The main objective of a cleaning programme is to control microbial activity. Although an adequate cleaning programme will eliminate nearly all the soil present, it will not destroy or remove all the microorganisms. This requires a second step - disinfection.
Disinfection is defined as: “the destruction of microorganisms but not usually bacterial spores; this does not necessarily involve killing all microorganisms, but reducing their number to a level not normally harmful to health. The term is applicable in a commercial context solely to the treatment of inanimate objects and materials”. This definition assumes that where total numbers of surviving microorganisms are small, undesirable types such as pathogens are less likely to be present in significant numbers.
Disinfection methods can be divided into two groups:
Non-chemical disinfection methods
Chemical disinfection methods
In many cases steam is very good for disinfection but it may be inconvenient or impractical due to the following reasons.
Providing steam is expensive and may cause materials to deteriorate and equipment to distort. If steam is used it will take considerable time to heat and cool equipment. It may cause baking-on of food and other residues. Visibility is reduced in the environment thus reducing the effectiveness of the sanitizing procedures. Finally it gives condensation problems.
Insufficient heating may result in the incubation of microorganisms in inaccessible parts of the machines and the equipment.
The most essential for an effective chemical disinfection programme is a clean surface. Consequently, to achieve microbial control the cleaning and disinfection programme must be thorough, compatible and totally effective.
Note: A disinfectant will not cover up faulty cleaning practices
The choice of chemical disinfectants is determined by the following considerations:
Public health regulations
Spectrum of effectiveness, i.e. ability to kill many types of microorganisms
Must be efficient under the conditions of use
Quick action
Presence or absence of organic matter
Corrosive properties
Type of processing area and type of surface to be disinfected
Non-toxic and gentle to the skin
Inexpensive
Must not affect the odour or flavour of the food processed on the equipment disinfected
Must be easily rinsed away and leave no toxic residues
Easy to dispense and to handle
Must be safe in use and must not affect the operators who use it.
Manufacturers offer a large number of disinfectants, each claimed to be the best on the market. Nevertheless the only ones suitable for the food industry contain chemicals of one of the following groups:
Chlorine and chlorine-releasing compounds
Quarternary ammonium compounds
Amphoteric (ampholytic) compounds
Phenolic compounds
Peracetic acid
Chlorine is the most effective disinfectant available and sodium (or calcium) hypochlorite is a cheap disinfectant commonly in use. The hypochlorites have a characteristic smell produced by free hypochlorous acid which is considered to be the germicidally active form of chlorine. A practical disadvantage of sodium hypochlorite is the risk of corrosion to all common metals (especially aluminium and galvanized iron), except perhaps high quality stainless steel.
The disinfectant properties of the hypochlorites depend on a number of factors:
| a) | the concentration of available chlorine, which should be 200–300 mg/l |
| b) | pH of the solution (to reduce the breakdown during storage, the hypochlorite solution should be maintained at pH 9–11) |
| c) | Temperature |
| a-c) | The hypochlorite is more efficient if the concentration and temperature are raised and/or the pH is lowered (8.3) |
| d) | Contact time |
| c-d) | To decrease the corrosive effect the temperature should not exceed 50–60°C and the contact time should not exceed 30–60 min. |
| e) | absence of organic material. Organic material consumes available chlorine and reduces the disinfecting capacity. |
When using hypochlorite it is very important to ensure that hypochlorites and acids are never mixed due to development of toxic gases which may cause severe damage to personnel.
The organic chlorine disinfectants are chloramines (chloramine T) and chloroisocyanurates.
Chloramine T should only be used where long exposure is practicable as its bactericidal activity is slow compared to hypochlorite. Chloramine T is more stable than hypochlorites not only in solution but also in powder form. It is also less irritating and less corrosive than hypochlorites.
The chloroisocyanurates are unstable both in solution and in the presence of non-ionic surface active agents.
The cationic synthetic surface active agents or quarternary compounds are excellent disinfectants. The quarternary ammonium compounds are free of odour and colour, are highly stable and have little corrosive action on metals when used in recommended concentrations. They are more effective against gram-positive than gram-negative bacteria. They are more active in the presence of small amounts of organic matter than any other class of disinfectants, but are inactivated by soaps, anionic detergents and inorganic polyphosphates. They are more expensive than hypochlorites.
Utensils and equipment should be thoroughly rinsed after applying these compounds as disinfectants because of possible toxic effects.
Amphoteric compounds are essentially alkyl or acyl amino acids. They combine detergent and disinfectant properties. They are of low toxicity, are non-corrosive and are expensive. They are effective against both gram-positive and gram-negative bacteria.
Phenolic compounds are not generally suitable for use in the food industry. Some halogenated phenol derivatives can be used in the meat industry. They are effective against spores, viruses, moulds and grampositive and gram-negative bacteria. They are corrosive and can irritate the skin of personnel.
A quite new disinfectant is a mixture of peracetic acid, hydrogen peroxide and acetic acid, which is stable and is effective against bacteria, spores, yeasts, moulds and viruses. The active agent is peracetic acid. The mixture is non-corrosive.
The choice of disinfectants will normally depend on several factors, one often being the supply situation when the more specific disinfectants are marketed by only a single or a few companies. Expenditure on disinfectants will also be important and the cost must be compared to the characteristics of the disinfectant before a choice is made. The previous cleaning programme should also be considered. A disinfectant will never assure a demanded hygienic level without previous cleaning.
The choice will often be hypochlorites which are inexpensive disinfectants with a good germicidal effect. Precautions to prevent corrosion of the surfaces and to prevent development of toxic gases, i.e. to prevent the possibility of mixing hypochlorites and acids, should be taken.
For emergency disinfection, i.e. when disinfection is required during processing because of animal disease (anthrax, foot and mouth disease etc.) the following disinfectants are recommended:
sodium hydroxide (caustic soda or lye) in a hot solution of approximately 2% for foot and mouth disease and 5% for anthrax.
(Avoid any splashes in eyes or on skin (caustic). If an accident happens wash clean water).
sodium hypochlorite solution: 0.5% available chlorine
hot water (90°C or more) or steam
Chloride of lime for lairages, stables and transport vehicles: approximately 5% solution.
