Microbiological hazards and safe process design 297 elements of the process contribute to safety and shelf-life, the shelf-life is restricted and any delay to await the results of microbiological testing uses-up shelf-life. HACCP involves the identification of realistic(microbiological)hazards, such as pathogenic agents and the conditions leading to their presence, growth or survival (HACCP is also used for the control of chemical and physical hazards the identification of specific requirements for the control of hazards and identification of process stages where this is achieved procedures and equipment to measure and document the efficacy of the controls that are an integral part of the HACCP system the documentation of limits and the actions required when these are exceeded For steps in the manufacturing process that are not recognised as CCPs of GMP/GHP provides assurance that suitable control and standards are applied. The identification and analysis of hazards within the HACCP programme ill provide information to interpret GMP/GHP requirements and direct training, calibration etc. for specific products or processes. The Microbiology and Food Safety Committee of the National Food Processors Association(NFPA 1993)has considered HACCP systems for chilled foods produced at a central location and distributed chilled to retail establishments. They used chicken salad as a model for proposing critical control points and give practical advice on HACCP planning, development of a production flow diagram, hazard identification, establishing critical limits, monitoring requirements; and verification procedures to ensure the HACCP system is working effectively. There are also USDA recommendations and outline HACCP flow diagrams for chilled food processes, cook-in-package and cook-then-package Snyder(1992) Risk analysis Ensuring the microbiological safety and wholesomeness of food requires the identification of realistic hazards and their means of control (risk assessment) The ability of a food producer to assess the impact of process, product and market changes on the level of risk and the type of hazard are important to the assurance of consistent standards of food safety. The effects of changes on risk and hazards need to be identified and can include the development of new products and processes, the use of different raw material sources, or the targeting of new customer groups, such as children. Food producers have always assessed these risks using either empirical or experiential approaches. As causal links have been established between food-borne illness and the presence, or activities (toxigenisis), of food-poisoning micro-organisms, so the control of specified microbial hazards has progressively become the means of ensuring food safety These practical approaches have now developed into formal systems with well defined procedures and are known as Microbiological Risk Assessment(MRA) They are described in Microbiological Risk Assessment; an interim report (ACDP, 1996)or by the Codex scheme(Figure l, Codex Alimentarius, 1996)
elements of the process contribute to safety and shelf-life, the shelf-life is restricted and any delay to await the results of microbiological testing uses-up shelf-life. HACCP involves: • the identification of realistic (microbiological) hazards, such as pathogenic agents and the conditions leading to their presence, growth or survival (HACCP is also used for the control of chemical and physical hazards) • the identification of specific requirements for the control of hazards and identification of process stages where this is achieved • procedures and equipment to measure and document the efficacy of the controls that are an integral part of the HACCP system • the documentation of limits and the actions required when these are exceeded. For steps in the manufacturing process that are not recognised as CCPs, the use of GMP/GHP provides assurance that suitable control and standards are being applied. The identification and analysis of hazards within the HACCP programme will provide information to interpret GMP/GHP requirements and direct training, calibration etc. for specific products or processes. The Microbiology and Food Safety Committee of the National Food Processors Association (NFPA 1993) has considered HACCP systems for chilled foods produced at a central location and distributed chilled to retail establishments. They used chicken salad as a model for proposing critical control points and give practical advice on HACCP planning; development of a production flow diagram, hazard identification, establishing critical limits, monitoring requirements; and verification procedures to ensure the HACCP system is working effectively. There are also USDA recommendations and outline HACCP flow diagrams for chilled food processes, cook-in-package and cook-then-package Snyder (1992). Risk analysis Ensuring the microbiological safety and wholesomeness of food requires the identification of realistic hazards and their means of control (risk assessment). The ability of a food producer to assess the impact of process, product and market changes on the level of risk and the type of hazard are important to the assurance of consistent standards of food safety. The effects of changes on risk and hazards need to be identified and can include the development of new products and processes, the use of different raw material sources, or the targeting of new customer groups, such as children. Food producers have always assessed these risks using either empirical or experiential approaches. As causal links have been established between food-borne illness and the presence, or activities (toxigenisis), of food-poisoning micro-organisms, so the control of specified microbial hazards has progressively become the means of ensuring food safety. These practical approaches have now developed into formal systems with well defined procedures and are known as Microbiological Risk Assessment (MRA). They are described in Microbiological Risk Assessment; an interim report (ACDP, 1996) or by the Codex scheme (Figure 1, Codex Alimentarius, 1996). Microbiological hazards and safe process design 297
