Cleaning and disinfection 407 the disinfectant and destroy its antimicrobial properties, and it is therefore ntial to remove all soil and chemical residues prior to disinfection Disinfectants should be used only within the pH range as specified by the manufacturer. Perhaps the classic example of this is chlorine, which dissociates water to form HOCI and the OCl ion. From pH3-7.5, chlorine is predominantly present as HOCl, which is a very powerful biocide, though the potential for corrosion increases with acidity. Above pH7.5, however, the majority of the chlorine is present as the OCl ion which has about 100 times less biocidal action than hocl In general, the higher the temperature the greater the disinfection. For most food manufacturing sites operating at ambient conditions(around 20C)or higher this is not a problem as most disinfectants are formulated(and tested) to ensure performance at this temperature. This is not, however, the case in the chilled food industry. Taylor et al.(1999) examined the efficacy of 18 disinfectants at both 10%C and 20%C and demonstrated that for some chemicals particularly quaternary ammonium based products, disinfection was much reduced at 10C and recommended that in chilled production environments, only products specifically formulated for low-temperature activity should be In practice, the relationship between microbial death and disinfectant concentration is not linear but follows a sigmoidal curve. Microbial populations are initially difficult to kill at low concentrations, but as the biocide concentration is increased, a point is reached where the majority of the population is reduced. Beyond this point the microorganisms become more difficult to kill(through resistance or physical protection) and a proportion may survive regardless of the increase in concentration. It is important, therefore, to use the disinfectant at the concentration as recommended by the manufacturer Concentrations above this recommended level may thus not enhance biocidal effect and will be uneconomic whilst concentrations below this level may significantly reduce biocidal action Sufficient contact time between the disinfectant and the microorganisms is rhaps the most important factor controlling biocidal efficiency. To be effective. disinfectants must find. bind to and transverse microbial cell envelopes before they reach their target site and begin to undertake the reactions which will subsequently lead to the destruction of the microorganism (Klemperer 1982). Sufficient contact time is therefore critical to give good results, and most general-purpose disinfectants are formulated to require at least five minutes to reduce bacterial populations by five log orders in suspension This has arisen for two reasons. Firstly five minutes is a reasonable approximation of the time taken for disinfectants to drain off vertical or near vertical food processing surfaces. Secondly, when undertaking disinfectant fficacy tests in the laboratory, a five-minute contact time is chosen to allow ease of test manipulation and hence timing accuracy. For particularly resistant organisms such as spores or moulds, surfaces should be repeatedly dosed to ensure extended contact times of 15-60 minutes
with the disinfectant and destroy its antimicrobial properties, and it is therefore essential to remove all soil and chemical residues prior to disinfection. Disinfectants should be used only within the pH range as specified by the manufacturer. Perhaps the classic example of this is chlorine, which dissociates in water to form HOCl and the OCl ion. From pH 3–7.5, chlorine is predominantly present as HOCl, which is a very powerful biocide, though the potential for corrosion increases with acidity. Above pH 7.5, however, the majority of the chlorine is present as the OCl ion which has about 100 times less biocidal action than HOCl. In general, the higher the temperature the greater the disinfection. For most food manufacturing sites operating at ambient conditions (around 20ºC) or higher this is not a problem as most disinfectants are formulated (and tested) to ensure performance at this temperature. This is not, however, the case in the chilled food industry. Taylor et al. (1999) examined the efficacy of 18 disinfectants at both 10ºC and 20ºC and demonstrated that for some chemicals, particularly quaternary ammonium based products, disinfection was much reduced at 10ºC and recommended that in chilled production environments, only products specifically formulated for low-temperature activity should be used. In practice, the relationship between microbial death and disinfectant concentration is not linear but follows a sigmoidal curve. Microbial populations are initially difficult to kill at low concentrations, but as the biocide concentration is increased, a point is reached where the majority of the population is reduced. Beyond this point the microorganisms become more difficult to kill (through resistance or physical protection) and a proportion may survive regardless of the increase in concentration. It is important, therefore, to use the disinfectant at the concentration as recommended by the manufacturer. Concentrations above this recommended level may thus not enhance biocidal effect and will be uneconomic whilst concentrations below this level may significantly reduce biocidal action. Sufficient contact time between the disinfectant and the microorganisms is perhaps the most important factor controlling biocidal efficiency. To be effective, disinfectants must find, bind to and transverse microbial cell envelopes before they reach their target site and begin to undertake the reactions which will subsequently lead to the destruction of the microorganism (Klemperer 1982). Sufficient contact time is therefore critical to give good results, and most general-purpose disinfectants are formulated to require at least five minutes to reduce bacterial populations by five log orders in suspension. This has arisen for two reasons. Firstly five minutes is a reasonable approximation of the time taken for disinfectants to drain off vertical or near vertical food processing surfaces. Secondly, when undertaking disinfectant efficacy tests in the laboratory, a five-minute contact time is chosen to allow ease of test manipulation and hence timing accuracy. For particularly resistant organisms such as spores or moulds, surfaces should be repeatedly dosed to ensure extended contact times of 15–60 minutes. Cleaning and disinfection 407
