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Quantitative microbiological risk assessment: principles applied to determining the comparative risk of salmonellosis from chicken products
International Biodeterioration & Biodegradation 50 (2002) 155 – 160
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Quantitative microbiological risk assessment: principles applied to determining the comparative risk of salmonellosis from chicken products Martyn H. Brown ∗ Microbiology Unit, Unilever Research Laboratory, Colworth House, Sharnbrook, Bedfordshire, MK44 1LQ, UK
Abstract Since causal links have been established between food-borne illness and particular microorganisms, it has been possible to assess the public health risks of their presence in foods and propose measures to ensure the safety of customers. E,ective control measures for food safety have been based on knowledge of the resistance of pathogenic microorganisms to the treatments used for preservation (e.g. acidi-cation or reduced water activity) or decontamination (e.g. pasteurisation or sterilisation). Using this principle, food safety has been managed informally and successfully within the food industry for many years. Recently, formal risk assessment schemes, for example from Codex Alimentarius, have developed and placed the elements of decision-making on suitable control measures into a formal framework with clearly identi-able stages. The output of the four stages of a formal risk assessment (hazard identi-cation, hazard characterisation, exposure assessment and risk characterisation) provides the basis for decisions on actions needed to control the identi-ed hazard. There are many di5culties in ensuring that risk assessments are realistic and accessible to potential users, in many cases their value is limited by the data available. The study reported here on control of Salmonella in poultry products illustrates that it is possible to produce a comparative risk assessment based on published data. This study is not a full quantitative risk assessment, but provide useful pointers for a risk manager. Di,erences are discussed between the exposure assessment data needed to propose controls for infectious or toxigenic pathogens. For infectious pathogens, the presence of viable and infectious microorganisms is itself the hazard, but for toxigenic microorganisms, absence or destruction of viable cells at ingestion does not in itself ensure the absence of toxin. For toxin hazards, exposure assessment needs to consider previous conditions that may have led to toxin formation and persistence, rather than just the level of microbes at ingestion. ? 2002 Published by Elsevier Science Ltd. Keywords: Microorganisms; Salmonella; Salmonellosis; Chicken; Risk assessment; Risk management; Toxigenic microorganisms
1. Introduction Ensuring the microbiological safety of food requires the identi-cation of realistic hazards and their means of control. product defects and food-poisoning with microbiological origins have been recognised for many years and may be ranked according to seriousness for the customer and producer. They range from complaints of low quality or spoiled product, through mild food-poisoning to serious cases of food-borne infection or intoxication requiring hospitalisation. Food producers have always assessed the risks of their products causing food-poisoning, using either empirical or experiential approaches. Nowadays food microbiologists ∗
Fax: +44-1234-222277. E-mail address: [email protected] (M.H. Brown).
0964-8305/02/$ - see front matter ? 2002 Published by Elsevier Science Ltd. PII: S 0 9 6 4 - 8 3 0 5 ( 0 2 ) 0 0 0 8 1 - 1
are usually responsible for assessing the impact of product and process changes on the risks of a product causing food-poisoning. Food producers have always used a simple, empirical scheme for assessing and managing risks: • • • • • •
What is wrong? Who knows and what is known about the topic? What are the options for control? Which one should be used for action, from the options? Who needs telling about our decision, and What will we do?
Causal links have been established between food-borne illness and the presence, or activities (toxigenisis) of bacteria (Table 1) allowing the management of speci-ed hazards
156
M.H. Brown / International Biodeterioration & Biodegradation 50 (2002) 155 – 160
Table 1 Examples of bacteria causing food-borne diseases
Bacillus cereus Brucella abortus Campylobacter jejuni Clostridium botulinum Clostridium perfringens Coxiella burnettii Escherichia coli Enteropath E.coli Listeria monocytogenes Mycobacterium bovis Salmonella Shigella Staphylococcus aureus Vibrio parahaemolyticus Vibrio vulni5cus Yersinia enterocolitica
Fig. 1. The Codex Almentarius risk assessment scheme.
