Ultimate Guide on Retort Processing

How to produce high quality, safe retort food?

What is retort food?

Ready to eat, shelf stable, retort sterilized food is completely cooked food, packed in airtight containers, which can be preserved at room temperature for a long period of time without the necessity of freezing, cooling or drying. Shelf stable products must be safe, and in no way hazardous to the health of the consumer. They should be nutritious while tasting good and, last but not least, be reasonably priced.

The sterilization technique must be designed to fit those demands. The first priority, of course, is product safety, but retention of high quality is almost equally important.

The sterilization value is calculated to determine the precise heating process to be applied (temperature and time) that is necessary to provide shelf stability to a food product inside a hermetically sealed container. A key factor in this is the ability of the organisms in the product to withstand the heat.

Heat process requirements

The purpose of the heat process is to preserve the pack contents by destroying the microorganisms and enzymes capable of activity in the food during subsequent normal storage. Additionally, the process may be designed to cook the food, thus developing the required characteristics of flavor, texture and color.

Establishment of the scheduled heat process

There are five key steps in the establishment of the scheduled heat process for foods in flexible packs:

1. 1

   Consideration of the degree of lethal heat required to achieve commercially stable products (establish the Scheduled Process / Operating Temperature).

2. 2

   Establishment of heat distribution throughout the heating vessel (Retort Validation – Temperature Distribution Study). Heat distribution tests must be performed to ensure that adequate lethality is delivered to the product.

3. 3

   Execution of heat penetration rate studies into the product, under simulated production conditions, in order to determine the critical factors and the approximate heat process cycle conditions required to achieve commercial sterility (Product Validation – Heat Penetration Study).

4. 4

   Consideration of the coldest spot in the container.

5. 5

   Final checking of the preliminary heat process schedule using the production heat processing equipment (Validation and Scheduled Process Establishment).

These are the five most important steps involved in retort processing of products in flexible containers. The explanation of each step is given below.

1. Consideration of the degree of lethal heat required (F value / F0 value) to achieve commercial sterility in low acid foods

The principles and practices of heat processing need to be applied to retort processing of products in flexible packages. At least the minimum botulinum heat process should be given for sterilized ambient products.

This is normally achieved through application of a heat process that has a value (F0 value) equivalent in lethal effect to not less than 3 minutes at 121.1°C, resulting in a product that is “commercially sterile”.

The following information is required:

• The types and number of bacterial spores / micro-organisms present in the product. (It is always assumed that Clostridium botulinum will be present and so should be targeted.)
• The heat resistance of the significant spores present in that particular product. This in turn is dependent upon the composition of the food, pH value, Aw (water activity) and levels of preservatives. For many products this information is not obtained directly, and assumptions on thermal death times of the spores present are made from published data.
• The expected storage conditions of the product. Temperatures higher than 45°C may lead to growth of thermophilic organisms which have survived the heat process. In this case a higher thermal process should be considered.

Degree of lethal heat required for pasteurized products

The choice of pasteurization treatment will depend on the expected microbial load of the raw materials and the desired quality of the final products. In order to design an effective microbial treatment, the following should be considered:

• The kind and number of pathogens likely to be in the food (pasteurization processes cannot be expected to destroy spore formers).
• Whether the pathogens are infective or produce toxins.
• Whether the environmental conditions in the food are likely to change after pasteurization during the proposed packaging, transportation and retail conditions, or as a result of consumer abuse, such that the stability of the product is lost and the pathogenic and spoilage organisms would then be able to grow.
• The thermal death characteristics of the most resistant pathogen at the range of temperatures and times likely to occur.
• Product formulation including Aw, salt content, pH and chemical preservatives.

