Commercial sterilization of low-acid food in hermetically sealed containers: retort systems

On this page

1. Introduction
2. Process design
3. Thermal processing equipment
4. Incoming material control
5. Empty container control
6. Product preparation and blending control
7. Filling control
8. Container closure control
9. Pre-thermal process control
10. Process control
11. Post-process control
12. Incubation
13. Deviations and corrective actions
14. Verification of product safety
15. References

1. Introduction

The following provides recommended preventive controls for the thermal processing of commercially sterile low-acid food in hermetically sealed packages (containers) stored at room temperature.

The safety of thermally processed low-acid food is dependent primarily upon four conditions:

1. The scheduled process is adequate to control Clostridium botulinum for the product and processing system;
2. The product adheres to the scheduled process parameters and heat is applied to all points in the product for a time and at a temperature sufficient to produce commercial sterility;
3. Container filling adheres to the scheduled process parameters, and container sealing prevents microorganisms from entering and contaminating the sterilized product; and
4. Post-process handling procedures protect the finished closures from contamination or damage – which can lead to leakage or re-contamination – through to distribution to the consumer.

2. Process design

Temperature distribution

A temperature distribution test identifies the location of the slowest heating point (the "cold spot") in the retort. This "cold spot" is taken into consideration when conducting subsequent heat penetration studies to establish the scheduled process.

• Conduct temperature distribution tests on retort systems under worst-case conditions during normal operation to ensure uniform temperature distribution throughout the retort and to establish operating procedures such as the vent schedule (steam retorts)
  • a retort system could consist of a single retort or a group of identically equipped retorts
  • refer to Temperature distribution test for steam-still retorts for guidance on important elements to consider when carrying out a temperature distribution test
• Conduct these tests upon installation, after making any changes that could negatively impact on heat transfer, and at a sufficient frequency (such as every 2-3 years) to ensure equipment wear has not affected its performance

Scheduled process

A scheduled process is determined using heat penetration studies or other equivalent scientific methods by persons having scientific knowledge and expertise (competency) in thermal processing, for example a process authority^Footnote 1. These tests are carried out using the most adverse conditions that could occur during normal production.

• For every low-acid food formula, container type and size, ensure there is a written description of the scheduled process, including:
  • the name of the processing expert responsible for its development
  • the formulation of the food
    • specify all ingredients and additives (concentration, type) and their amounts
  • all critical factors and the associated critical limits
  • the accepted scientific methods used to establish the scheduled process (such as microbial thermal death time data, process calculations based on product heat penetration data, inoculated packs), including the details of the experimental methods
• Never alter a scheduled process except at the direction of a processing expert

Thermal process design records

• Keep records of the determination of the scheduled process, including any heat penetration studies and temperature distribution tests

3. Thermal processing equipment

• Use thermal processing equipment that is designed, constructed, installed and maintained to deliver the requirements of the scheduled process. For example, equip each retort with an accurate:
  • independent temperature measuring device (indicating thermometer)
  • time/temperature recording device (to provide a permanent record of the thermal treatment in relation to time)
  • pressure gauge
  • temperature controller
• Take measures to prevent unauthorized adjustments to critical equipment settings, such as by restricting access

Refer to the following documents for more information on thermal processing equipment specifications:

• Regulatory requirements: Preventive controls, section 6.0 contains guidance on the general design, installation, maintenance and calibration requirements for equipment, including thermal processing equipment
• Code of hygienic practice for low-acid and acidified low-acid canned foods, Codex Alimentarius Commission (CAC/RCP 23-1979, sections 7.5.3.6 and 7.6)
• Guidance on resistance temperature detectors used as indicating thermometers for retort vessels

4. Incoming material control

The ability of a thermal process to reduce the microbial load to an acceptable level is influenced by the number of bacteria present in the product before heat treatment begins.

• Inspect incoming raw materials, ingredients, and containers upon receipt to ensure that they are suitable for processing
  • refer to Incoming ingredients, materials and non-food chemicals for guidance on preventive control measures for incoming materials, for example written specifications and supplier approval
• Minimize microbial contamination and growth in raw materials and ingredients prior to thermal processing

Commonly used containers include rigid containers such as cans and glass jars, and semi-rigid or flexible containers such as pouches and cartons. A container must be able to withstand the thermal process and maintain its hermetic seal for the shelf life of the product.