as the means of ensuring food with a known level of risk. Description of the link between a speci-c pathogen (e.g. Salmonella) and food-poisoning is essential for any risk assessment, because it is used to de-ne the dose-response of the target consumers and the severity of illness, the prevalence and level of the agent in raw materials and the e,ects of processing and use on the numbers eventually ingested. 2. Development of formal risk assessment Since the 1980s, crude traditional tools for determining the risks associated with identi-ed hazards (risk assessment) have developed into formal systems with well-de-ned stages and procedures. These have become known as microbiological risk assessment (MRA), and are described in Microbiological risk assessment: An Interim Report (ACDP, 1996) or by the Codex scheme (Codex Alimentarius, 1996) depicted in Fig. 1. The aim of all MRA schemes is to reduce risk by: • identifying realistic microbiological hazards and characterising them according to the severity of their e,ects on consumers,
• examining the impact of raw material contamination, processing and use on the level of risk, and • communicating clearly and consistently, via the output of the study, the level of risk to the consumer. Users of the output of a risk assessment should include all those involved in the design, production and consumption of food, and therefore it must be in a form that they can understand. When risk assessment is put together with risk management (actions to eliminate or minimise risk) and risk communication (information on a risk and on the decisions taken to combat a risk) a risk analysis is produced (ACDP, 1996). 3. Background to this risk assessment Because food companies are often in a state of change with respect to microbiological risks and hazards, the ability to assess risks and the impact of process, product and market changes on the level of risk are important means of ensuring consistent standards of food safety. Changes which a,ect risk can include the development of new products and processes, di,erent raw material sources, and target customer groups, e.g. products intended for children. Where the behaviour of microorganisms or the doseresponse of consumers can be described by mathematical models, numerical information can be processed by a computer to provide quantitative or comparative risk assessments. This study of the risks of salmonellosis from chicken products has two purposes, -rstly to use existing mathematical models to provide a quantitative risk assessment (QRA) tool within the framework and de-nitions proposed for risk analysis and assessment by Codex Alimentarius (1996), and secondly to provide a transparent, model-based QRA which would allow e,ective risk management and communication within a food manufacturing business. 4. Stages in this risk assessment It is essential that any risk assessment is preceded by clear formulation of the problem. This leads to a statement of purpose, or scope, similar to that preceding a Hazard Analysis Critical Control Point (HACCP) study (Fig. 2). In this example, the trigger is ‘evaluation of control options’ and the purpose is ‘to provide a quantitative microbiological risk assessment (QRA) of salmonellosis from frozen poultry products’ using computer-based models according to the Codex Alimentarius stages. 5. Hazard identi"cation Frozen poultry products may be cooked either in the factory, or in the home, or both. The raw poultry meat which is a starting material may contain Salmonella (Mead, 1982).
M.H. Brown / International Biodeterioration & Biodegradation 50 (2002) 155 – 160
Fig. 2. Deriving the statement of purpose.
Therefore, salmonellosis may be acquired by ingesting undercooked products which contain infective cells (ACMSF, 1996). This model-based MRA integrates models describing: occurrence and distribution of Salmonella in the raw material, the incidence (% contaminated) and the concentration of agent (log10 normal (Kilsby and Pugh, 1981) and other distributions were used to characterise the mean and variability of concentration of Salmonella). 6. Exposure assessment Exposure assessment describes the degree of exposure to the hazard (Salmonella) likely to occur from consumption of the product. It is de-ned by the portion size and level of Salmonella in the portion. The residual level in turn depends on the numbers of Salmonella entering the heating process, the heating characteristic of the product—determined by its size, shape and weight and the heating conditions used either in-factory or in-home. The combination of the heating time and temperature (heating process) and the rate of heat penetration into the product unit combine to deliver a heat treatment with a certain predictable lethal e,ect on Salmonella. This e,ect (log reductions from the initial level) can be calculated if the heat sensitivity of Salmonella at various temperatures (D and z values) and the product’s thermophysical properties are known. The very simple thermophysical models used can predict the e,ect on lethality of di,erent cooking temperatures, alterations to length of the thermal path and heat transfer to the surface and within the product. This model can sum the lethal e,ects of two heating stages (in-plant and in-home) with di,erent methods of heating, because it is recognised that microbiological safety for this type of product is not necessarily guaranteed by manufacturing processes, but can be a joint responsibility with the customer (Notermans et al., 1996). This makes the consumer part of the process of ensuring that the end product is safe an therefore an assessment of their e,ect is an essential part of the risk assessment.
