The use of heat treatment as a method of food preservation has ancient roots. The aim of using heat is to eliminate, minimise or inhibit micro‐organism and enzymic activity that, if it was allowed to proliferate, would make the food unsafe or otherwise unfit for human consumption. The term ‘thermal preservation’ denotes the use of heat over a prescribed period of time to render a food suitable for consumption and is a means to extend product durability and shelf life. Examples of heat preservation include canning, cooking, boiling, pasteurising, sterilising and blanching amongst others. However, it should be noted that whilst some heat preservation can address vegetative bacteria a given process may not affect thermophilic micro‐organisms, heat‐tolerant bacterial toxins or microbiological spores. This chapter is not written as a comprehensive guide to heat preservation, but instead to give a general outline of the subject of heat‐preserved foods in the context of food manufacture.
43.1 Heat treatment is a method of preservation that will prevent or delay product deterioration or spoilage and inhibit the growth of pathogenic organisms. There are several methods of heat treatment, including canning, pasteurisation, ultra‐heat treatment and sterilisation.
43.2 Heat treatment processes within the manufacturing environment must be designed to ensure that the foods subject to such treatment are microbiologically stable and growth of micro‐organisms is prevented under subsequent storage conditions, within the manufacturing process, and during the period in which the foods are intended to be suitable for storage and consumption.
43.3 Commercially sterile food is food that has undergone a heat treatment that will destroy all vegetative pathogens and organisms capable of growth or causing spoilage in the food under standard storage conditions, for example low‐acid canned food or aseptically packed product.
43.4 All low‐acid foods having in any part of them a pH value of 4.5 or above and intended for storage under non‐refrigerated conditions must be subjected to the minimum botulinum process, that is, one that will reduce the probability of survival of Clostridium botulinum spores by at least 12 decimal reductions, unless the formulation or water activity, or both, of the food is such that it can be demonstrated that growth of strains or forms of the organism cannot occur. The scheduled heat process required to achieve commercial sterility will be in excess of the minimum botulinum process (Fo = 3), as many organisms associated with the spoilage and economic loss of heat‐preserved foods have greater heat resistance than C. botulinum. In addition to achieving commercial sterility, the scheduled heat process may be further extended for specific reasons, for example to soften the bones in canned fish or to tenderise or texturise meats.
43.5 Low‐acid foods packed in acidic carrying fluids or whose natural pH value has been otherwise lowered by the controlled addition of acids, should be subjected to at least a minimum botulinum process unless it can be established that the equilibrium pH, including the pH value at the cores of particles, is less than 4.2 within 4 hours of the end of the thermal process, thus providing a safety margin for non‐uniformity of acidification.
43.6 The scheduled heat process is the key document for the manufacture of all heat‐processed foods. In the UK, it is addressed by a number of guidelines available from Campden BRI.1
43.7 The Statutory Instruments of minimum scheduled heat treatments for milk, semi‐skimmed and skimmed milk, milk‐based drinks and cream include the Dairy Products (Hygiene) Regulations 1995, SI 1995 No. 1086, as amended, for ice cream in SI 1959 No. 734, as amended in 1962, 1963, 1982, 1985, 1990 and 1995, and for liquid egg in the Egg Products Regulations 1993, SI 1993 No. 1520.
43.8 For efficient heat treatments, the physical, chemical and microbiological characteristics of each specific product need to be identified since these are critical factors that can affect the rate of heat penetration or heat distribution. These, therefore, need to be taken into account for plant design and processing regimes. Products should be defined in terms of their densities, rheological properties, water activity (aw), pH, temperature, thermal properties, pressures, specific heats, microbial loading, gas content and corrosive properties. Electrical properties are critical when using electrical energy for sterilisation. The processing parameters set must be validated to demonstrate that they are capable of delivering safe food (see 43.26 and Chapter 3).
43.9 Discrete particles should be defined additionally in terms of size, shape, concentration and swelling or shrinkage. Dry ingredients should be thoroughly dispersed and wetted. Non‐condensable gases should be disengaged.
43.10 Scheduled heat processes are derived by measuring the rate of heat penetration into the product and integrating the time/temperature exposure, hence determining the total lethal heat that the product receives within a particular process regime. For low‐acid foods, or foods otherwise designated as requiring at least a minimal botulinum process, ‘worst‐case’ assumptions involving the lowest temperature reached and shortest retention time at it should be made for C. botulinum spore destruction kinetics (see 43.4).
