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Optimising the processing of flexible containers
7 Optimising the processing of flexible containers M. L. Seiboth and G. H. Shaw, Ellab UK Limited, UK
7.1
Introduction: challenges in processing flexible containers
The processing of food products by the application of heat to a sealed container offers a very simple mechanism by which the food can be preserved and stored for a significantly increased shelf-life. The two key parameters that allow this preservation technique to be successful are: (i)
The application of a scheduled process appropriate to the individual product and the target shelf-life under given storage conditions that is to be achieved. Such a process will be denoted in terms of a length of time at which the product must be held at a process temperature which, in some cases, allied with the intrinsic characteristics of product such as pH and water activity, ensures a sufficient level of microbiological kill of a target micro-organism. Whilst this aspect of thermal processing is beyond the direct scope of this chapter and will not be covered in detail, it cannot be ignored and will be considered in Section 7.6. (ii) The integrity of the packaging, with the container being hermetically sealed to ensure that post-process contamination cannot occur. The methodology and technology of creating seals and seams is covered in Chapter 4. In combination, the correct delivery of these two key parameters will allow the production of a safe product that will keep. However, the environment that is
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required for the application of (i) presents severe challenges to achieving (ii), with the processing environment being particularly challenging for plastics materials. During the thermal process, the packaging will be exposed to an external pressure that builds up within the processing vessel, in addition to internal pressures that can be generated by components within the product itself. These pressures provide a source of stresses to the packaging that can affect the integrity of the packaging. For the most traditional format of packaging, the metal can, the design of the container has evolved to allow for pressure differentials that are likely to be experienced at typical process temperatures in saturated steam during the process. It is therefore the ‘newer’ packaging formats, the flexible retort pouch and the semi-rigid trays with film lids, that can be affected and lose integrity during a process. This chapter will consider the challenges to the thermal process authority in establishing a process for a flexible or semi-rigid packaging format. The equipment that is available will be discussed and the methodology behind its application will be considered within a case study example. Whilst this will focus on the behaviour of the packaging, it is important to consider how manipulating this may impact on both the operation of the processing vessel and the heating of the product within the container, and these will be covered in Section 7.6.
7.2
Processing of flexible containers
The retortable pouch and plastic tray offer food manufacturers and retailers a visually more appealing and versatile packaging format than the traditional metal can in which to produce and sell heat-preserved foods. Each format offers greater apparent convenience to the consumer with a short cook in a microwave producing a quick meal or a component thereof. For the food manufacturers, the retort technology, and in particular the control systems, have developed from the early saturated steam retorts. For a saturated steam retort, the pressure profile during a process would be of a fixed regime showing a correlation to the temperature within the vessel. Developments in control systems and retort designs allow pressure to be controlled independently of temperature. Retort systems known as ‘overpressure’ retorts, such as those manufactured by Lagarde and Steriflow SAS, permit a controlled input of air to be added to the processing vessel during a cook cycle, thus letting the user determine the pressure to which packs are exposed during a process. This ability to control pressure allows the processor to be able to limit an expansion of the packaging and, in turn, stresses on the seams of the packaging. It is important that an understanding is made of the changes that occur with the application of heat to the product and container during the process. The major change will be an increase in internal pressure causing expansion of the packaging that can be attributed to a number of factors:
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expansion of headspace gases. expansion and release of gases entrapped within the food product. generation of water vapour. release of gases due to reduced solubility of dissolved gases at higher temperatures. thermal expansion of the solid and liquid phases of the food.
The amount of expansion that can take place within a processed food package depends on: • the temperature of the food at the point when the pack is closed. • the amount of vacuum drawn on the pack at the point of closing, e.g. mechanical vacuum seamers. • the mechanical resistance of the pack. • the external balancing pressure. An additional problem for the processor will be that the pack will significantly soften during heating. The challenge, therefore, when choosing a pack format for a heating application or establishing a heat process for a selected pack, is to minimise the stresses upon the packaging. In the sector of the food industry that produces metal cans, the need for tailored pressure profiles has largely been avoided by careful design of the metal can. Cans are designed to withstand the pressure differentials that they are likely to experience at typical process temperatures in saturated steam. However, weaker packaging formats for heat-preserved foods, such as jars, semi-rigid plastic pots/trays, pouches or even cans with easyopen features, puts greater emphasis onto the food processors to design optimum pressure profiles during cooking that give maximum protection to the packaging. A further hurdle arises with the desire to reduce the thickness of packaging materials. Overpressure retorts offer the capability to ramp pressures up and down during the process to optimally match the pressure changes taking place inside the packs. Such tailored pressure profiles can be used for processing relatively weak packaging formats that simply would not be possible under fixed external pressure regimes.
7.2.1 The importance of thermal stresses for packaging performance When exposed to pressure differentials, packaging materials will be subjected to stress, and eventually to permanent deformation. Metal cans have evolved to withstand high internal pressures (possibly up to 6 or 7 bars). Can ends are designed to dome as the internal pressure increases and, in doing so, relieve some of the stress on the seals. However, if the internal pressure becomes high enough, then permanent deformation of the ends occurs and this can take the form of ‘peaks’. The reverse can occur, for example during the cooling process, where the external pressure remains high after the internal can pressure drops in response to product cooling. In this case, panelling will occur, which is the inward collapse of the relatively weak sides of the can. With semi-rigid trays and bowls, the effects of excessively high or low pressure
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differentials are similar to those with cans, but unlike cans there is a significant risk that the lids could burst open in the retort. It might be said that semi-rigid constructions require greater pressure control because they are highly sensitive to both low and high external pressure conditions. This is because the lidding films can be pushed inward as well as outward. Pouches are possibly a little easier to work with when defining pressure profiles than semi-rigid formats, because they are less sensitive to high external pressures. If the product is not compressible and does not require free space for mixing in a rotary process (e.g. rice), then application of a high overpressure should control pack expansion.