157
or target consumers to infection (e.g. normal or susceptible) and the e,ect of cooking (in the factory or home) on concentration of the agent in the product. This programme allows users to produce risk estimates without extensive data on the dose-response within populations, since they can specify the dose-response in terms of median responses and interquartile range. Actual data on infection rate can only come from national or international epidemiological studies, whereas data on the numbers and distribution of hazardous agents will always be associated with speci-c commodities producted or harvested under certain conditions. 8. Risk characterisation This programme facilitates comparative risk assessment and risk communication, because it can show how a range of variables inMuences the risks of Salmonella infection associated with the consumption of a cooked, frozen poultry product. Product, process and other variables, can be altered to produce comparative risk assessments based on di,erent sets of values. This system concentrates on a single factor, the heating stage, controlled during manufacture or by the customer, rather than the sequence of production stages considered by Buchanan and Whiting (1996). But it can take account of the e,ect of changes in population sensitivity, raw material quality, and cooking regime on the -nal risk. The e,ects of growth during storage and re-contamination of the product are not considered. Where possible the values used for predictions have been based on published or experimental data. To facilitate communication of decision on risk and their basis, the models are visualised and information is presented graphically (Fig. 3), to allow transparent communication with both customers and sta,. Conditions or properties may be changed by means of displays and slider controls on a computer screen, to encourage users to assess the e,ect of changes in processing, markets or materials on risk. For convenience this model-based QRA was handled a in statistical analysis system with a graphical interface (SAS=AF version 6.11, SAS Institute Inc., Cary, NC, USA). Target users include product developers, buyers, quality assurance and manufacturing personnel. Where risk decisions are communicated e,ectively, risk management practices can be readily implemented, consistent standards applied, and dangerous changes may be stopped. Implementation of an e,ective process of communication and understanding on a consistent, scienti-c, and yet practical basis is a continuing dilemma. The screen presentation is shown below and the mathematics are given in Brown et al. (1998).
7. Hazard characterisation
9. Discussion
This quanti-es the risks of infection after product consumption. To do this it links the sensitivity of populations
The risk assessment model could be improved in several ways.
158
M.H. Brown / International Biodeterioration & Biodegradation 50 (2002) 155 – 160
Fig. 3. The risk characterisation screen.
• Heating. The heat transfer model used in this program is relatively simple (Carslaw and Jaeger, 1959). The use of di,erent coe5cients relevant to other products or processes and a more Mexible model, which could take into account freezing, thawing, and several process steps would improve its Mexibility and range of application. • Thermal inactivation models. Accuracy would be improved by the use of thermal inactivation=survival models which accounted for injury to microbial cells caused by heating or storage and took account of any consequent change in infectivity. For models covering multi-stage processes (Buchanan and Whiting, 1996) thermal inactivation models need to take account of growth and the cumulative e,ects of injury during a sequence of process steps. Whiting and Buchanan (1997) have used the Monte-Carlo simulation to assess the cumulative risks of a process (pasteurized egg manufacture), based on process and environmental input values. Their approach, taking into account variability described by a frequency distribution, produces a di,erent and less conservative assessment of risk than one based on taking the worst case at each stage. • Dose response and population data. Categorising of the seriousness of hazards and quantifying risk is hampered by the paucity of scienti-c data on the sensitivity of consumers to food-borne pathogens. In many cases this has prevented even comparative risk assessments. Judgement-Encoding Methodology has been used to quantify the uncertainty of judgements based on dose– response curves (Martin et al., 1995). However, there is no substitute for scienti-c data.
10. Further application of the risk assessment model Infectious pathogens. This framework can be used to make risk assessments for a range of infectious pathogens (e.g. salmonellae, Listeria monocytogenes, Escherichia coli O157), if their fate is controlled by a single process stage (heating) and there are appropriate models for infection, heating and death kinetics (e.g. Blackburn et al., 1997). Toxigenic microbes. This program has been built to deal with the risks associated with non-toxigenic pathogens. However, a common source of foodborne diseases (Todd, 1978) is ingestion of performed microbial toxins rather than infection. For these pathogens the dose-response is not linked directly to the number of microorganisms present when the product is consumed, but to the quantity of toxin. The Codex approach can still be used to assess the risks of toxigenesis and di,erence and similarities between the factors requiring consideration are shown in Fig. 4. The hazard identi-cation step for a toxin QRA would identify the toxin from one (or more) of the known toxigenic microorganisms (e.g. Staphylococcus aureus, Clostridium botulinum). The dose-response would describe the sensitivity of a population to that toxin, rather than to the microorganism producing it. The dose response would therefore be based on the weight of toxin per unit body weight to cause illness, rather than numbers of microorganisms. Hazard characterisation would be linked to the pathogenicity of the toxin, the dynamics of intoxication and the distribution of susceptibility in the population of consumers to the toxin. Exposure assessment would have to deal with both numbers of microorganisms and the kinetics and boundaries for
M.H. Brown / International Biodeterioration & Biodegradation 50 (2002) 155 – 160
Fig. 4. Risk factors di,erentiating infectious and toxigenic microorganisms (Information summarised from ICMSF, 1996).
