3. UNIT OPERATIONS


3.1 Raw Material Handling
3.2 Pre-treatment
3.3 Pre-cooking
3.4 Filling
3.5 Sealing
3.6 Retorting

3.6.1 Retort operating procedures for cans
3.6.2 Retort operating procedures for glass

3.7 Post-process Handling

3.7.1 Chlorination and cooling water quality
3.7.2 Post-process hygiene and sanitation

3.8 Final Operations

3.8.1 Container damage during handling and storage
3.8.2 Rate of cooling
3.8.3 Temperature of storage


Despite the wide range of canned fishery products that is available, there are relatively few Operations which are unique to one manufacturing process. For instance, the correct pre-process handling techniques and refrigerated storage conditions of all fish for canning have much in common. (In fact. there will be very little difference in the handling methods for fish destined for canning, and the handling of the same species destined for refrigerated and/or frozen food chains). Similarly, with can seaming, with retort operating procedures, and with post-process handling of containers, the methods adopted are !independent of the type of the product. The purpose of retorting (i.e., to achieve a shelf-stable and safe product by the application of lethal heat) remains the same for all canned fishery products despite there being considerable differences in. say, the severity of processes for abalone and salmon. So too with post-process handling of containers; since the dangers that arise from mishandling canned shrimp are not significantly different from those arising from mishandling canned mussels, or any canned fishery product. it is understandable that there are common guidelines which discourage manual handling of all processed wet containers, and recommend that all retort cooling water be chlorinated. This generalized view of the unit operations in fish canning is simplistic, but nevertheless, it provides a framework for identifying those points in the process where control systems can be implemented to ensure that critical stages of production are effectively monitored. The following is therefore a summary of the major operations in fish canning - a more detailed discussion of the production of each of the main commercial canned fishery items is in Chapter 4.

3.1 Raw Material Handling

There is a direct, and unavoidable, relationship linking raw material quality and end product quality, and this holds as much for the production of canned fish as it does for fish which is bought fresh and prepared in the home. Because handling conditions immediately after catching are responsible for the rapid loss of the "as-fresh" quality, the quality of canned fish suffers whenever the raw material is temperature abused and/or physically damaged between catching and thermal processing. This means that the quality criteria considered desirable by cannery management when they assess their raw materials, ought be the same as those chosen by consumers when they purchase fresh fish. This is not to overlook the fact that, in many cases, fish for canning can be trimmed to remove bruises and other localized flesh defects; however, the provision of trimming operations does not justify the use of fish which has reached an advanced stage of spoilage resulting from poor post-harvest handling and/or storage. Thus, the handling techniques that are recommended for refrigerated and frozen storage of fish apply equally well to fish that is to be canned. As the quality of fish deteriorates from the moment of death, all that can be hoped for by good handling is to retard the rate at which undesirable, quality degrading, changes occur. Techniques which are recommended for the rapid inhibition of temperature related spoilage in freshly caught fish for canning include:

Irrespective of which of the above techniques is adopted, the attn of cold or frozen storage prior to canning is to ensure that fish are received in a condition enabling manufacture of a commercial quality product displaying the desirable sensory attributes which are characteristic of the canned species.

For greater detail regarding handling of fish see the following publications:

FAO Fisheries Circular Nos.:

Other factors important in the handling of the raw material include observation of hygienic practices, to avoid excessive contamination with, and proliferation of, spoilage microorganisms, and elimination of rodents, insects, birds or other vermin. Safe guards to control cross-contamination can be particularly important in warm climatic zones where ambient temperatures often are above 30C, and therefore favourable for the rapid growth and multiplication of bacteria. This means that those canneries allowing their frozen stock to thaw while exposed on the factory floor during the day, and often overnight, do so to the detriment of end-product; quality, and under extern circumstances at the risk of pre-process and/or under-processing spoilage.

Ideally canners will receive fish of uniform and good quality so that the finished product is of a constant standard, however as this is not always possible, it. is often necessary to grade fish prior to canning. Grading systems may be for size and/or any of the sensory attributes which reflect fish freshness and ultimately end-product quality.

3.2 Pre-treatment

Pre-treatment covers the range of operations during which the product. is prepared for canning. Examples of pre-treatment, include, gutting, washing, nobbing, filleting, shucking, shelling (peeling), cutting, brining and dipping. Each of these steps has the common objective of bringing the raw material closer to the size, form or composition required for retorting. Given the advances made with mechanization in fish handling, most of these operations call be carried out using semi-automatic: pr automatic equipment. While mechanization usually means greater production speeds, common advantages of manual operations include, higher yields and greater versatility, plus a greater opportunity for continuous in-process inspection procedures. The benefits accruing from manual operations must be weighed against the costs of labour. In developed countries, where labour costs are relatively high.. there is a tendency to use machines rather than rely on manual operations; but in developing countries, because labour is comparatively cheap, there is a greater dependence on a large labour force.

Each of the pre-treatments listed previously is referred to, in the context of the canning process of which it forms part, in Chapter 4. First, however, some general introductory comments.

