Rev. sci. tech. Off. int. Epiz., 1995,14 (2), 343-363

Disinfection of food production areas J.T. HOLAH *

Summary: Disinfection, other than by heat, is ineffective unless all surfaces have previously been thoroughly cleaned to remove interfering materials. Cleaning is therefore extremely important as part of a two-stage cleaning and disinfection (sanitation) programme. The author describes the principles of sanitation, the chemicals and equipment involved, and the programme of events to be followed. For food products of 'low risk' (in terms of stable shelf life and safety), traditional sanitation programmes are adequate and in some cases disinfection may not be required. However, disinfection is essential for 'high-risk' food products, but this cannot be effectively undertaken without due consideration of hygienic design and possible cross-contamination. To ensure continued satisfactory performance of a sanitation programme, routine assessments should be undertaken.

KEYWORDS: Biofilms - Cleaning - Disinfection - Food hygiene - Food production - Hygienic design - Sanitation.

INTRODUCTION

As detai led elsewhere (21), sani tat ion is under t aken primarily to remove all undesirable mater ia l (food residues, microorganisms, foreign bodies and cleaning chemicals) from surfaces, in an economical manner and to a level at which any residues remaining are of minimal risk to the quality or safety of the product. Such undesirable material , generally referred to as 'soil ' , can be derived from normal product ion, spillages, line-jams, equipment maintenance , packaging or general environmenta l contamination (dust and dirt).

The products of the food industry fall into two broad categories according to the perceived risk. 'Low-risk' products include 'ambient shelf stable' products and products which require subsequent cooking prior to eating. 'High-risk' products include chilled products with a short shelf life and products which require no cooking prior to consumption.

For low-risk products, correctly-undertaken traditional sanitation programmes are cost-effective, easy to manage and can both increase product quality and reduce the risk of microbial and foreign body contaminat ion. This article is in tended to provide sufficient practical background knowledge to enable such a thorough sani ta t ion programme to be undertaken.

For high-risk products with intrinsic demands for higher hygiene s tandards , traditional sanitation programmes have been shown to be unsuitable in some cases. The

* Head, Food Hygiene Department, Campden Food and Drink Research Association, Chipping Campden, Glos GL55 6LD, United Kingdom.

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section on 'Requirements for sanitation of high-risk food production areas' may provide sufficient understanding of cross-contamination issues to ensure successful sanitation programmes which do not result in food product contamination.

SOILS

Throughout the production period, debris builds up on surfaces, requiring subsequent removal and control by sanitation programmes. This debris may result from normal production, spillages, line-jams, maintenance, packaging or general dust and dirt, and may include food residues, microorganisms and foreign matter. In practical terms, a soil is anything in the wrong place at the wrong time (e.g. peas on a conveyor during production are 'product', but after production or on the floor they are 'soil').

A successful sanitation programme requires knowledge of the nature of the soil to be removed. The product residues are readily observed and may be characterised by their chemical composit ion (e.g. carbohydra te , fat or prote in) . In addit ion, different processing and/or environmental factors affecting the same product soil may lead to a variety of cleaning problems, primarily dependent on moisture levels and temperature. Higher temperatures for product soils usually entail a delay in initiating the sanitation programme (i.e. the drier and more baked the soil becomes, the more difficult it is to remove).

Microorganisms can either be incorporated into the soil or can attach to surfaces and form layers or biofilms. The fundamental means of microorganism a t tachment to surfaces are well known (9, 12, 37, 44). Al though a t tachment to food product ion surfaces is not well documented (10,14, 22, 24, 33, 38, 49, 55), some reviews on this subject have been published (11,50).

In other disciplines, biofilms are envisaged as biological growths on surfaces - consisting (sometimes) of higher organisms, a mult i tude of microbial cells and extracellular polymers - which develop with time into thick biological films. However, in the food processing industry, the time frame for biofilm development is usually relatively short and varies with respect to tempera ture , nutr ient supply and presence of antimicrobial agents. The term biofilm is therefore more associated with the surface attachment and growth of microorganisms. The type of microbial species present is also extremely important . Relatively high levels of non-spoilage or non-pathogenic microorganisms on surfaces may be tolerated, but the presence of food pathogens (e.g. Salmonella spp. or Listeria spp.) would generally be unacceptable.