Fig. 5. Requirements to assess the risks from infectious and toxigenic pathogens.
their toxigenisis and the accumulation of toxin from raw material to consumption (Fig. 5). Similar elements (e.g. incidence, distribution and fate of the toxigenic microorganisms) are required for exposure assessment as for infectious pathogens. However, additional information speci-c to toxigenic microorganisms is required to describe the production and fate of the toxin during processing, storage and consumer use. This involves knowledge of the kinetics and boundaries of microbial toxin production, which are dependent on the condition in (e.g. pH, water activity, competing microbes) and around the food (e.g. temperature and PO2 ) plus information on the stability and persistance of the toxin within the product and the likely e,ect of cooking. There is only general data on the thermal denaturation of toxins in food. The output of a toxin QRA would be unchanged: expected number of people intoxicated for a given number of portions, however the source of harm will be the toxins rather than the microorganisms directly. 11. Conclusions and issues Quantitative Risk assessment is an important tool for evaluating and communicating the impact of raw material quality, processing and changes on food safety. The answers from any QRA are only as good as the input data and at
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present, lack of data makes QRA di5cult to implement. In many cases, even causal links between microbial behaviour and harmful e,ects have not been reliably established. All components of risk assessment are currently limited by the availability of information. There are few clinical and epidemiological studies on the e,ects of pathogens, and surveillance data with valid causal links is scarce. The use of benchmark populations, as in this example, may partly overcome the lack of epidemiological data. Limited investigation of microbial physiological and ecology limit our understanding of the features of food systems which promote or inhibit microbial growth, survival and toxigenisis. Clearly, infectious and toxigenic pathogens need models of di,erent complexity to describe their behaviour and hence the hazards they can present within the food chain. To make QRA work, a robust, accepted processes for risk assessment combined with good quality kinetic data on the target microorganisms is needed (NRC, 1983). Commonly, information on analogous microorganisms, situations or contexts is used for both risk assessment and communication. Although the scienti-c aspects of risk assessment can be tackled using the Codex methodology, e,ective communication of conclusions and recommendations on risk management remains a major problem, the means of communication should in principle be dictated by the objective of communication. The Codex approach is aimed at description or characterisation of the risk, rather than being focused on providing data for decision making. The key issues preventing e,ective microbiological risk assessment remain • uncertainty (i.e. lack of relevant data), • variability (i.e. data available indicates that the variability of a feature may limit e,ective assessment of the risks associated with it), • accessibility (i.e. the data or conclusions are not available in a form that allows their use by decision-makers), and • misuse=miscommunication (i.e. risk assessments are not presented in a clear and unambiguous way, and at worst including such emotive phrases as ‘completely safe’). With acknowledgements to my co-authors of the original paper published in the Journal of Food Protection, viz. Kenneth Davies, Christelle Billon, Carol Adair and Peter McClure.
References Advisory Committee on the Microbiological Safety of Food (ACMSF) 5, 1996. Report on Poultry Meat. HMSO, London. Advisory Committee on Dangerous Pathogens (ACDP) 1996. Microbiological Risk Assessment: an interim report. HMSO, London. Blackburn, C. de W., Curtis, L.M., Humpheson, L., Billon, C.M.-P., McClure, P.J., 1997. Development of thermal inactivation models for Salmonella enteritidis and Escherichia coli O157:H7 with temperature,
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M.H. Brown / International Biodeterioration & Biodegradation 50 (2002) 155 – 160
pH and NaCl as controlling factors. International Journal of Food Microbiology, in press. Brown, M.H., Davies, K., Billon, M-P., Adair, C., McClure, P.J., 1998. Quantitative microbiological risk assessment: principles applied to determining the comparative risk of salmonellosis from chicken products. Journal of Food Proteins 61, 1446–1453. Buchanan, R.L., Whiting, R.C., 1996. Risk assessment and predictive microbiology. Journal of Food Protein (Suppl.), 31–36. Carslaw, H.S., Jaeger, J.C., 1959. Conduction of Heat in Solids, 2nd Edition. Oxford University Press, London. Codex Alimentarius Commission, 1996. Principles and Guidelines for the Application of Microbiological Risk Assessment. CX=FH 96=10 1996, Food and Agriculture Organization of the United Nations, Rome, Italy. International Commission on Microbiological Safety of Food (ICMSF), 1996. Micro-organisms in Foods 5. Microbiological Speci-cations of Food Pathogens. ICMSF.ISBN O 412 47350 X. Blackie Academic & Professional. Kilsby, D.C., Pugh, M.E., 1981. The relevance of the distribution of micro-organisms within batches of food to the control of