All of the pre-treatments (particularly those in which flesh is cut.), ought be carried out under conditions of good manufacturing practice; which means that the rudimentary steps of process hygiene should be implemented. Satisfactory control of contamination from operating surfaces, from viscera or from raw materials, is achievable with regular cleaning (i.e., by washing the product and cleaning the line and ancillary equipment) and/or by limiting the duration of exposure at temperatures suitable for growth of spoilage microorganisms.

3.3 Pre-cooking

Pre-cooking is usually carried out in steam, water, oil, hot air or smoke, or a combination of these. It serves a number of related functions:

As pre-cooking conditions affect yield and sensory quality it is important that they be regulated. An excessive treatment tends to reduce yields, whereas inadequate pre-cooking means that the purpose of the treatment is not achieved. Pre-cooking conditions are usually established through pilot trials in which centre temperatures of the product at the completion of a "satisfactory" process are measured, or alternatively, the time (at pre-cooking temperature) required to bring about the desired effect is determined.

Pre-cooking can be combined with a dipping process, particularly for products which require additives to impart flavour or colour, or in order to modify texture through the surface action of brines. Dips may be a source of contamination, and if so, their quality should be monitored so that they can be changed when necessary.

Construction of a time-temperature plot for the product as it moves down the processing line is a simple technique for highlighting those potential danger areas, where delays in production can adversely affect the microbiological status of the product. At stages where the combination of temperature and time favour rapid microbial growth (e.g., immediately after pre-cooking before the product has cooled), process control points can be established and monitoring systems set up, so that the manufacturer can take corrective action should product quality appear to be at risk.

In some cases (e.g., when steaming tuna, when blanching abalone, or when boiling crabs prior to picking) pre-cooking will precede packing the product into containers for subsequent sealing and retorting. There are also processes in which the product is Picked into cans prior to pre-cooking. An example of the latter is in the manufacture of Mediterranean style canned sardines, which are packed and then heated in two-stage flash cookers (the fish are steamed and then dried in a continuous operation). Cans are drained of condensate and drip, filled with oil or sauce and then sealed and retorted.

3.4 Filling

Whether filling operations are manual or automatic it is most important that fill weights, and fill temperatures for hot fill products, are monitored because both affect the rate of heat transfer to the SHP of the can during retorting. In processes which go beyond the minimum botulinum cook (Fo = 2.8 min) variations in fill weight and/or temperature are not likely to have public health significance; however in processes where target Fo values are recognized as close to the minimum for safety from botulism (e.g. , Fo = 2.8 to 3.0 min), even small variations in fill temperature or fill weight can have significant effects on the adequacy of the process. Because filling can be critical to product safety, it is imperative that it be carried out under strict control.

Apart from the need for containers to appear full, headspace is necessary so that thermal expansion, caused by heating the product from filling temperature to processing temperature, does not result in an excessive build-up of pressure and damage to the hermetic seal. Under normal circumstances seams withstand the strains generated by internal pressure, however, in extreme cases this causes permanent deformation (known as peaking. or buckling) of the can end. Peaking is unacceptable, as it carries with it an associated risk that the seam in the vicinity of the damage will leak and permit ingress of contaminants, particularly during cooling when the cans draw a vacuum. (Excessive pressure build up in glass containers during thermal progressing will usually dislodge the cap.)

Since peaking is a consequence of excessive internal pressure in the can, it can be prevented by controlling a number of factors other than headspace, these include:

3.5 Sealing

Techniques for formation and the evaluation of the hermetic seals with metal cans, glass containers and laminated systems are described in Chapter 2. Central to the success of the entire fish canning industry is the ability of canners to form hermetically sealed containers whether they be made of metal, glass or laminates of plastic and/or plastic and foil. Failure in this critical operation will mean that product safety and shelf stability is at risk. Given the potentially serious implications of seal failure and post-process contamination, manufacturers must be sure that their operations are strictly monitored at regular intervals throughout the entire production. Once sealing machines have been adjusted, suitably trained personnel must confirm their: satisfactory performance by examination of sealed containers. There is ah abundance of literature available from packaging material and sealing machine suppliers recommending methods of seal formation and criteria for their evaluation. In several countries regulatory authorities have published procedures for the evaluation of seal adequacy; the purpose of this is to ensure that not only local manufacturers have guidelines to follow, but also so that foreign manufacturers can comply with the requirements of the country to which they are exporting.

Since formation of sound hermetic seals is critical, it is essential that records confirming compliance with GMP guidelines are completed during production, and maintained after release of the product. If in the event of a product recall there are no permanent records, manufacturers run the considerable risk of being unable to demonstrate that their operations were in control, and that due care was taken to assure the safety of the finished product.

It is important that sealed containers be indelibly coded with details of the production date and time, product codes, the manufacturing plant. and any other information that is necessary to identify the origin and nature of the product.

3.6 Retorting

Procedures for developing and controlling delivery of thermal process schedules are outlined in Chapter 1, and descriptions of available retorting systems are in Chapter 5. However, no matter how well the scheduled process has been formulated, nor how great the capital expenditure for buying top quality equipment, these efforts will be wasted if there is human error in delivery of the process.