298 Chilled foods The overall aim of risk analysis is to reduce risk by identifying realistic microbiological hazards and characterising them according to severity 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 When risk assessment is put together with risk communication(distribution of information on a risk and on the decisions taken to combat a risk)and used to promote sound risk management(actions to eliminate or minimise risk), a risk analysis is produced(ACDP 1996) Stages in a risk assessment Clear formulation of a problem is an essential prelude to risk assessment. A process or ingredient change, the emergence of a new pathogen or a change public concern may trigger a risk assessment over a hazard and this can lead to the review of control, factory layout or sourcing options or the revision of cooking instructions The first stage of the assessment is to identify the hazard(hazard identifica- tion). For example, concern may be over the presence of salmonella in a product, as ingesting products containing infective cells may cause salmonellosis. The virulence, incidence and concentration) and its prevalence in raw materials chances of causing harm are governed by many factors specific to the hazard Exposure assessment describes the likely exposure of customers to the hazard, based on the size of the portion consumed and the impact of prior manufacturing etc. on the quantity of the infectious agent (i.e. Salmonellae) present(and infectious) at consumption. For a cooked product, exposure will depend on the numbers of salmonella entering the heating process, the heating haracteristics of the product and the heating conditions used either in-factory or in-home, these combine to determine the number of pathogens surviving at onsumption. If the heat sensitivity of salmonella and the products heat treatment are known, then numbers likely to survive can be estimated. For many chilled foods (e.g. burgers or flash-fried poultry products), microbiological safety 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 for ensuring that the end product is safe, and therefore an assessment of their effect an essential part of a risk assessment Quantification of the risks of infection after product consumption is known as hazard characterisation. It links the sensitivity of consumers to infection (i.e usually making use of expert opinion or knowledge of the dose-response within populations) with the concentration of the agent in the portion. The output of these three stages is a risk characterisation, which describes for a certain consumer the risks of (Salmonella) infection associated with the consumption of a particular product, sourced and manufactured under specified conditions
The overall aim of risk analysis is to reduce risk by • identifying realistic microbiological hazards and characterising them according to severity • 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. When risk assessment is put together with risk communication (distribution of information on a risk and on the decisions taken to combat a risk) and used to promote sound risk management (actions to eliminate or minimise risk), a risk analysis is produced (ACDP 1996). Stages in a risk assessment Clear formulation of a problem is an essential prelude to risk assessment. A process or ingredient change, the emergence of a new pathogen or a change in public concern may trigger a risk assessment over a hazard and this can lead to the review of control, factory layout or sourcing options or the revision of cooking instructions. The first stage of the assessment is to identify the hazard (hazard identification). For example, concern may be over the presence of salmonella in a product, as ingesting products containing infective cells may cause salmonellosis. The chances of causing harm are governed by many factors specific to the hazard (its virulence, incidence and concentration) and its prevalence in raw materials. Exposure assessment describes the likely exposure of customers to the hazard, based on the size of the portion consumed and the impact of prior manufacturing etc. on the quantity of the infectious agent (i.e. Salmonellae) present (and infectious) at consumption. For a cooked product, exposure will depend on the numbers of salmonella entering the heating process, the heating characteristics of the product and the heating conditions used either in-factory or in-home, these combine to determine the number of pathogens surviving at consumption. If the heat sensitivity of salmonella and the product’s heat treatment are known, then numbers likely to survive can be estimated. For many chilled foods (e.g. burgers or flash-fried poultry products), microbiological safety 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 for ensuring that the end product is safe, and therefore an assessment of their effect an essential part of a risk assessment. Quantification of the risks of infection after product consumption is known as hazard characterisation. It links the sensitivity of consumers to infection (i.e. usually making use of expert opinion or knowledge of the dose-response within populations) with the concentration of the agent in the portion. The output of these three stages is a risk characterisation, which describes for a certain consumer the risks of (Salmonella) infection associated with the consumption of a particular product, sourced and manufactured under specified conditions. 298 Chilled foods