7.3
Setting up an overpressure profile
In order to minimise the stresses to a packaging material, it is necessary to be able to set up an overpressure profile to run in parallel with the thermal process that is cooking the product within the container. An investigation of the behaviour of the packaging under processing conditions and the establishment of a suitable overpressure profile should be undertaken in the early stage of the development of a new product or packaging format. It will be rare for this work to be undertaken on a production retort system and, in general, a pilot retort system will be utilised that is a scaled-down version of the production retort or retorts that will eventually be used for the product. It is important that the pilot retort has a programmable control system that allows a degree of manual intervention on the retort pressure profile, so that during a trial run the pressure can be manipulated to an optimum level at each stage of the process. Should such a control system not be available, trial runs will need to be carried out with a review of the traces from the chart recorder being undertaken in between runs, to assess the changes to the retort program required for the following run. This is an iterative process with the programme nearing the optimum with each new trial. Sometimes the expected improvement does not occur at each iterative step and this often reflects a lack of consistency in the pack preparation conditions, for example, there is pack-to-pack variation in the vacuum being pulled at closure.
7.4
Equipment for establishing an overpressure profile
The preceding sections have highlighted that there are two parameters for which a physical measurement can be taken during a process cycle that can indicate how the packaging may be behaving. These are the physical movement, or deflection, of the packaging material and a value of pressure, which can be measured both externally to the pack and internally. Temperature within the retort system may also be recorded but this will not be having a direct influence on pack behaviour. Indirectly, retort temperature will be a factor as it will impact on the rate of heating of the product and so on the rate of expansion of gases held within.
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The requirement to monitor and record pressure and/or deflection during a process means that the thermal process authority given the task of establishing an over pressure profile needs to have access to appropriate equipment.
7.4.1 Pressure sensors The acquisition of pressure measurements during a process cycle can be successfully achieved using one of two types of datalogging equipment. (i)
A real-time pressure sensor from Ellab A/S, which uses an oil-filled capillary tube to transmit pressures from a pack to a strain gauge transducer located outside of the retort, which in turn is connected to an E-Val Flex datalogger. The key advantage offered by this type of sensor is the availability of realtime data, allowing adjustments to be made mid-process to the pressure within the retort system. This could reduce the number of practical runs needed to develop the correct over-pressure profile for a packaging format. (ii) Pressure measuring wireless data acquisition units such those from TMI Orion or Ellab A/S (Fig. 7.1). These units are ‘cable free’, being small wireless devices that are programmed and downloaded from a reader station connected to a PC computer. The memory in such devices allows the user to collect around 14 000 individual sets of data within a working temperature range of –20 to + 140 °C. The small size (approximately 15 × 40 mm for an Ellab Tracksense Micro pressure device) gives the advantage that they can be put inside containers and have a minimal disruption to the product within. The small size is also of benefit for test containers going through continuous cookers such as reel and spiral retort systems and in batch rotary retorts. For reel and spiral applications, the time interval at which data can be gathered could be important, because faults on these cookers may occur in the can transfer valves through which cans travel in seconds. The major drawback with such devices is that they are memory dataloggers and cannot generate any real-time information. The thermal process technologist must therefore wait until the completion of a run before being able to see the impact of any adjustments that have been made to a retort profile. Using pressure measuring devices, it is possible to measure the pressure on the inside and outside of a container in order to determine the differential and therefore the potential for pack expansion. Where there is no pressure differential, there is no driving force for pack surface movement. The bigger the differential, the more likely it is that there will be adverse stresses being applied to the packaging. Pressure differential measurements are used for determining pressure profile requirements for metal cans, but the approach is less useful for semi-rigid or flexible containers. For containers that exhibit a high degree of flexibility, the tendency is for the internal pack pressure to equilibrate quickly with the external pressure, so pressure measured inside and outside the container will be similar. Likewise, the pressures required to cause deformations in weak plastic packs are so small that the sensitivity of the two pressure measurement sensors would need to
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Fig. 7.1 Ellab Tracksense Micro Pressure Datalogger. With dimensions of only 15 × 40 mm, this device is ideal for recording pressure within a flexible container during a thermal process.
be very high to get useful results. As this method may not be suited to some flexible packaging types, alternative methodologies for developing an overpressure profile should be considered. 7.4.2 Pack deflection measurement Where pressure differential measurements are not possible, direct measurement of the movement or deflection of the pack surface offers a good alternative and is often the preferred method within industry for establishing an overpressure profile. This approach utilises a movement transducer directly placed in contact with the pack surface so that either pack expansion or contraction can be monitored using a real-time datalogger system. A commercial system of this type from Ellab uses a spring-loaded stainless steel probe with 18mm of travel for measurement. The probe can either be placed directly in contact with the pack surface or put in mechanical contact through an appropriate holding device (Fig. 7.2). It is important that the deflection device does not adversely influence the movement of the pack surface so care must be taken to ensure that this is not the case. A key advantage of this system is that it allows real-time data to be acquired, so when used in conjunction with a real-time pressure sensor (see Section 7.4.1) and a standard thermocouple, a complete overview of the processing conditions can be acquired. This type of system is limited to batch applications because the cables make it unsuitable for continuous cookers. It is possible to take deflection measurements from batch rotary retorts by restraining the measured container and holding the deflection sensor in a fixed position relative to it. The data can be passed out of the retort through a thermocouple slip-ring assembly. Possibly the most difficult aspect of using deflection in rotary environments is interpretation, because the
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Fig. 7.2 Ellab EVal Flex Deflection Transducer. Shown here in an experimental rig for a pouch product, the deflection transducer provides a mechanism for correctly establishing a correct overpressure profile.