Degree of lethal heat required for REPFEDs (refrigerated pasteurized foods of extended durability)

As per international guidelines, for chilled foods (below 8°C) with an expected shelf life greater than 10 days in sealed packs, where psychrotrophic Clostridium botulinum is the organism of concern, it is recommended that one or more of the following control hurdles be imposed:

• A heat process of 90°C for 10 minutes or equivalent lethality (z value 9).
• A pH of 5 (or less) throughout the food and throughout all components of complex foods.
• A minimum salt level of 3.5% in the aqueous phase throughout the food and throughout all components of complex foods.
• An Aw of 0.97 or less throughout the food and throughout all components of complex foods.
• A combination of heat and preservative factors which can be shown consistently to prevent growth and toxin production by psychrotrophic Clostridium botulinum. (Psychrotrophic microorganisms are microorganisms that can grow in the temperature range of -5°C to 40°C, and may include Clostridium botulinum, Listeria monocytogenes and Yersinia enterocolitica.)

Where a heat process is applied, this should be followed by prompt cooling to prevent outgrowth of spore-forming organisms that will survive a pasteurization process of 90°C for 10 minutes. In all cases process validation must be carried out correctly and expert advice should be sought.

2. Establishment of heat distribution throughout the heating vessel (retort)

Heat distribution tests must be performed to ensure that adequate lethality is delivered to the product.

A heat distribution (temperature distribution) test is defined as a test performed to study the uniformity in lethality throughout the retort, with emphasis on identifying the position in the retort that potentially results in the lowest lethality and is the slowest to reach the scheduled processing temperature.

Heat distribution tests must be carried out on each tray configuration. Heat distribution should be tested on installation of a retort and then repeated every 3 years to confirm there is no change in performance of the retort. Other reasons for performing heat distribution testing include the following:

• Engineering work, such as change to services and repair work.
• Change to pack size and shape.
• Change to crate design and loading pattern.

3. Determination of heat penetration within the product

A heat penetration test is conducted to determine the heating and cooling characteristics at the coldest point of a given product, in a given container, under specified process conditions, usually in the coldest zone of the retort (as determined by the heat distribution study). To read more, see the guidelines for performing heat penetration trials for establishing thermal processes in batch retort systems.

Locating a temperature monitoring device at the slowest heating point is very difficult when using a container without a fixed geometry. It is therefore necessary to define the most adverse pack geometry which could occur in production, to arrange the product within the pouch to take up that configuration, and to locate the temperature measuring device at the slowest heating point in that pack. In many instances it may be necessary to use a non-metallic mold to tie the temperature monitoring device in the most adverse position.

The temperature measuring probe may need to be mounted properly to hold it at the slowest heating point inside the container. Whichever system is used, it is important that it does not speed up the rate of heat transfer to the slowest heating point by allowing heat to be conducted through or along the positioning or monitoring equipment faster than through the food product. The heat penetration test should simulate actual production conditions and should not block the actual heat transfer process.

It is essential that the number of heat penetration rate trials carried out is sufficient to determine the variation in results, which should be taken into account when setting the scheduled heat process.

4. Production heat process trials

It is good practice to carry out confirmatory checks of the provisional scheduled process in the production heat processing equipment. The racking system and its handling, the positioning of the packs within the heat processing vessel, the heating medium composition and its flow rate, and the precise temperature / pressure / time cycle used can all affect the rate of heat penetration.

These trials should be carried out using a fully loaded heat process vessel but, in addition, there should be some trials to establish the effect of processing a partial load. Heat processing partial loads will affect the flow of the heat transfer medium, the retort come-up time and cooling conditions. When using water as the heat transfer medium, it will also affect the quantity of water needed to fill the retort.

The results of the heat penetration trials in the pilot scale and production equipment should form the basis of the scheduled heat process used in production. Documentation should include specifications for all factors required to assure the adequacy of the scheduled heat process. Suitable thermal processing studies should be performed to ensure adequate processing.

4.1 Heat processing equipment and instrumentation

There are no agreed standards for the design of equipment for heat processing food in flexible packs. To know more about accurate and reliable heat processing equipment for retort validation and process monitoring applications, see heat penetration sensors and fittings.

4.2 Services to heat process vessels

Particular attention should be paid to the attainment of uniform distribution of steam and to the avoidance of direct impingement of steam onto pouches, which could result in some pouch delamination.

