This Section is prepared by Dr. Susanne Knochel
Water used for food processing is one of the important critical control points. This is true for water used as an ingredient, for water used as final rinse when cleaning equipment or water which is in any way likely to come into contact with the product. Most often it is just stated that the water should meet drinking water standards and both supply and quality are mostly taken for granted. However, local standards may vary somewhat or may even be absent. The quality of the source water differs enormously from place to place as does the water treatment. The control exerted by the local regulatory authorities may also differ greatly depending on the local situation. Lastly, in-plant problems may sometimes render potable water unfit as drinking water at the final point of use.
So how can acceptable drinking water quality be defined ? What is the rationale behind these guidelines ? And what can the food processors do ?
A universally accepted list of standards for biological and physico-chemical parameters for drinking water does not exist.
WHO issued an excellent book called “Guidelines for drinking water quality”, Vol 1, 2, and 3 (WHO 1984b). Volume 1 deals with the guideline values, Volume 2 contains monographs on each contaminant, and Volume 3 gives information on how to handle the water supplies in small, rural communities. In this book, WHO recognizes that very stringent standards cannot be used universally as this may severely limit the availability of water and instead, a range of guideline values for more than 60 parameters have been elaborated. A general review of the standards employed by WHO, EEC, Canada and USA is given by Premazzi et al. (1989). It is recognized that e.g. most of the rural wells all over the world would have difficulties meeting all the guideline values suggested. It goes without saying that all the parameters cannot be monitored so selection and priorities must be made based on hazard analysis and feasibility. Most nations (or in some cases even individual provinces) have their own guidelines or standards. The basic microbiological guideline values, however, do not differ so much from place to place. Below are the microbiological parameters and guideline values suggested by WHO (Table 6.1) and EEC (Table 6.2)
Table 6.1. Microbiological criteria (guidelines) for drinking water quality (WHO 1984b).
|Organism in 100 ml1)||Guideline value||Remarks|
|Piped water supplies|
|Treated water entering the distribution system|
|fecal coliforms coliform||0||turbidity < 1 NTU; for disinfection with chlorine, Ph preferably < 8.0, free chlorine residual 0.2–0.5 mg/1 following 30 min (minimum) contact|
|Water in the distribution system|
|coliform||in 95% of samples examined throughout the year -|
|organisms||0||in the case of large supplies when sufficient samples are examined.|
|coliform||in an occasional sample but not in consecutive|
1) Multiple tube techniques (MPN procedure) and the membrane filtration technique havebeen considered as capable of yielding comparable information.
|Maximum admissible concentration (MAC)|
|Parameters||Results: volume of the sample (ml)||Guide level (GL)||Membrane filter method||Multiple tube method (MPN)|
|Total bacteria counts||11)||101)|
|for water supplied for human consumption||12)||1002)|
1) Incubation at 37°C
2) Incubation at 22°C
In the case of water used for food production, it is of vital importance that these microbiological guideline values should be met since potentially pathogenic bacteria are capable of multiplying rapidly if they are introduced into foodstuffs making even initially low and non-infectious doses of bacterial pathogens, a hazard.
Disinfectant residuals should be monitored where possible and periodic verifications of the bacteriological quality should be conducted. Turbidity, colour, taste and odour are also easily monitored parameters. If there are local problems with chemical constituents (e.g. fluoride, iron) or contaminants from industry or agriculture (e.g. nitrate, pesticides, mining waste) these should hopefully be monitored and dealt with by the water suppliers.
Water treatments vary from region to region depending on the water sources available. While groundwater from sedimentary aquifers has undergone extensive filtration the water from hard rock aquifers or surface water sources should be filtered as part of the water treatment in order to decrease the content of particulates, microorganisms and organic and inorganic matter.
Parasites are removed to a large extent by filtration. The levels of bacteria and virus also decrease markedly and the removal mechanisms are both filtration and adsorption. The cation concentration influences adsorption, i.e., increasing concentrations give rise to increased adsorption. Ca2+ and Mg2+ seem to be especially efficient. These small cations will decrease the repulsive forces between the soil particles and the microorganisms. Iron oxides also have a high affinity for viruses as well as bacteria. Ferric hydroxide impregnated lignite has even been suggested as a local filtration/adsorption media (Prasad and Chaudhuri 1989).