Research at the Campden Food and Drink Research Association has demonstrated that , given complementary conditions of t empera ture and a nutr ient source, microorganisms will attach to and grow on a majority of the materials used in food manufacture, including stainless steel, aluminium, high-density nylon, polypropylene, polycarbonate and Polyvinylchloride. Field studies have also demonstrated that surface coverings in excess of 10 7 cells/cm 2 are readily apparent (50). Therefore , hygienic conditions per t inent to food processing surfaces are not concerned with whether microorganisms can grow on surfaces or be found in product soils, but ra ther with whether their numbers can be reduced to a satisfactory level by sanitation programmes. This is especially true for the high-risk environments in which disinfection often plays a crucial role.

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SANITATION PRINCIPLES

Soiling of surfaces is a natural process which reduces the free energy available within a system. To undertake a sanitation programme, energy must therefore be added to the soil to reduce both soil particle/soil particle and soil par t ic le /equipment surface interactions (8, 13, 28, 31 , 34, 45). In addit ion, energy is requi red to remove microorganisms remaining after the cleaning phase or to render these non-viable. Within the sanitation programme, the following stages have been identified (20,28,31):

a) The cleaning solution wets and penet ra tes both the soil and the equipment surface.

b) The cleaning solution reacts with both the soil and the surface to facilitate a number of processes: peptisation of organic materials, dissolution of soluble organics and minerals, emulsification of fats, and the dispersion and removal from the surface of solid soil components.

c) The dispersed soil is prevented from re-depositing back onto the cleansed surface.

d) The disinfectant solution wets residual microorganisms to facilitate dispersion and surface removal, or the reaction with cell membranes and/or penetration of the microbial cell to produce a biocidal or biostatic action. Dependent on the disinfectant practice chosen, this may be followed by dispersion of the microorganisms from the surface.

To accomplish these four stages, sanitation programmes utilise a combination of the four major energy factors described below. The proportion of these four factors varies for each sanitation programme, and if the use of one energy source is restricted, this shortfall may usually be balanced by increasing energy inputs from the other factors, as follows:

- mechanical (or kinetic) energy - chemical energy - temperature (or thermal energy) - time.

Mechanical or kinetic energy is used to physically remove soils and may include manual scraping, brushing and wiping, automated scrubbing (physical abrasion) and pressure-jet washing (fluid abrasion) . Of these four factors, physical abrasion is regarded as the most efficient in terms of energy transfer (41), while the efficiency of fluid abrasion and the effect of impact pressure have also been described (2,19,50).

In cleaning, chemical energy is used to break down soils to facilitate removal, and to disperse and suspend the soils in solution to aid rinsability. In chemical disinfection, chemicals react with microorganisms remaining on surfaces after cleaning to reduce their viability or affect their growth rate. The influence of detergency in cleaning and disinfection has been described (1,14,15,35,39,47,50).

The chemical effects of cleaning and disinfection increase linearly with temperature and approximately double for every 10°C rise. For fatty soils, temperatures above the melting point of the fats are used to break down and emulsify these deposits to aid removal.

For cleaning processes using mechanical, chemical and thermal energies, increasing the duration of action usually increases the efficacy of the process. When extended time periods can be employed in sanitation programmes (e.g. soak-tank operations), other energy inputs can be reduced (e.g. reduced detergent concentration, lower temperature

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or less mechanical brushing). Alternatively, for soils which are difficult to remove, cleaning efficiency in soak tanks is often improved by using higher temperatures than could be used manually and increasing the duration of soaking.

The retention of soil in cracks and the difficulty of cleaning inaccessible areas mean that routine cleaning operations are never 100% efficient (20). Therefore, over a course of multiple soiling/cleaning cycles, soil deposits (potentially including microorganisms) will accumulate. As soil accumulates, cleaning efficiency will decrease and, for a period, soil deposits may grow exponentially (Fig. 1). The time scale for such soil accumulation will vary between processing applications and can range from hours (e.g. in heat exchangers) to, typically, several days or weeks. In practice, the build-up of soil is controlled by 'periodic' cleaning (16). Periodic cleans return the soil accumulation to an acceptable base level (Fig. 1) and involve increasing cleaning time and/or energy input (e.g. higher temperatures, alternative chemicals or extensive dismantling of equipment, typically at weekends).

