Chemical disinfectants are defined by their mode of action and can be broadly split into two groups, oxidising and non-oxidising.
Oxidising disinfectants such as Sodium Hypochlorite, Peracetic Acid and Hydrogen Peroxide attack all cellular material and stop the micro-organism from functioning. Unfortunately they will also attack any food soiling residues, this will essentially neutralise the disinfectant and reduce its efficacy.
Non-oxidising disinfectants such as Quaternary Ammonium Compounds (QACs), Biguanides and Amphoterics are more subtle in their operation. They penetrate the cell wall and disturb the phospho-lipid molecules which make up the cell membrane; they then block metabolic pathways needed for the organism to survive and /or cause it to leak vital chemicals.
Sodium Hypochlorite
Sodium hypochlorite is very effective against all types of micro-organism (including spores). It is inexpensive in comparison to other disinfectants and it is unaffected by hard water. The efficacy of a solution is dependent on its pH.
Care should also be taken with storage and stock rotation of Sodium Hypochlorite. The product will naturally lose available chlorine (by conversion to sodium chloride – not chlorine gas) and should generally be used within 3 months of manufacture. If stored in direct sunlight the rate of loss of available chlorine increases. It should also be stored separate from other chemicals, particularly acids, as lethal chlorine gas will be released if these two products are mixed.
Ideally Sodium Hypochlorite should be automatically dosed via a chlorine compatible dosing unit to ensure that it is used at the correct strength.
Chlorine Dioxide
Chlorine Dioxide is mainly used for the disinfection of water systems but is being utilised for surface disinfection in CIP. It is produced by mixing under controlled conditions:
- Chlorine with chlorite
- Acid with hypochlorite and chlorite
- Acid with chlorite
- Or using a stabilised form of chlorine dioxide with a low pH acid activator
All of the above methods must be done by using a specialised dosing system with safety precautions built in. The main advantages that chlorine dioxide has in the treatment of water systems are:
- Broad spectrum of activity at low concentrations
- Rapid microbiological killing action
- Dosing is reliably and automatically controlled
- It does not readily react with organics to form ecotoxic and bioaccumulative by-products
- It is much less tainting than chlorine
- It is approved for potable water use (0.5mg/ltr causes no objectionable taste, whereas the same concentration of chlorine would). It will typically be used at 0.3mg/ltr in water treatment.
Hydrogen Peroxide
Hydrogen Peroxide works in the same manner as Sodium Hypochlorite but may not be as effective against as many micro-organisms. It is used predominantly in the beverage and brewing sectors, as it is low foaming and ideal for Clean In Place systems. Typically a 0.03% solution will give 100 ppm Hydrogen Peroxide, at this level of concentration it may not require rinsing from the surface. Used at elevated temperatures, hydrogen peroxide becomes more effective. It is not safe to use on aluminium, zinc, tin or their alloys.
Peracetic Acid
Peracetic acid (PAA) products are used mainly in the beverage and brewing sectors as they are low foaming, effective against all types of bacteria and can be used at very low concentrations and temperatures. Typically two concentrations are commercially available; these have activities of 5% and 15% peracetic acid. Used at the recommended concentration (e.g. 0.1 - 0.4%) a 5% active product will give 50-200 ppm Peracetic Acid which would not typically be rinsed off but allowed to slowly decompose to acetic acid, oxygen and water. As with other oxidising disinfectants PAA should not be used on soft metals. Some peracetic acid formulations are approved by DEFRA for use against specific animal diseases.
Iodophors
Iodophors are expensive but very effective disinfectants having both detergent and disinfectant properties. They are produced by dissolving iodine in an acid medium together with surfactants. Iodophors have several advantages including:
- The ability to kill a wide range of organisms at low temperatures.
- Short contact time.
- The ability to cope with soiling/hard water.
However, even at the correct concentration they can cause taint if left on a surface without rinsing. They may also turn some plastics brown and should not be used on soft metals.
Iodophors are however ideal in dirty applications such as farm gate foot baths and drive-over disinfectant blankets, indeed Iodophor based disinfectants have DEFRA approvals for many farm based diseases. Iodophors are now very rarely used in food manufacturing applications.
Non-Oxidising Disinfectants:
Quaternary Ammonium Compounds (QAC’s or quats)
QAC’s (or quats) are the most widely used biocides in food or beverage businesses. Properly formulated products are effective against gram positive bacteria and against gram negative bacteria, but less effective against spores, moulds and fungi. QAC based disinfectants are stable and generally taint free. They may be inactivated by hard water, organic material and some plastics. To improve the effectiveness of QACs, formulations often include sequestrants and non-ionic detergents.
Amphoterics
Amphoterics are excellent disinfectants but can be expensive. They have low toxicity, relatively non corrosive, tasteless, odourless and used at approx 1%. They are high foaming and are unsuitable for use with machines and high velocity sprays. Formulations often have similar co-formulants to QAC based products.
Biguanides
These are cationic bactericides similar in function to QAC’s. Their main use within the food industry revolves around the disinfection of evaporator units. These types of disinfectants are formulated to provide a tenacious gel film when they come into contact with moisture. This gel then slowly releases the biocide and detergent.
Alcohols
Where there is a requirement for light cleaning and disinfection in an essentially ‘dry’ area, such as a Bakery, then the use of an alcohol based spray/wipe product becomes very useful. This is typically a blend of alcohol, QAC and possibly mild detergent additives formulated to provide good disinfection in lightly soiled conditions therefore reducing the amount of water used. The product flashes dry after application, thus removing the need for an undesirable wipe-dry operation. Care must be taken because of the flammability of alcohols such as ethanol and iso-propanol, two of the alcohols commonly used in these products.
Alcohol disinfectants have also been adapted for use a post wash hand disinfectant. 60 to 70% Alcohols are fast acting.
Effects of Time & Concentration
Contact time and concentration are two of the most vital factors that can affect the performance of a terminal disinfectant. Although some disinfectants are effective within minutes, in most cases it is recommended that they receive at least 15-20 minutes contact time. Failure to allow the recommended contact time could result in an ineffective reduction of micro-organisms on the disinfected surface. Disinfectants should always be used at the manufacturer’s recommended concentration and an even coverage of the surface is vital.
Attention should be paid when applying disinfectants to horizontal surfaces after rinsing; the presence of pools of water will dilute the disinfectant solution.