underlying trends in expansion/contraction of the container have to be separated from the ‘noise’ of the movement of the sensor and product under the effect of gravity, with each rotation of the retort basket. An alternative to a real-time system for rotary processes would be to use a wireless deflection device such as the TMI Orion Nanovacq Deformation datalogger. As with the real-time device, a steel sensor is placed in contact with the pack surface, but for this device the movements of the sensor are recorded to a memory chip for downloading on completion of the test run. For either of these types of sensor, the user is required to interpolate any improvements to the pressure profile that might be required manually. An alternative to this approach is to set up a feedback loop between the sensor and the retort control system. Within such a set-up, a software program determines from the output of the deflection sensor as to whether the pressure is too high or too low, and makes adjustments accordingly by either opening the retort vent valve to reduce pressure, or by opening the retort compressed air valve to increase retort pressure. Such a set up necessitates a retort that has services (vent and air valve) that can be configured in this manner. One retort manufacturer, Steritech (Saverne, France) produces a deflection sensor for use with their retort control system, whilst the Ellab CMC 92 datalogger system may be utilised on other retort systems where possible.
7.4.3 A viewing window A viewing window in the retort system, if available, is probably the oldest method for tailoring pressure profiles for packs in retorts. The approach is simply to look through the window to judge the level of pack expansion or contraction taking place and adjust the pressure profile accordingly. Viewing is made significantly easier by having a powerful light to illuminate the retort interior. Although the
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approach lacks sophistication, it does offer some real benefits because the human eye can judge deformation on all pack surfaces, whereas, for example, deflection measurements are limited to those surfaces to which sensors are applied. This can be important because, in complex pack formats with surfaces of different rigidities, the control action required on one surface may be entirely wrong for another part of the pack. To take an extreme example, it is possible to measure deflection from a flexible lid on a semi-rigid pot and find that no matter how much external pressure is applied, the outward expansion of the lid cannot be restrained. Further investigation might show that the pot itself has been crushed inwards, so pushing the lid out further with every increment in pressure. A window is a useful tool for static retorting applications but cannot be used for rotary retorts unless the rotation speed is slow or a high-speed video camera is used. Sometimes, a static test in a retort window is used as a preliminary test for a rotary process. In this case, the transfer of the pressure profile from the static to the rotary mode is facilitated by taking heat penetration measurements inside the container. With these data, it is possible to equate the pressure required at each pack internal temperature in the static mode to the pressure required at the same inpack temperature in rotary mode, though this may not be considered an exact science.
7.5
Case study
This example considers an industrial trial that was undertaken to improve the overpressure profile and reduce the number of pack rejects that the company was experiencing. The product tested was a simple ready meal type product in a semirigid tray with a film lid. This was processed in a static batch retort that used rain water as the heating medium. The first step of the trial was to acquire data for the existing conditions to identify the deflection that was taking place under the retort pressure profile then in place. A pilot retort was setup with an Ellab CMC 92 datalogging system that utilised a real-time deflection sensor to monitor the movement taking place of the film lid, and a real-time pressure sensor and a thermocouple to record the retort conditions. The data generated can be seen in Fig. 7.3. As can be seen from the profiles, the programme in place incorporated a single-step ramp up for the pressure within the retort, with this reaching a hold set value of 0.8 bar at the same time as the retort reaches the hold set point temperature of 105 °C. The ramp up time from 0.0 to 0.8 bar is approximately eight minutes. The impact that such a quick increase in retort pressure has upon the pack can be clearly seen. The deflection profile records a negative change, with the film lid being pushed in in excess of 10 mm during the ramp up phase. The deflection is actually so great that it goes beyond the recording range of the sensor, as indicated by the flat bottom that the deflection trace records for the first 12 minutes of the process. At this point in the process, the gases within the pack are expanding, which leads to the film lid being pushed out. The pressure profile is now shown to be incorrect at the end of
Fig. 7.3
Case study: initial retort run with incorrect retort pressure profile and excessive stresses to the container.
Fig. 7.4
Case study: revised retort run with adjusted retort pressure profile and reduced stresses to the container.
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the hold phase as the pack expands beyond the zero start position to beyond the positive range of the sensor and again a flat plateau is recorded as the film lid deflection continues to increase at the end of the hold phase. It can therefore be seen from the first set of data that the packs processed using this retort pressure profile are being stressed both positively and negatively, with total movement of the film lid being in excess of the 18 mm range of the deflection sensor. The two key areas of the process that need adjustment to the pressure profile are the ramp up and start of the hold phase where excessive pressure is being recorded, and the end of the hold phase where the programmed pressure value is insufficient. To reduce the deflection that the film lids are being subjected to, the company changed the retort pressure profile to incorporate a gradual ramp up of pressure for the duration of the process, with a maximum value of 1.0 bar at the end of the hold phase. A second test run was carried out and the data shown in Fig. 7.4 generated. It can be seen that, whilst the film lid is still showing some movement, it is considerably reduced from the first run and, more importantly, that changes are gradual, minimising the stresses on the seals. The range of deflection recorded during the hold phase has now been reduced to 10 mm, or half the amount seen with the original pressure profile. This case study example shows how the use of commercially available datalogging equipment can very easily identify areas of the retort program that are leading to excessive stressing of the packaging. The data generated can be easily interpreted and improvements implemented that reduce the stresses and the volume of reject product that a company may be producing.