In steam/air retorts, pressure control should be by steam and air additions, excess pressure or temperature being controlled by venting. Precautions should be taken to ensure that localized high concentrations of air cannot occur in the heat process vessel.

In water/air overpressure retorts there should be a separate air inlet above the water level for the control of pressure. This is in addition to any air introduced with the steam.

It is important that any water that comes into contact with a product or packaging is of the highest standard required for drinking water. Frequent microbiological testing of cooling water is required. Where heating and cooling water comes into direct contact with the pack, the water must either be of potable quality or have undergone disinfection with suitable biocides such as chlorine or bromine.

Consideration should be given to the water used for indirect cooling of packs, for example in the case of a leaking heat exchanger. It is also necessary to filter recirculated water to prevent any build-up of contaminants or gross debris. These filters should be cleaned periodically, and it may also be necessary to drain and refill the water in the recirculating system. Build-up of impurities and debris could increase microbial load, having an effect on the disinfection efficacy.

4.3 Temperature measurement

The master temperature indicating instrument (MTI) can be either a mercury-in-glass (MIG) or a platinum resistance thermometer (PRT), manufactured and regularly checked as per international standards, or an alternative instrument of proven reliability. The positioning of the MTI is critical and will depend upon the heat processing vessel design. It should be sited to monitor the lowest temperature point of relevance to the heating medium in contact with the containers, and positioned in the path of the air/steam flow.

4.4 Heat process control

There should be automatic control of heat process temperature and pressure during the hold period at the heat process temperature. The complexity of operation of most heat vessels used for the heat processing of flexible packs makes it preferable for the whole heat process cycle to be automated.

4.5 Overpressure control

The application of an overpressure is essential in order to control thermal expansion of entrapped gases within the sealed pack. Inadequate overpressure will lead to ballooning of pouches. Ballooning increases the pressure on pouch seals and could lead to bursting of pouches. It can also lead to a pattern on the pouch where it has come into contact with the tray surface. Loss of uniformity in the process temperature at any stage during processing may lead to the overpressure being inadequate and cause bursting or ballooning of packs. Heat penetration may also be affected.

The overpressure required to prevent excessive expansion of pouches during retorting will depend upon the amount of gas in the pack and the heating temperature. To establish the correct overpressure profile, packs can either be processed in a retort with a window, or a differential pressure study can be performed, which covers measurement of retort chamber pressure and the internal pressure developed in sealed flexible packages.

Using these methods, it can be detected when the pouch is starting to expand excessively. The pressure being applied to the pack can then be increased until the expansion is under control (this is most easily accomplished in a retort with an element of manual control). The resulting pressure profile must be recorded, and this record can then be used as the basis for an automated pressure control profile in production.

During the come-up period of the heat process cycle, the pack headspace heats rapidly relative to the pack contents, and consequently the overpressure should be increased to its specified maximum as the vessel temperature increases. At heat processing temperature, the overpressure should be controlled within ±1 psi. During the cooling period, the headspace will cool and contract quite rapidly, and again the overpressure can be reduced with vessel temperature. To protect the flexible containers from damage, the pressure outside the pack should balance the pressure inside.

4.6 Circulation of heat transfer medium

It is an essential requirement, when packs are not heat processed in a pure saturated condensing vapor, that the heat transfer medium is circulated. The aim of circulation is the attainment of uniform temperature distribution within the stack of pouches and the provision of a uniform heat transfer rate to all pack surfaces.

The general tolerance adopted for a uniform temperature distribution is +1°C, -0.5°C (+2°F, -1°F) of the heat process temperature. This should ideally be obtained within the first 10 per cent of the heat process hold time and maintained for the rest of the heat process hold period. Because the rate of circulation of the heating medium is critical in most installations, there should be a means of checking the rate during each cycle of a batch retort.