The disinfection efficiency is greatly affected by type of disinfectant, type and state of microorganism, water quality parameters such as turbidity (or suspended solids), organic matter, some inorganic compounds, Ph and temperature. The “hardness” of the water may indirectly influence disinfection since deposits may harbour microorganisms and protect them from cleaning agents and disinfectants.
By far the most widespread disinfectant is chlorine but also chloramines, chlorine dioxide, ozone and UV light are being used in some instances. Chlorine is cheap and available in most places and monitoring free residual levels is simple. It is desirable to maintain a free residual chlorine level of 0.2–0.5 mg/l in the distribution system (WHO 1984b). For sanitation of clean equipment, up to 200 mg/l is used. To avoid corrosion lower concentrations of 50–100 mg/l and longer contact times (10–20 minutes) are often used. Chloramines are more stable but less bacteriocidal and much less efficient towards parasites and virus than chlorine. Chlorine dioxide is, if anything, more microbicidal than chlorine, especially at high Ph, but there is concern with regards to the by-products. In the case of ozone and UV light there is no residual to monitor. Ozone seems to be very efficient towards protozoa. The efficiency of UV disinfection decreases markedly if there is any turbidity or dispersed organic matter and problems are often encountered due to lack of lamp maintenance.
In the case of most disinfectants, the order of sensitivity is:
vegetative bacteria > viruses > bacterial spores, acid-fast bacteria and protozoan cysts
The sensitivity varies within groups and even within species. Our indicator bacteria are unfortunately among the more sensitive microorganisms and the presence of e.g. fecal coliforms in treated, disinfected water is therefore a very clear indication that the water contains potentially pathogenic microorganisms while the absence of such indicator bacteria do not guarantee pathogen-free water.
Bacteria from nutrient-poor media as well as otherwise stressed bacteria may also exhibit greatly increased resistance. Some of the effects mentioned on the efficiency of free chlorine are illustrated in Table 6.3.
If microbes are associated with granular material or other surfaces the effect of a disinfectant such as chlorine decreases drastically. Attachment of Klebsiella pneumonia to glass surfaces may for example increase the resistance to free chlorine 150-fold (Sobsey 1989).
Organic matter may react and “consume” disinfectants such as chlorine and ozone and the presence will also interfere with UV light. The chloramines are less susceptible to organic matter.
Ph is important in disinfection with chlorine and chlorine dioxide with greater inactivation at low Ph in the case of chlorine and greater inactivation at high Ph in the case of chlorine dioxide (Sobsey 1989).
In general, higher temperatures result in increased inactivation rates.
|Organism||Water||Cl2residues, mg/l||Temperature, °C||Ph||Time, min.||Reduction %||C*t1)|
|E. coli + GAC5)||CDF||1.5||4||?||60||<<10||>>60|
1) C*t product of disinfectant concentration (C) in mg/l and
2) BDF = buffered demand free
3) ND = no datacontact time (t) in minutes for 99% inactivation (mod.a. Sobsey 1989)
4) CDF = Chlorine demand free
5) GAC = granular activated carbon
The use of non-potable water may be necessary for water conservation purposes or desirable because of cost. The water may e.g. be surface water, sea water or chlorinated water from can cooling. Relatively clean water such as chlorinated water from can cooling operations may be used for washing cans after closing before heat treatment, for transporting raw materials before processing (after the water has cooled off), for initial washing of boxes, for cooling of compressors, for use in fire protection lines in non-food areas and for fuming of waste material. It is absolutely necessary that potable and non-potable water should be in separate distribution systems which should be clearly identifiable. If potable water is used to supplement a non-potable supply the potable source must be protected against valve leaking, back-pressure e.g. by adequate air-gaps (Katsuyama and Strachan 1980). Back-flow due to sudden pressure differentials or blockage of pipes have unfortunately occurred in many systems.
Potentially contaminated water such as coastal water or surface water should not be used at the production premises but may, if aesthetically acceptable, be used for removing waste material in places where no contact to food is possible.
The responsible person should have continuously updated reference drawings of the pipe system and the authority to remove dead-ends. Especially in cases where a plant has undergone many changes, the piperuns may become more and more complicated over the years. The person should also be in contact with the local waterworks and the authorities in order to be informed of special events (repairs, pollution accidents or other changes).
