Facilities Inspection Manual
13 - Process Determinations for Thermal Processes

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Subject 1 Thermal Process Control Policy for Federally Registered Canneries

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1. Scope

This document outlines the regulations, policies and procedures governing the control of thermal processes for the commercial sterilization of low-acid1 and acidified low-acid canned foods. It explains thermal processing controls that are to be followed by registered canneries which are in addition to the general requirements for registration of establishments covered in Chapter 2, Subject 1; Chapter 5, Subject 2 and Chapter 6, Subject 2 of this manual.

2. Authorities

Fish Inspection Act, R.S.C., 1985, c. F-12; Part I, Fish Inspection Regulations (FIR), C.R.C., 1978, c. 802.

Section 34 (FIR)

Canned fish shall be sterilized by a method approved by the President of the Agency.

3. Definitions

Acidified Low-Acid Food: a low-acid food that has been treated in a manner, acid(s) or acid food(s) are added, so that all components have attained an equilibrium pH of 4.6 or below by the time the thermal process is completed.

Come-up Time: the time, including vent time, which elapses between the introduction of the heating medium into the closed retort and the time when the temperature in the retort reaches the required processing temperature.

Commercial Sterility of Canned Fish: the condition obtained in a canned fish product which has been processed by the application of heat, alone or in combination with other treatments, to render the food free from viable forms of microorganisms, including spores, capable of growing in the foods at temperatures at which the food is normally designed to be held during storage and distribution.

Such a process is designed to result in the reduction of the reference organism, Clostridium botulinum, by 12 log (12D concept). This value may not ensure the destruction of all spoilage organisms. It is the processor's responsibility to determine which critical factors will be used to ensure destruction of the pathogen C. botulinum as well as spoilage organisms.

Can: means any hermetically sealed container.

Canned Fish: means any fish that is sealed in a can and is sterilized.

Control Measure: an action performed to eliminate a hazard or reduce it to an acceptable level.

Corrective Action: the procedure that is to be followed whenever a deviation from a critical limit in a HACCP plan occurs or whenever the results of monitoring procedures in respect of a prerequisite program plan or regulatory action point plan show that there is non-compliance with the Fish Inspection Regulations.

Critical Control Point: a point in a process operation at which control is to be applied in order to prevent or eliminate a hazard or reduce it to an acceptable level.

Critical Factor: physical and chemical factors that can influence the thermal response of a product to a thermal process, the variation of which may influence the scheduled process, including container, product, retort and processing conditions

Critical Limit: the maximum or minimum value to which a hazard must be controlled at a critical control point.

Deviation: failure to deliver the scheduled thermal process, meet critical factors related to the delivery of the thermal process, or critical limits relating to the process.

Deviation Procedure: documented set of corrective actions that are implemented when a process deviation occurs.

Documentation: the physical or electronic record of the procedures or activities that are to be followed as they relate to the thermal process. Documentation explains what controls are in place and how these controls are delivered. They include but are not limited to written formulae, procedures or specifications used by the processor or required by a manufacturer.

Equilibrium pH: the condition attained in an acidified low-acid food product in which there is no further change in the pH of any of the components.

Heat-Penetration Tests: scientific experiments conducted to determine heating and cooling behavior of a product/package combination, processed in a specific retort system, in order to establish safe thermal processes that will result in commercially sterile product or to evaluate process deviations. Chapter 13, Subject 3 contains a protocol for carrying out heat-penetration studies.

Hermetically Sealed Container: a container designed and intended to be secure against the entry of microorganisms, including spores.

Incubation: tests in which the thermally processed product is kept at a specific temperature for a specific period of time in order to determine if outgrowth of micro-organisms or other problems occur under tested conditions.

Initial Temperature: the product temperature of the coldest container to be processed at the time the sterilization cycle begins.

Inoculated Pack: a test pack used in scientific experiments wherein microorganisms to be targeted by the thermal process are added to a substrate (product) to confirm the adequacy of a theoretical process.

Lethality: F represents the time intercept from a thermal-death time curve (log tgm vs T) at T ' Tx. The F value is often referred to as the process lethality and it is the equivalent time in minutes, at a specific temperature, required to reduce the bacterial load of a target organism whose z value is known. The sterilizing value of a process is generally expressed as an F0 value which is equivalent to the number of minutes required to destroy a specific number of organisms with a z value of 10°C (18°F), at 121.1°C (250°F).

Low-Acid Food: a food where any component of the product has a pH greater than 4.6 and a water activity greater than 0.85.

Minimum Initial Temperature: the lowest temperature in a container for which the thermal process was established.

Objective Evidence: information which can be proven true, based on facts obtained through observation, measurement, test or other means.

Process Authority: means any person or organization that has been recognized by the Agency as being competent in developing and evaluating thermal processes. This would include competency in the following areas:

  • considerable knowledge concerning product characteristics, critical factors relating to the thermal process and the effect the commercial equipment and procedures will have on the heating and cooling characteristics of the product and the delivery of the thermal process;
  • experience in conducting studies relating to thermal processing of food, such as heat-penetration and temperature-distribution studies, and thermal-death time and validation studies and the application of other scientific methods relating to thermal processing;
  • the ability to evaluate data generated by scientific studies and tests in order to document: the effectiveness of the thermal process relating to the production of safe and commercially sterile product; and, that testing has been carried out to identify all possible factors which could affect the heating characteristics of the product and the safety of the final product.

Process Verification: written confirmation from a thermal process specialist or process authority that the calculated lethality from the use of a non-standardized process achieved commercial sterility or that the use of a standardized process resulted in commercial sterility.

Records: observations, measurements and other data written by the processor, or recorded by means of monitoring equipment to document the adherence to critical limits, critical factors, or other process requirements.

Retort: a pressure vessel designed for thermally processing food, packed in hermetically sealed containers, by an appropriate heating medium and where necessary with super-imposed pressure.

Scheduled Process: the thermal process alone or in combination with critical factors, and verified by the thermal process specialist or process authority, for a given product formulation, container type and size and thermal processing system to achieve commercial sterility of the product.

Standardized process: a thermal process, that has been published and subject to peer review, based on generally accepted scientific principles, and designed to produce a commercially sterile product.

Temperature-Distribution Study: test(s) performed to determine the time, temperature or other parameters that must be met to ensure uniform temperature is established in the retort system.