microbiological hazards from foods. Journal Applied Bacteriology 51, 345–354. Martin, S.A., Wallsten, T.S., Beaulieu, N.D., 1995. Assessing the risk of microbial pathogens: application of a judgement-encoding methodology. Journal of Food Proteins 58, 289–295. Mead, G.C., 1982. Microbiology of poultry and game birds. In: Brown, M.H. (Ed.), Meat Microbiology. Applied Science Publishers, London, pp. 67–101. National Research Council, 1983. Risk Assessment in the Federal Government: Managing the Process. National Academy Press, Washington, DC. Notermans, S., Mead, G.C., Jouve, J.L., 1996. Food products and consumer protection: a conceptual approach and a glossary of terms. International Journal of Food Microbiology 30, 175–185. Todd, E.C.D., 1978. Foodborne disease in six countries—a comparison. Journal of Food Proteins 41, 559–565. Whiting, R.C., Buchanan, R.L., 1997. Development of a quantitative risk assessment model for Salmonella enteritidis in pasteurised liquid eggs. International Journal of Food Microbiology 36, 111–125.
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Quantitative microbiological risk assessment: principles applied to determining the comparative risk of salmonellosis from chicken products Martyn H. Brown ∗ Microbiology Unit, Unilever Research Laboratory, Colworth House, Sharnbrook, Bedfordshire, MK44 1LQ, UK
Abstract Since causal links have been established between food-borne illness and particular microorganisms, it has been possible to assess the public health risks of their presence in foods and propose measures to ensure the safety of customers. E,ective control measures for food safety have been based on knowledge of the resistance of pathogenic microorganisms to the treatments used for preservation (e.g. acidi-cation or reduced water activity) or decontamination (e.g. pasteurisation or sterilisation). Using this principle, food safety has been managed informally and successfully within the food industry for many years. Recently, formal risk assessment schemes, for example from Codex Alimentarius, have developed and placed the elements of decision-making on suitable control measures into a formal framework with clearly identi-able stages. The output of the four stages of a formal risk assessment (hazard identi-cation, hazard characterisation, exposure assessment and risk characterisation) provides the basis for decisions on actions needed to control the identi-ed hazard. There are many di5culties in ensuring that risk assessments are realistic and accessible to potential users, in many cases their value is limited by the data available. The study reported here on control of Salmonella in poultry products illustrates that it is possible to produce a comparative risk assessment based on published data. This study is not a full quantitative risk assessment, but provide useful pointers for a risk manager. Di,erences are discussed between the exposure assessment data needed to propose controls for infectious or toxigenic pathogens. For infectious pathogens, the presence of viable and infectious microorganisms is itself the hazard, but for toxigenic microorganisms, absence or destruction of viable cells at ingestion does not in itself ensure the absence of toxin. For toxin hazards, exposure assessment needs to consider previous conditions that may have led to toxin formation and persistence, rather than just the level of microbes at ingestion. ? 2002 Published by Elsevier Science Ltd. Keywords: Microorganisms; Salmonella; Salmonellosis; Chicken; Risk assessment; Risk management; Toxigenic microorganisms
1. Introduction Ensuring the microbiological safety of food requires the identi-cation of realistic hazards and their means of control. product defects and food-poisoning with microbiological origins have been recognised for many years and may be ranked according to seriousness for the customer and producer. They range from complaints of low quality or spoiled product, through mild food-poisoning to serious cases of food-borne infection or intoxication requiring hospitalisation. Food producers have always assessed the risks of their products causing food-poisoning, using either empirical or experiential approaches. Nowadays food microbiologists ∗
Fax: +44-1234-222277. E-mail address: [email protected] (M.H. Brown).
0964-8305/02/$ - see front matter ? 2002 Published by Elsevier Science Ltd. PII: S 0 9 6 4 - 8 3 0 5 ( 0 2 ) 0 0 0 8 1 - 1
are usually responsible for assessing the impact of product and process changes on the risks of a product causing food-poisoning. Food producers have always used a simple, empirical scheme for assessing and managing risks: • • • • • •
What is wrong? Who knows and what is known about the topic? What are the options for control? Which one should be used for action, from the options? Who needs telling about our decision, and What will we do?