A quality monitoring scheme could consist of a schematizised plan of all the sampling points and a checklist for each point describing what to examine and why, the frequency, who takes the sample, who does the analysis, what is the limit (value, tolerance) and what to do in case of deviation (Poretti 1990). If the water is obviously polluted there is of course no reason to wait for analytical results. The sampling frequency and the range of parameters will vary with the circumstances and the needs and possibilities of the specific plant. A minimum program may for example consists of monitoring free chlorine daily and total counts plus coliforms on a weekly basis and a special, more intense monitoring program to be used after repairs, when using new water supplies etc.
The technical procedures describing the analyses for the common indicator organisms are given in standard textbooks. The WHO “Guidelines for drinking-water Quality”, vol. 3 (WHO 1984b) mentions some methods and equipment suitable for small, rural supplies. The values used by the company should refer to the specific method employed and the recommendations should include how to sample (tap flow, volume, sampling vessel, labelling etc.) and how to handle and examine the sample. Even though the commonly used methods for detecting e.g. fecal coliforms are standard analyses faulty handling of the samples often occurs. Samples should be processed within 24 hours or less and be kept cool, but not frozen (preferably below 5°C), and in the dark. The impact of sunlight can be very dramatic causing false negative results (Knochel 1990).
If chlorination is used for disinfection, monitoring of the free chlorine level is the simplest way of checking the water treatment and should be performed most often (e.g. on a daily basis). Simple laboratory methods are described by WHO (1984b) and commercial dipsticks are now available for on-the-spot measurements (e.g. Merckoquant Chlor 100 from Merck). The microbiological indicator parameters may be checked less frequently. If disinfection systems leaving no residuals are being used, checking the equipment should be done regularly. The performance of the systems may be monitored at weekly intervals using indicator bacteria measurements.
This Section is prepared by Professor Mogens Jakobsen
Cleaning and disinfection belong to the most important operations in today's food industries. Numerous and costly cases of food spoilage and unacceptable contamination with pathogenic bacteria has been traced back to failures or insufficiencies of these procedures.
The standards of hygiene required to avoid such problems are variable. In a plant, packaging products processed for safety (e.g. by heat treatment) requirements will be very strict whereas handling of fresh chilled fish with a short shelf life and which is cooked before consumption, will be less demanding.
Factors like housekeeping, personal hygiene, training and education, plant layout, design of equipment and machines, characteristics of materials selected, the maintenance and general condition of the plant can easily become more important than the actual cleaning and disinfection. For optimal use of resources and to ensure the microbiological quality of foods, it is important that all such factors are addressed when deciding on cleaning and disinfection procedures.
In some cases it may even be best to avoid cleaning and disinfection, because more harm than good can be done. As an example, this applies for dust accumulated on pipes and constructions unless time allows for a complete removal. Further, as another example, dry areas should always be kept dry and cleaning will then be limited to vacuuming if available, or sweeping, brushing etc.
It follows from the above that for each particular food plant or operation, implementation of cleaning and disinfection procedures is a project on its own where specialists, internal or external, should be consulted.
Cleaning and disinfection will be processes like any other plant operation, and they should be equally documented and so should the corresponding process control i.e. the control of cleaning and disinfection respectively. If a HACCP concept is applied, these procedures should be treated as Critical Control Points (CCPs). If a Quality System like ISO 9000 is in operation, they should be integrated in the System as illustrated in the previous chapter of this book. Responsible management realizes that these procedures are integrated parts of production and poor hygienic condition in food processing plants will primarily be caused by management lack of knowledge and commitment.
For the whole process, three distinct operations are involved i.e.
i) preparatory work; ii) cleaning and iii) disinfection. They are clearly distinct operations but linked firmly together in the way that the final result will not be acceptable, unless all three are carried out correctly. Table 6.4 shows the various steps, which will be included in a complete cycle.
Table 6.4. Steps included in the complete cycle of preparatory work, cleaning, disinfection and control.
Remove food products, clear the area for bins, containers etc.
Dismantle equipment to expose surfaces to be cleaned. Remove small equipment, parts and fittings to be cleaned in a specified area. Cover sensitive installations, to protect them against water etc.
Clear the area, machines and equipment for food residues by flushing with water (cold or hot) and by using brushes, brooms etc.