4.7 Heat process records

There should be a continuous record of heat process temperature / pressure / time that is checked before product is released for sale. Records should be kept of readings from the MTI, the heat process vessel pressure gauge and the heating medium circulation rate indicator. All data should be recorded against the product code and/or appropriate details specifying the particular production lot.

4.8 Unloading the retort

At the end of the heat process cycle, the retort operator checks the recorder trace and log to ensure that no process deviations have occurred, and then removes the baskets from the retort. It is advisable to keep a dedicated wired or wireless sensor at the cold spot of each basket. In case of any deviation observed, the operator must add a status deviation note marked with the retort number and cycle, and inform their supervisor, who must check and agree the status of that basket of product. The supervisor initials the status deviation note and, if it is agreed that the crate has been processed, adds the post-process status note.

Baskets / crates should not be unloaded unless the sensor associated with that basket indicates the achievement of the required operating temperature and appropriate batch markings, or the countersigned process status deviation note is present. A prominent notice to this effect should be displayed in the unloading area.

The wired / wireless sensor may be retained and sufficient records made to enable each pallet load of final product to be traced back to specific crate numbers and retort processes. If the required temperature or F0 value is not achieved for the products at the cold spots, that load should be re-processed or discarded to eliminate the risk of rejection.

At the end of a production period, the independent records of numbers of crates filled, numbers sterilized and numbers unloaded should be reconciled and checked by a designated person.

The design of the loading / unloading system should ensure the complete segregation of unsterilized and sterilized crates. Furthermore, in normal operation, the system should only allow crates to be unloaded once they have received the scheduled thermal process.

An appropriate and relevant system should also be employed when the loading system is automated.

Any pouches found away from their allocated position, damaged, or contained within a crate where layers have moved prior to or during the retort (cook and cool) process should be destroyed, together with those pouches whose process may have been lessened by their movement.

5. Post-process handling of retorted pouches

The objective of post-process handling is to take wet, heat-processed pouches from a retort vessel and convey them, ready for further packing, whilst compromising neither the integrity of the pouches, the product in those pouches, nor any secondary packaging the pouches are packed in. A risk assessment should be carried out to confirm that the post-process handling is satisfactory when designing a process.

Seals are generally weakest immediately after processing, and great care must be taken to prevent post-process contamination.

Studies have shown that it is important for the packs to be dried as quickly as possible. This is because moisture in contact with the pouch acts as an effective carrier medium and may lead to increased risk of spoilage of the contents if the hermetic pouch integrity has been affected.

5.1 Handling

One of the most difficult areas of pouch product manufacture is post-process manual handling. Undoubtedly the safest procedure is to ensure that pouches are not manually handled once processed, reflecting the well-established rule of canned product manufacture. However, this is impractical for a lot of manufacturers, often because the flexible nature of the pouch does not lend itself to ease of automatic handling.

The solution to this dilemma is the correct application of risk assessment for that product and process. Unlike a can, a correctly sealed and heat-treated pouch will remain sealed to microbiological ingress throughout the post-process handling. Indeed, the requirement of drying the pouches as quickly as possible may well override the need to avoid their manual handling. Therefore, if manual handling is required, the Critical Control Points must be designed with this in mind, thus ensuring that handling pouches by hand will not result in post-process contamination.

Studies have shown that wet manual handling of deliberately punctured post-processed pouches that are then quickly dried has a relatively small effect on recontamination rates compared to storing these pouches wet. Furthermore, manual handling of these pouches once they are dry has negligible effects.

If manual handling of wet pouches is required in the process, then the following points need to be ensured during this phase of the manufacturing process:

• The environment, protective clothing and hands are kept clean.
• Contact with the pouch is minimal.
• Pouches are subsequently dried as quickly as possible.

Even if the pouches are not being manually handled, it is important that surfaces are designed and maintained to avoid pouch damage and that they are kept hygienic.

5.2 Drying

A holding period will allow the evaporation of water from the surface of the pouch. A study can be performed to determine the rate of pouch drying, to determine at what stage the pouch is sufficiently dry to enable packing.