Microbiological hazards and saf gn cess desig 299 To facilitate communication of decisions on risk and their basis information must be accessible to the management, customers and staff. Where risk lecisions or conclusions are communicated effectively, risk management prac tices can be readily implemented, consistent standards applied and dangerous hanges may be stopped. Implementation of an effective process for ommunication and understanding on a consistent, scientific, and yet practical basis is an unsolved problem. Risk assessment has been reviewed (Jaykus 1996) and applied to specific problems, listeriosis(Miller et al. 1997), the role of indicators(Rutherford et al. 1995)and links with HACCP(Elliot et al. 1995) Precautionary principle Generally, the actions taken to protect public health are based on sound science. However. from time to time decisions have to be taken in an area of scientific uncertainty, for example if the prevalence or severity of a new pathogen is unknown. Any decisions made using the precautionary principle should control the perceived health risks without resorting to excessively restrictive control measures and should be proportionate to the severity of the food safety problem An example is the measures taken to control the presence of E coli o157: H7 vegetables(by pasteurisation) or salad crops(by disinfection during washing)or by the proposal of Good Agricultural Practices(De Roever 1998), when alence of the pathogen is unknown and the illness caused severe. The resulting actions need to have been derived in an understandable and justifiable and any assumptions and uncertainties need to be clear. Most of all the ction in risk achieved must be acceptable to all the parties involved Decisions arising in this way should be regarded as temporary awaiting further information that will allow a more reliable risk assessment and lead to the appropriate control measures, as described above 11.2.4 Processes Cooking Cooking indicates that a process step delivers or has delivered sufficient heat cause all parts of a food to reach the required sensory quality and should have caused a significant reduction in the numbers of any infectious pathogens that may be present. A 6-log reduction in numbers of infectious pathogens, or 70C for two minutes, is usually considered to be a diligent minimum target(see "Safe process design below ) Some cooking stages may deliver considerably more heat than this, for example those involving prolonged periods of boiling. If the ooking stage is used as a pasteurization step, then recontamination must be prevented. It is important that cooking specifications distinguish between heat treatments(i.e. the conditions within a food)and the process conditions needed to provide the heat treatment. For example, for a given heat treatment, process conditions will vary according to the diffusion of heat through the product, its dimensions and the transfer of heat to or through its surface
To facilitate communication of decisions on risk and their basis, information must be accessible to the management, customers and staff. Where risk decisions or conclusions are communicated effectively, risk management practices can be readily implemented, consistent standards applied and dangerous changes may be stopped. Implementation of an effective process for communication and understanding on a consistent, scientific, and yet practical basis is an unsolved problem. Risk assessment has been reviewed (Jaykus 1996) and applied to specific problems, listeriosis (Miller et al. 1997), the role of indicators (Rutherford et al. 1995) and links with HACCP (Elliot et al. 1995). Precautionary principle Generally, the actions taken to protect public health are based on sound science. However, from time to time decisions have to be taken in an area of scientific uncertainty, for example if the prevalence or severity of a new pathogen is unknown. Any decisions made using the precautionary principle should control the perceived health risks without resorting to excessively restrictive control measures and should be proportionate to the severity of the food safety problem. An example is the measures taken to control the presence of E.coli O157: H7 in vegetables (by pasteurisation) or salad crops (by disinfection during washing) or by the proposal of Good Agricultural Practices (De Roever 1998), when prevalence of the pathogen is unknown and the illness caused severe. The resulting actions need to have been derived in an understandable and justifiable way and any assumptions and uncertainties need to be clear. Most of all the reduction in risk achieved must be acceptable to all the parties involved. Decisions arising in this way should be regarded as temporary awaiting further information that will allow a more reliable risk assessment and lead to the appropriate control measures, as described above. 11.2.4 Processes Cooking Cooking indicates that a process step delivers or has delivered sufficient heat to cause all parts of a food to reach the required sensory quality and should have caused a significant reduction in the numbers of any infectious pathogens that may be present. A 6-log reduction in numbers of infectious pathogens, or 70ºC for two minutes, is usually considered to be a diligent minimum target (see ‘Safe process design’ below). Some cooking stages may deliver considerably more heat than this, for example those involving prolonged periods of boiling. If the cooking stage is used as a pasteurization step, then recontamination must be prevented. It is important that cooking specifications distinguish between heat treatments (i.e. the conditions within a food) and the process conditions needed to provide the heat treatment. For example, for a given heat treatment, process conditions will vary according to the diffusion of heat through the product, its dimensions and the transfer of heat to or through its surface. Microbiological hazards and safe process design 299