Apply the cleaning agent and use mechanical energy (e.g. pressure and brushes) as required.
Rinse thoroughly with water to completely remove the cleaning agent after the appropriate contact time, (residues may completely inhibit the effect of disinfection).
Control of cleaning.
Sterilization by chemical disinfectants or heat.
Rinse the sterilant off with water after the appropriate contact time. This final rinse is not needed for some sterilants e.g. H2,O2, based formulations which decompose rapidly.
After the final rinse, equipment is reassembled and allowed to dry.
Control of cleaning and disinfection.
In some cases it will be good practice to re-disinfect (e.g. with hot water or low levels of chlorine) just before production starts.
In this phase, the processing area is cleared of remaining products, spills, containers and other loose items. Machines, conveyors etc. are dismantled so that all locations, where microorganisms can accumulate become accessible for cleaning and disinfection. Further electrical installations and other sensitive systems should be protected against water and the chemicals used.
Before use of the cleaning agent, a gross food debris removal procedure should be carried out by brushing, scraping or similar. All surfaces should be further prepared for the use of cleaning agents by a pre-rinse activity preferably with cold water which will not to coagulate proteins. Hot water may be used to remove fat or sugars in cases, where protein is not present in significant amounts.
Completion of the preparatory work should be checked and recorded as any other process, to ensure the quality of the complete cycle of cleaning and disinfection.
Cleaning is undertaken to remove all undesirable materials (food residues, microorganisms, scales, grease etc.) from the surfaces of the plant and the process equipment, leaving surfaces clean, as determined by sight and touch and with no residues from cleaning agents.
Microorganisms present will either be incorporated in the various materials or they attach to the surfaces as biofilms. The latter will not be removed completely by cleaning, but experience has shown that a majority of the microorganisms will be removed. However, there will still be some left to be inactivated during the disinfection.
The effectiveness of a cleaning procedure in general depends upon:
The type and amount of material to be removed.
The chemical and physio-chemical properties of the cleaning agent (such as acid or alkali strength, surface activity, etc.) at the concentration, temperature and exposure time used.
The mechanical energy applied e.g. turbulence of cleaning solutions in pipes, stirring effect, impact of water jet, “elbow-grease”, etc.
Condition of the surface to be cleaned.
Some surfaces e.g. corroded steel and aluminum surfaces can simply not be cleaned which means that disinfection also becomes very inefficient. The same applies for other surfaces e.g. wood, rubber etc. The preferred material obviously will be high quality stainless steel.
The types of residues to be removed in food plants, will mainly be the following:
Organic matter, such as protein, fat and carbohydrate. These are most effectively removed by strongly alkaline detergents (especially caustic soda, NaOH). Further, it is found that combinations of acid detergents (especially phosphoric acid) and non-ionic surfactants are effective against organic matter.
Inorganic matter, such as salts of calcium and other metals. In beer stone, milk stone, etc., salts are encrusted with protein residues. These are most effectively removed by acid cleaning agents.
Biofilms, formed by bacteria, moulds, yeast and algae can be removed by cleaning agents that are effective against organic matter.
Most cleaning agents work faster and more effectively at higher temperatures, so it can be profitable to clean at a high temperature. Cleaning is often carried out at 60–80°C in areas where it pays energy wise to use such high temperatures.
Water is used as a solvent for all cleaning and sterilizing agents, and also for intermediate rinses and final rinse of equipment.
The chemical and microbiological quality of the water is therefore of decisive importance for the efficiency of the cleaning procedures as already described in a previous section of this chapter. In principle, water used for cleaning must be potable.
Hard water contains a large amount of calcium and magnesium ions. When the water is heated, calcium and magnesium salts corresponding to the temporary hardness will precipitate as insoluble salts. Also, some cleaning agents, especially alkalis, can precipitate calcium and magnesium salts.
Apart from reducing the effectiveness of detergents hard water leads to the formation of deposits or scales. Scales which can be formed in several other ways are not only unsightly but objectionable of several reasons:
They harbour and protect microorganisms.
They reduce the rate of heat exchange on heat exchanger surfaces. This could lead to underprocessing, underpasteurization or understerilization.
The presence of scales tends to increase corrosion.
