Level 3 internal barriers separating manufacturing processes

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The final set of barriers to contamination are those within the factory itself. Two levels of barriers are required:

• The first level to separate processing from non-processing areas.

• The second to separate 'high-risk' from 'low-risk' processing areas.

The design of any food-processing area must allow for the accommodation of five basic requirement, i.e.

• Raw materials and ingredients.

• Processing equipment.

• Staff concerned with the operation of such equipment.

• Packaging materials.

• Finished products.

All other requirements should be considered as secondary to these five basic requirements and, wherever possible, must be kept out of the processing area. These secondary requirements are:

• Structural steel framework of the factory.

• Service pipework for water, steam and compressed air; electrical conduits and trunking; artificial lighting units; and ventilation ducts.

• Compressors, refrigeration/heating units and pumps.

• Maintenance personnel and equipment associated with any of these services.

Ashford (1986) suggests building a 'box within a box' by creating insulated clean rooms within the structural box of the factory, with the services and control equipment located in the roof void above the ceiling. Equipment and ductwork are suspended from the structural frames and access to all services is provided by catwalks, as shown diagrammatically in Fig. 6.3. This arrangement, if properly undertaken, eliminates a major source of contamination from the process area.

Recessed lighting

Recessed lighting

Recessed lighting

Recessed lighting

Local drops to specific equipment n

High-Care Production Area (Internal walls not shown)

Fig. 6.3 Basic design concepts - the separation of production from services and maintenance operations.

Within the overall manufacturing area, a further, final set of barriers is required between 'high-risk' and 'low-risk' processing areas. High-risk (high-hygiene or high-care) areas may be broadly defined as areas processing food components that have undergone a decontamination or preservation process and where there is a risk of product recontamination between decontamination/preservation and a final process, for example pack sealing, which removes the immediate risk of further contamination. In contrast, low-risk areas refer to those processes dealing with food components that have not yet undergone a decontamination/ preservation process. Some experts make a further distinction, for example, between 'high-risk areas' (HRAs) and 'high-care areas' (HCAs). The UK Chilled Food Association, for example, uses both terms (Anon., 1997a). In general the requirements in both these type of areas dealing with decontaminated product are the same. It is important also to note that the distinction between high- and low-risk areas does not mean that lower overall standards are acceptable in 'low-risk' areas, for example raw material reception or final product storage or distribution. Unsatisfactory practices in 'low-risk' areas may put greater pressure on the barriers separating the two, either increasing the level of initial contamination or increasing the risk of recontamination, for example through poor storage or damage to the packaging of the final product.

The final barrier between high- and low-risk processing areas is composed of a number of sub-barriers designed to control contamination from a number of routes:

• The point at which the product leaves the preservation/decontamination process and enters the high-risk area.

• The movement of other materials in to and out of the high-risk area (e.g. waste, packaging).

• The movement of employees and equipment in to and out of high-risk areas.

Some of these potential sources of contamination may be controlled by appropriate procedures, for example governing movement of personnel and materials, which are discussed in the following chapter. The principal areas where hygienic design is the critical factor are:

• The interface between preservation/decontamination and the high-risk area.

• Appropriate facilities to support the movement of personnel in to and out of the high-risk area.

6.4.1 The interface between preservation/decontamination and high-risk areas

Decontamination/preservation equipment must be designed such that as far as is possible a solid, physical barrier separates the low- and high-risk areas. Where it is not physically possible to form a solid barrier, air spaces around the equipment should be minimised and the low-/high-risk floor junction should be fully sealed to the highest possible height. The fitting of devices that provide heat treatment within the structure of a building presents two main difficulties. Firstly, the devices have to be designed to load product on the low-risk side and unload on the high-risk side. Secondly, the maintenance of good seals between the heating device surfaces, which cycle through expansion and contraction phases, and the barrier structure, which may have a different thermal expansion, is problematical. Of particular concern are ovens:

• Some ovens have been designed such that they drain into the high-risk area. This is unacceptable since it may be possible for any pathogens present on the surface of product to be cooked to fall to the floor through the melting of the product surface layer (or exudates on overwrapped product) at a temperature that is not lethal to the pathogen. The pathogen could then remain on the floor or in the drain of the oven in such a way that it could survive the cook cycle. On draining, the pathogen would then subsequently drain into the high-risk area. Pathogens have been found at the exit of ovens in a number of food factories.

• Problems have occurred with leakage from sumps under the ovens into the high-risk area. There can also be problems in sump cleaning where the use of high-pressure hoses can spread contamination into the high-risk area.