• Source containers that meet the criteria identified in the scheduled process and that can withstand the strain of subsequent handling steps
  • refer to Guidance for food establishments concerning construction materials and packaging materials and non-food chemicals for recommended options to ensure containers are suitable

5. Empty container control

Empty container handling

• Control empty container handling to minimize damage and to prevent the use of damaged, defective or contaminated containers, for example
  • handle and transfer containers in a manner that minimizes damage and contamination, such as from forklifts, conveyors and transfer points
  • prior to use, monitor containers for damage that could compromise the final integrity (such as cracks and fractures) and contamination (such as glass, rust and foreign material)

Container cleaning

• Ensure that only clean containers are filled, for example
  • immediately prior to filling, clean rigid containers in an inverted position such as by an air, water jet or suction system
  • any water or steam used is potable
  • any detergents or sanitizers used are suitable for this purpose
    • refer to Guidance for food establishments concerning construction materials and packaging materials and non-food chemicals
• Protect clean containers to prevent contamination prior to filling, for example
  • don't leave unprotected containers on the line between the cleaner and the filler at breaks, during clean-up or extended downtime
  • have a glass breakage control procedure for lines that use glass containers and for other glass breakage occurrences such as glass thermometer bulbs.

Empty container records

• Keep records of empty container usage by lot and of controls of container handling damage and/or visual monitoring of empty containers
• Keep records to demonstrate control of broken glass

6. Product preparation and blending control

Critical factor control

• Control critical factors specified in the scheduled process and formulation during preparation and blending to minimize food safety hazards

  Examples of critical factor controls related to product preparation and blending are:

  • size control, such as dicing, grinding, slicing
  • temperature treatment control, such as heating, blanching, defrosting and cooling
  • moisture control, such as rehydration and concentration (viscosity, Brix)
  • proportioning control, such as weighing, volumetric control (metering)
  • pH/acidity control, such as pH measurement and titratable acidity for pectinization
  • additives such as preservatives and firming agents
• Control time and temperature during preparation, blending and holding of in-process materials to prevent conditions that could result in excessive microbial growth or in the production of enterotoxin by Staphylococcus aureus

Product preparation and blending records

• Keep records to demonstrate control of product preparation and blending, for example
  • records showing adherence to critical limits specified in the formula
  • records for critical factors specified in the scheduled process

7. Filling control

Filling of containers

• Control all filling factors specified in the scheduled process, for example:
  • fill temperature
  • ingredient distribution
  • orientation of products in the containers and handling
  • volume and weight control, for example proportion weights, fill weights, head space
  • type and size of container
  • packing medium
• Protect product from contamination during filling
• Keep seam or seal areas as clean and dry as possible to ensure a proper closure, for example ensure that
  • filler drip does not contaminate seam or seal areas
  • product is not overfilled onto the sealing area
• When filling glass containers, control breakage in the filling area, for example
  • evaluate areas that may contaminate the product with glass
  • check the alignment and timing
  • inspect filling area regularly and remove any broken glass
  • monitor for a possible broken glass containers, such as by using a gap detector (counts the number of containers entering and exiting the equipment; any discrepancy indicates a container failure)
  • tear down equipment to the extent necessary to remove the potential for glass contamination, for example seal, rubbers, valve blow outs, valve dismantling

Filling records

• Keep records to demonstrate adequate control of critical factors in filling specified in the scheduled process, for example
  • temperature control records, such as filler bowl temperature recorder charts and/or manual records
  • ingredient distribution control records, such as visual checks for product orientation
  • weight/volume control records, such as portion weights from multi-phase fillers, total weights, head space

Filled unsealed container handling

• Ensure that unsealed containers are conveyed from the filler to the closing equipment in a manner that prevents breakage, damage, or contamination of the container, the product, or the sealing/seaming area, for example
  • don't leave containers on the line between the filler and the closing equipment during breaks, cleaning, or extended downtime
  • maintain the filling area and equipment in a sanitary condition
  • have a procedure for dealing with damaged or broken containers (pouches, glass jars)

Extraneous material contamination control

• Monitor equipment (metal detectors, magnets, etc.) during processing to ensure it detects and removes extraneous material
• Test equipment regularly to ensure it is operating in accordance with the equipment manufacturer's specifications

8. Container closure control

Container closure

• Operate, maintain, routinely check and adjust closing equipment to ensure a tight and secure seal
  • follow the equipment manufacturer's instructions
  • fit and adjust sealing and closing equipment for each type of container and cover used

Metal can seaming

• Routinely adjust and maintain seaming equipment to give correct seam contours and to prevent seam problems
  • take into account any variations in container materials, plate thickness and temper when setting up the seamer
  • inspect ends for signs of damage before loading them into the seaming equipment