408 Chilled foods Ideally, disinfectants should have the widest possible spectrum of activity against microorganisms, including bacteria, fungi, spores and viruses, and this should be demonstrable by means of standard disinfectant efficacy tests. The range of currently available disinfectant test methods was reviewed by Reybrouck (1998) nd fall into two main classes, suspension tests and surface tests. Suspension tests are useful for indicating general disinfectant efficacy and for assessing environmental parameters such as temperature, contact time and interfering matter such as food residues. In reality however, microorganisms disinfected on food contact surfaces are those that remain after cleaning and are therefore likely to be adhered to the surface. A surface test is thus more appropriate a number of authors have shown that bacteria attached to various surfaces are generally more resistant to biocides than are organisms in suspension haliwal et al. 1992, Frank and Koffi 1990, Holah et al. 1990a, hugo et al 1985, Le Chevalier et al. 1988, Lee and Frank 1991, Ridgeway and Olsen 1982, Wright et al. 1991, Andrade et al. 1998, Das et al. 1998). In addition, cells growing as a biofilm have been shown to be more resistant(Frank and Koffi 1990, Lee and Frank 1991, Ronner and Wong 1993). The mechanism of resistance in attached and biofilm cells is unclear but may be due to physiological differences such as growth rate, membrane orientation changes due to attachment and the formation of extracellular material which surrounds he cell. Equally, physical properties may have an effect e.g. protection of the ells by food debris or the material surface structure or problems in biocide diffusion to the cell/material surface. To counteract such claims of enhanced surface adhered resistance, it can be argued that in reality, surface tests do not consider the environmental stresses the organisms may encounter in the processing environment prior to disinfection(action of detergents, variations in temperature and pH and mechanical stresses) which may affect susceptibilit Both suspension and surface tests have limitations, however, and research based methods are being developed to investigate the effect of disinfectants against adhered microorganisms and biofilms in-situ and in real time. Such methods have been reviewed by Holah et al.(1998) In Europe, CEN TC 216 is currently working to harmonise disinfectant testing and has produced a number of standards. The current food industry disinfectant test methods of choice for bactericidal and fungicidal action in suspension are EN 1276(Anon. 1997)and EN 1650(Anon 1998a)respectively and food manufacturers should ensure that the disinfectants they use conform to hese standards as appropriate. A harmonised surface test is expected in 2000 Because of the limitations of disinfectant efficacy tests, however, food manufacturers should always confirm the efficacy of their cleaning and disinfection programmes by field tests either from evidence supplied by the chemical company or from in-house trials As well as having demonstrable biocidal properties, disinfectants must also be safe(non-toxic)and should not taint food products. Disinfectants can enter food products accidentally e.g. from aerial transfer or poor rinsing, or deliberately e.g. from 'no rinse status'disinfectants, The practice of rinsing or
Ideally, disinfectants should have the widest possible spectrum of activity against microorganisms, including bacteria, fungi, spores and viruses, and this should be demonstrable by means of standard disinfectant efficacy tests. The range of currently available disinfectant test methods was reviewed by Reybrouck (1998) and fall into two main classes, suspension tests and surface tests. Suspension tests are useful for indicating general disinfectant efficacy and for assessing environmental parameters such as temperature, contact time and interfering matter such as food residues. In reality however, microorganisms disinfected on food contact surfaces are those that remain after cleaning and are therefore likely to be adhered to the surface. A surface test is thus more appropriate. A number of authors have shown that bacteria attached to various surfaces are generally more resistant to biocides than are organisms in suspension (Dhaliwal et al. 1992, Frank and Koffi 1990, Holah et al. 1990a, Hugo et al. 1985, Le Chevalier et al. 1988, Lee and Frank 1991, Ridgeway and Olsen 1982, Wright et al. 1991, Andrade et al. 1998, Das et al. 1998). In addition, cells growing as a biofilm have been shown to be more resistant (Frank and Koffi 1990, Lee and Frank 1991, Ronner and Wong 1993). The mechanism of resistance in attached and biofilm cells is unclear but may be due to physiological differences such as growth rate, membrane orientation changes due to attachment and the formation of extracellular material which surrounds the cell. Equally, physical properties may have an effect e.g. protection of the cells by food debris or the material surface structure or problems in biocide diffusion to the cell/material surface. To counteract such claims of enhanced surface adhered resistance, it can be argued that in reality, surface tests do not consider the environmental stresses the organisms may encounter in the processing environment prior to disinfection (action of detergents, variations in temperature and pH and mechanical stresses) which may affect susceptibility. Both suspension and surface tests have limitations, however, and research based methods are being developed to investigate the effect of disinfectants against adhered microorganisms and biofilms in-situ and in real time. Such methods have been reviewed by Holah et al. (1998). In Europe, CEN TC 216 is currently working to harmonise disinfectant testing and has produced a number of standards. The current food industry disinfectant test methods of choice for bactericidal and fungicidal action in suspension are EN 1276 (Anon. 1997) and EN 1650 (Anon. 1998a) respectively and food manufacturers should ensure that the disinfectants they use conform to these standards as appropriate. A harmonised surface test is expected in 2000. Because of the limitations of disinfectant efficacy tests, however, food manufacturers should always confirm the efficacy of their cleaning and disinfection programmes by field tests either from evidence supplied by the chemical company or from in-house trials. As well as having demonstrable biocidal properties, disinfectants must also be safe (non-toxic) and should not taint food products. Disinfectants can enter food products accidentally e.g. from aerial transfer or poor rinsing, or deliberately e.g. from ‘no rinse status’ disinfectants. The practice of rinsing or 408 Chilled foods