FiG.1

Build-up of soilage and/or microorganisms in food production areas A indicates the curve followed if periodic cleaning returns soil accumulation to an acceptable level (vertical sections on the A curve represent the effets of periodic

cleaning), while B represents the situation with routine cleaning alone

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CLEANING CHEMICALS

No single cleaning agent is able to perform all the functions necessary to a successful cleaning programme, and a cleaning solution or detergent is therefore blended from a typical range of characteristic components:

- water - surfactants - inorganic alkalis - inorganic and organic acids - sequestering agents.

The range and purpose of chemicals used has been extensively documented (5,17, 18,19,26,27,31,43), and only the principles are outlined below.

Potable water is the base ingredient of all 'wet ' cleaning systems, and provides the cheapest transport medium for rinsing and dispersing soils. Water has dissolving powers to remove soluble ionic compounds (e.g. salts and sugars), helps to emulsify fats at temperatures above their melting point and, in pressure-jet cleaning, can be used as an abrasive agent. However, water used on its own is a poor 'wetting' agent and cannot dissolve non-ionic compounds.

Organic surfactants (surface-active or wetting agents) are composed of a long non-polar (hydrophobic) chain or tail and a polar (hydrophilic) head. Surfactants are classified as anionic, cationic or non-ionic, depending on their ionic charge in solution. Anionics and non-ionics are more commonly used than cationic surfactants. Amphipolar molecules aid cleaning by reducing the surface tension of water (thus increasing 'wettability') and by emulsifying fats.

Alkalis are useful cleaning agents, as they are inexpensive, and are able to break down proteins ( through the action of hydroxyl ions) and saponify fats. At higher concentrat ions, alkalis may also be microbicidal. Alkal ine detergents may be chlorinated to aid the removal of proteinaceous deposits, but chlorine is not an effective biocide at alkaline pH. The main disadvantages of alkalis are the potential to precipitate hard water ions, the formation of scums with soaps and poor rinsability.

Acids have minor detergency properties, although they are very useful in solubilising carbonate and mineral scales (including hard water salts) and proteinaceous deposits. Acids also have microbicidal properties.

Sequester ing agents (chelating agents) are used to prevent the precipi ta t ion of mineral ions by forming soluble complexes with these ions. The primary use of these agents is in the control of hard water ions, and they are added to surfactants to aid dispersion capacity and rinsability.

A general-purpose food detergent could therefore contain the following elements:

- a strong alkali to saponify fats - weaker alkali 'builders' or 'bulking' agents - surfactants to improve wetting, dispersion and rinsability - sequestrants to control hard water ions.

The detergent should ideally be safe, non-taint ing, non-corrosive, stable and environmentally friendly. The choice of cleaning agent will depend on the nature and solubility characteristics of the soil to be removed, and these are summarised for a range of food products in Table I.

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TABLE I Solubility characteristics and recommended cleaning products for a range of soil types

(Modified from 17)

Soil types Solubility characteristics Recommended cleaning products

Sugars, organic acids, salt Water soluble Mildly alkaline detergent

High protein foods (meat, Water soluble, alkali soluble, Chlorinated alkaline poultry, fish) slightly acid soluble detergent

Starchy foods, tomatoes, fruit Partly water soluble, alkali soluble Mildly alkaline detergent

Fatty foods (fat meat, Water insoluble, alkali soluble Mildly to strongly alkaline butter, margarine, oils) detergent

Heat precipitated water hardness, Water insoluble, alkali insoluble, Acidic detergent milk stone, protein scale acid soluble

Chemicals employed during the cleaning stage are responsible for the removal not only of soil but also of the majority of microorganisms present. Cleaning may remove 2-6 log orders of microorganisms (20,40,46), and therefore it is vital both to purchase a good-quality formulated cleaning product and to emphasise the importance of ensuring a successful cleaning operation.

Although most microbial contamination is removed in the cleaning phase, sufficient viable microorganisms may remain on the surface to warrant the application of a disinfectant. Disinfection is under taken to reduce further the surface populat ion of viable microorganisms, via removal or destruction, and/or to prevent surface microbial growth during the inter-production period. Heat is the best disinfectant, as it penetrates into surfaces, is non-corrosive, is non-selective to microbial types (higher temperatures are required for spores), is easily measured and leaves no residue (28). However, for open surfaces, the use of hot water or steam is uneconomical, hazardous or impossible, and reliance is therefore placed on chemical biocides.

The types of disinfectants suitable for use in the food industry have been described elsewhere (21). This section is therefore restricted to discussion of the factors which control the efficiency of disinfectants. These can be summarised as follows:

- interfering substances (primarily organic matter) - pH - concentration - contact time.