43.11 All new products or changes in the manufacturing operations, formulation and, if appropriate, containers for existing packs should be fully evaluated as to their effect on the rate of heat transfer through the product before commercial production is undertaken. Effective validation must be undertaken to ensure that such modifications, and the associated changes to processing conditions that are required, are effective. This activity must be recorded and records maintained.
43.12 Products not given at least a minimum botulinum process, or that are not otherwise shelf stable at ambient temperature, should carry clear instructions on storage conditions, including maximum storage temperatures.
43.13 All containers should be indelibly marked with a code indicating at least the place and date of production. Further information on the time of production and the production line can be very valuable. Empty containers or reel stock should be checked on receipt to ensure that they comply with the agreed recommendations for the product. They should be stored and handled to prevent their becoming contaminated or damaged and the integrity of the container thereby affected. Containers conveyed into a filling machine should be clean and flawless.
43.14 All batch and continuous processing equipment should be fitted with direct reading (indicating) temperature probes and automatic time and temperature recording instruments. These should be calibrated at designated intervals (see Chapter 34). Records of calibration and processing records, including data from temperature monitoring and automatic recording devices, should be kept for at least 3 years from the date of production depending on the duration of the product or customer requirement. Monitoring may be undertaken by on‐line or quality control personnel. Personnel undertaking monitoring need to be aware of the critical nature of such monitoring and the actions to be taken in the instance that there is a product or process failure identified at a product safety critical control point (see Chapter 3).
43.15 Food processing equipment should be internally clean and disinfected, correctly assembled, free from ‘dead legs’ and with control systems that have instruments of reproducible accuracy that are routinely calibrated. Procedures must be in place to address the actions to be taken in the event of equipment calibration failure (see Chapter 34).
43.16 Water used for manufacturing purposes, including that used in making up products or likely to come into direct contact with the product, must be of potable quality and free from any:
- substances in quantities likely to cause harm to health;
- substances at levels capable of causing accelerated internal corrosion of metallic containers and closures or causing taints; and
- harmful micro‐organisms.
To maintain the required microbiological standards, the water should, if necessary, be chlorinated or otherwise adequately treated (see 43.32).
43.17 Mechanical unloading and handling systems should be designed so that:
- dryers can be located as early as possible in the conveying system;
- there is minimum contact between container closures and the conveyor surfaces;
- abuse of containers is kept to a minimum; and
- conveyor surfaces must be easily cleaned and disinfected as frequently as is necessary to maintain proper standards of hygiene (see Chapter 21).
43.18 Regular microbiological surveillance, including swab tests, should be made on conveyors and other contact surfaces to establish the effectiveness of sanitation programmes (see Chapter 21).
43.19 To reduce the potential for cross‐contamination between high‐risk and low‐risk areas (HRAs and LRAs), the post‐process area and its personnel should be segregated from other sections, particularly from the preparation area of the plant. In the instance of manufacturing units that process raw and cooked meats, strict segregation of personnel should be undertaken, including segregation of welfare facilities, designated staff and designated clothing, tools and equipment.
43.20 Processed containers should be loaded into clean dry cartons or other suitable outer packaging material and stored in areas specifically allocated for this purpose. Care should be taken to minimise any damage to processed containers that could compromise seal integrity.
43.21 Samples of finished packs may be incubated as part of the overall food safety and quality control programme. However, the acceptability of the manufacturing operations should not be judged solely on the results of such tests, but rather that in most situations they are forms of verification and provide cumulative and retrospective confirmation of the efficacy of the process control and hygiene operations.
43.22 It is essential to ensure product integrity after processing and handling. Suitable precautions should be taken to ensure that package structures, including seals and seams, are free from defects that would otherwise impair their effectiveness as microbiological barriers, and that the food does not cause breakdown of the packaging materials with which it is in contact.
43.23 The products should be suitably protected for handling and stacking in storage. They should bear identification marks that are traceable to the production records derived from the manufacturing process (see Chapter 14).
43.24 Transport may present risks to package integrity caused, for example, by tilt and vibration, in respect of which correct handling procedures should be adopted (see Chapter 32).
43.25 Every effort should be made to take into account factors that may affect the continuing integrity of the products after they leave the immediate control of the producer. This, for example, includes handling during transportation, in wholesale and retail outlets and by the consumer. Labels warning against the use of case hooks and careless use of case opening knives should be adopted.