Thermal Process: the thermal treatment required to achieve commercial sterility and is quantified in terms of time and temperature.

Thermal-Process Specialist: person(s) or organization having expert knowledge of thermal-processing requirements for foods in hermetically sealed containers, having access to facilities for making such determinations, and designated by the cannery to determine the scheduled thermal process(es) and vent schedule(s). The thermal-process specialist is responsible for:

  • establishing the thermal process and identifying all critical factors;
  • establishing the vent schedule;
  • assuring the retort system is capable of delivering the thermal process; and
  • analyzing process deviations and providing the processor with appropriate corrective actions.

Time Lapse: the time between sealing containers filled with product and retorting.

Underprocessed Product: product that has been thermally processed but not all of the requirements specified of the scheduled process have been met.

Unprocessed Product: product that has been sealed in containers but has not yet been subjected to a thermal process.

Venting: means the complete removal of air from steam retorts through the vents by the introduction of steam, or other appropriate methods, prior to the attainment of the sterilization temperature.

Vent Schedule: a schedule indicating a specific period of time and a specific temperature that must be achieved in order to effectively remove air from the retort, so that a uniform sterilizing temperature can be obtained throughout the retort. The vent schedule is determined by analyzing data generated during a temperature distribution study.

Verification: confirmation by examination and provision of objective evidence that specified requirements (standard) have been fulfilled.

Water Activity: the ratio of the water vapor pressure of a food to the vapor pressure of pure water at the same temperature and pressure.

4. Policy

4.1 No thermal process shall be used to process canned fish in a federally registered establishment until a Quality Management Program (QMP) plan has been developed and documented and the processor's system verification has been accepted by the Fish, Seafood and Production Division of the Canadian Food Inspection Agency (CFIA) for the specific canned fish product.

4.2.1 The following information must be in the processor's QMP and available for review by the CFIA:

  1. management roles and responsibilities (recommended information);
  2. product and process information;
  3. the product description, which must identify those product attributes and characteristics as described in Section 2 of the Fish Inspection Regulations that are important in ensuring a safe and acceptable canned fish product;
  4. the process flow diagram, which must outline all of the production steps and assist in identifying those steps that are important in processing a safe canned fish product meeting all regulatory requirements;
  5. a Prerequisite Plan;
  6. a Regulatory Action Point Plan; and
  7. a Hazard Analysis Critical Control Point (HACCP) Plan.

4.2.2 The following is a list of the type of information that must be maintained in the QMP file:

  1. name and address of the thermal-process specialist, or the process authority;
  2. product preparation and formulation;
  3. container type and size;
  4. vent schedule (time and temperature) for the cannery's specific retort system;
  5. the process time, process temperature, and cooling procedure for the specific canned fish product being processed;
  6. heat-penetration data relating to the canned fish product, or a letter from the cannery's thermal-process specialist or process authority;
  7. temperature-distribution study(s) for the retort system and a retort survey (a Cannery Retort Survey is included in Appendix A);
  8. method of container loading of the retort;
  9. written verification of the thermal process to be used by the processor, provided by the thermal-process specialist or process authority for standardized and non-standardized thermal processes;
  10. non-standardized thermal process: written documentation expressing the minimum lethality being delivered by the thermal process in order to achieve commercial;
  11. standardized thermal process: written verification provided by the thermal process specialist or process authority that the process produces a commercial sterile product;
  12. all critical factors related to achieving commercial sterility must be identified to ensure the adequacy of the thermal process;
  13. test conditions used to design the thermal process.

This list is not all inclusive as there may be other information which is relevant to a particular process and that must be recorded in the file.

4.3 The vent schedule shall be based on temperature-distribution studies performed under the supervision of a thermal-process specialist or process authority. The vent schedule shall identify the minimum time and temperature required for a specific retort installation to reach uniform temperature. The vent schedule shall specify the testing conditions and all critical factors that will impact on the retort system reaching uniform temperature. Consideration should be given to steam-header pressure, divider hole size/spacing, valve settings, container loading, maximum number of retorts being vented at one time, and other steam operations that may impact on venting.

4.4.1 The Fish, Seafood and Production Division recognizes Bulletin 26L (Thermal Processes for Low-Acid Foods in Metal Containers) published by the Grocery Manufacturers Association (GMA) as containing standardized processes. When using a standardized process from Bulletin 26L, the processor will not have to report the lethality (F0) being delivered by the process.

However, the processor must have a thermal-process specialist or process authority verify in writing that the standardized process being used commercially by the processor satisfies all of the process design parameters and critical factors that have been identified with the product being thermally processed, and renders it commercially sterile. Commercial sterility is not defined in the regulations in terms of a sterilizing value (F0) but it is internationally accepted that a minimum sterilizing value (F0) of 3 minutes is required to render a low-acid food microbiologically safe. It is the processor's responsibility to determine which critical factors will be used to ensure destruction of the pathogen C. botulinum as well as spoilage organisms and based on such information, a sterilizing value (F0) above 3 may be necessary. Written verification provided by the thermal-process specialist or process authority is to be placed in the processor's QMP file.

4.4.2 If an unstandardized process is used, the processor must have on file documentation supporting the design and development of the thermal process. The thermal-process specialist or process authority must verify in writing that the process being used commercially by the processor delivers a commercially sterile product and report the minimum process lethality (F0), delivered by the process. Commercial sterility is not defined in the regulations in terms of a sterilizing value (F0) but it is internationally accepted that a minimum sterilizing value (F0) of 3 minutes is required to render a low-acid food microbiologically safe. It is the processor's responsibility to determine which critical factors will be used to ensure destruction of the pathogen C. botulinum as well as spoilage organisms and based on such information, a sterilizing value (F0) above 3 may be necessary to achieve commercial sterility.

4.4.3 All critical factors related to the product, as specified by the thermal-process specialist, must be monitored and controlled as part of the cannery's QMP. The processor must maintain records to demonstrate that these critical factors are being controlled.

4.5 A temperature-distribution test must be conducted to verify the effectiveness of the vent schedule when changes are made to the retort, steam supply piping or to ancillary equipment that may affect temperature distribution. The equipment must also be inspected by the thermal-process specialist, in accordance with the requirements of Chapters 5.2 and 6.2 of this Manual, before production commences. All relevant documentation verifying the vent schedule must be available for review.