The efficiency of all disinfectants is reduced in the presence of organic matter for two main reasons: chemical reaction and spatial non-reaction. Organic material may react non-specifically with the disinfectant such that the disinfectant loses its biocidal potency (this is particularly t rue for oxidative biocides). Other interfering substances (e.g. cleaning chemicals) may react chemically with the disinfectant and destroy its antimicrobial proper t ies ; for example, alkaline detergents will disable cationic quaternary ammonium compounds. In a non-reactive way, organic material may form a spatial barrier protecting microorganisms from the effects of disinfectants (e.g. soil containing microorganisms may be left in a crevice into which the disinfectant cannot penetrate). It is therefore essential to remove all soil during a comprehensive cleaning phase, and to remove all chemical residues via thorough rinsing, prior to disinfection.

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Disinfectants may be affected by the p H of the water used for dilution, and only water within the p H range specified by the manufacturer should be used. For example, chlorine dissociates in water to form HOC1 and the OCI ion. At pH 3-7.5, chlorine is predominantly present as H O C 1 or 'free chlorine' , which is a very powerful biocide. However, above pH 7.5 the majority of the chlorine is present as the OC1" ion, which has approximately 1% of the biocidal action of HOC1. Chlorinated alkaline detergents should therefore not be considered biocidal by virtue of the chlorine content alone.

To be effective, disinfectants must find, bind to and traverse microbial cell envelopes before reaching their target site and beginning to undertake the reactions which will subsequently lead to the destruction of the microorganism (30). Sufficient contact time is therefore critical to ensure disinfection, and most general-purpose disinfectants are formulated to require at least five minutes to reduce bacterial populations in suspension by five log orders. Five minutes is usually chosen as being representative of the time for which most disinfectants remain on non-horizontal food processing surfaces, although some biocides (including amphoterics and quaternary ammonium compounds) may attach to the surface to prolong contact time, and are claimed by the manufacturers to be 'surface active'. When microbial problems have been associated with a food product, food manufacturers may enhance disinfection by increasing contact time through the use of soak tanks or repeated surface dosing.

The relat ionship between microbial dea th and disinfectant concentrat ion is not linear but follows a typical sigmoidal death curve. Microbial populations are difficult to kill at low biocide concentrations, but increasing the concentration leads to a point at which the majority of the population succumbs. Beyond this point, the microorganisms become more difficult to kill ( through resistance or physical protect ion) and a propor t ion may survive regardless of increases in concentrat ion. It is therefore important that the disinfectant is used at the concentrat ion recommended by the manufacturer. Changes to this concentration may not enhance effects; application of the disinfectant will never give rise to 'sterile' surfaces.

CLEANING EQUIPMENT

Cleaning and disinfection may be performed manually using simple tools (e.g. brushes or cloths). This form of cleaning is not susceptible to mechanical breakdown or error, is easily directed and is cost-effective for small areas. For operative safety, low levels of chemicals and low tempera tures are used, and the major energy input is mechanical (e.g. the effort of the opera to r ) . As the area of open surface requir ing cleaning and disinfection increases, manual cleaning becomes uneconomical with respect to time and labour, and onlyllight levels of soiling can be removed economically by this method. This is due to the labour cost, which usually represents 75% of the total cost of the sanitation programme. For most food companies, the cost of addit ional labour resources is prohibitive. \

Specialist equipment therefore becomes necessary in the cleaning of larger areas, to rinse surfaces, dispense chemicals and/or provide mechanical energy. Chemicals may be applied as low-pressure mists, foams or gels, while mechanical energy is provided by high- and low-pressure water jets or electrically-powered scrubbing brushes (5; 19, 36). Alternatively, dismantled equipment and production utensils may undergo manual removal of gross soil, and may then be cleaned and disinfected automatically in tray or

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tunnel washers. As with soak tank operat ions, high levels of chemical and thermal energy can be used to cope with the majority of soils.

The differences between the mist, foam and gel techniques reside in varying abilities to maintain detergent/soil/surface contact time. For all three techniques, mechanical energy can be varied by the use of high- or low-pressure water rinses, while temperature effects are minimal for open-surface cleaning.

Mist spraying of chemicals is under taken using small hand-pumped containers , knapsack sprayers or pressure washing systems at low pressure. Misting 'wets ' non-horizontal surfaces; only small quanti t ies can be applied, as otherwise chemical solutions would quickly run off. This technique results in a useful contact t ime of < 5 min. The tendency for aerosols to be formed using this technique (an operative safety hazard) means that only weak chemicals can be applied and the use of misting is therefore limited to light soiling. However, misting is the method most commonly used for applying disinfectants to cleaned surfaces.