The formation of scales can be reduced by addition of chelating and sequestering agents, which bind calcium and magnesium in insoluble complexes. However, it is advisable to prevent precipitations by softening the water before it is used for cleaning. Softening can be effectively achieved by ion exchange, in which the calcium and magnesium ions are replaced by sodium ions, the salts of which are soluble. A modern, and more costly, method of softening water is by means of reverse osmosis.
Microbiological purity of water to be used for final rinse must be beyond reproach. If not, it will in some cases be acceptable to include low levels of chlorine i.e. a few ppm.
The ideal detergent would be characterized by the following properties:
It possesses sufficient chemical power to dissolve the material to be removed.
It has a surface tension low enough to penetrate into cracks and crevices; it should be able to disperse the loosened debris and hold it in suspension.
If used with hard water, it should possess water softening and calcium salt dissolving properties to prevent precipitation and build-up of scale on surfaces.
It rinses freely from the plant, leaving this clean and free from residues, which could harm the products and affect sterilization negatively.
It does not cause corrosion or other deterioration of the plant. It is recommended always to check by consulting the supplier of machines etc.
It is not hazardous for the operator.
It is compatible with the cleaning procedure being used, whether manual or mechanical.
If solid, it should be easily soluble in water and its concentration easily checked.
It complies with legal requirements concerning safety and health as well as biodegradability.
It is reasonably economical to use.
A detergent with all these characteristics does not exist. So one must, for each individual cleaning operation, select a compromise by choosing a useable cleaning agent and water treatment additives so that the combined detergent has the properties that are most important for the procedure concerned.
When choosing a cleaning agent, one can pick either a ready-mixed factory product, which has the desired properties, or it can be homemade following the guidelines given in Table 6.5. In this case it must be assured that the components are mutually compatible.
Table 6.5 (from Lewis 1980) shows important characteristics of the cleaning agents most commonly used in the food industry.
|Categories of aqueous cleaners||Approximate concentrations for use (%, w/v)1)||Examples of chemical used2)||Functions||Limitations|
|Clean water||100||Usually contains dissolved air and soluble minerals in small amounts||Solvent and carrier for soils, as well as chemical cleaners||Hard water leaves deposit on surfaces. Residual moisture may allow microbial growth on washed surfaces.|
|Strong alkali||1–5||Sodium hydroxide Sodium orthosilicate Sodium sesquisilicate||Detergents for fat and protein. Precipitate water hardness||Highly corrosive. Difficult to remove by rinsing. Irritating to skin and mucous membranes.|
|Mild alkali||1–10||Sodium carbonate Sodium sesquisilicate Trisodium phosphate Sodium tetraborate||Detergents. Buffers at Ph 8.4 or above Water softeners||Mildly corrosive. High concentrations are irritating to skin|
|Inorganic acid||0.5||Hydrochloric Sulphuric Nitric Phosphoric Sulphamic||Produce Ph 2.5 or below Remove inorganic precipitates from surfaces||Very corrosive to metals, but can he partially inhibited by anti-corrosive agents. Irritating to skin and mucous membranes|
|Moderately corrosive, but can be inhibited by various anti-corrosive compounds|
|Anionic wetting agents||0.15 or less||Soaps Sulphated alcohols Sulphated hydrocarbons Aryl-alkyl polyether sulphates Sulphonated amides Alkyl-arylsuphonated||Wet surfaces Penetrate crevices and woven fabrics Effective detergents Emulsifiers for oils, fats, waxes, and pigments Compatible with acid or alkaline cleaners and may be synergistic||Some foam excessively Not compatible with cationic wetting agents|
|Non-ionic wetting agents||0.15 or less||Polyethenoxyethers condensates Amine-fatty acid condensate||Excellent detergents for oil. Ethylene oxide-fatty acid agents to control foam||May be sensitive to acids Used in mixtures of wetting|
|Cationic wetting agents||0.15 or less||Quaternary ammonium||Some wetting effect Antibacterial action||Not compatible with anionic wetting agents|
|Sequestering agents||Variable (depending on hardness of water)||Tetrasodium pyrophosphate Sodium tripolyphosphate Sodium hexametaphosphate Sodium tetrapolyphosphate Sodium acid pyrophosphate Ethylenediaminetetra-acetic acid (sodium salt) Sodium gluconate with or without 3% sodium hydroxide||Form soluble complexes with metal ions such as cal- cium, magnesium and iron to prevent film formation on equipment and utensils See also strong and mild alkalis above||Phosphates are inactivated by protracted exposure to heat Phosphates are unstable in acid Solution|