Replacement of a steam spreader with an identical spreader would not require additional testing, but replacement of a pipe with a different diameter or a change in hole size or spacing would require a temperature distribution test to validate the change(s). New valves would be accepted providing the processor could demonstrate that the valves had the same flow coefficients (Cv value).

4.6 The CFIA shall audit all retort installations and scheduled thermal processes. The Canadian Food Inspection Agency shall also review the names and qualifications of the thermal-process specialist or process authority used by the processor. The results of the retort audit shall be recorded on the Cannery Retort Survey (Appendix A) and this form will become part of the cannery QMP audit.

4.7 In the event of a process deviation, the processor shall be responsible under the QMP to have a procedure in place to effectively control the product; evaluate the deviation to ensure that potential health and safety hazards have been addressed and commercial sterility has been achieved; and to take product action as necessary. Product shall be held for evaluation and disposition by the thermal-process specialist or process authority when the critical factors of a scheduled process are not being met by the processor.

4.8 Heat-penetration and temperature-distribution studies being carried out in registered establishments, to develop scheduled processes or vent schedules, must be performed under the direction of a thermal-process specialist or process authority. All relevant data associated with these tests is to be documented in the QMP file.

5. Forms/Documents

The following worksheets are provided for optional use:

  • Cannery Retort Survey Report
  • Cannery Retort Survey Report - Detailed

Subject 2 Guidelines for Temperature Distribution Studies when Processing in Steam-still Retorts Excluding Crateless Retorts

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1. Introduction

These guidelines have been formulated jointly by Agriculture Canada, Fisheries and Oceans Canada and Health Canada. They represent important elements to be considered when carrying out a temperature distribution study for any product to be thermally processed in steam-still retorts excluding crateless retorts.

When appropriate, temperature distribution studies will be evaluated by these departments using the elements given in these guidelines. Only persons experienced and knowledgeable on thermal processing in steam-still retorts should carry out and evaluate the results of such studies.

2. Application

Temperature distribution studies should be done to: develop or validate a venting schedule; to locate cold or slow heating zones in preparation for heat-penetration studies; in the case of new installations; and for any changes to an installation which may influence the temperature distribution in the product zone. Examples are: changes to steam spreaders, decreased steam pressure in lines, changes to the product loading patterns, changes to the basket and/or dividers, etc.

3. Inventory of the Thermal Processing System

Prior to the selection of the test retort(s) a survey should be made of the following:

3.1 Lay-out Diagram

A detailed diagram identifying all equipment for which the use of steam is required (including the numbering system used to identify each retort) and the steam supply line arrangement should be made as prescribed in this section. (Note that it is recommended that all steam lines from the main line to the retort(s) be clearly identified in the diagram from those steam lines feeding other equipment).

3.2 Steam Supply to the Retorts

3.2.1 Boiler(s) Capacity (psi or kPa)

Record potential and actual settings, amount of steam developed and available, i.e., pounds or kilograms of steam produced per unit time.

3.2.2 Retort Header Pressure

It is important to insure that adequate steam pressure and volume is being delivered to the retort(s). This measurement should be taken when maximum operational demand is made on the steam supply.

3.2.3 Headers, Manifolds, Lines and Valves

Record pipe size and length, valve size and types, of the main steam line from the boiler(s) immediately before the pressure/steam regulator to the retort(s).

3.2.4 All Connecting Steam Lines Other than to the Retort

Record size of all connecting steam lines to the main steam line noting other equipment using steam (e.g., blanchers, exhaust boxes, etc.).

3.3 Retort(s)

A detailed diagram of each retort, including associated operational equipment as identified below, should be made. Where identical retort configurations exist, one diagram is sufficient. The designated retort number(s) must be shown on the diagram. The system should include the full manifold system.

3.3.1 Retort shell

Record retort type and internal dimensions. For vertical retorts, note the presence of centring guides and/or baffle plates.

3.3.1 Retort Crates

Record maximum number of crates used in each run as well as their design and dimensions.

3.3.3 Steam Supply from Pressure/Steam Regulator to Retort

Record pipe sizes, valve type and sizes, pressure/steam regulators or reducers and all pipe fittings including steam by-pass lines and steam spreaders (shape, pipe size, length, location; number, size and location of holes in pipe).

3.3.4 Steam Control

Record type of controller (i.e., pressure to air, temperature to air) and location of sensor.

3.3.5 Air System for Controls (if applicable)

Record size of air compressors, air dryer capacity, filter type and location(s). Include the line pressure that must be maintained for operation of the controls and how this pressure is controlled.

3.3.6 Other Piping and Required Equipment

Record the following information:

  1. Vents: location, length and size of pipes, also type and size of valves
  2. Vent manifold or manifold headers: location, length and size of all pipes, connecting pipes, and valve(s) type(s) and size, where applicable.
  3. Bleeders, mufflers: location, number, size and construction
  4. Drains: location and size. In addition, note where they drain and whether they are open to the atmosphere.
  5. Water supply (if applicable): location and size of pipes, valve type and size.
  6. Air supply (if applicable): location and size of pipes, valve type and size, and the available air pressure.
  7. Temperature-indicating device (Mercury-in-glass (MIG) thermometer or equivalent): location of the sensing point in the retort and date/year when it was last calibrated.
  8. Temperature controller: sensing point location in the retort.
  9. Pressure gauge: location of the sensing point in the retort and date/year it was last calibrated.
  10. Additional piping or equipment such as condensate removal systems, etc.

3.3.7 Recording Device

Note type of recording device (recorder or recorder/controller). For more information consult section of the Recommended Canadian Code of Hygienic Practice for Low-acid and Acidified Low-Acid Foods in Hermetically Sealed Containers (Canned Foods).

3.4 Loading Equipment

Record the following information:

  1. Container size, loading configuration and maximum number of containers per layer or per basket (scramble pack).
  2. Maximum number of baskets in each retort.
  3. Hole size and spacing of the basket base plate.
  4. Determine the percent open area of the base plate and separator sheets if used in the crates or baskets. Where separator sheets are located over a base plate, they should be positioned to reflect the worst case scenario.

Note: It is important to document the survey findings correctly in order to enable a proper evaluation before selecting the test retort(s). The documented survey should be maintained on company's file and updated when necessary.