• Where the floor of the oven is cleaned, cleaning should be undertaken in such a way that cleaning solutions do not flow from low-risk areas to high-risk areas. Ideally, cleaning should be from the low-risk area with the high-risk area door closed and sealed. If cleaning solutions have to be drained into the high-risk area, or in the case of ovens that have a raining water cooling system, a drain should be installed immediately outside the door in the high-risk area.

6.4.2 Personnel

Within the factory building, provision must be made for adequate and suitable staff facilities and amenities for changing, washing and eating. There should be lockers for storing outdoor clothing in areas that must be separate from those for storing work clothes. Toilets must be provided and must not open directly into food-processing areas, all entrances of which must be provided with handwash-ing facilities arranged in such a way that their ease of use is maximised.

In high-risk operations, personnel facilities and requirements must be provided in a way that minimises any potential contamination of high-risk operations. The primary sources of potential contamination arise from the operatives themselves and from low-risk operations. This necessitates further attention to protective clothing and, in particular, special arrangements and facilities for changing into high-risk clothing and entering high-risk areas. Best practice with respect to personnel hygiene is continually developing and has been recently reviewed by Guzewich and Ross (1999), Taylor and Holah (2000) and Taylor et al. (2000).

High-risk factory clothing does not necessarily vary from that used in low risk in terms of style or quality, though it may have received higher standards of laundry, especially related to a higher temperature process, sufficient to reduce microbiological levels significantly. Indeed some laundries now operate to the same low-/high-risk principles as the food industry such that dirty laundry enters 'low risk', is loaded into a washing machine that bridges a physical divide, is cleaned and disinfected and exits into 'high risk' to be dried and packed.

All clothing and footwear used in the high-risk area is colour coded to distinguish it from that worn in other parts of the factory and to reduce the chance that a breach in the systems would escape early detection. High-risk footwear should be captive to high-risk areas, i.e. it should remain within high-risk areas, operatives changing into and out of footwear at the low-/high-risk boundary. This has arisen because research has shown that boot baths and boot washers are unable adequately to disinfect low-risk footwear such that they can be worn in both low and high risk and decontaminated between the two (Taylor et al., 2000). In addition, boot baths and boot washers can both spread contamination via aerosols and water droplets that, in turn, can provide moisture for microbial growth on high-risk floors. Bootwashers were, however, shown to be very good at removing organic material from boots and are thus a useful tool in low-risk areas both to clean boots and help prevent operative slip hazards.

The high-risk changing room should provide the only entry and exit point for personnel working in or visiting the area and is designed and built both to house the necessary activities for personnel hygiene practices and to minimise contamination from low-risk areas. In practice, there are some variations in the layout of facilities of high-risk changing rooms. This is influenced by, for example, space availability, product throughput and type of products, which will affect the number of personnel to be accommodated and whether the changing room is a barrier between the low- and high-risk operatives or between operatives arriving from outside the factory and high risk. Generally higher construction standards are required for low-/high-risk barriers than outside/high-risk barriers because the level of potential contamination in low risk, both on the operatives hands and in the environment, is likely to be higher (Taylor and Holah, 2000). In each case, the company must evaluate the effectiveness of the changing-room layout and procedure to ensure the high-risk area and products prepared in it are not being put at risk. This is best undertaken by a Hazard Analysis Critical Control Point (HACCP) approach, so that data are obtained to support or refute any proposals regarding the layout or sequence.

Research at the Campden and Chorleywood Food Research Association (CCFRA) has also proposed the following hand hygiene sequence to be used on entry to high-risk areas (Taylor and Holah, 2000). This sequence has been designed to maximise hand cleanliness, minimise hand transient microbiological levels, maximise hand dryness yet at the same time reduce excessive contact with water and chemicals that may both lead to dermatitis of the operatives and reduce the potential for water transfer into high-risk areas.

1. Remove low-risk or outside clothing.

2. Remove low-risk/outside footwear and place in designated 'cage' type compartment.

3. Cross over the low-risk/high-risk dividing barrier.


5. Put on in the following order:

• high-risk captive footwear;

• hair net - put on over ears and cover all hair (plus beard snood if needed) - and hat (if appropriate);

• overall (completely buttoned up to neck).

6. Check dress and appearance in the mirror provided.

7. Go into the high-risk production area and apply an alcohol-based sanitiser.

8. Draw and put on disposable gloves, sleeves and apron, if appropriate.

A basic layout for a changing room in shown in Fig. 6.4 and has been designed to accommodate the above hand hygiene procedure and the following requirements:

• An area at the entrance to store outside or low-risk clothing. Lockers should have sloping tops.