Foams can be genera ted and applied by the en t rapment of air in high-pressure systems or by the addition of compressed air in low-pressure equipment. Foams work on the principle of forming a layer of bubbles above the surface to be cleaned. The outer film of the bubble holds the cleaning chemical and, as the bubbles collapse, the surface is wetted with fresh chemical solution. The crucial element in foam generation is for the bubbles to collapse at the correct rate: too fast and the contact time will be minimal; too slow and the surface will not be bathed with fresh chemical solution. When the rate is correct, contact times of approximately 15 min are possible on vertical surfaces.

The use of thixotropic gels has recently been introduced; these are typically fluid at high and low concentrat ions but become thick and gelatinous at a concentrat ion of 5-10%. These gels are easily mixed and applied through high- and low-pressure systems, foaming equipment , or por table electric pump units . In contrast to foams, gels physically attach to the surface, and when applied properly (so that the correct 'gelling' properties are obtained) gels will remain surface-bound almost indefinitely.

Foams and gels are more viscous than mists and are less prone to aerosol formation, therefore allowing the use of more concentrated detergents . These techniques are therefore able to cope with higher levels of soils than misting although, in some cases, rinsing of surfaces may require large volumes of water. Foams and gels are popular with operators and management, as a more consistent application of chemicals is possible and it is easier to spot areas which have been 'missed'.

Mists, foams and gels are removed from surfaces by low-pressure hoses operating at mains water pressure or by high-pressure/low-volume systems. Pressure washing systems may be mobile units (in which water is typically pumped at pressures of up to 100 bar [107 Pa] through a 15° nozzle), wall-mounted units (serving one or more outlets) or centralised units (where one unit may supply many outlets via a ring main). Water jets confer high mechanical energy, can be used on a wide range of equipment and environmental surfaces, are not limited to flat surfaces, will pene t ra te into surface irregularities, and are able to mix and apply sanitation chemicals.

Mechanical scrubbers for use on floors, walls and other surfaces include traditional floor scrubbers, scrubber/driers (automats) which vacuum up the cleaning solution, water-driven attachments to high-pressure systems, and electrically-operated small-diameter brushes. Contact time is usually limited with these techniques (although this time can be increased), but the combination of detergency and high mechanical input

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allows these systems to tackle most types and levels of soil. As with other automatic techniques, mechanical scrubbers are popular with opera tors and the only real limitation is that food processing areas have not traditionally been designed for the use of such equipment. However, this can be amended in new or refurbished areas.

SANITATION PROGRAMME

Sanitation programmes are designed to enable efficient use of water and chemicals, to allow selected chemicals to be used under optimum conditions, to ensure the safety of operators, machinery and products, to be easily managed, and to reduce manual labour and cleaning costs. In this way, an adequa te level of sanitat ion will be achieved economically and with due regard to environmental pollution.

The sani tat ion programme forms par t of a cleaning or sani tat ion schedule. The schedule should clearly illustrate each stage of the cleaning and disinfection process, all pertinent information on safety, and the key inspection points and means of assessment. A typical cleaning schedule includes the following:

- description of all chemicals to be used (together with hazard code, in-use concentration, method of preparation, storage conditions and location, and amount for use)

- information on sanitat ion equipment (type, use instructions, set parameters [pressure, nozzle type, etc.], maintenance instructions and location)

- description of the equipment to be cleaned, need for maintenance fitters, and procedures for dismantling and reassembly of equipment

- full description of the cleaning process, frequency and requirement for periodic measures

- staff requirements and responsibilities

- key points for assessment of the sanitat ion procedure and description of evaluation procedures.

The principle stages involved in a typical sanitation programme are described below.

Step 1: Production periods

Production staff should be encouraged to operate good cleanliness practices during production and leave their work stations in a reasonable condition (soil left on process lines is wasted product!) . Such attention by production staff facilitates the job of the sanitation team, thus improving the quality and safety of the product.

Step 2: Preparation

After production, machinery should be switched off, and electrical and other sensitive systems should be protected from water or chemical ingress. Equipment should be dismantled as required, and unwanted utensils and equipment removed. Dismantled equipment should be stored on racks, tables or soak tanks - not on the floor!