300 Chilled foods Pasteurisation Pasteurisation is a processing stage involving heat. It is designed to bring about consistently a predictable reduction in the numbers of specified types of micro- organism in a food or ingredient. The safety and shelf-life requirements should determine the minimum severity of any pasteurisation stage, and the processes used may not necessarily heat ingredients sufficiently to give the required sensory quality or destroy all the micro-organisms present. The effectiveness of a particular heat treatment may be altered by various factors, including the preservation system employed in a particular food. For example, the heat resistance of bacteria and their pores is generally increased by low water activity, but decreased by low pH. Gaze and Betts(1992) have produced an overview of types of pasteurisation process and their microbiological targets. They also use the example of a pre-cooked chille product to provide advice on process design and on manufacturing control points based on published heat resistance and growth data. Minimum pasteurisation severe, being targeted at the more heat-resistant bacteria causin e most are more product(see Gaze and Betts 1992 and CFDRA 1992) P-value Pasteurisation values (P-values) specify the effectiveness of a pasteurisation heat treatment. They are used to indicate the equivalent heat treatment corresponding to a specified heating time at a stated reference temperature c-value Z-value is an empirical value, quoted in temperature(C or F degrees), and used for calculating the increase or decrease in temperature that is needed to alter by a factor of 10 the rate of inactivation of a particular micro-organism. It assumes that the kinetics of microbial death at constant temperature is exponential (i.e. "log-linear) Although is fundamental to the calculation of sterilisation process equivalence, it should be used with extreme caution for pasteurisation processes, as the death kinetics of many types of micro-organisms are not log-linear; especially where vegetative micro-organisms are concerned and when hea tting rates are low For instance,"heat adaptation'may occur. This may even raise the resistance of the micro-organism during the heating process(Mackey and Derrick 1987). " Shoulders and" on survivor curves are more commonly seen( Gould 1989). In practice, the validity of the concept is therefore particularly limited at low temperatures such as those involved in pasteurisation. Consequently, where there are other factors such as preservatives interacting with the heat treatment, or if processes are designed to cause large log reductions(i.e. in excess of ten thousand-fold), then tailing of the survivor curves may become particularly important. Actual or challenge trials should be undertaken to establish confidently safe processes Re-heating The customer usually does re-heating and it is a procedure intended by the manufacturer only to ensure the optimum culinary quality of a product
Pasteurisation Pasteurisation is a processing stage involving heat. It is designed to bring about consistently a predictable reduction in the numbers of specified types of microorganism in a food or ingredient. The safety and shelf-life requirements should determine the minimum severity of any pasteurisation stage, and the processes used may not necessarily heat ingredients sufficiently to give the required sensory quality or destroy all the micro-organisms present. The effectiveness of a particular heat treatment may be altered by various factors, including the preservation system employed in a particular food. For example, the heat resistance of bacteria and their spores is generally increased by low water activity, but decreased by low pH. Gaze and Betts (1992) have produced an overview of types of pasteurisation process and their microbiological targets. They also use the example of a pre-cooked chilled product to provide advice on process design and on manufacturing control points based on published heat resistance and growth data. Minimum pasteurisation processes should be targeted at foodborne pathogens, but in practice most are more severe, being targeted at the more heat-resistant bacteria causing spoilage in the product (see Gaze and Betts 1992 and CFDRA 1992). P-value Pasteurisation values (P-values) specify the effectiveness of a pasteurisation heat treatment. They are used to indicate the equivalent heat treatment corresponding to a specified heating time at a stated reference temperature. z-value z-value is an empirical value, quoted in temperature (C or F degrees), and used for calculating the increase or decrease in temperature that is needed to alter by a factor of 10 the rate of inactivation of a particular micro-organism. It assumes that the kinetics of microbial death at constant temperature is exponential (i.e. ‘log-linear’). Although z is fundamental to the calculation of sterilisation process equivalence, it should be used with extreme caution for pasteurisation processes, as the death kinetics of many types of micro-organisms are not log-linear; especially where vegetative micro-organisms are concerned and when heating rates are low. For instance, ‘heat adaptation’ may occur. This may even raise the resistance of the micro-organism during the heating process (Mackey and Derrick 1987). ‘Shoulders’ and ‘tails’ on survivor curves are more commonly seen (Gould 1989). In practice, the validity of the z concept is therefore particularly limited at low temperatures, such as those involved in pasteurisation. Consequently, where there are other factors such as preservatives interacting with the heat treatment, or if processes are designed to cause large log reductions (i.e. in excess of ten thousand-fold), then tailing of the survivor curves may become particularly important. Actual or challenge trials should be undertaken to establish confidently safe processes. Re-heating The customer usually does re-heating and it is a procedure intended by the manufacturer only to ensure the optimum culinary quality of a product. 300 Chilled foods