7.6
Implementing pressure profiles
In the event of experimental trials producing an amendment to the retort pressure profile, it is important that consideration is given as to whether or not this has an effect on other aspects of the process. One key area that should be reviewed is the performance of the retort system in the form of temperature distribution trials. Within such trials, thermocouples or wireless dataloggers will be positioned throughout a retort load of product to record the environment temperatures within the load and identify any areas of slow heating within the retort system. If the pressure profile is changed, which will in turn change the physical shape of the containers of the retort, there might be an impact on the movement of the retort heating media. This will be a particular issue within systems such as a Lagarde steam/air retort, which utilises a large fan to move a mixture of steam and air through the retort load. Once it has been ascertained that either no change in retort performance has occurred, or that a change has occurred and the variation and position within the retort has been identified, testing should also be carried within the product itself. Heat penetration testing will consider the rate of heating within a container and in turn the microbiological kill that is taking place during the thermal process. For taking measurements within a flexible or semi-rigid container, it is important to ensure that the fittings used for locating either a thermocouple
Optimising the processing of flexible containers
Fig. 7.5
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Example of a fitting for data acquisition in a flexible packaging that ensures correct location of the temperature probe within the sample.
or a wireless datalogger within the container do not restrict any movement of that container during the process. This is of particular importance for flexible pouches, where the packaging material itself cannot support the temperature measuring device within the pouch at the cold spot of the product. In such a case, a custom-made fitting such as that produced by Ellab A/S and shown in Fig. 7.5 should be used. The positioning the test heat penetration samples is critical, and ideally these should be at the cold zone identified by the temperature distribution testing. If this is not possible, any variation of processing conditions should be allowed for within modelling software, such as the CCFRA CTemp program or the TechniCAL Calsoft program.
7.7
Future trends
Optimising the processing of flexible containers will be driven by advances and new trends in the packaging sector. As new technologies allow the development of new packaging formats and thinner materials for existing designs, there will be an on-going demand for safe processes to be established; processes that ensure stresses experienced by containers as a result of the thermal process are kept to a minimum. The technology and equipment currently commercially available from companies such as Ellab A/S and TMI Orion offer process technologists the capability and flexibility needed to test the various packaging formats. As with standard temperature datalogging, it could be suggested that the ‘Holy Grail’ of a retortable, wireless, transmitting device would be of benefit for the recording of both pressure and deflection measurements. Whilst developments to attain and produce such a temperature logger continue, it is questionable as to how much actual practical benefit such a pressure or deflection device would offer, with only testing of rotary processes showing clear-cut advantages in eliminating the requirement for a slip ring assembly.
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7.8
In-pack processed foods
Sources of further information and advice
For companies entering into the processing of flexible packaging for the first time, the input of a knowledgeable consultant could be invaluable. For companies in Europe, organisations such as the Campden and Chorleywood Food Research Association offer pilot facilities, along with the equipment and expertise to develop new processes. In addition to pilot facilities, CCFRA offer valuable information in the form of published guidelines and reports on completed research projects (see below for more detail). Datalogging equipment manufacturers such as Ellab A/S and TMI Orion can offer technical support and advice on utilising their individual products for given applications, whilst retort manufacturers or their agents in a particular country can advise on the suitability of their systems for a given process. A valuable source of information for thermal processing as a whole is the Institute for Thermal Processing Specialists (IFTPS), who offer guidance and support to the industry through annual meetings and protocols put together by Institute members, based on their practical experience. Details for all the companies or organisations mentioned in this chapter are as follows: Ellab A/S, Krondalvej 9, DK – 2610 Roedovre, Denmark Tel: +45 44 52 05 00 Website: www.ellab.com Campden and Chorleywood Food Research Association (CCFRA), Station Road, Chipping Campden, Gloucestershire GL55 6LD, UK Tel: +44 1386 842000 Website: www.campden.co.uk TMI Orion, Parc Industriel et Technologique de la Pompignane, Rue de la Vieille Poste, 34055 MONTPELLIER cedex 1, France Tel: +33 4 99 52 67 10 Website: www.tmi-orion.com/index.htm Societe Lagarde, Z.I. Les Plaines - N° 5 bis, 26780 Malataverne, France Tel: +33 4 75 90 58 58 Website: www.lagarde-autoclaves.com Steriflow SAS, 32, Rue de Cambrai, 75019 Paris, France Tel: +33 1 40 37 08 45 Website: www.steriflow.com Steritech/ECPS SA, ZA du Kochersberg, 1, Rue de Furchhausen, 67700 Saverne, France Tel : +33 3 88 71 04 33 Website: www.steritech.fr Institute for Thermal Processing Specialists (IFTPS), 304 Stone Road West, Suite 301, Guelph, ON N1G 4W4, Canada Tel: +1 519 824 6774 Website: www.iftps.org Some useful publications of the Campden and Chorleywood Food Rearch Association are:
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CCFRA Guideline 50 ‘Guidelines on good manufacturing practice for heat processed flexible packaging’ (2006). CCFRA Review 46 ‘Review of the integrity of heat processed containers through manufacturing and distribution’ (2004). CCFRA Guideline 17 ‘Guidelines for establishing heat distribution in batch overpressure retort systems’ (1997).