• A barrier to divide low- and high-risk floors. This is a physical barrier such as a small wall (approximately 60 cm high), that allows floors to be cleaned on either side of the barrier without contamination by splashing, etc., between the two.

• Open lockers at the barrier to store low-risk footwear.

• A stand on which footwear is displayed/dried.

• An area designed with suitable drainage for bootwashing operations. Research has shown (Taylor et al., 2000) that manual cleaning (preferably during the cleaning shift) and industrial washing machines are satisfactory bootwashing methods.

Food Factory Changing Room Designs
Fig. 6.4 Schematic layout for a high-risk changing room.

• Hand wash basins to service a single hand wash. Handwash basins must have automatic or knee/foot operated water supplies, water supplied at a suitable temperature (that encourages hand washing) and a waste extraction system piped directly to drain. It has been shown that hand wash basins positioned at the entrance to high-risk areas, which was the original high-risk design concept to allow visual monitoring of hand wash compliance, gives rise to substantial aerosols of staphylococcal strains that can potentially contaminate the product.

• Suitable hand-drying equipment, e.g. paper towel dispensers or hot-air dryers, and, for paper towels, suitable towel disposal containers.

• Access for clean factory clothing and storage of soiled clothing. For larger operations this may be via an adjoining laundry room with interconnecting hatches.

• Interlocked doors are possible such that doors only allow entrance to high-risk areas if a key stage, e.g. hand washing, has been undertaken.

• Closed-circuit television (CCT) cameras as a potential monitor of hand wash compliance.

• Alcoholic hand rub dispensers immediately inside the high-risk production area.

There may be the requirement to site additional handwash basins inside the high-risk area if the production process is such that frequent hand washing is necessary. As an alternative to this, Taylor et al. (2000) demonstrated that cleaning hands with alcoholic wipes, which can be done locally at the operative's work station, is an effective means of hand hygiene.

The air is an important potential source of pathogens and the intake into the high-risk area has to be controlled. Air can enter the high-risk area via a purpose-built air-handling system or can enter into the area from external uncontrolled sources (e.g. low-risk production operations). For high-risk areas, the goal of the air-handling system is to supply suitable filtered fresh air, at the correct temperature and humidity, at a slight overpressure to prevent the ingress of external air sources.

The cost of the air-handling systems is one of the major costs associated with the construction of a high-risk area, and specialist advice should always be sought before embarking on an air-handling design and construction project. Following a suitable risk analysis, it may be concluded that the air-handling requirements for high-care areas may be less stringent, especially related to filtration levels and degree of overpressure. Once installed, any changes to the construction of the high-risk area (e.g. the rearrangement of walls, doors or openings) should be carefully considered as they will have a major impact on the air-handling system.

Air quality standards for the food industry were reviewed by a CCFRA Working Party and guidelines were produced (Brown, 1996). The design of the air-handling system should consider the following issues:

• Degree of filtration of external air.

• Overpressure.

• Temperature requirements.

• Local cooling and barrier control.

• Humidity requirements.

• Installation and maintenance.

Air flows

The main air flows within a high-risk area are shown in Fig. 6.5. A major risk of airborne contamination entering high-risk areas is from low-risk processing operations, especially those handling raw produce, which is likely to be contaminated with pathogens. The principal role of the air-handling system is thus to provide filtered air to high-risk areas with a positive pressure with respect to low-risk areas. This means that wherever there is a physical break in the low-/high-risk barrier, e.g. a hatch, the air flow will be through the opening from high to low risk. Microbial airborne levels in low-risk areas, depending on the product and processes being undertaken, may be quite high (Holah et al., 1995) and overpressure should prevent viable pathogenic microorganisms entering high-risk areas.

In addition to providing a positive over-pressure, the air-flow rate must be sufficient to remove the heat load imposed by the processing environment (processes and people) in maintaining the desired temperature in the high-risk area. It must also provide operatives with fresh air. Generally 5-25 air changes per hour are adequate, though in a high-risk area with large hatches/doors that are frequently opened up to 40 air changes per hour may be required.

Joint work undertaken since 1995 by CCFRA and the Silsoe Research Institute, sponsored by the UK Ministry of Agriculture, Fisheries and Food (MAFF, now DEFRA), has looked at the control of airborne microbial contamination in high-risk food production areas. The work has resulted in the production of a best practice guideline on air flows in high-risk areas published by MAFF in 2001 (Anon., 2001a). The work has centred on the measurement of both air flows and airborne microbiological levels in actual food factories. Computational fluid dynamics (CFD) models have been developed by Silsoe to predict air and particle (including microorganism) movements. The work has led to innovations in two key areas:

• Firstly, the influence on air flows of air intakes and air extracts, secondary ventilation systems in, e.g., washroom areas, the number of hatches and doors and their degree of openings and closing, can readily be visualised by CFD. This has led to the redesign of high-risk areas, from the computer screen, such that air-flow balances and positive pressures have been achieved.