Step 3 : Gross soil removal

All loosely-adherent or gross soil should be removed manually by brushing, scraping, shovelling or vacuum suction, etc. Wherever possible, soil on floors and walls should be picked up rather than hosed into drains.

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Step 4: Pre-rinse

Surfaces should be rinsed from the top downwards with low-pressure cold water, to remove loosely-adherent small debris. Hot water may be used for fatty soils, but too high a temperature may coagulate proteins, making these more difficult to remove.

Step 5: Cleaning

A selection of cleaning chemicals, tempera tures and mechanical energy sources should be used to remove adhered soils.

Step 6: Intermediate rinse

Soil which has been detached and dispersed by cleaning operations, and residues of cleaning chemicals, should be removed from surfaces by rinsing with low-pressure cold water.

Step 7: Disinfection

If necessary, depending on the food product and process involved, chemical disinfectants (or occasionally heat) are applied to remove and/or reduce the viability of remaining microorganisms to a level of no significant risk.

Step 8: Final rinse

Disinfectant residues should be removed by rinsing away with low-pressure cold water of known potable quality.

Step 9: Inter-production cycle conditions

Procedures should be under taken to prevent the growth of microorganisms on product contact surfaces prior to the commencement of the next production process. This may include procedures to control microbial growth (e.g. the removal of excess water and/or equipment drying) or prevent cross-contamination from other sources (e.g. the use of screens when cleaning adjacent production lines).

Step 10: Periodic practices

Procedures should be undertaken at given time periods (e.g. weekly or monthly) to clean equipment more thoroughly than by daily cleaning. This normally involves addit ional dismantling of equipment and/or the application of increased cleaning energy. Periodic practices also include the cleaning of areas/items which are usually cleaned less frequently (e.g. ceilings and overhead fittings).

REQUIREMENTS FOR SANITATION OF HIGH-RISK FOOD PRODUCTION AREAS

The requirement to control specific pathogens (particularly Salmonella and Listeria) in high-risk food product ion areas has led to the examination of routes of product contamination which, although not specific to such manufacture, are particularly critical during the production of high-risk foods. The two major factors have been identified as hygienic design and cross-contamination.

Hygienic design

Hygienic design is fundamental to the control of equipment and environmental contamination. Good hygienic design prevents the retention of product outside the

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main product flow during processing and the re ten t ion of product soils (including microorganisms) after cleaning. Poor hygienic design is often characterised by rough surfaces, crevices and dead spaces, which can retain product residues. Equipment and other environmental surfaces which retain product after cleaning cannot be effectively disinfected (except by heat), and therefore contamination cannot be controlled.

The principles of hygienic design are currently being established in Europe through a European Standards Technical Committee (CEN/TC 153) and an independent group, the European Hygienic Design of Equipment Group. Several articles on this subject have also been published (3, 4, 6, 29, 52, 53). The areas of hygienic design generally agreed to be of the most common concern for equipment and (where appropriate and using the same principles) environmental surfaces are as follows:

- construction materials - surface finish - joints - fasteners - drainage - internal angles and corners - dead spaces - bearings and shaft seals - instrumentation - doors, covers and panels - controls.

Hygienic design cannot be examined in full detail within the scope of this article but, as an example, the importance of design can be demonstrated for meat slicing machines (Fig. 2). Several cases have occurred in the recent past in which Listeria monocytogenes has contaminated sliced meat products as a result of inadequate cleaning and disinfection of slicing equipment. This was due to both inadequate dismantling and poor hygienic construction (54).

Metal /metal or metal/plastic joints may be sufficiently tight to prevent the accumulation of product residues, but will allow the entry of microorganisms which will thus be protected from subsequent sanitation programmes. Figure 3 shows a schematic drawing of a shear edge and a guide bar which guides meat onto the blade. Both of these sections of the slicer are in contact with every piece of meat sliced, and when these were dismantled further than normally required by the sani tat ion p rogramme, L. monocytogenes was found at the points marked 'A' . When these areas were dismantled on a regular basis for sanitation, problems associated with L. monocytogenes in the product were greatly reduced.

Figure 4 shows a range of fasteners of poor hygienic construction, characterised by dead spaces (A) , metal /metal contact points (B) and crevices (C). In each case, L. monocytogenes has been isolated from such areas and is difficult or impossible to control via sanitation. Contamination control can be ensured only by replacing these unhygienic fasteners with those of good hygienic design, as illustrated in Figure 5.