3.5 Selection of Test Retort(s)

All information required in section 3 above must be taken into consideration when selecting the test retort(s). The retort(s) selected should represent the worst possible condition that could influence the delivery of the venting procedure. Note that under certain conditions (i.e., when the plumbing and equipment configuration is not identical for all retorts), it may be necessary to carry out a temperature-distribution study of a number of retorts in a system in order to determine which one represents the worst case.

Where all plumbing and equipment configurations are identical, it is generally advisable to select as the worst possible case the retort which is located at the end of the steam line. However, this is not always the case. This is an area where the knowledge and experience of the specialist supervising the study are of upmost importance.

4. Test Equipment

4.1 Data Logger

Note if data logger has a sufficient number of channels to monitor adequately and record temperatures during the temperature-distribution study.

4.2 Thermocouples

Note if thermocouples and lead wires, or other temperature-measuring devices used are of an appropriate type, size, length and number to adequately monitor the temperatures within the retort.

4.3 Temperature-Indicating Device(s)

Note which type used (Mercury-in-glass thermometer or other) see 3.3.6 item 8.

4.4 Pressure-Indicating Device(s)

Note which type used (if required) see 3.3.6 item 9.

4.5 Stuffing Box (packing gland)

Note if diameter is sufficient to accommodate number of lead wires (if thermocouples are used as the temperature measuring device) and specify its location on the retort.

5. Standardization of Test Equipment

5.1 Retort Mercury-in-glass (MIG) Thermometer (or equivalent temperature-indicating device)

The MIG shall conform with section of the Recommended Canadian Code of Hygienic Practice for Low-acid and Acidified Low-Acid Foods in Hermetically Sealed Containers (Canned Foods). Prior to performing a temperature-distribution test, the MIG thermometer (or equivalent) shall be certified by a recognized authority as meeting the stated accuracy according to specifications, such as set out by the National Research Council of Canada (NRC), and calibrated. If it has been calibrated and certified in the past 12 months, then it should not have to be done again unless there is doubt as to its accuracy.

5.2 Temperature-Measurement System (e.g., data logger, thermocouples, extension wires or other temperature-measuring devices (TMD), etc.)

  1. Prior to conducting a temperature-distribution test, standardization of test equipment (see Section 4) must be performed using the test retort selected. All leads, extensions and connections should be assembled as they will be used under actual operational conditions.
  2. Place one or more TMDs in close proximity of the known accurate retort MIG thermometer probe (or equivalent). Care should be taken not to inhibit steam flow past the thermometer probe (or equivalent).
  3. The retort is brought up to the temperature to be used during the temperature-distribution tests and the entire system is allowed to run for 10 minutes after equilibrium is reached.
  4. All TMDs should be standardized at the intended retort operational temperature. Thus a variance amongst the TMDs to be used can be identified and those which vary by more than 0.3°C (0.5°F) from the standard thermometer should be discarded. The range of all thermometers should be no more than 0.6°C (1°F). After correction factors have been incorporated, all TMDs should give the same reading.
  5. In order to meet the above calibration criteria, consideration must be given to minimizing errors due to variables inherent in any component of the temperature-measuring system. For example, the use of thermocouple wire from the same spool is recommended to make all thermocouple leads and extensions.

6. Placement of the Temperature Measuring Devices in the Retort

A minimum of 12 TMDs (or equivalent) should be used. However, the number of TMDs depends upon many factors, for example, size of the retort chamber zone, container size, number and configuration in the baskets, etc.

TMDs shall be placed in the following locations in the retort vessel:

  1. In close proximity to the MIG thermometer probe (or equivalent).
  2. In close proximity to the temperature controller probe. If this probe is in close proximity to the thermometer probe, this location is not necessary.
  3. Guidance as to the placement of TMDs in the product zone can be obtained from the design of the retort and the steam supply and distribution system as well as the loading pattern in the baskets or crates. However, location of cold zones does not always follow logic, specially when determining a venting schedule which requires freedom from steam/air pockets. This is an area where the knowledge and experience of the specialist supervising the study are of upmost importance.

As a general guidance it is recommended to place TMDs in the following manner:

3a. For Vertical Retorts:

Temperatures should be measured in the middle of each basket at the top, centre and bottom. If more thermocouples are available, points along the edge at the top and bottom of each basket may be measured. If still more thermocouples are available, other points around the periphery of the basket may be measured.

3b. For Horizontal Retorts:

In this type of retort the product is usually in cars. In a horizontal retort thermocouples should be located in the middle of the basket at the top, centre and bottom of each car. If more thermocouples are available, they should be located at the centre of the outside of the four sides of the car.

Note: A schematic diagram of the placement of all TMDs within the retort and covering all three dimensions should become part of information recorded for the temperature-distribution tests.

4. For determining the initial temperature (IT), TMDs should be placed in a sufficient number of medium-filled testing containers. Generally two containers have been found to be acceptable. Alternatively, a hand-held thermometer may also be used to make that determination. Ideally all containers in the retort should be equilibrated to a previously identified IT.

7. Preparing the Test Crates or Baskets with Containers

  1. Select the container size processed in the retorts, usually the smallest, that will yield the worst-case situation for the operation.
  2. The product that has the highest heat absorption rate (convection heating) processed in the retorts should be used. Water may be used in the cans in place of product.
  3. Containers are placed in the crates or baskets in a manner that is equivalent to the worst-case situation under the commercial operation. If separator or divider sheets are used between the layers of containers, the sheets having the smallest percent total open area shall be used for testing.

8. Temperature-distribution Test

8.1 Set-up

  1. Review the retort survey
  2. Initial Temperature (IT):

    The initial temperature is usually determined from the container having the lowest temperature. When determining the test IT, the range of initial temperatures to be encountered during normal commercial operation should be taken into account and the coldest IT be selected.

8.2 Critical Items

The following are critical and should be monitored and recorded during the test.

  1. Controller temperature set point.
  2. Initial temperature (IT).
  3. Retort steam header pressure.
  4. Time steam on or "0" time.
  5. Time when the drain is closed, if it is open during a portion of the vent.
  6. Time that vent is closed, retort temperature at the time the vent is closed as determined by the reference temperature-measuring device (TMD).
  7. Time when the reference temperature-measuring device reaches the processing temperature.
  8. Time when the controller (if applicable) advances to the "cook" cycle in the program or when the cook begins.
  9. Reference temperature-measuring device readings at sufficient intervals, including the time it reaches the processing temperature.