Causal links have been established between food-borne illness and the presence, or activities (toxigenisis) of bacteria (Table 1) allowing the management of speci-ed hazards
156
M.H. Brown / International Biodeterioration & Biodegradation 50 (2002) 155 – 160
Table 1 Examples of bacteria causing food-borne diseases
Bacillus cereus Brucella abortus Campylobacter jejuni Clostridium botulinum Clostridium perfringens Coxiella burnettii Escherichia coli Enteropath E.coli Listeria monocytogenes Mycobacterium bovis Salmonella Shigella Staphylococcus aureus Vibrio parahaemolyticus Vibrio vulni5cus Yersinia enterocolitica
Fig. 1. The Codex Almentarius risk assessment scheme.
as the means of ensuring food with a known level of risk. Description of the link between a speci-c pathogen (e.g. Salmonella) and food-poisoning is essential for any risk assessment, because it is used to de-ne the dose-response of the target consumers and the severity of illness, the prevalence and level of the agent in raw materials and the e,ects of processing and use on the numbers eventually ingested. 2. Development of formal risk assessment Since the 1980s, crude traditional tools for determining the risks associated with identi-ed hazards (risk assessment) have developed into formal systems with well-de-ned stages and procedures. These have become known as microbiological risk assessment (MRA), and are described in Microbiological risk assessment: An Interim Report (ACDP, 1996) or by the Codex scheme (Codex Alimentarius, 1996) depicted in Fig. 1. The aim of all MRA schemes is to reduce risk by: • identifying realistic microbiological hazards and characterising them according to the severity of their e,ects on consumers,
• examining the impact of raw material contamination, processing and use on the level of risk, and • communicating clearly and consistently, via the output of the study, the level of risk to the consumer. Users of the output of a risk assessment should include all those involved in the design, production and consumption of food, and therefore it must be in a form that they can understand. When risk assessment is put together with risk management (actions to eliminate or minimise risk) and risk communication (information on a risk and on the decisions taken to combat a risk) a risk analysis is produced (ACDP, 1996). 3. Background to this risk assessment Because food companies are often in a state of change with respect to microbiological risks and hazards, the ability to assess risks and the impact of process, product and market changes on the level of risk are important means of ensuring consistent standards of food safety. Changes which a,ect risk can include the development of new products and processes, di,erent raw material sources, and target customer groups, e.g. products intended for children. Where the behaviour of microorganisms or the doseresponse of consumers can be described by mathematical models, numerical information can be processed by a computer to provide quantitative or comparative risk assessments. This study of the risks of salmonellosis from chicken products has two purposes, -rstly to use existing mathematical models to provide a quantitative risk assessment (QRA) tool within the framework and de-nitions proposed for risk analysis and assessment by Codex Alimentarius (1996), and secondly to provide a transparent, model-based QRA which would allow e,ective risk management and communication within a food manufacturing business. 4. Stages in this risk assessment It is essential that any risk assessment is preceded by clear formulation of the problem. This leads to a statement of purpose, or scope, similar to that preceding a Hazard Analysis Critical Control Point (HACCP) study (Fig. 2). In this example, the trigger is ‘evaluation of control options’ and the purpose is ‘to provide a quantitative microbiological risk assessment (QRA) of salmonellosis from frozen poultry products’ using computer-based models according to the Codex Alimentarius stages. 5. Hazard identi"cation Frozen poultry products may be cooked either in the factory, or in the home, or both. The raw poultry meat which is a starting material may contain Salmonella (Mead, 1982).
M.H. Brown / International Biodeterioration & Biodegradation 50 (2002) 155 – 160
Fig. 2. Deriving the statement of purpose.