|Abrasives||Variable||Volcanic ash Seismotite Pumice Feldspar Silica flour Steel wool3) Metal of plastic 'chlore balls3) Scrub brushes||Removal of dirt from surfaces with scrubbing Can be used with detergents for difficult cleaning jobs||Scratch surfaces Particles may become imbedded in equipment and later appear in food Damage skin of workers|
|Chlorinated compounds||1||Dichlorocyanuric acid Trichlorocyanuric acid Dichlorohydantoin||Used with alkaline cleaners to petizing of proteins and minimize milk deposits||Not germicidal because of high Ph Concentrations vary depending on the alkaline cleaner and conditions of use|
|Amphoterics||1.2||Mixtures of a cationic amine salt or a quaternary ammonium compound with an anionic carboxy compound, a sulfate ester, or a sulfonic acid||Loosen and soften charred food residues on ovens or other metal and ceramic surfaces||Not suitable for use on food contact surfaces4)|
|Enzymes||0.3 -1||Proteolytic enzymes||Digest proteins and other complex organic soils||Inactivated by heat Some people become hyper-sensitive to the commercial preparations.|
1) Concentration of cleaning agent in solution as applied to equipment
2) Some regulatory agencies require prior approval
3) Steel wool and metal 'chlore balls' should not be used on food plant
4) Some amphoteric disinfectants are used on food contact surfaces
The various steps shown in Table 6.4 and including sterilization, represents the most comprehensive procedure for manual cleaning and disinfection or Clean Out of Place (COP). It is suitable for modern plants. For cleaning liquid handling plants like breweries and dairies Clean In Place (CIP) Systems will be used, based on circulation by pumping of water, cleaning agents and disinfectants. In principle the two systems will be similar.
In most factories, a combination of COP and CIP will be used. Use of CIP may be limited to part of the plants or even to a particular machine. However, regardless of the type and size of food production the general principles behind the complex cycle shown in Table 6.4 should be kept in mind and applied to ensure effective cleaning and disinfection.
The frequency of cleaning and disinfection will vary from several times during the working day i.e. at every major break to once every day, at end of production, or even less frequent. Sometimes disinfection will not be included e.g. in areas to be kept dry and for environments with materials which cannot be disinfected or premises unsuitable for disinfection. In such cases cleaning is still very important for the general appearance and hygienic condition of the plant or premises and the general attitude towards hygiene of the employees.
As mentioned earlier, effective cleaning is a prerequisite for an efficient disinfection. This indicates the importance of controlling cleaning. As described in Table 5.18 in the previous Chapter the most important control is visual inspection and other rapid tests to demonstrate the following important results of cleaning:
That all cleaned surfaces are visibly clean.
That all surfaces by feeling are free from food residues, scales and other materials and by smelling free from undesirable odours.
Further, the concentrations and Ph-values of cleaning agents, temperatures, if hot cleaning is used, and contact times should be monitored and registered. Ph measurements, or similar testing, of rinse water may be used to ensure that the cleaning agent is removed so that it will not interfere with the disinfectant.
These controls are all rapid and allow immediate decisions to be made as to whether cleaning should be repeated, partly or completely, or to proceed to the process of disinfection. All controls etc. shall be registered as part of the Quality System.
At this stage, microbiological control serves no real purpose. Firstly biofilms and surviving microorganisms are likely to be present and secondly, reliable rapid methods are not available.
Traditionally, the terms “disinfection” and “disinfectants” are used to describe procedures and agents used in food industries to ensure a microbiologically acceptable standard of hygiene. This practice will be followed although it is realized that the procedures and agents described will rarely introduce ‘sterility’ i.e. total absence of viable microorganisms.
Disinfection can be effected by physical treatments such as heat, U.V. irradiation, or by means of chemical compounds. Among the physical treatments, only heat shall be described.
The use of heat in the form of steam or hot water is a very safe method and a widely used method of disinfection. The most commonly used chemicals for disinfection are:
Chlorine and chlorine compounds.
Peracetic acid and hydrogen peroxide.
Quaternary ammonium compounds.
Table 6.6 summarizes the characteristics for some of these disinfectants and for the use of steam.