7.1
Introduction: challenges in processing flexible containers
The processing of food products by the application of heat to a sealed container offers a very simple mechanism by which the food can be preserved and stored for a significantly increased shelf-life. The two key parameters that allow this preservation technique to be successful are: (i)
The application of a scheduled process appropriate to the individual product and the target shelf-life under given storage conditions that is to be achieved. Such a process will be denoted in terms of a length of time at which the product must be held at a process temperature which, in some cases, allied with the intrinsic characteristics of product such as pH and water activity, ensures a sufficient level of microbiological kill of a target micro-organism. Whilst this aspect of thermal processing is beyond the direct scope of this chapter and will not be covered in detail, it cannot be ignored and will be considered in Section 7.6. (ii) The integrity of the packaging, with the container being hermetically sealed to ensure that post-process contamination cannot occur. The methodology and technology of creating seals and seams is covered in Chapter 4. In combination, the correct delivery of these two key parameters will allow the production of a safe product that will keep. However, the environment that is
Optimising the processing of flexible containers
103
required for the application of (i) presents severe challenges to achieving (ii), with the processing environment being particularly challenging for plastics materials. During the thermal process, the packaging will be exposed to an external pressure that builds up within the processing vessel, in addition to internal pressures that can be generated by components within the product itself. These pressures provide a source of stresses to the packaging that can affect the integrity of the packaging. For the most traditional format of packaging, the metal can, the design of the container has evolved to allow for pressure differentials that are likely to be experienced at typical process temperatures in saturated steam during the process. It is therefore the ‘newer’ packaging formats, the flexible retort pouch and the semi-rigid trays with film lids, that can be affected and lose integrity during a process. This chapter will consider the challenges to the thermal process authority in establishing a process for a flexible or semi-rigid packaging format. The equipment that is available will be discussed and the methodology behind its application will be considered within a case study example. Whilst this will focus on the behaviour of the packaging, it is important to consider how manipulating this may impact on both the operation of the processing vessel and the heating of the product within the container, and these will be covered in Section 7.6.
7.2
Processing of flexible containers
The retortable pouch and plastic tray offer food manufacturers and retailers a visually more appealing and versatile packaging format than the traditional metal can in which to produce and sell heat-preserved foods. Each format offers greater apparent convenience to the consumer with a short cook in a microwave producing a quick meal or a component thereof. For the food manufacturers, the retort technology, and in particular the control systems, have developed from the early saturated steam retorts. For a saturated steam retort, the pressure profile during a process would be of a fixed regime showing a correlation to the temperature within the vessel. Developments in control systems and retort designs allow pressure to be controlled independently of temperature. Retort systems known as ‘overpressure’ retorts, such as those manufactured by Lagarde and Steriflow SAS, permit a controlled input of air to be added to the processing vessel during a cook cycle, thus letting the user determine the pressure to which packs are exposed during a process. This ability to control pressure allows the processor to be able to limit an expansion of the packaging and, in turn, stresses on the seams of the packaging. It is important that an understanding is made of the changes that occur with the application of heat to the product and container during the process. The major change will be an increase in internal pressure causing expansion of the packaging that can be attributed to a number of factors:
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expansion of headspace gases. expansion and release of gases entrapped within the food product. generation of water vapour. release of gases due to reduced solubility of dissolved gases at higher temperatures. thermal expansion of the solid and liquid phases of the food.
The amount of expansion that can take place within a processed food package depends on: • the temperature of the food at the point when the pack is closed. • the amount of vacuum drawn on the pack at the point of closing, e.g. mechanical vacuum seamers. • the mechanical resistance of the pack. • the external balancing pressure. An additional problem for the processor will be that the pack will significantly soften during heating. The challenge, therefore, when choosing a pack format for a heating application or establishing a heat process for a selected pack, is to minimise the stresses upon the packaging. In the sector of the food industry that produces metal cans, the need for tailored pressure profiles has largely been avoided by careful design of the metal can. Cans are designed to withstand the pressure differentials that they are likely to experience at typical process temperatures in saturated steam. However, weaker packaging formats for heat-preserved foods, such as jars, semi-rigid plastic pots/trays, pouches or even cans with easyopen features, puts greater emphasis onto the food processors to design optimum pressure profiles during cooking that give maximum protection to the packaging. A further hurdle arises with the desire to reduce the thickness of packaging materials. Overpressure retorts offer the capability to ramp pressures up and down during the process to optimally match the pressure changes taking place inside the packs. Such tailored pressure profiles can be used for processing relatively weak packaging formats that simply would not be possible under fixed external pressure regimes.
7.2.1 The importance of thermal stresses for packaging performance When exposed to pressure differentials, packaging materials will be subjected to stress, and eventually to permanent deformation. Metal cans have evolved to withstand high internal pressures (possibly up to 6 or 7 bars). Can ends are designed to dome as the internal pressure increases and, in doing so, relieve some of the stress on the seals. However, if the internal pressure becomes high enough, then permanent deformation of the ends occurs and this can take the form of ‘peaks’. The reverse can occur, for example during the cooling process, where the external pressure remains high after the internal can pressure drops in response to product cooling. In this case, panelling will occur, which is the inward collapse of the relatively weak sides of the can. With semi-rigid trays and bowls, the effects of excessively high or low pressure
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differentials are similar to those with cans, but unlike cans there is a significant risk that the lids could burst open in the retort. It might be said that semi-rigid constructions require greater pressure control because they are highly sensitive to both low and high external pressure conditions. This is because the lidding films can be pushed inward as well as outward. Pouches are possibly a little easier to work with when defining pressure profiles than semi-rigid formats, because they are less sensitive to high external pressures. If the product is not compressible and does not require free space for mixing in a rotary process (e.g. rice), then application of a high overpressure should control pack expansion.