Therefore, salmonellosis may be acquired by ingesting undercooked products which contain infective cells (ACMSF, 1996). This model-based MRA integrates models describing: occurrence and distribution of Salmonella in the raw material, the incidence (% contaminated) and the concentration of agent (log10 normal (Kilsby and Pugh, 1981) and other distributions were used to characterise the mean and variability of concentration of Salmonella). 6. Exposure assessment Exposure assessment describes the degree of exposure to the hazard (Salmonella) likely to occur from consumption of the product. It is de-ned by the portion size and level of Salmonella in the portion. The residual level in turn depends on the numbers of Salmonella entering the heating process, the heating characteristic of the product—determined by its size, shape and weight and the heating conditions used either in-factory or in-home. The combination of the heating time and temperature (heating process) and the rate of heat penetration into the product unit combine to deliver a heat treatment with a certain predictable lethal e,ect on Salmonella. This e,ect (log reductions from the initial level) can be calculated if the heat sensitivity of Salmonella at various temperatures (D and z values) and the product’s thermophysical properties are known. The very simple thermophysical models used can predict the e,ect on lethality of di,erent cooking temperatures, alterations to length of the thermal path and heat transfer to the surface and within the product. This model can sum the lethal e,ects of two heating stages (in-plant and in-home) with di,erent methods of heating, because it is recognised that microbiological safety for this type of product is not necessarily guaranteed by manufacturing processes, but can be a joint responsibility with the customer (Notermans et al., 1996). This makes the consumer part of the process of ensuring that the end product is safe an therefore an assessment of their e,ect is an essential part of the risk assessment.
157
or target consumers to infection (e.g. normal or susceptible) and the e,ect of cooking (in the factory or home) on concentration of the agent in the product. This programme allows users to produce risk estimates without extensive data on the dose-response within populations, since they can specify the dose-response in terms of median responses and interquartile range. Actual data on infection rate can only come from national or international epidemiological studies, whereas data on the numbers and distribution of hazardous agents will always be associated with speci-c commodities producted or harvested under certain conditions. 8. Risk characterisation This programme facilitates comparative risk assessment and risk communication, because it can show how a range of variables inMuences the risks of Salmonella infection associated with the consumption of a cooked, frozen poultry product. Product, process and other variables, can be altered to produce comparative risk assessments based on di,erent sets of values. This system concentrates on a single factor, the heating stage, controlled during manufacture or by the customer, rather than the sequence of production stages considered by Buchanan and Whiting (1996). But it can take account of the e,ect of changes in population sensitivity, raw material quality, and cooking regime on the -nal risk. The e,ects of growth during storage and re-contamination of the product are not considered. Where possible the values used for predictions have been based on published or experimental data. To facilitate communication of decision on risk and their basis, the models are visualised and information is presented graphically (Fig. 3), to allow transparent communication with both customers and sta,. Conditions or properties may be changed by means of displays and slider controls on a computer screen, to encourage users to assess the e,ect of changes in processing, markets or materials on risk. For convenience this model-based QRA was handled a in statistical analysis system with a graphical interface (SAS=AF version 6.11, SAS Institute Inc., Cary, NC, USA). Target users include product developers, buyers, quality assurance and manufacturing personnel. Where risk decisions are communicated e,ectively, risk management practices can be readily implemented, consistent standards applied, and dangerous changes may be stopped. Implementation of an e,ective process of communication and understanding on a consistent, scienti-c, and yet practical basis is a continuing dilemma. The screen presentation is shown below and the mathematics are given in Brown et al. (1998).
7. Hazard characterisation
9. Discussion
This quanti-es the risks of infection after product consumption. To do this it links the sensitivity of populations
The risk assessment model could be improved in several ways.
158
M.H. Brown / International Biodeterioration & Biodegradation 50 (2002) 155 – 160
Fig. 3. The risk characterisation screen.
• Heating. The heat transfer model used in this program is relatively simple (Carslaw and Jaeger, 1959). The use of di,erent coe5cients relevant to other products or processes and a more Mexible model, which could take into account freezing, thawing, and several process steps would improve its Mexibility and range of application. • Thermal inactivation models. Accuracy would be improved by the use of thermal inactivation=survival models which accounted for injury to microbial cells caused by heating or storage and took account of any consequent change in infectivity. For models covering multi-stage processes (Buchanan and Whiting, 1996) thermal inactivation models need to take account of growth and the cumulative e,ects of injury during a sequence of process steps. Whiting and Buchanan (1997) have used the Monte-Carlo simulation to assess the cumulative risks of a process (pasteurized egg manufacture), based on process and environmental input values. Their approach, taking into account variability described by a frequency distribution, produces a di,erent and less conservative assessment of risk than one based on taking the worst case at each stage. • Dose response and population data. Categorising of the seriousness of hazards and quantifying risk is hampered by the paucity of scienti-c data on the sensitivity of consumers to food-borne pathogens. In many cases this has prevented even comparative risk assessments. Judgement-Encoding Methodology has been used to quantify the uncertainty of judgements based on dose– response curves (Martin et al., 1995). However, there is no substitute for scienti-c data.