The hygiene implications of the design and use of cleaning equipment should be carefully considered and have been described by Holah et al. (23). Unpublished work conducted at the Campden Food and Drink Research Association has shown that in high-risk areas, cleaning equipment is the most likely source of environmental contamination with Listeria spp. and other pathogenic microorganisms, and that such

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FIG. 2

Schematic side elevation of a typical orbital slicing machine

equipment , by the nature of its use, provides an efficient means of transferring contaminat ion throughout the food processing environment. After use, therefore, cleaning equipment should be thoroughly cleaned, dried and, if appropriate, disinfected.

The importance of hygienic design cannot be overstated. Regardless of the quality of the design and execution of the sanitation programme, contamination will never be controlled unless the areas in which soils may be present are exposed. Persons responsible for the management of sanitation programmes in high-risk food production areas must become familiar with the principles of hygienic design to ensure safe and wholesome food products.

Cross-contamination

The potent ia l for cleaning equipment to disperse microbial and physical contamination by the formation of aerosols has been reported (23,25), and all cleaning equipment tested was shown to produce viable bacterial aerosols from experimentally-contaminated test surfaces.

Given a typical food-contact surface height of 1 m, both high-pressure/low-volume (HPLV) and low-pressure/high-volume (LPHV) techniques were shown to disperse a significant level of aerosol to this height and therefore should not be used during production periods. Reducing water pressure or changing the impact angle made little difference to the degree of aerosol spread, and dispersal to heights of > 1 m was achieved under all test condit ions. However, other techniques (including floor scrubbers, mechanically-driven brushes and manual techniques) were acceptable for use in 'clean-as-you-go' operat ions (which are to be encouraged) , as the chance of contaminating products is low when using such methods. After production, HPLV and LPHV techniques may be safely used (and are likely to be the appropriate choice), but

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FIG. 3

Schematic drawing of a shear edge (upper) and a guide bar (lower) to illustrate examples of unhygienic dismountable joints (A)

sufficient t ime must be allowed between cleaning and disinfection to ensure that aerosols have settled onto surfaces.

Microbial aerosols can also be generated by automated cleaning and disinfection systems (such as tray washers) and these should not be sited directly in high-risk production areas without physical barriers. Neither manual cleaning stations nor hand­washing facilities are thought to produce significant aerosol levels.

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A: dead space B: metal/metal contact point C: annular crevice

FIG. 4

Examples of unhygienic fasteners

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FIG. 5

Examples of hygienic fasteners

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To minimise the risks of cross-contamination within the sanitation programme, a sanitat ion sequence should be established for each processing area. A sani tat ion sequence determines the order in which the product contact surfaces (equipment) and environmental surfaces (walls, floors, drains, etc.) are sanitised, so that disinfected product contact surfaces are not recontaminated by cleaning aerosols . A typical sanitation sequence would be as follows:

a) Remove gross soil from production equipment. b) Remove gross soil from environmental surfaces. c) Rinse equipment and environmental surfaces (usually to a minimum height of 2 m

for walls) from top to bottom, and flush to drain. d) Clean environmental surfaces. e) Rinse environmental surfaces. f) Clean equipment. g) Rinse equipment. h) Disinfect equipment. i) Rinse equipment.

If the principles of cross-contaminat ion are unders tood by cleaning staff and management, it should be possible to undertake a sanitation programme to minimise the level of microbial and physical contamination on all product-contact surfaces within the food processing area prior to the commencement of production.

MONITORING OF SANITATION

Sanitation programmes may be monitored immediately by sensory evaluation, and historically (if the sensory evaluat ion is satisfactory) by microbiological methods . Sensory evaluations are used as a process control to immediately rectify obvious shortfalls in sanitation, while microbiological assessments are typically used to ensure compliance with microbial standards and optimise sanitation procedures. Methods have been developed to rapidly assess microbial surface populations and/or soil residuals in a time relevant to process control (usually taken as less than 15-20 min). This time scale is sufficient to allow a decision to be made on whether the sanitation programme should be repeated.

Sensory evaluation involves visual inspection of surfaces under good lighting, smelling for product or offensive odours, and feeling for greasy or encrusted surfaces. For some product soils, residues are observed more clearly by wiping the surface with paper tissues. If no product residues are detected, microbiological techniques may be used; these have been extensively reviewed (7, 32, 42) and involve the removal or sampling of microorganisms from surfaces (using sterile cotton or alginate swabs, sponges or r inses), which are then cul tured using s tandard agar plating methods . Alternatively, microorganisms may be sampled directly onto self-prepared or commercial ('dip slides') agar contact plates.