7.3
Setting up an overpressure profile
In order to minimise the stresses to a packaging material, it is necessary to be able to set up an overpressure profile to run in parallel with the thermal process that is cooking the product within the container. An investigation of the behaviour of the packaging under processing conditions and the establishment of a suitable overpressure profile should be undertaken in the early stage of the development of a new product or packaging format. It will be rare for this work to be undertaken on a production retort system and, in general, a pilot retort system will be utilised that is a scaled-down version of the production retort or retorts that will eventually be used for the product. It is important that the pilot retort has a programmable control system that allows a degree of manual intervention on the retort pressure profile, so that during a trial run the pressure can be manipulated to an optimum level at each stage of the process. Should such a control system not be available, trial runs will need to be carried out with a review of the traces from the chart recorder being undertaken in between runs, to assess the changes to the retort program required for the following run. This is an iterative process with the programme nearing the optimum with each new trial. Sometimes the expected improvement does not occur at each iterative step and this often reflects a lack of consistency in the pack preparation conditions, for example, there is pack-to-pack variation in the vacuum being pulled at closure.
7.4
Equipment for establishing an overpressure profile
The preceding sections have highlighted that there are two parameters for which a physical measurement can be taken during a process cycle that can indicate how the packaging may be behaving. These are the physical movement, or deflection, of the packaging material and a value of pressure, which can be measured both externally to the pack and internally. Temperature within the retort system may also be recorded but this will not be having a direct influence on pack behaviour. Indirectly, retort temperature will be a factor as it will impact on the rate of heating of the product and so on the rate of expansion of gases held within.
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The requirement to monitor and record pressure and/or deflection during a process means that the thermal process authority given the task of establishing an over pressure profile needs to have access to appropriate equipment.
7.4.1 Pressure sensors The acquisition of pressure measurements during a process cycle can be successfully achieved using one of two types of datalogging equipment. (i)
A real-time pressure sensor from Ellab A/S, which uses an oil-filled capillary tube to transmit pressures from a pack to a strain gauge transducer located outside of the retort, which in turn is connected to an E-Val Flex datalogger. The key advantage offered by this type of sensor is the availability of realtime data, allowing adjustments to be made mid-process to the pressure within the retort system. This could reduce the number of practical runs needed to develop the correct over-pressure profile for a packaging format. (ii) Pressure measuring wireless data acquisition units such those from TMI Orion or Ellab A/S (Fig. 7.1). These units are ‘cable free’, being small wireless devices that are programmed and downloaded from a reader station connected to a PC computer. The memory in such devices allows the user to collect around 14 000 individual sets of data within a working temperature range of –20 to + 140 °C. The small size (approximately 15 × 40 mm for an Ellab Tracksense Micro pressure device) gives the advantage that they can be put inside containers and have a minimal disruption to the product within. The small size is also of benefit for test containers going through continuous cookers such as reel and spiral retort systems and in batch rotary retorts. For reel and spiral applications, the time interval at which data can be gathered could be important, because faults on these cookers may occur in the can transfer valves through which cans travel in seconds. The major drawback with such devices is that they are memory dataloggers and cannot generate any real-time information. The thermal process technologist must therefore wait until the completion of a run before being able to see the impact of any adjustments that have been made to a retort profile. Using pressure measuring devices, it is possible to measure the pressure on the inside and outside of a container in order to determine the differential and therefore the potential for pack expansion. Where there is no pressure differential, there is no driving force for pack surface movement. The bigger the differential, the more likely it is that there will be adverse stresses being applied to the packaging. Pressure differential measurements are used for determining pressure profile requirements for metal cans, but the approach is less useful for semi-rigid or flexible containers. For containers that exhibit a high degree of flexibility, the tendency is for the internal pack pressure to equilibrate quickly with the external pressure, so pressure measured inside and outside the container will be similar. Likewise, the pressures required to cause deformations in weak plastic packs are so small that the sensitivity of the two pressure measurement sensors would need to
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Fig. 7.1 Ellab Tracksense Micro Pressure Datalogger. With dimensions of only 15 × 40 mm, this device is ideal for recording pressure within a flexible container during a thermal process.
be very high to get useful results. As this method may not be suited to some flexible packaging types, alternative methodologies for developing an overpressure profile should be considered. 7.4.2 Pack deflection measurement Where pressure differential measurements are not possible, direct measurement of the movement or deflection of the pack surface offers a good alternative and is often the preferred method within industry for establishing an overpressure profile. This approach utilises a movement transducer directly placed in contact with the pack surface so that either pack expansion or contraction can be monitored using a real-time datalogger system. A commercial system of this type from Ellab uses a spring-loaded stainless steel probe with 18mm of travel for measurement. The probe can either be placed directly in contact with the pack surface or put in mechanical contact through an appropriate holding device (Fig. 7.2). It is important that the deflection device does not adversely influence the movement of the pack surface so care must be taken to ensure that this is not the case. A key advantage of this system is that it allows real-time data to be acquired, so when used in conjunction with a real-time pressure sensor (see Section 7.4.1) and a standard thermocouple, a complete overview of the processing conditions can be acquired. This type of system is limited to batch applications because the cables make it unsuitable for continuous cookers. It is possible to take deflection measurements from batch rotary retorts by restraining the measured container and holding the deflection sensor in a fixed position relative to it. The data can be passed out of the retort through a thermocouple slip-ring assembly. Possibly the most difficult aspect of using deflection in rotary environments is interpretation, because the
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Fig. 7.2 Ellab EVal Flex Deflection Transducer. Shown here in an experimental rig for a pouch product, the deflection transducer provides a mechanism for correctly establishing a correct overpressure profile.