Heating at suitably high temperatures for a suitably long time is the safest method for killing microorganisms. The velocity with which heat killing occurs depends on temperature, humidity, type of microorganism and the environment in which the microorganisms occur during heat treatment. If microorganisms are entrapped in scales or other substances, they are protected and not even heating may be effective. It is important to recall the kinetics for heat inactivation of microorganisms:
logCt = logCo - K x t,
where Co = original population of living microorganisms (initial viable count) and Ct = total surviving after time t. K is a constant (= slope of the straight line) and depends on the microorganism concerned and the experimental conditions. K is described as the death rate. It is seen that the number of surviving microorganisms at time ‘t’ is determined by the initial level of infection, as well as the death rate constant and the heating time.
Circulation of hot water (about 90°C) is very effective. The water should be circulated for at least 20 minutes after the temperature of the return water has risen to 85°C or more. Obviously steaming is equally effective when applicable.
|Steam||Chlorine||Iodophores||QAC/QUATS surfactants||Acid anionic|
|Effective against||Gram-positive bacteria (lactics, clostridia, Bacillus, Staphylococcus)||Best||Good||Good||Good||Good|
|Gram-negative bacteria (E. coli, Salmonella, psychrotrophs)||Best||Good||Good||Poor||Good|
|Affected by hard water||No||(No)||Slightly||Some are||Slightly|
|Irritative to skin||Yes||Yes||Yes||No||Yes|
|Affected by organic matter||No||Most||Somewhat||Least||Somewhat|
|Incompatible with:||Materials sensitive to high temperature||Phenols, amines, soft metals||Starch, silver||Anionic wetting agents, soaps.||Cationic surfactants and alkaline detergents|
|Stability of use solution||Dissipates rapidly||Dissipates slowly||Stable||Stable|
|Stability in hot solution (greater than 66°C)||Unstable, some compounds stable||Highly usable (best used below 45° C)||Stable||Stable|
|Leaves active residue||No||No||Yes||Yes||Yes|
|Tests for active residue chemical||Unnecessary||Simple||Simple||Simple||Difficult|
|Maximum level permitted by USDA and FDA w/o rinse||No limit||200 ppm||25 ppm||25 ppm|
|Effective at neutral Ph||Yes||Yes||No||No||No|
With the use of chemical disinfectants, the death rate for microorganisms depends, among other things, upon the agent's microbicidal properties, concentration, temperature and Ph as well as the degree of contact between disinfectant and microorganisms. Good contact is obtained, e.g. by stirring, turbulence, smooth surfaces and low surface tension. As with heat disinfection, different microorganisms show different resistance to chemical sterilants. It is also so that contamination by inorganic or organic matter can reduce the death rate considerably. As mentioned before an effective disinfection can only be obtained after an effective cleaning. The desirable plant disinfectant would be characterized by the following properties:
It has sufficient anti-microbial effect to kill the microorganisms present in the available time and should have a sufficiently low surface tension to ensure good penetration into pores and cracks.
It rinses freely from the plant, leaving this clean and free from residues which could harm the products.
It must not lead to development of resistant strains or any surviving microorganisms.
It does not cause corrosion or other deterioration of the plant. It is recommended that the suppliers of machines etc. be asked before chlorine or other aggressive disinfectants are taken into use.
It is not hazardous to the operator.
It is compatible with the disinfection procedure being used, whether manual or mechanical.
If solid, it should be easily soluble in water.
Its concentration is easily checked.
It is stable for extended storage periods.
It complies with legal requirements concerning safety and health as well as biodegradability.
It is reasonably economical in use.
It will often be necessary to combine sterilants with additives in order to obtain the required properties.
To prevent development of resistant strains of microorganisms it can be advantageous to change from one type of sterilant to another from time to time.
This is especially advisable when quaternary ammonium compounds are used.
Among the most used sterilants the following shall be described briefly.
Chlorine is one of the most effective and widely used disinfectants. It is available in several forms like sodium hypochlorite solutions, chloramines and other chlorine containing organic compounds. Gaseous chlorine and chlorine dioxide are also used.
Chlorinated sterilants at a concentration of 200 ppm free chlorine are very active and also with some cleaning effect. The disinfectant effect is considerably decreased when organic residues are present.
