw connections effecting the seal compression directly (Fig. 36). If a connection or fastening must be made with screws in the product area, poor design of screws and nuts creating crevices, grooves, or dead areas must be avoided (Fig. 4). Hygienic design of equipment with enclosed threads requires screw heads or nuts appropriate to mechanical or in-place cleaning. Metal to metal contact must be avoided by the use of metal-backed elastomer gaskets (Fig 5).

Fig. 3: Disrnountable joints. (a) From overlapped screw joints hazards arise from crevices between the sheet edges, poorly designed exposed nuts or screw heads and un-sealed threads; (b) appropriate design uses sealed edges of overlapped sheets, controlled seal compression (by pins or bolts) and screwjoints on the reverse side to the product; (c) for optimum design flanged sheets with sealed edges and screw joints on the reverse side to the product can be applied.

Fig. 4: Hazards due to unhygienic design of screws exposed to product are caused by metal to metal contact (a), crevices and gaps (b) or dead spaces (c).

Fig. 5: Hygienic design of screw joints. (a) The exposed domed head is easily cleanable and the metal backed gasket is used to seal the thread; (b) ifapplicable, any risk can be avoided by using a stud welded on the non product side.

Drainability. Food containing equipment (tanks, containers, vessels, troughs, reservoirs, hoppers, bins, chutes) with discharge openings must be fully self drainable as demonstrated by examples of Fig. 6c - 6f.

Fig. 6: Drainability of equipment principles. (a) and (b) Discharge outlets above the lowest level of equipment prevent selfdraining; (c), (d), (e), and (a demonstrate self drainable design with discharge outlets at the lowest level, sloped bottoms, and well rounded corners.

For good drainability and cleanability sharp corners must be avoided. They must be properly radiussed (see also 3). Horizontal surfaces must have a slope of more than 3" towards the outlet (Fig. 7). Equipment which can be tipped for discharging must also have well rounded corners, be fully drainable, and easily cleanable (Fig. 8).

Fig. 7: Drainability of corners and walls. (a) Sharp corners and horizontal walls can not easily be cleaned or drained; (b) to avoid any hazards corners must be well radiussed and horizontal surfaces must be sloped.

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