8.3 Important Items

In addition, the following points are important and are highly recommended to be monitored and recorded during the test.

  1. Time when the temperature-recording device reaches the processing temperature set point.
  2. Retort pressure gauge (optional) readings, at sufficient intervals.

8.4 Conducting the Test

  1. The data logger should record the temperature of each TMD just prior to "steam on" and at sufficient intervals - not to exceed one minute - throughout the test. The data logger record shall become part of the test records.
  2. Critical items (see 8.2) should be recorded, as required, at intervals of sufficient frequency to describe and verify retort operating parameters during the test. These records shall become part of the test records and shall include the temperature-recording chart(s).
  3. The test should extend for at least ten minutes after the retort control systems have stabilized and a definite temperature profile has been established for all TMDs.
  4. In the absence of a maintenance system, each retort should be tested every two years under the worst-case scenario.

8.5 Required Parameters for the Determination of a Vent Schedule

  1. On the basis of the data accumulated during the performance of temperature-distribution testing on steam-still retorts, excluding crateless retorts, a vent schedule should specify as a minimum the following critical parameters:
    1. Vent time ("steam on" to vent closed).
    2. Vent temperature (when the vent valve is closed).
    3. Where appropriate, minimum initial temperature (IT).
    4. Use of any opening in the retort (other than the vent valve) during the vent period to increase vent capacity.
    5. Time and temperature when the drain is closed if it is opened during a portion of the vent.
  2. For a vent schedule to be determined successfully, it should be based on a minimum of three (3) repeatable runs, and conducted under "worst-case" conditions. "Repeatable" means that all three (3) runs, conducted under the same test conditions, must show that adequate temperature distribution is achieved.

For more information on vents and venting system refer to sections and of the Recommended Canadian Code of Hygienic Practice for Low-acid and Acidified Low-acid Foods in Hermetically Sealed Containers (Canned Foods).

Subject 3 Protocol for Carrying out Heat-Penetration Studies

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Various methods and equipment may be employed in order to collect accurate heat-penetration data. The overall objective of these guidelines is to recommend procedures for carrying out heat penetration studies for establishing thermal processes necessary to produce commercially sterile foods packaged in hermetically sealed containers. The following recommendations are to be considered voluntary guidelines. While this does not preclude the application of other methods and equipment for collecting heat-penetration data, these guidelines have been developed by consensus of the Institute for Thermal Processing Specialists (IFTPS) and should be given serious consideration for adoption as methodology by individuals performing heat-penetration studies.

1. Nomenclature

Retort come-up time is the time between the start of heating and the time when the retort reaches processing temperature (at times referred to as CUT)
Process time is the time from the end of the come-up period to the end of heating (at times referred to as the operator's process time)
Container center or coldspot temperature (at times referred to as CT)
Retort temperature (at times referred to as RT)
Cooling water temperature (at times referred to as CW)

2. Terminology

2.1 Ballast Containers: Containers may be required to fill the retort during heat-penetration studies to simulate production retort conditions. Type, shape and size of containers should be the same as used for the intended process. Material used for filling containers could be the test product, or any suitable material having heating characteristics similar to that of the test product, or in some circumstances, water.

2.2 Cooling Time: Time required following the introduction of the cooling medium to decrease the internal temperature of the product to a specified value, commonly 35 to 45°C (95 to 110°F).

2.3 Critical Factors: Physical and chemical factors that can influence the thermal response of a product to a thermal process, the variation of which may influence the scheduled process, including: container, product, retort and processing conditions.

2.4 Fill, Drain and Net Weights: Fill weight is the weight of solids prior to processing; drain weight, the weight of solids after processing; and net weight, the weight of all product in a container.

2.5 Heat-Penetration Curve: Plot of the logarithmic difference between either retort temperature and product temperature (heating curve) or product temperature and cooling medium temperature (cooling curve) versus time.

2.6 Mercury-in-Glass Thermometer (MIG): Generally used as the retort reference temperature device and regulated for that application by government agencies in some countries. Other temperature-measuring devices may be calibrated against a MIG retort thermometer which has been calibrated against a traceable temperature standard.

2.7 Resistance-Temperature Detector (RTD): Thermometry system based on the positive change in resistance of a metal-sensing element (commonly platinum) with increasing temperature.

2.8 Temperature-Measuring Device (TMD): Device used for measuring temperature, including: thermometers, thermocouples, RTDs and thermistors.

2.9 Thermistor: TMD manufactured from semiconductor materials which exhibits large changes in resistance proportional to small changes in temperature. Thermistors are more sensitive to temperature changes than thermocouples or RTDs and are capable of detecting relatively small changes in temperature.

2.10 Thermocouple: TMD composed of two dissimilar metals which are joined together to form two junctions. When one junction is kept at an elevated temperature as compared to the other, a small thermoelectric voltage or electromotive force (emf) is generated which is proportional to the difference in temperature between the two junctions.

3. Design of a Heat-penetration Study

The purpose of a heat-penetration study is to determine the heating and cooling behaviour of a product/package combination in a specific retort system for the establishment of safe thermal processes and evaluating process deviations. The study must be designed to adequately and accurately examine all critical factors associated with the product, package and process which affect heating rates. Numbers of containers per test run, and number of test runs to account for statistical variability are important and discussed in sections 5.11 and 5.12. Before commencing a heat-penetration study, an evaluation of retort temperature and heat transfer uniformity, at times referred to as a heat or temperature distribution study (IFTPS, 1992), should have been completed. A goal in conducting these studies is to identify the worst-case temperature response expected to occur in commercial production as influenced by the product, package and process.

4. Factors Affecting Heating Behaviour

Several product, process, package and measurement-related factors can contribute to variations in the time-temperature data gathered during a heat-penetration test. Establishment of a process requires expert judgement and sound experimental data for determining which factors are critical and the effect of changing those factors both within and beyond established critical limits. The list of items addressed in this section is extensive, but should not be assumed to cover all possible factors. Quantitative data on variability should be recorded where appropriate and all pertinent data should be documented to better understand and account for possible variations in heat-penetration behaviour.

4.1 Product:

4.1.1 Product formulation and weight variation of ingredients should be consistent with worst-case production values. Changes in formulation may necessitate a new heat-penetration study.

4.1.2 Fill weight used for heat-penetration studies should not be less than the maximum declared on the process schedule. Excess product may be expressed as percent overfill.