Microbiological hazards and safe process design 301 Depending on the product design, such a process may or may not provide adequate heating for safety; this is especially true where products are designed for microwave reheating. A re-heating process should, therefore, only be recommended to the customer if the food has effectively been freed of hazardous contaminants during processing and has remained so during any further processing, packaging, distribution and shelf-life. The chemical and physical characteristics of a ready meal, including saltiness, type of tray, and geometry and layout of components, affect microwave heating uniformity and rate Ryynanen and ohlsson(1996)heated four-component chilled ready meals in a domestic microwave oven and found that arrangement and geometry of omponents and type of tray mainly affected heating uniformity. Where microwaves are being used for cooking their effectiveness should be validated; a suitable method uses alginate beads containing micro-organisms with a known heat resistance(Holyoak et al. 1993) Coolin Cooling reduces the product temperature after factory cooking. Its aim should be to ensure that the product spends the minimum possible time in the temperature range allowing the growth of hazardous bacteria, i.e. between 55C and 10C Cooling rates are often specified in legislation; for example, in the EEC Meat and Meat Products Directive 77/99, prepared meals must be cooled to below 10C within two hours of cooking. Evans et al.(1996) have highlighted the importance of cooling and the mandatory requirements that exist in the UK for cook-chill products(Anon. 1982)These guidelines recommend that 80mm trays should be chilled to below 10C in 2.5 hours, between 10 and 40mm they should be chilled to below 3C in 1.5 hours. Assuming that surface freezing is to be avoided and a simple, single-stage operation used, only a 10-mm deep tray can be chilled within these time limits Cooling of liquids or slurries may be done in line, using heat exchangers Where batch cooling of solids and slurries is done in containers the size of the container or the quantity of product should not be so large that rapid cooling is not possible. Product depths exceeding 10-15 cm should not be used because above this thickness conduction of heat to the surface rather than removal of heat from the surface, will limit the rate of cooling of the bulk of the product and ay allow microbial growth When warm or hot material is loaded into containers for air cooling. the materials of construction of containers will exert a significant effect on cooling rates, thick-walled plastic containers cooling considerably more slowly than metal ones The design of chillers, especially their air distribution pattern, air velocity and temperature and the way that product containers are packed into them, will control ooling rates. Racking or packing systems should allow the flows of cold air over the container surfaces so that cooling rates are maximised. Special attention should be paid to hygiene, and the control of condensation in chillers, as this is a ajor potential source of recontamination with Listeria if condensation is
Depending on the product design, such a process may or may not provide adequate heating for safety; this is especially true where products are designed for microwave reheating. A re-heating process should, therefore, only be recommended to the customer if the food has effectively been freed of hazardous contaminants during processing and has remained so during any further processing, packaging, distribution and shelf-life. The chemical and physical characteristics of a ready meal, including saltiness, type of tray, and geometry and layout of components, affect microwave heating uniformity and rate. Ryynanen and Ohlsson (1996) heated four-component chilled ready meals in a domestic microwave oven and found that arrangement and geometry of components and type of tray mainly affected heating uniformity. Where microwaves are being used for cooking their effectiveness should be validated; a suitable method uses alginate beads containing micro-organisms with a known heat resistance (Holyoak et al. 1993). Cooling Cooling reduces the product temperature after factory cooking. Its aim should be to ensure that the product spends the minimum possible time in the temperature range allowing the growth of hazardous bacteria, i.e. between 55ºC and 10ºC. Cooling rates are often specified in legislation; for example, in the EEC Meat and Meat Products Directive 77/99, prepared meals must be cooled to below 10ºC within two hours of cooking. Evans et al. (1996) have highlighted the importance of cooling and the mandatory requirements that exist in the UK for cook-chill products (Anon. 1982) These guidelines recommend that 80mm trays should be chilled to below 10ºC in 2.5 hours, between 10 and 40mm they should be chilled to below 3ºC in 1.5 hours. Assuming that surface freezing is to be avoided and a simple, single-stage operation used, only a 10-mm deep tray can be chilled within these time limits. Cooling of liquids or slurries may be done in line, using heat exchangers. Where batch cooling of solids and slurries is done in containers, the size of the container or the quantity of product should not be so large that rapid cooling is not possible. Product depths exceeding 10–15 cm should not be used because above this thickness conduction of heat to the surface, rather than removal of heat from the surface, will limit the rate of cooling of the bulk of the product and may allow microbial growth. When warm or hot material is loaded into containers for air cooling, the materials of construction of containers will exert a significant effect on cooling rates, thick-walled plastic containers cooling considerably more slowly than metal ones. The design of chillers, especially their air distribution pattern, air velocity and temperature and the way that product containers are packed into them, will control cooling rates. Racking or packing systems should allow the flows of cold air over the container surfaces so that cooling rates are maximised. Special attention should be paid to hygiene, and the control of condensation in chillers, as this is a major potential source of recontamination with Listeria if condensation is Microbiological hazards and safe process design 301