In‐container Heat Treatment
43.26 The scheduled heat process should take into account all critical factors that may affect the rate of heat transfer in the containers. It should be established by competent and properly trained personnel using accepted scientific methods. The scheduled heat process should be validated during the product and process design phased and then revalidated in the event of changes or amendments to the original process or product design. Records giving full details of how the scheduled heat process was established and confirmed as appropriate through validation should be retained permanently on file.
43.27 Product preparation and filling operations, for example setting of headspace, that may affect the integrity of the container closure or the rate of heat penetration should be carefully controlled.
43.28 The efficiency of the closing or sealing operation should be checked before processing begins and kept under constant control to ensure the integrity of the container closure or seal. Records of quality control assessments should be kept for a designated period from the date of production depending on the duration of the product and customer requirements (see Chapter 13).
43.29 Washing of filled and closed containers, should, if required, be undertaken before the heat process is given.
43.30 Temperature distribution tests should be undertaken on all processing equipment and a venting schedule established for each type of processing unit. All changes to the services of the manufacturing site, e.g. air or gas supply, water supply, etc., and the layout of such services in relation to the processing equipment or alterations in the method of loading batch retorts should be fully evaluated for their effect on the temperature distribution and adequacy of the venting schedule.
43.31 For batch retorts, there should be a method of checking that no batch of containers has bypassed the heat process.
43.32 Potable water, including water for cooling containers, should be free from harmful micro‐organisms and meet minimum quality standards and present no risk of product contamination (see Chapter 20). Borehole water run directly to retorts or municipal mains water may be used, provided that is meets micro‐biological and quality standards. All water used for cooling should be sufficiently chlorinated with an adequate contact time (20 minutes or more, depending on the pH) before use such that free residual chlorine (FRC) can be detected in the water after it has cooled the containers. Tests for FRC should be undertaken at a frequency derived by risk assessment, ideally the testing should be through continuous monitoring. It is not common practice to use other chemical disinfectants to sanitise water for cooling purposes, but where approved materials are contemplated for use they should achieve the same APC quality standard and have a residual present in active form after the cooling cycle. Any changes must be fully validated as part of the food safety management process (see Chapter 3).
43.33 Cooled containers should not be manually handled while still wet.
Heat Treatment Followed by Aseptic Packaging
43.34 Aseptic technology is more complex than in‐container sterilisation and needs to be thoroughly understood by the processor. It involves the production of a commercially sterile hermetically sealed package of food by pre‐sterilising the food, followed by cooling, filling and sealing it in sterile containers under scheduled processes, which must be applied in order to achieve and maintain commercial sterility. Assurance of attaining these standards comes from in‐process controls. Retrospective quality control analysis can only indicate the degree of success or failure in maintaining the required standards. The microbiological objective is to achieve and maintain commercial sterility.
43.35 Preproduction sterilisation of all food contact surfaces should take into account any location that may be slow to attain the set temperature. Non‐food contact surfaces may be chemically treated to achieve sterilisation. Chemicals that may be used include hydrogen peroxide. The integrity of the total system should be checked at this stage.
43.36 Aseptic fillers should be cleanable, sterilisable and capable of being isolated by a separating sterile barrier and with control systems that have instruments of reproducible accuracy, which are routinely calibrated. Aseptic filling machines may be affected in their performance by dust or soil in the external environment, and therefore careful consideration should be given to their location. A filling environment should have internal surfaces suitable for cleaning and microbiological sterilisation. Any inflow of sterile air or gas should have suitable flow, velocity and direction. With laminate packaging, the operation of the sealing equipment is also critical in ensuring that the packaging is adequately sealed to prevent air or bacterial ingress. Quality control personnel and/or machine operators should ensure that the machines have been set up correctly and the resultant sealing meets specified requirements before production commences.
43.37 Aseptic fillers should be appropriate for both product and packaging. Product temperature should also be compatible with the packaging material used. Control over dispense volume is critical since this may affect the integrity of the aseptic seal of the packages or the internal stress caused by the product in the finished packages.
43.38 During start‐up, standby or shutdown of an aseptic filling machine, special conditions may apply, for example in the supply of decontaminated air or gas, application of heat or application of chemicals. Care must be taken to ensure that all personnel health and safety procedures are followed and any fire risk is minimised.
43.39 Prepacking sterilisation of a product needs to take into account the critical factors to be considered when determining a scheduled heat process (43.8–43.11).