4.1.3 Solids content should be measured for nonhomogeneous products both before and after processing. Solids content deposited in a sieve should be weighed and expressed as a percentage of total weight. Note: Addition of compressed or dehydrated ingredients may result in increased drained weight.

4.1.4 Consistency or viscosity of semi-liquid or liquid components should be measured before and after processing. Flow behaviour will change with type and concentration of thickening agent (starch, gums, etc.), temperature and shear rate. Changes may be reversible or irreversible which may be important when reprocessing product.

4.1.5 Size, shape and weight of solid components should be measured before and after processing.

4.1.6 Integrity and size of solid component clusters may change during processing and affect temperature sensor placement in the product and coldspot location.

4.1.7 Methods of product preparation prior to filling should simulate commercial practice. For example, blanching may cause swelling, matting or shrinkage which could influence heat-penetration characteristics.

4.1.8 Product matting or clumping may change heat-penetration characteristics and influence coldspot location. Also, caution should be exercised with sliced products which may stack together during processing.

4.1.9 Rehydration of dried components, either before or during processing, is a critical factor which may influence heat-penetration behaviour, as well as process efficacy with respect to spore inactivation. Details of rehydration procedures should be recorded during the heat-penetration study.

4.1.10 Product may heat by convection, conduction or mixed convection/conduction depending on its physical properties. Some foods exhibit complex (broken) heating behaviour. Product may initially heat by convection, then due to a physical change in the product, change to conduction-heating behaviour. For example, for products such as soups which contain starch, the change in heating behaviour may be due to starch gelatinization at a particular temperature. Small variations in product formulation or ingredients may cause the transition from convection to conduction heating to occur at a different temperature and related time. Special care should be taken to identify and control specific product and process variables related to the heating rates of these products.

4.1.11 Additional product characteristics such as salt content, water activity, pH, specific gravity, concentration of preservatives, and methods of acidification may influence heat transfer or microbiological resistance and should be recorded.

4.2 Container:

4.2.1 Manufacturer and brand name information should be recorded in case information related to filling, sealing or processing is required.

4.2.2 Container type (metal cans, glass jars, retort pouches, semi-rigid containers), size and dimensions should be recorded.

4.2.3 Nesting of low profile containers can influence heating behaviour. Heat-penetration studies on jumble-loaded retorts (no racks or dividers) should include tests conducted on stacks of nested cans as well as single cans.

4.2.4 Container vacuum and headspace should be recorded for rigid containers. For flexible and semi-rigid containers the volume of residual gases in the container should be determined. Entrapped gases may create an insulating layer in the container causing a shift in the coldspot location and a decrease in the heating rate. Controlled overpressures during processing have been found to reduce these effects.

4.2.5 Maximum thickness of flexible packages (pouches) has a direct relationship to the coldspot temperature history with thicker packages heating more slowly. Heat-penetration studies should be carried out at the maximum specified package thickness.

4.2.6 Container orientation (vertical or horizontal) within the retort may be a critical factor for some product/package combinations and should be studied where appropriate. Changes in container orientation may also influence vent schedules and come-up time.

4.2.7 Postprocessing examination of test containers for abnormalities should be conducted with special emphasis on the slowest and fastest heating containers. It is strongly recommended that flexible packages be carefully examined following processing to identify the thermocouple junction location. If the intended sensing location has shifted, it is likely that heat-penetration data collected are not reliable.

4.3 Method of Fill:

4.3.1 Fill temperature of the product should be controlled. It will affect the initial temperature which may influence some heat-penetration parameters (lag factor, retort come-up period). This may constitute a critical control point for a process, particularly for products which exhibit broken heating behaviour.

4.3.2 Fill and net weights may influence heating rates both in still and rotary cooks. Information on variability may be found in statistical process control and product quality control records.

4.3.3 In most cases, controlling headspace by determining net weight is not sufficient due to possible variations in the specific gravity of the food product. Care should be taken to avoid incorporation of air which would affect the headspace vacuum. In rotary processes, container headspace is a critical control point since the headspace bubble helps mix the product during agitation.

4.4 Closing or Sealing: Closing or sealing equipment should provide a strong, hermetic seal which is maintained during the thermal process. Vacuum in cans and jars for most canned foods is recommended to be between 35-70 kPa (10-20 in-Hg) measured at room temperature. Vacuum is affected by variables such as: headspace, product temperature, entrapped air, and vacuum efficiency of the closing equipment. Some products such as vegetables vacuum-packed in cans may have a minimum vacuum as a critical control point. For others packed in flexible or semi-rigid containers, vacuum setting will influence the residual air content in the package, also constituting a critical control point.

4.5 Retort System: The type of retort system used may have a significant influence on the heating rates of products processed in the retort. Results from a heat-penetration test should be reported with reference to the retort type and conditions existing at the time of testing.

4.5.1 Retort come-up time should be as short as possible, consistent with obtaining satisfactory temperature distribution. Laboratory size retorts may be used for development work on heat-penetration behaviour. Results will be conservative when the smaller retorts have shorter come-up times and cool more quickly than production retorts. After development, the thermal process should, if physically possible, be verified in an appropriate production retort.

4.5.2 Racking systems may be used to separate layers of cans or jars, constrain the expansion of semi-rigid and flexible containers, provide support and circulation channels for thin profile containers, and ensure maximum pouch thickness is not exceeded. Care should be taken to understand the influence of a specific rack design on retort performance and heat transfer to containers.

4.5.3 Still batch-retort systems vary in operation based on: type of heating medium (steam, steam/air, water immersion, water spray); orientation of the retort (vertical, horizontal); method of heating medium agitation (fans, pumps, air injection); and other factors which may influence the heating behaviour.

4.5.4 Rotational batch retort systems (axial, end-over-end) are designed to rotate (or oscillate) entire baskets of product during processing. Container agitation may provide faster rates of heat penetration to the container coldspot as compared to still cooks. However, while this is true for some containers, it may not be so for all containers within a load and caution must be exercised to identify the slowest heating containers. This may entail a detailed can position study. It is recommended that during initial testing, data be collected at small time increments (15 s) particularly for viscous fluids where the coldspot may move in relationship to a fixed thermocouple during rotation, producing erroneous results. Slip-ring connectors should be cleaned and thermocouple calibration verified at regular intervals. Critical factors in these retorts include: headspace, product consistency, solids to liquid ratio, initial temperature, container size, rotational speed and radius of rotation.