10. Further application of the risk assessment model Infectious pathogens. This framework can be used to make risk assessments for a range of infectious pathogens (e.g. salmonellae, Listeria monocytogenes, Escherichia coli O157), if their fate is controlled by a single process stage (heating) and there are appropriate models for infection, heating and death kinetics (e.g. Blackburn et al., 1997). Toxigenic microbes. This program has been built to deal with the risks associated with non-toxigenic pathogens. However, a common source of foodborne diseases (Todd, 1978) is ingestion of performed microbial toxins rather than infection. For these pathogens the dose-response is not linked directly to the number of microorganisms present when the product is consumed, but to the quantity of toxin. The Codex approach can still be used to assess the risks of toxigenesis and di,erence and similarities between the factors requiring consideration are shown in Fig. 4. The hazard identi-cation step for a toxin QRA would identify the toxin from one (or more) of the known toxigenic microorganisms (e.g. Staphylococcus aureus, Clostridium botulinum). The dose-response would describe the sensitivity of a population to that toxin, rather than to the microorganism producing it. The dose response would therefore be based on the weight of toxin per unit body weight to cause illness, rather than numbers of microorganisms. Hazard characterisation would be linked to the pathogenicity of the toxin, the dynamics of intoxication and the distribution of susceptibility in the population of consumers to the toxin. Exposure assessment would have to deal with both numbers of microorganisms and the kinetics and boundaries for
M.H. Brown / International Biodeterioration & Biodegradation 50 (2002) 155 – 160
Fig. 4. Risk factors di,erentiating infectious and toxigenic microorganisms (Information summarised from ICMSF, 1996).
Fig. 5. Requirements to assess the risks from infectious and toxigenic pathogens.
their toxigenisis and the accumulation of toxin from raw material to consumption (Fig. 5). Similar elements (e.g. incidence, distribution and fate of the toxigenic microorganisms) are required for exposure assessment as for infectious pathogens. However, additional information speci-c to toxigenic microorganisms is required to describe the production and fate of the toxin during processing, storage and consumer use. This involves knowledge of the kinetics and boundaries of microbial toxin production, which are dependent on the condition in (e.g. pH, water activity, competing microbes) and around the food (e.g. temperature and PO2 ) plus information on the stability and persistance of the toxin within the product and the likely e,ect of cooking. There is only general data on the thermal denaturation of toxins in food. The output of a toxin QRA would be unchanged: expected number of people intoxicated for a given number of portions, however the source of harm will be the toxins rather than the microorganisms directly. 11. Conclusions and issues Quantitative Risk assessment is an important tool for evaluating and communicating the impact of raw material quality, processing and changes on food safety. The answers from any QRA are only as good as the input data and at
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present, lack of data makes QRA di5cult to implement. In many cases, even causal links between microbial behaviour and harmful e,ects have not been reliably established. All components of risk assessment are currently limited by the availability of information. There are few clinical and epidemiological studies on the e,ects of pathogens, and surveillance data with valid causal links is scarce. The use of benchmark populations, as in this example, may partly overcome the lack of epidemiological data. Limited investigation of microbial physiological and ecology limit our understanding of the features of food systems which promote or inhibit microbial growth, survival and toxigenisis. Clearly, infectious and toxigenic pathogens need models of di,erent complexity to describe their behaviour and hence the hazards they can present within the food chain. To make QRA work, a robust, accepted processes for risk assessment combined with good quality kinetic data on the target microorganisms is needed (NRC, 1983). Commonly, information on analogous microorganisms, situations or contexts is used for both risk assessment and communication. Although the scienti-c aspects of risk assessment can be tackled using the Codex methodology, e,ective communication of conclusions and recommendations on risk management remains a major problem, the means of communication should in principle be dictated by the objective of communication. The Codex approach is aimed at description or characterisation of the risk, rather than being focused on providing data for decision making. The key issues preventing e,ective microbiological risk assessment remain • uncertainty (i.e. lack of relevant data), • variability (i.e. data available indicates that the variability of a feature may limit e,ective assessment of the risks associated with it), • accessibility (i.e. the data or conclusions are not available in a form that allows their use by decision-makers), and • misuse=miscommunication (i.e. risk assessments are not presented in a clear and unambiguous way, and at worst including such emotive phrases as ‘completely safe’). With acknowledgements to my co-authors of the original paper published in the Journal of Food Protection, viz. Kenneth Davies, Christelle Billon, Carol Adair and Peter McClure.
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