Glass capping

• Prevent damage or breakage of glass containers during the capping process and ensure a hermetic seal, for example
  • protect the cap/lid hopper from glass contamination
  • inspect capper area regularly for broken glass
  • adjust closing equipment to avoid chipping either the finish or glass edge
  • remove broken glass from capping equipment and surroundings

Pouch sealing

• Control the critical parameters during pouch closure to ensure a hermetic seal, for example
  • use properly formed and undamaged pouches
  • ensure pouches are properly placed (aligned) within the jaws of the sealing machine
  • use compatible sealing material
  • ensure sealing surfaces are flat, smooth and parallel
  • ensure the sealing tool holds the surfaces together at the proper pressure and temperature, and for the proper time

Vacuum-packed foods

• Control container vacuum to meet the requirements of the scheduled process, to protect the hermetic seal, to prevent permanent distortion of container ends during processing and to maintain concave ends during distribution and storage, for example
  • control all factors affecting vacuum, such as fill temperature, air exhaustion by steam or mechanical vacuum, fill weight/volume and head space to prevent permanent seam distortion
  • monitor container vacuum to ensure adherence to the requirements of the scheduled process

Visual and destructive examinations

These examinations should be performed only by individuals that are competent in identifying container defects.

• Visually monitor sealed containers at a sufficient frequency to identify and control defects that may compromise the hermetic seal, for example
  • check for external defects on at least one sealed container from each closing head every 30 minutes or as recommended by the container or sealing equipment manufacturer
• Conduct additional visual examinations and non-destructive measurements following a major jam, after an adjustment, or at start-up following a prolonged shut-down
• Conduct destructive examinations on sealed containers at a sufficient frequency to identify and control defects that may compromise the hermetic seal, for example
  • check for sealing defects on at least one container from each closing head every 4 hours (double seams, caps), every 2 hours (heat seals), or as recommended by the container or sealing equipment manufacturer
• Adjust closure heads as needed based on the results of the visual and destructive examinations

Refer to the container and/or sealing equipment manufacturer's instructions for closure specifications and guidance on how to conduct these evaluations. Other resources include the CFIA's Metal can defects manual and the Flexible retort pouch defects manual, as well as section 7.4.8 of the Codex Code of hygienic practice for low-acid and acidified low-acid canned foods (CAC/RCP 23-1979).

Container closure control records

• Keep records to demonstrate adequate control of the container closure operation, for example
  • Visual Examination Records
    • code lot
    • date and time of evaluation
    • examination results
    • corrective actions taken
  • Destructive Examination Records
    • code lot
    • date and time of closure evaluation
    • container closure specifications
    • container closure measurements
    • corrective actions taken

9. Pre-thermal process control

Sealed container handling

• Control container handling systems to minimize damage that could compromise container integrity
  • identify and eliminate points where container damage could occur, such as container transfer points, areas where containers abruptly change speed or direction, controls to stop conveyors when containers are stationary and synchronization of container handling systems
  • where necessary, clean filled containers to remove product residues from the outside of the container (reduces the risk of external corrosion)
  • handle pouches in a manner to prevent puncturing by sharp objects or abrasion from equipment
  • minimize the handling of pouches
  • avoid excessive flexing, folding, movement and vibration of pouches
  • ensure that the mechanical coding process does not cause damage to the containers, and that the codes are legible and permanent

Maximum time lapse

• Control the elapsed time between sealing of containers and the start of thermal processing to ensure adherence to factors critical to product safety
  • Generally, elapsed time should not exceed 1 hour unless
    • a longer time has been determined to be safe by an expert in thermal processing
    • sealed product is held at temperatures that will not permit the growth of microorganisms that could impact on the safety of the process (that is, less than 4°C or greater than 65°C)
    • the heat transfer characteristics of the product are not affected
• If the maximum lapse time is exceeded, treat this as a process deviation and hold the product for safety evaluation by an expert in thermal processing

Retort loading

• Control the loading of baskets to meet the requirements of the scheduled process and to prevent damage to the containers
  • adhere to the loading requirements specified in the temperature distribution test and process schedule, for example, use of dividers (type, layering, hole configuration), product orientation, number of layers per basket, pouch placement
  • ensure there is no nesting of cans or pouch overlap unless it has been included in the temperature distribution test and scheduled process
  • minimize impact damage during loading, for example
    • do not use damaged baskets or handle baskets roughly
    • adequately cushion containers during scrambled/jumble loading
• keep pouch racks free of rust and scale to prevent pin holes in pouches

Traffic control and process indicators

• Control the movement of processed and unprocessed product to prevent unprocessed product from by-passing the retort, to prevent co-mingling of unprocessed containers with processed containers, and to ensure that each basket goes to the appropriate retort
  • maintain physical separation between closing and retorting
  • immediately destroy (puncture) any containers of unknown process status
  • where there are multiple retorts and products requiring different processes, have a system of identification to verify that each basket has been directed to the correct retort, such as a tag system
  • if batch thermal treatment is applied, use a temperature-sensitive indictor to visually indicate that the containers have been thermally treated

10. Process control

Retort systems should only be operated by, or under the continuous supervision of, an individual competent in thermal processing.