underlying trends in expansion/contraction of the container have to be separated from the ‘noise’ of the movement of the sensor and product under the effect of gravity, with each rotation of the retort basket. An alternative to a real-time system for rotary processes would be to use a wireless deflection device such as the TMI Orion Nanovacq Deformation datalogger. As with the real-time device, a steel sensor is placed in contact with the pack surface, but for this device the movements of the sensor are recorded to a memory chip for downloading on completion of the test run. For either of these types of sensor, the user is required to interpolate any improvements to the pressure profile that might be required manually. An alternative to this approach is to set up a feedback loop between the sensor and the retort control system. Within such a set-up, a software program determines from the output of the deflection sensor as to whether the pressure is too high or too low, and makes adjustments accordingly by either opening the retort vent valve to reduce pressure, or by opening the retort compressed air valve to increase retort pressure. Such a set up necessitates a retort that has services (vent and air valve) that can be configured in this manner. One retort manufacturer, Steritech (Saverne, France) produces a deflection sensor for use with their retort control system, whilst the Ellab CMC 92 datalogger system may be utilised on other retort systems where possible.
7.4.3 A viewing window A viewing window in the retort system, if available, is probably the oldest method for tailoring pressure profiles for packs in retorts. The approach is simply to look through the window to judge the level of pack expansion or contraction taking place and adjust the pressure profile accordingly. Viewing is made significantly easier by having a powerful light to illuminate the retort interior. Although the
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approach lacks sophistication, it does offer some real benefits because the human eye can judge deformation on all pack surfaces, whereas, for example, deflection measurements are limited to those surfaces to which sensors are applied. This can be important because, in complex pack formats with surfaces of different rigidities, the control action required on one surface may be entirely wrong for another part of the pack. To take an extreme example, it is possible to measure deflection from a flexible lid on a semi-rigid pot and find that no matter how much external pressure is applied, the outward expansion of the lid cannot be restrained. Further investigation might show that the pot itself has been crushed inwards, so pushing the lid out further with every increment in pressure. A window is a useful tool for static retorting applications but cannot be used for rotary retorts unless the rotation speed is slow or a high-speed video camera is used. Sometimes, a static test in a retort window is used as a preliminary test for a rotary process. In this case, the transfer of the pressure profile from the static to the rotary mode is facilitated by taking heat penetration measurements inside the container. With these data, it is possible to equate the pressure required at each pack internal temperature in the static mode to the pressure required at the same inpack temperature in rotary mode, though this may not be considered an exact science.
7.5
Case study
This example considers an industrial trial that was undertaken to improve the overpressure profile and reduce the number of pack rejects that the company was experiencing. The product tested was a simple ready meal type product in a semirigid tray with a film lid. This was processed in a static batch retort that used rain water as the heating medium. The first step of the trial was to acquire data for the existing conditions to identify the deflection that was taking place under the retort pressure profile then in place. A pilot retort was setup with an Ellab CMC 92 datalogging system that utilised a real-time deflection sensor to monitor the movement taking place of the film lid, and a real-time pressure sensor and a thermocouple to record the retort conditions. The data generated can be seen in Fig. 7.3. As can be seen from the profiles, the programme in place incorporated a single-step ramp up for the pressure within the retort, with this reaching a hold set value of 0.8 bar at the same time as the retort reaches the hold set point temperature of 105 °C. The ramp up time from 0.0 to 0.8 bar is approximately eight minutes. The impact that such a quick increase in retort pressure has upon the pack can be clearly seen. The deflection profile records a negative change, with the film lid being pushed in in excess of 10 mm during the ramp up phase. The deflection is actually so great that it goes beyond the recording range of the sensor, as indicated by the flat bottom that the deflection trace records for the first 12 minutes of the process. At this point in the process, the gases within the pack are expanding, which leads to the film lid being pushed out. The pressure profile is now shown to be incorrect at the end of
Fig. 7.3
Case study: initial retort run with incorrect retort pressure profile and excessive stresses to the container.
Fig. 7.4
Case study: revised retort run with adjusted retort pressure profile and reduced stresses to the container.
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the hold phase as the pack expands beyond the zero start position to beyond the positive range of the sensor and again a flat plateau is recorded as the film lid deflection continues to increase at the end of the hold phase. It can therefore be seen from the first set of data that the packs processed using this retort pressure profile are being stressed both positively and negatively, with total movement of the film lid being in excess of the 18 mm range of the deflection sensor. The two key areas of the process that need adjustment to the pressure profile are the ramp up and start of the hold phase where excessive pressure is being recorded, and the end of the hold phase where the programmed pressure value is insufficient. To reduce the deflection that the film lids are being subjected to, the company changed the retort pressure profile to incorporate a gradual ramp up of pressure for the duration of the process, with a maximum value of 1.0 bar at the end of the hold phase. A second test run was carried out and the data shown in Fig. 7.4 generated. It can be seen that, whilst the film lid is still showing some movement, it is considerably reduced from the first run and, more importantly, that changes are gradual, minimising the stresses on the seals. The range of deflection recorded during the hold phase has now been reduced to 10 mm, or half the amount seen with the original pressure profile. This case study example shows how the use of commercially available datalogging equipment can very easily identify areas of the retort program that are leading to excessive stressing of the packaging. The data generated can be easily interpreted and improvements implemented that reduce the stresses and the volume of reject product that a company may be producing.