43.40 In continuous‐flow sterilisation, the control system should ensure that the correct product sterilisation temperature and holding time are achieved. It should be designed to fail‐safe in the event of malfunction, and under‐processed food should be routed out of the system and not transferred to the aseptic storage or filling section. The plate packs on pasteurisers should be routinely checked for integrity and to ensure that there are no pinholes in the plates. The relative pressure of the product and the cooling water should be reviewed to ensure that the risk of cooling water contamination through a pinhole/crack is minimised.
43.41 Direct heating systems may use electrical energy instead of steam. The energy and heat transfer mechanisms are different and should be understood by those responsible for the process design, the control system and its operation.
43.42 During continuous‐flow product cooling there should be an absence of microbiologically contaminating leaks.
43.43 Equipment used for batch sterilisation of product should have sensors, indicators, controllers and recorders for temperature and pressure. Bulk mixing patterns are important where they relate to temperature distribution. There should be a capability of aseptic transfer of product and an absence of microbiologically contaminating leaks.
43.44 Any methods used for microbiological decontamination of packaging should be compatible with the packaging materials and also take into account the microbiological loads. The methods should be designed using proven procedures. When scheduled processes for microbiological sterilisation of packaging materials depend on synergistic effects, these are critical factors that should then be monitored.
43.45 Heat‐based processes depend on suitable temperature, pressure, humidity, time of exposure and extent of air venting, and all processes should be formally validated and then revalidated at routine intervals. Monitoring and verification activities should be adopted to ensure that critical limits as well as target levels and tolerances have been complied with during processing.
43.46 Chemical applications depend on type, concentration, dose, temperature, coverage and contact time, and all processes should be formally validated and then revalidated at routine intervals. Monitoring and verification activities should be adopted to ensure that critical limits as well as target levels and tolerances have been complied with during processing.
43.47 Ultraviolet radiation has biocidal activity that is dependent on wave and band frequency, power, distance, reflectance, temperature, exposure time and age of lamp. Dust particles may have a shielding effect. Ionising radiations exert sterilising effects depending on the dose, therefore applications of this nature must be validated and then revalidated at routine intervals. Monitoring and verification activities should be adopted to ensure that critical limits as well as target levels and tolerances have been complied with during processing.
43.48 Packaging materials that are microbiologically decontaminated by the foregoing methods should be protected from recontamination.
43.49 Packaging materials may require special conditions of storage and must be stored and handled to minimise damage that can affect packaging integrity, especially with laminate packaging.
43.50 Temperatures and humidity should be controlled within acceptable limits, and additional safeguards should be applied where pre‐sterilised packaging materials or packages are used.
43.51 EU Regulation No. 2073/2005 on the Microbiological Criteria for Foods (as amended by EU Regulation No. 1441/2007) applies to all UK food businesses involved in processing, manufacturing, handling and distributing food. The Regulation introduces two types of microbiological criteria:
- Food safety criteria are used to assess the safety of a given product or batch and apply throughout the shelf‐life. Food safety criteria must be met and if they are not then the manufacturer cannot place the food on the market or if the food is already distributed the manufacturer must withdraw or recall the batch(es) involved from the market (see Chapter 27). Corrective and preventive action must then be implemented as deemed appropriate to prevent a reoccurrence and ensure that food products comply with prescribed food safety criteria in the future (see Chapter 28).
- Process hygiene criteria help show that the production processes are operating correctly at designated stages of manufacturing and handling. If a process hygiene criterion is exceeded then this should lead to a review of manufacturing procedures to improve process hygiene to the levels required. The review should also include consideration of whether there is a need for the product to be reprocessed, withdrawn or recalled. Any actions taken must be recorded (see Chapter 28).
The microbiological criteria outlined above can be used by the food manufacturer to both validate and also verify the food safety management system (FSMS) that has been adopted and implemented, and to demonstrate that the food safety plan is effective (see Chapter 3).
43.52 Except for carcasses, minced meat, meat preparations and mechanically separated meat, where minimum sampling frequencies are specified in UK legislation, EU Regulation No. 2073/2005 does not specify minimum requirements for sampling and testing nor does it require a manufacturer to undertake positive release, i.e. to wait for the test results from the product and process monitoring that has been carried out before releasing the associated product to the market. However, the food manufacturer is required to demonstrate that their FSMS is both appropriate and effective. An appropriate sampling and testing protocol must be included within the FSMS and include both food safety and process hygiene criteria. See Annex 1 of EU Regulation No. 2073/2005 for more details.2