• Secondly, the CFD models allow the prediction of the movement of airborne microorganisms from known sources of microbial contamination, e.g. operatives. This has allowed the design of air-handling systems which provide

Exhaust air

Exhaust air

Clothing For High Risk Areas Food
Fig. 6.5 Schematic diagram showing the air flows within a high-risk or high-care production area.

directional air that moves particles away from the source of contamination, in a direction that does not compromise product safety.


To aid the performance of the air-handling system, it is also important to control potential sources of aerosols, generated from personnel, production and cleaning activities, in both low- and high-risk areas. Filtration of air is a complex matter and requires a thorough understanding of filter types and installations. The choice of filter will be dictated by the degree of microbial and particle removal required and filter types are described in detail in the CCFRA guideline (Brown, 1996).


To be effective, the pressure differential between low- and high-risk areas should be between 5 and 15 pascals. The desired pressure differential will be determined by both the number and size of openings and also the temperature differentials between low- and high-risk areas. For example, if the low-risk area is at ambient temperature (20 °C) and the high-risk area at 10 °C, hot air from the low-risk area will tend to rise through the opening while cold air from the high-risk area will tend to sink through the same opening, causing two-way flow. The velocity of air through the opening from the high-risk area may need to be 1.5m/s or greater to ensure one-way flow is maintained.


The choice of relative air humidity is a compromise between operative comfort, product quality and environmental drying. A relative humidity of 55-65% is very good for restricting microbial growth in the environment and increases the rate of equipment and environment drying after cleaning operations. Low humidities can, however, cause drying of the product with associated weight and quality loss, especially at higher air velocities. Higher humidities maintain product quality but may give rise to drying and condensation problems that increase the opportunity for microbial survival and growth. A compromise target humidity of 60-70% is often recommended, which is also optimal for operative comfort.

Chilled foods operations

Chilled foods manufacturers have traditionally chosen to operate their high-risk areas at low temperatures, typically around 10-12°C, both to restrict the general growth of microorganisms in the environment and to prevent the growth of some (e.g. Clostridium perfringens) but not all (e.g. Listeria) food pathogens. Chilling the area to this temperature is also beneficial in reducing the heat uptake by the product and thus maintaining the chill chain. Moreover, chilled food manufacturers have to ensure that their products meet legislative requirements such as those governing temperature control in food processing in the UK (Anon., 1995) as well as those imposed by their retail customers. However, there is a need to balance these requirements with operator comfort. Recommendations on achieving an appropriate balance are provided, for example, by Guidance on Achieving

Reasonable Working Temperatures and Conditions during Production of Chilled Foods (Brown, 2000).

Another joint CCFRA/Silsoe, MAFF-sponsored, project, has examined the use of localised cooling with the objectives of:

• Providing highly filtered (H11-12), chilled air directly over or surrounding product. This could reduce the requirement to chill the whole of the high-risk area to 10°C (13 °C would be acceptable), and reduce the degree of filtration required (down to H8-9). The requirement for positive pressure in the low-risk area is paramount, however, and the number of air changes per hour would remain unchanged.

• Using the flow of the air to produce a barrier that resists the penetration of aerosol particles, some of which would contain viable microorganisms.

An example of such a technology has been reported in Burfoot et al. (2000). Equipment and installation

Air is usually supplied to high-risk areas by either ceiling grilles or textile ducts (socks), usually made from polyester or polypropylene to reduce shrinkage. Ceiling grilles have the advantage that they are cheap and require little maintenance but have limitations on velocity and flow rate without high noise levels or the potential to cause draughts. With respect to draughts, the maximum air speed close to workers to minimise discomfort through 'wind chill' is 0.3 m/s. Air socks have the ability to distribute air, at a low draught-free velocity with minimal ductwork connections, though they require periodic laundering and spare sets are required. Ceiling-mounted chillers that cool and recirculate the air are suitable for high-care operations only if additional air supplies are used to maintain positive pressures.

Finally, air-handling systems should be properly installed such that they can be easily serviced and cleaned and, as part of the commissioning programme, their performance should be validated for normal use. The ability of the system to perform in other roles should also be established. These could include dumping air directly to waste during cleaning operations, to prevent air contaminated with potentially corrosive cleaning chemicals entering the air-handling unit, and recirculating ambient or heated air after cleaning operations to increase environmental drying.

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