7.6
Implementing pressure profiles
In the event of experimental trials producing an amendment to the retort pressure profile, it is important that consideration is given as to whether or not this has an effect on other aspects of the process. One key area that should be reviewed is the performance of the retort system in the form of temperature distribution trials. Within such trials, thermocouples or wireless dataloggers will be positioned throughout a retort load of product to record the environment temperatures within the load and identify any areas of slow heating within the retort system. If the pressure profile is changed, which will in turn change the physical shape of the containers of the retort, there might be an impact on the movement of the retort heating media. This will be a particular issue within systems such as a Lagarde steam/air retort, which utilises a large fan to move a mixture of steam and air through the retort load. Once it has been ascertained that either no change in retort performance has occurred, or that a change has occurred and the variation and position within the retort has been identified, testing should also be carried within the product itself. Heat penetration testing will consider the rate of heating within a container and in turn the microbiological kill that is taking place during the thermal process. For taking measurements within a flexible or semi-rigid container, it is important to ensure that the fittings used for locating either a thermocouple
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Fig. 7.5
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Example of a fitting for data acquisition in a flexible packaging that ensures correct location of the temperature probe within the sample.
or a wireless datalogger within the container do not restrict any movement of that container during the process. This is of particular importance for flexible pouches, where the packaging material itself cannot support the temperature measuring device within the pouch at the cold spot of the product. In such a case, a custom-made fitting such as that produced by Ellab A/S and shown in Fig. 7.5 should be used. The positioning the test heat penetration samples is critical, and ideally these should be at the cold zone identified by the temperature distribution testing. If this is not possible, any variation of processing conditions should be allowed for within modelling software, such as the CCFRA CTemp program or the TechniCAL Calsoft program.
7.7
Future trends
Optimising the processing of flexible containers will be driven by advances and new trends in the packaging sector. As new technologies allow the development of new packaging formats and thinner materials for existing designs, there will be an on-going demand for safe processes to be established; processes that ensure stresses experienced by containers as a result of the thermal process are kept to a minimum. The technology and equipment currently commercially available from companies such as Ellab A/S and TMI Orion offer process technologists the capability and flexibility needed to test the various packaging formats. As with standard temperature datalogging, it could be suggested that the ‘Holy Grail’ of a retortable, wireless, transmitting device would be of benefit for the recording of both pressure and deflection measurements. Whilst developments to attain and produce such a temperature logger continue, it is questionable as to how much actual practical benefit such a pressure or deflection device would offer, with only testing of rotary processes showing clear-cut advantages in eliminating the requirement for a slip ring assembly.
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7.8
In-pack processed foods
Sources of further information and advice
For companies entering into the processing of flexible packaging for the first time, the input of a knowledgeable consultant could be invaluable. For companies in Europe, organisations such as the Campden and Chorleywood Food Research Association offer pilot facilities, along with the equipment and expertise to develop new processes. In addition to pilot facilities, CCFRA offer valuable information in the form of published guidelines and reports on completed research projects (see below for more detail). Datalogging equipment manufacturers such as Ellab A/S and TMI Orion can offer technical support and advice on utilising their individual products for given applications, whilst retort manufacturers or their agents in a particular country can advise on the suitability of their systems for a given process. A valuable source of information for thermal processing as a whole is the Institute for Thermal Processing Specialists (IFTPS), who offer guidance and support to the industry through annual meetings and protocols put together by Institute members, based on their practical experience. Details for all the companies or organisations mentioned in this chapter are as follows: Ellab A/S, Krondalvej 9, DK – 2610 Roedovre, Denmark Tel: +45 44 52 05 00 Website: www.ellab.com Campden and Chorleywood Food Research Association (CCFRA), Station Road, Chipping Campden, Gloucestershire GL55 6LD, UK Tel: +44 1386 842000 Website: www.campden.co.uk TMI Orion, Parc Industriel et Technologique de la Pompignane, Rue de la Vieille Poste, 34055 MONTPELLIER cedex 1, France Tel: +33 4 99 52 67 10 Website: www.tmi-orion.com/index.htm Societe Lagarde, Z.I. Les Plaines - N° 5 bis, 26780 Malataverne, France Tel: +33 4 75 90 58 58 Website: www.lagarde-autoclaves.com Steriflow SAS, 32, Rue de Cambrai, 75019 Paris, France Tel: +33 1 40 37 08 45 Website: www.steriflow.com Steritech/ECPS SA, ZA du Kochersberg, 1, Rue de Furchhausen, 67700 Saverne, France Tel : +33 3 88 71 04 33 Website: www.steritech.fr Institute for Thermal Processing Specialists (IFTPS), 304 Stone Road West, Suite 301, Guelph, ON N1G 4W4, Canada Tel: +1 519 824 6774 Website: www.iftps.org Some useful publications of the Campden and Chorleywood Food Rearch Association are:
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CCFRA Guideline 50 ‘Guidelines on good manufacturing practice for heat processed flexible packaging’ (2006). CCFRA Review 46 ‘Review of the integrity of heat processed containers through manufacturing and distribution’ (2004). CCFRA Guideline 17 ‘Guidelines for establishing heat distribution in batch overpressure retort systems’ (1997).