4.5.5 Continuous retort systems may move containers through the processing vessel along a spiral track located at the outside circumference of a horizontal retort shell or be carried through a hydrostatic retort in chain driven flights. Regardless of the configuration, it becomes difficult or impossible to use thermocouples to collect heat-penetration data in these systems. Data may be obtained using self-contained temperature measurement and data storage modules in the commercial vessel or by using process simulators.

5. Temperature Measurement and Data Acquisition

5.1 Data Acquisition System: Accuracy and precision of the data acquisition system (datalogger) used for heat-penetration studies will affect temperature readings. Dataloggers are typically comprised of a multi-channel temperature-measuring and digital-data-output system. Calibration of a data-acquisition system should include verification of the data-acquisition rate, since errors in the time base would result in erroneous data.

5.2 Type of Thermocouple: The most common TMDs used in thermal processing are duplex Type T (copper/constantan) thermocouples with Teflon insulation. Common configurations are flexible wires (20-, 22- or 24-gauge) and rigid needle types. Details on thermocouples and connecting units are available in Bee and Park (1978) and Pflug (1975).

5.3 Type of Connectors and Associated Errors: Connectors used in a thermocouple circuit are fittings attached to a thermocouple within which electrical connections are made. Several types of connectors are available for specific applications and thermocouple type. Caution must be exercised to avoid certain sources of error which may be associated with the use of connectors and extension wires. These include: disparity in thermal emf between thermocouples, connectors and extension wires; temperature differences between two wire junctions; and reversed polarity at the thermocouple-extension wire junction. Thermocouple connectors should be cleaned frequently with metal cleaner to assure good electrical contact and prevent errors in thermocouple readings. Similar concerns should be addressed when using RTDs and thermistors.

5.4 Thermocouple Calibration: Thermocouples should be calibrated against a traceable calibration standard (thermometer, RTD, thermistor). Inaccuracies in temperature measurements may result in errors in process evaluation; hence, frequent calibration is essential to provide reliable data. Factors affecting calibration include: worn or dirty slip-rings; improper junctions; metal oxidation; multiple connectors on one lead, and inadequate datalogger cold junction compensation. As a consequence, thermocouples should be calibrated in place as part of the complete data-acquisition system. Some precautions when using thermocouple-based data-acquisition systems include: minimizing multiple connections on the same wire; cleaning all connections; grounding the thermocouples and recording device; slitting thermocouple outer insulation outside the retort to prevent flooding of datalogger or data recording device (see NFPA, 1985, or ASTM, 1988 for illustrations); and using properly insulated thermocouple wires.

5.5 Positioning of Thermocouple in the Container: The method of inserting a thermocouple into a container should result in an airtight, watertight seal which should be verified after testing. Thermocouple sensing junctions should be positioned in the slowest heating component of the food product and situated in the slowest heating zone within the container. During insertion of the thermocouple, caution must be taken to avoid physical changes to the product. Also, the method employed for mounting the thermocouple into the container should not affect the container geometry which could influence heat-penetration characteristics. Flexible or rigid thermocouples may be inserted into rigid, flexible and semi-rigid containers using compression fittings or packing glands. For flexible containers, NFPA (1985) provides illustrations of thermocouple positioning into a solid particulate and several thermocouple positioning devices to ensure the thermocouple remains in a fixed position within the container. The most appropriate device for a particular application will depend upon the product, racking system, container type and sealing equipment. Leakage may be detected by weighing the container before and after processing to determine changes in gross weight. If there is leakage caused by improperly mounted thermocouples, data collected for that container should be discarded. Note: Ecklund (1956) reported correction factors for heat-penetration data to compensate for errors associated with the use of non-projecting, stainless steel receptacles. While not reported in the literature, this may also be a concern with other fittings.

5.6 Type and Placement of Containers: The type and size of container used in the heat-penetration study should be the same as that used for the commercial product. The racking and loading of rigid (cans), semi-rigid (trays and cups) and flexible (pouches) containers should simulate commercial practice. Test containers should be placed at the slowest heating location in the retort, as determined by temperature and heat transfer distribution studies.

5.7 Temperature of the Heating Medium: TMDs should be positioned so as to prevent direct contact with racks or containers and identified according to their specific location in the retort. A minimum of two thermocouples is recommended for retort temperature measurement: one situated close to the sensing bulb of the retort MIG thermometer, the other located near the test containers. In addition, at least one thermocouple should be placed near the sensor for the temperature controller when that location is remote from the location of the MIG thermometer bulb.

5.8 Retort Pressure: Overpressure conditions during processing will influence package expansion by constraining the expansion of headspace gases. This may be beneficial by improving heat transfer to food in flexible and semi-rigid containers or detrimental by restricting the size of the headspace bubble in rotary processes. For steam/air retorts, overpressure conditions are also related to the steam content of the heating medium at a particular processing temperature which may influence heat transfer conditions within the retort. In addition, cooling without overpressure may result in depressurization within a container upon collapse of steam at the end of a process, leading to accelerated decreases in temperature for fluid foods.

5.9 Coldspot Determination: The location of the slowest heating or coldspot in a container is critical to establishing a process. For a conduction heating product in a cylindrical can with minimal headspace, the geometric center of the can is considered to be the slowest heating spot. Generally, if a larger headspace is included, the coldspot may shift closer to the top of the can due to the insulating effect of the headspace which may be significant if the height-to-diameter ratio of the can is small. The coldspot location in vertically oriented cylindrical cans containing products which heat by natural convection may be near the bottom of the container. Products which exhibit broken heating behaviour may have a coldspot which migrates during heat processing as the physical properties of the product change. The use of containers with different geometries or constructed from different materials may have differing effects on coldspot locations. A coldspot-location study should be completed to determine the slowest heating location for a specific product/package/process combination. Usually, the coldspot location will be determined from a series of heat-penetration tests employing several containers with thermocouples inserted at different locations. Alternatively, more than one thermocouple per container may be used; however, multiple thermocouples may influence heating behaviour, especially for products in smaller containers. In all cases, care should be taken to determine the "worst case" anticipated during production. Careful judgement, based on a number of preliminary experiments, must be exercised to ensure the coldspot location has been identified.