The compounds dissolved in water will produce hypochlorous acid, HOCI, which is the active sterilizing agent, acting by oxidation. In solution it is very unstable, particularly in acid solution where oxic chlorine gas will be liberated. Furthermore, solutions are more corrosive at low Ph.
Unfortunately, the germicidal activity is considerably better in acid solution than in alkaline, thus the working Ph should be chosen as a compromise between efficiency and stability. Organic chlorinated sterilants are generally more stable but require longer contact times.
When used in the proper range of values (200 ppm free chlorine), chlorinated sterilants in solutions at ambient temperatures are non-corrosive to high quality stainless steel but they are corrosive to other less resistent materials.
Iodophors contain iodine, bound to a carrier, usually a non-ionic compound, from which the iodine is released for sterilization. Normally the Ph is brought down to 2–4 by means of phosphoric acid. Iodine has its maximum effect at this Ph range.
Iodophors are active disinfectants with broad antimicrobial spectrum just like chlorine. They are inactivated by organic material. Concentrations corresponding to approx. 25 ppm free iodine will be effective.
Commercial formulations are often acidic making them able to dissolve scales. They can be corrosive depending on the formulation and they should not be used above 45°C as free iodine may be liberated. If residues of product and caustic cleaning agents are left in dead legs and similar places, this may in combination with iodophors cause very unpleasant “phenolic” off-flavours.
Hydrogen peroxide and peracetic acid are effective sterilants acting by oxidation and with a broad antimicrobial spectrum. Diluted solutions may be used alone or in combination for disinfection of clean surfaces. They lose their activity more readily than other sterilants in the presence of organic substances and they rapidly loose their activity with time.
Quaternary ammonia compounds are cationic surfactants. They are effective fungicides and bactericides but often less effective against Gram negative bacteria. To avoid development of resistant strains of microorganisms, these compounds should only be used alternating with the use of other types of disinfectants.
Due to their low surface tension, they have good penetrating properties and for the same reason, they can be difficult to rinse off.
If quats come into contact with anion-active detergents, they will precipitate and become inactivated. Mixing or successive use of these two types of chemicals must therefore be avoided.
Ampholytic sterilants have properties similar to quaternary ammonia compounds.
Control of disinfection will be the final control of the complete cycle of cleaning and disinfection. Provided cleaning has been controlled effectively as described above control of disinfection will be effective when the following conditions are met:
Control of time and temperature conditions for disinfection by heat.
Control of active concentrations of chemical disinfectants.
Control that all surfaces to be disinfected are covered by the disinfectant.
Control of contact time.
The above controls should be documented and the observations reported and registered as required in standard Quality Systems.
Microbiological testing and control serve the purpose of verification. Various techniques are available but none are ideal and they are not “real time” methods which is highly desirable for control of cleaning and disinfection. Overnight incubation is too late to correct critical situations.
However, if conducted at regular intervals and planned to cover all critical points, useful information from microbiological control can be accumulated with time. Various methods are used and shall be mentioned briefly.
Swab testing. This is the most usual technique and one of the better ones. By use of a sterile swab of cottonwool, part of the disinfected surface is swabbed, and the bacteria transferred to the swab is transferred to a diluent for determination of colony forming units in standard agar substrates. Swabs are especially useful in places, where other control methods can only be used with difficulty i.e. pockets, valves etc.
Final rinse water. Membrane filtration of rinse water and incubation on agar substrate is a very sensitive technique for control of CIP systems as well as other cleaning and disinfection systems, where a rinse can be applied.
Direct surface plates. In these methods petri dishes or contact slides with selective or general purpose agar media are applied to the surface to be examined, followed by incubation and counting of colony forming units. These techniques can only be applied to plane surfaces, which is a limiting factor.
Bioluminometric assay of ATP. This is almost a “real time” method giving the answer within minutes. It is very sensitive and can be combined with swabbing for collection of microorganisms from surfaces. The method is rather nonspecific, and it may not be able to distinguish between microorganisms and food residues. However, if applied under defined conditions it may prove useful and superior to the conventional methods, because it provides the answer in minutes.
Regardless of the technique used, it is valuable to know from the verification analyses that the system was working, when it was established. There is also a value in knowing trends as expressed in the verification results recorded. The objective of studying trends and conducting the microbiological control of cleaning and disinfection, obviously will be, to take corrective action before loss of control of products or processes occur.