The following may be considered:

1. 1

   To aid drying, the process of cooling should be designed so that the internal average temperature of the sterilized or pasteurized product is at or just below 40°C at the end of cooling. (With a static heat-treatment process, the warmest temperature point of the pack can be higher than 40°C to achieve an overall average of 40°C.)

2. 2

   If the retort is a batch rotary model, tilting the basket will remove a significant amount of the excess water.

3. 3

   The crates or baskets must be removed from the retort as soon as possible. The inside of a retort immediately after a cycle will have a very high humidity and is therefore a very poor environment for drying.

4. 4

   It is important that unloading of the retort proceeds as smoothly as possible. Careful attention must be paid to the alignment of basket carriers, by effective installation and maintenance. Failure to do so will result in physical damage to pouches and unintentional manual handling of the pouches.

5. 5

   Ideally the baskets may also be tilted, especially if this has not been possible inside the retort.

6. 6

   Consideration of the environment in which the pouches are now handled is very important. It should have good airflow, low relative humidity and an ambient temperature.

7. 7

   Baskets should be destacked to allow pouches to be exposed to the desired dry conditions. Removing the pouches from the layer trays themselves may be necessary to dry the surfaces in direct contact with the tray.

8. 8

   Further methods of removing water are the use of air knives and surfactant sprays to decrease the water tension.

6. Quality assurance checks – post processing

Before the packs are distributed, it is important to ensure that the integrity of the pack is good and the product is safe. Frequency and sampling will be an in-house decision.

Testing may include:

• Visual inspection
• Correct printing
• Correct coding
• Maximum headspace volume
• Seal strength – a test to check the strength required to separate the seal of the package.
• Compression test
• Burst test – a test in which compressed air is introduced into the flexible container until structural failure results. The reading obtained indicates seal strength.
• Drop test – the ability to be dropped at selected orientations and from selected heights without imparting rotation and other influences.
• Dye penetration – injection of a test solution into the sealing area of a pack to determine any openings in the sealed area.
• Delamination
• Incubation – a test in which containers are held for a suitable time at a temperature specific for the growth of the microorganisms under investigation, in order to ascertain whether microbial growth occurs.

7. Outer packaging

It is important to minimize microbial contamination and physical damage to the packs following processing and during distribution and retailing. Therefore, outer packaging is required to protect the product and can be based on paperboard, plastic or a combination of both.

Outer packaging should:

• Protect and contain the pack from abuse.
• Be an interference fit, both width and lengthways, for a filled processed pouch to prevent excessive movement and flexing.
• Have an outer wrap that protects the product from vibration abuse.
• Be tamper proof.
• Have appropriate identification labelling.

7.1 Transit packaging

Transit packaging requirements will depend on the method of transportation, the distribution systems and display requirements. The following points should be considered:

• The transit protection should be afforded by the outer wrap or the outer case, or a combination of both. Unless rigid outer wraps are used, the transit case should be capable of taking all stacking and handling abuse. The use of shrink-wrap as a transit case is only acceptable if the immediate outer wraps give sufficient physical protection.
• Alignment of pouches within the outer case is critical. The outer case should be specified in this respect, and carry a clear indication of the correct transit position. Hard vacuum packs of solid products are less critical with respect to transit alignment, provided that each pack is an interference fit in all dimensions within its outer wrap.
• Palletization and stacking of cases should be specified to minimize the risk of compressing pouch contents.

7.2 Conditions during transit packaging

Atmospheric conditions during transit should be considered. Pouches should not be subjected to extremes of temperature. At temperatures around 0°C there may be a danger of reduced flex crack resistance, and above 45°C there may be a risk of growth from thermophilic organisms which have survived the heat process (unless the heat treatment is designed to remove thermophilic organisms with higher F0 values).

There is also the danger that water or high humidity may reduce the strength of the outer case, which should be specified or protected accordingly.

For testing the retort itself, see retort qualification testing. For the equipment requirements under US FDA norms, see what makes a retort FDA compliant.