A range of rapid techniques is available (20) but most of these are either research tools or are too expensive for routine use, and only the adenosine triphosphate (ATP) technique is regularly used. This technique is based on the assessment of levels of ATP present in animal, p lant and microbial cells, via an enzyme-linked system which produces light in proportion to the concentration of ATP present. Surfaces are sampled by swabbing or rinsing (48, 51), and addition of suitable reagents enables measurement

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of the level of ATP in approximately 5 min using a luminometer. In the majority of applications, analysis of total ATP is preferred on the assumption that any residues, soil or microorganisms should have been removed.

CONCLUSION

When under t aken correctly - and following a strict sani tat ion p rogramme determined by the hygiene requirements for the relevant food product - cleaning and disinfection re turn food product ion surfaces to a condit ion of minimum risk for subsequent food product ion. The acceptable level for numbers of microorganisms remaining on a surface after cleaning will depend on the food product, the process, the 'risk a rea ' , the level of microorganisms present before cleaning, and the degree of sanitation undertaken. Given initial levels of 10 5 organisms/cm 2, recent studies in high-risk food production areas have shown reductions of approximately 5 log orders by the sanitation programme (20), and thus levels of < 10 organisms/cm 2 should be obtained.

Finally, even the best technical sanitat ion p rogramme is only as good as the opera tors . Sanitat ion staff must therefore be adequately t ra ined, and senior management must take full responsibility for the successful operation of the sanitation programme as, ultimately, failures in the programme usually reflect poor management.

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DÉSINFECTION DES ZONES DE PRODUCTION ALIMENTAIRE. - J.T. Holah.

Résumé: Toute désinfection (sauf la désinfection thermique) est inefficace lorsque les surfaces n'ont pas d'abord été parfaitement nettoyées afin d'éliminer ce qui peut gêner l'action du produit désinfectant. Le nettoyage est par conséquent extrêmement important dans le cadre d'un programme en deux étapes : nettoyage et désinfection (assainissement). L'auteur décrit les principes de l'assainissement, les produits chimiques et les équipements utilisés ainsi que la suite des opérations à effectuer. En ce qui concerne les produits alimentaires à « faible risque » (du fait de leur durée de conservation et de leur innocuité), les programmes d'assainissement traditionnels sont adéquats, et la désinfection n'est pas toujours indispensable. Cependant, la désinfection est essentielle pour les produits alimentaires à « haut risque », mais pour y procéder efficacement il faut avoir bien pris en compte la conception hygiénique des équipements et les problèmes de contaminations croisées éventuelles. Afin de s'assurer de la réussite durable d'un programme d'assainissement, il convient de procéder à des vérifications de routine.

MOTS-CLÉS : Assainissement - Biofilms - Conception hygiénique des équipements - Désinfection - Hygiène alimentaire - Nettoyage - Production alimentaire.

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DESINFECCIÓN EN LAS ÁREAS DE PRODUCCIÓN ALIMENTARIA. - J.T. Holah.

Resumen: Toda desinfección, salvo la desinfección térmica, es ineficaz si no se ha efectuado previamente una limpieza absoluta de todas las superficies de

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manera de eliminar las sustancias que podrían interferir en la acción del desinfectante. La limpieza es por lo tanto, sumamente importante en el marco de un programa de saneamiento (que comporta ambas etapas: limpieza más desinfección). El autor describe los principios del saneamiento, los productos químicos y equipos que deben usarse y la serie de operaciones que se han de efectuar. En el caso de productos alimentarios de «bajo riesgo» (por su período de conservación y su inocuidad), los programas de saneamiento tradicionales son adecuados y en ciertos casos la desinfección no resulta indispensable. En cambio, es esencial desinfectar en el caso de productos alimentarios de «alto riesgo», pero para hacerlo de manera eficaz es necesario tener muy en cuenta la higiene en el momento de diseñar los equipos y las posibilidades de contaminaciones cruzadas. Por último, para garantizar una continuidad eficiente de los programas de saneamiento, conviene proceder a evaluaciones de rutina.

PALABRAS CLAVE: Biofilmes - Desinfección - Diseño higiénico de los equipos - Limpieza - Producción alimentaria - Protección alimentaria -Saneamiento.

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