Initial temperature

• Control the temperature of the contents of the coldest container at the start of the process to ensure that it is no lower than the minimum specified in the scheduled process
  • monitor the temperature of the contents of the coldest container with sufficient frequency to ensure that it does not drop below the specified initial temperature
  • use the same procedure for determining the initial temperature as that used in the heat penetration test

Operating procedures

• Control thermal process operations by having written operating procedures that incorporate the critical factors identified the scheduled process
  • include venting, processing and cooling procedures
  • include procedures for monitoring critical factors
  • include clear instructions for handling deviations
  • train operators on these procedures

Vent control

• Establish venting schedules based on the temperature distribution test
• Control venting to ensure adherence to the conditions under which the temperature distribution test was conducted
  • ensure the vent schedule meets both the minimum time and temperature requirements
  • adhere to the specifications from the temperature distribution test, such as loading configuration, steam manifold pressure, and number of retorts being vented at one time

Process time control

• Control process time to ensure that the minimum time requirements of the scheduled process are met
  • monitor and control process time using an accurate timing device, such as an analog or digital wall clock
  • when more than one timing device is used , synchronize them
  • where the timing device does not indicate seconds, add a one minute safety factor to the venting time and to the process
  • time the process only after the retort has been properly vented and the scheduled process temperature has been achieved
  • check that the time on the recorder chart agrees with the time of day indicated on the accurate timing device

Process temperature control

• Control the process temperature to meet the minimum temperature requirements of the scheduled process
  • monitor the indicating thermometer temperature at the start of the process and at least once during the process to verify that the minimum temperature is achieved and maintained
  • compare the indicating thermometer temperature with the recorder temperature at least once during the process after temperature equilibrium has been achieved
  • ensure that the recording thermometer does not vary by more than 0.5°C (1°F) from the indicating thermometer and at no time reads higher than the indicating thermometer
  • ensure that the recording chart produces a continuous record of the temperature. In the absence of the continuous record (ink skips), adequately document that the temperature was maintained
  • use the correct chart for the recorder (the number on the chart matches that specified by the instrument manufacturer)

Thermal process control records

• Keep records of time, temperature and other pertinent details for each retort load (or code lot, for continuous retorts) to demonstrate the safety of each application of the scheduled process. For example, record
  • product name
  • production volume
  • equipment used for the thermal process, including any maintenance or modifications to the equipment
  • container size and type
  • minimum initial temperature
  • any parameters of the thermal process, such as start and end times, temperature, and pressure
  • deviations from the scheduled process and corrective actions
  • other relevant processing data
• Keep these records for three years from the time of processing

Refer to Record keeping for guidance on how to ensure the integrity of recorded processing data, including data captured via electronic recording systems.

11. Post-process control

Cooling controls

• Control cooling to minimize post-process contamination, thermophilic growth and container damage
  • control pressure in the retort to prevent excessive pressure differentials between the interior and exterior of the container that may result in paneling, buckling, seam distortion and bursting (follow the container and closure manufacturers' instructions)
  • cool containers as rapidly as possible to below 40°C to avoid thermophilic spoilage
  • control the cooling of cans and jars to retain enough residual heat to rapidly evaporate water for corrosion prevention
  • control the cooling rate of glass jars to prevent thermal shock

Cooling water control

• Control the cooling water conditions to minimize the potential of post-process bacterial contamination, for example
  • treat recycled cooling water with a bactericide prior to use
  • ensure that an effective residual level of bactericide is maintained throughout the cooling system (for example, monitor free available chlorine level to ensure it is 0.5 to 2.0 ppm)
  • conduct monitoring at the location(s) in the cooling system where the bactericide level is the lowest
  • maintain cooling systems to prevent buildup of organic and inorganic materials which may reduce the effectiveness of the bactericide

Cooling water records

• Keep records to demonstrate control of recycled cooling water quality and bactericide treatment, for example
  • Water treatment records
    • date and time of analysis
    • concentration of residual bactericide and pH
    • method of analysis
    • code
    • product
    • analyst
    • retort system/sample site
• Keep these records for three years from the time of processing

Container handling and drying

• Control container handling and drying systems to minimize damage and/or post-process bacterial contamination.