5.10 Initial Product Temperature: Measurement of initial product temperature should be taken immediately prior to testing.

5.11 Number of Containers per Test Run: A heat-penetration test should evaluate at least 10 working thermocouples from each test run (NFPA, 1985). If the retort cannot accommodate this quantity, the number of replicate test runs should be increased.

5.12 Number of Test Runs: Replication of heat-penetration test runs is important in order to obtain results which account for run-to-run, product, container and process variability. After initial coldspot-determination tests are completed and all critical factors have been determined, at least two full replications of each test are recommended. Should results from these tests show variation, a minimum of a third test is recommended. Variation in the results is expected and quite common, especially for products which are non-homogeneous or exhibit complex heating behaviour. Variability is generally evaluated based on plots of the heating and cooling curves and/or lethality calculations and should be considered when identifying or predicting the slowest heat behaviour of a process

6.0 Summary of Documentation

The following provides a summary of details which may be incorporated in a checklist and documented in their entirety or partially as deemed appropriate for a specific study. Other factors not listed in this section may also be relevant.

6.1 Pre-test Documentation:

  • 6.1.1 Product Characteristics
    • Product name, form or style, and packing medium
    • Product formulation and weight distribution of components
    • Net weight and volume
    • Consistency or viscosity of the liquid component
    • Size, shape and weight of solid components
    • Size of solid component clusters
    • pH of solid and liquid components
    • Methods of preparation prior to filling (ingredient mixing methods, special equipment)
    • Matting tendency
    • Rehydration of components
    • Acidification procedures
    • Other characteristics (% solids, density, etc.)
  • 6.1.2 Container Description
    • Container material (brand name and manufacturer)
    • Type, size and inside dimensions
    • Container test-identification code
    • Maximum thickness (flexible container)
    • Gross weight of container
    • Container nesting characteristics
    • Slowest heating or coldspot location in container
  • 6.1.3 Data-Acquisition Equipment and Methodology
    • Identification of datalogging system
    • Thermocouple and connector plugs maintenance
    • Thermocouples and connectors numbered
    • Electrical ground checked
    • Thermocouples placed in heating medium and readings compared with a reference TMD
    • Type, length, manufacturer and identification code of thermocouples and connectors
    • Thermocouple location in container
    • Positioning technique for thermocouple
    • Calibration data for each thermocouple
  • 6.1.4 Fill Method
    • Fill temperature of product
    • Fill weight of product
    • Headspace
    • Filling method (comparison to commercial process)
  • 6.1.5 Sealing Operations
    • Type of sealing equipment
    • Time, temperature, pressure and vacuum settings (if applicable)
    • Gas evacuation method
    • Can vacuum
    • Volume of residual gases in flexible containers
  • 6.1.6 Retort System
    • Retort system: still or rotary (end-over-end, axial, oscillatory)
    • Reel diameter (number of container positions) and rotational speed
    • Can-position study data for batch rotary retorts
    • Heating medium (steam, steam/air, water immersion, water spray) and flow rate
    • Circulation method for water or overpressure media
    • Temperature distribution records
    • Retort venting schedule
    • Retort identification number
  • 6.1.7 Loading of Retort
    • Loading or racking system details
    • Location of test containers in retort (slowest heating zone)
    • Container orientation
    • Location of thermocouples for retort temperature measurement
    • Use of ballast containers to ensure fully loaded retort (some retort systems)
    • Selected time interval for data-logging system
  • 6.1.8 Additional Information
    • Date
    • Test identification
    • Processor and location
    • Individual(s) performing heat-penetration test

6.2 Test-Phase Documentation:

  • 6.2.1 Test run identification
  • 6.2.2 Initial temperature of product at the start of heating
  • 6.2.3 Time heating starts
  • 6.2.4 Time vent closed and temperature, if applicable
  • 6.2.5 Temperature indicated on MIG thermometer
  • 6.2.6 Time retort reaches set point temperature (tc)
  • 6.2.7 Pressure from a calibrated pressure gauge or transducer
  • 6.2.8 Time process begins
  • 6.2.9 Time cooling begins (pressure cooling, if applicable)
  • 6.2.10 Time cooling ends
  • 6.2.11 Rotation speed (if applicable)
  • 6.2.12 Cooling water temperature
  • 6.2.13 Any process irregularities or inconsistencies

6.3 Post-Test documentation:

  • 6.3.1 Container net and gross weight check for leakage
  • 6.3.2 Thickness of container
  • 6.3.3 Location of the thermocouple and whether or not it is impaled in a food particle
  • 6.3.4 Measurement of container vacuum (rigid metal and glass) or residual air content (flexible and semi-rigid containers)
  • 6.3.5 Post-processing product characteristics: syrup strength, appearance, viscosity, headspace, drained weight, pH, consistency, shrinkage, matting, clumping
  • 6.3.6 Container location and orientation (jumble pack)

7. Literature Cited

ASTM. 1988. Standard Guide for Use in the Establishment of Thermal Processes for Foods Packaged in Flexible Containers. F 1168-88. American Society for Testing and Materials, Philadelphia, PA.

Bee, G.R. and Park, D.K. 1978. Heat-penetration measurement for thermal-process design. Food Technol. 32(6): 56-58.

CFPRA. 1977. Guidelines for the Establishment of Scheduled Heat Processes for Low-Acid Foods. Technical Manual No. 3. Campden Food Preservation Research Association, Chipping Campden, Gloucestershire, UK.

Ecklund, O.F. 1956. Correction factors for heat penetration thermocouples. Food Technol. 10(1): 43-44.

IFTPS. 1992. Temperature Distribution Protocol for Processing in Steam-Still Retorts, Excluding Crateless Retorts. Institute for Thermal Processing Specialists, Fairfax, VA.

NFPA. 1985. Guidelines for Thermal Process Development for Foods Packaged in Flexible Containers. National Food Processors Association, Washington, DC.

Pflug, I.J. 1975. Procedures for Carrying Out a Heat-Penetration Test and Analysis of the Resulting Data. University of Minnesota, Minneapolis, MN.

Prepared by the Committee on Heat Penetration, Institute for Thermal Processing Specialists. Approved for publication November 10, 1995

Institute For Thermal Processing Specialists
P.O. Box 2764 Fairfax, VA 22031-0764
Telephone: (703) 591-1108 Fax: (703) 591-5903

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