  Cans and Glass

  • tilt containers to remove excess water
  • until the container vacuum is achieved and the compound has established proper sealing integrity:
    • handle containers in a manner to prevent contact with unsanitary surfaces, for example, employee contact, water splash from floors, sprays, aerosols, condensation and unsanitary equipment
    • handle containers in a manner to minimize impact damage and damage caused by equipment, for example, cable burns and cuts, abrasion, dents, seam damage
    • dry containers as soon as possible after processing

  Pouches

  • minimize handling
  • avoid excess flexing, folding, movement and vibration
  • handle pouches in a manner to prevent puncturing and other damage
  • secondary protective packaging can help to minimize pouch damage
    • if secondary packaging occurs elsewhere, use extreme care when transporting pouches to the packaging location

Post-process container integrity verification

• Monitor the integrity of processed containers, for example
  • pass canned product through low vacuum detection equipment
    • test this equipment periodically to verify it is working properly
  • conduct visual and destructive examinations on a random selection of containers from a lot (see Metal can defects – identification and classification and Flexible retort pouch defects manual – identification and classification)
  • incubation testing (see next section)

Post-process container integrity records

• Keep records of all post-process container integrity evaluations to demonstrate the integrity of processed containers

12. Incubation

Incubation testing

Incubation testing is the holding of samples of hermetically sealed, processed foods at a specified temperature for a specified period of time, before examination for signs of spoilage. It is useful when establishing a scheduled process (validation), to help diagnose spoilage issues and for verification purposes.

• Incubate the containers at a temperature and for a time that permits spoilage organisms (if present) to grow and cause detectable spoilage, for example
  • 37°C for 10 to 14 days to detect mesophiles
  • 55°C for 5 days to detect thermophiles
  • other appropriate temperature and time combinations
• Conduct post incubation tests to detect signs of spoilage (for example, loss of vacuum, pH change)

Incubation testing records

• Keep records of incubation time, temperature and the results of any post incubation tests for three years from the time of processing

13. Deviations and corrective actions

Deviation control

• Have procedures in place to identify, isolate and evaluate products when critical limits are not achieved, and when other defects occur which could affect product safety, for example
  • have a system for identifying deviations when they occur
  • isolate, clearly mark and control all affected product back to the point of last control
  • have an expert in thermal processing evaluate thermal process deviations for potential food safety risks
  • ensure personnel are adequately trained to conduct manual sorting of suspects lots for leakage

Corrective action

• Take effective corrective action following any deviation to ensure the safety of the product and to prevent recurrence of the deviation
  • investigate to determine the cause of the deviation
  • take measures to prevent recurrence of the deviation
  • verify the effectiveness of the corrective action
• Notify the CFIA immediately if the investigation determines that unsafe product is in distribution and take immediate action to protect consumers

Deviation and corrective action records

• Keep records to demonstrate the control of deviations and the effectiveness of corrective actions taken, for example

  Deviation/Hold

  • product/code
  • date produced/held/released
  • reason for the hold
  • amount of product held
  • amount of product sorted
  • results of evaluation/sort, for example amount analyzed, analysis report of the number and nature of defects
  • signature of personnel responsible for hold and evaluation
  • disposition of held product
  • signed authorization for disposition

  Corrective Action

  • cause of deviation identified
  • corrective action taken to correct deficiency
  • assessment of effectiveness of corrective action
  • date
  • signature of person responsible
• Keep these records for three years from the time of processing

14. Verification of product safety

• Use supplementary methods to verify the effectiveness of controls affecting product safety, for example
  • sampling and testing of in-process and finished products for the presence of controlled hazards
  • sampling and analysis to verify that specific manufacturing controls are effective, for example microbiological checks of treated cooling water, microbiological checks of surfaces coming into contact with hot, wet cans
  • independent, external audits
  • internal audits
  • incubation testing of finished products
  • screening, dud detection, weight verification
  • analysis of consumer complaint trends
  • vendor audits

15. References

CFIA references

• Evidence showing a control measure is effective
• Flexible retort pouch defects – Identification and classification
• Guidance on resistance temperature detectors used as indicating thermometers for retort vessels
• Heat-penetration studies
• Metal can defects – Identification and classification
• Packing food in containers
• Temperature distribution test for steam-still retorts

Other references

• Codex Alimentarius Commission, Code of hygienic practice for low-acid and acidified low-acid canned foods (CAC/RCP-23-1979)
• Institute for Thermal Processing Specialists, Guidelines for conducting thermal processing studies (02-13/14) (English only)