3.6.1 Retort operating procedures for Cans

In order to reduce the risks of operational errors, it is customary to adopt standard procedures for retort operation. In some countries the regulatory authorities require that supervisors of retort operations in those plants manufacturing low-acid canned foods shall have successfully completed a specialized training course in the principles of thermal process control. One of the objectives of these courses is to provide retort supervisors with standard operating procedures which will reduce the risk of error being made through ignorance or carelessness.

As a guide, and as a means of standardising procedures, it is recommended that retort operations be classified according to the five sequential steps shown in Table 6. Also shown is a checklist of key points for each stage in the retorting procedure. The five stages of retorting are intended to apply specifically for processing cans (loaded in retort baskets) in steam and pressure cooling in conventional retorts; the sequence will need to be adapted if processing glass (see sections 3.6.2 and 5.3.7) or if processing cans in crateless retorts (see section 5.3.6). As discussed previously, GMP regulations require that the retort operator must record on the retort log sheet (see Figure 5) all processing details for each batch processed.

Table 6 The five stages of retorting a/ and key point checklist

Stage

Checklist

1. Preparation and loading: Is the retort drained?
Are all containers removed?
Are air and water injects closed?
Are cans loaded and the process commenced within one hour of filling?
Are heat Sensitive indicators attached to retort baskets?
2. Venting: Is all the air removed?
Does indicating thermometer register retort temperature of > 103 C?
3. Come-up: Is it > 10 min for fully laden retort?
4. Processing: Is retort at scheduled operating; temperature for the scheduled process time?
Is process timing, commenced when retort reaches operating temperature?
If there are any deviations from the scheduled process are containers from the batch isolated?
Is there agreement between scheduled process time and thermograph record of process tittle?
Are bleeders open during the process?
Is condensate drain open and operating?
5. Cooling: Is steam removed from retort before cooling water enters?
Does the cooling water fill the retort within 10 min?
Is the retort pressure cooled to prevent cans peaking?
Is the pressure cooling controlled to prevent panelling?
Is the cooling water of suitable micro-biological quality? b/
Is cooling water chlorinated so that there is a detectable level of free available chlorine at the completion of cooling?
Are cans rapidly cooled to centre temperatures > 40 C?
Are there procedures to preclude manual handing; of wet containers?

a/ These guidelines are based on the operation of a static batch retort in which heating is with saturated steam and cooling is with an over-riding air pressure

b/ As a guide, suitable retort cooling water win have no detectable coliforms in 100 ml samples taken monthly, and have a total aerobic colony count of < 100 organisms/ml for samples taken weekly

3.6.2 Retort operating procedures for glass

Although many of the key points identified in Table 6 will apply equally to processing cans and to processing glass in water, it is important to make clear the distinction between the two systems. The two features about retorting glass that make the operation different from that for cans are the use of water as the heating medium and the need for over pressure. It is common practice when using vertical retorts to lower the baskets into pre-heated water. Pre-heating the water reduces the time required to bring the entire system up to operating temperature, and it also prevents thermal shock breakage that could follow if hot filled jars were immersed in cold water. The temperature of the water must be strictly controlled so that it does not exceed that of the product, otherwise the partial vacuum holding the cap in place may be lost, or sufficiently reduced for the seal to loosen or vent: if struck. Another reason for controlling the water temperature is that if permitted to fall, it will cause a drop in the initial product temperature and possibly lead to underprocessing.

In horizontal retorts, because the baskets cannot be added directly into pre-heated water, it is necessary to load the retorts while empty and then add the water. If possible the water should be pre-heated so that it: is added at approximately the same temperature as the product.

Overpressure is required to hold the caps in place during processing and in the early stages of cooling so that the total pressure in the vessel always exceeds that inside the container. Although the most commonly used technique to generate overpressure is to introduce air through the steam spreaders and/or into the headspace above the water level, some systems rely on steam which is added through an independently controlled steam supply feeding through the top of the retort. Two advantages in using air overpressure are that when entering through the steam spreaders it assists agitation and helps maintain uniform temperature, and secondly, it helps reduce the knocking that often occurs when adding steam to cool water. Without overpressure the pressure generated inside the container, by heating the contents, would eventually cause the seal to vent or the cap to be displaced. The overpressure required is affected by a number of inter-related factors; these are the headspace in the container, the product fill temperature, the vacuum at the time of sealing and the temperature of processing. In most cases it is sufficient to have between 70 and 105 kPa overpressure. This means that when sterilizing in water at 115.6 C, the total pressure in the retort will be that due to the steam (i.e., 68-70 kPa) plus an additional 70 to 105 kPa for the overpressure; whereas when the retort temperature is 121.1 C the total pressure in the retort will be that due to the steam which heats the water to 121.1 C (i.e., 103-105 kPa) plus a further 70 to 105 kPa for the air overpressure. Shown in Figure 22 is a simplified drawing showing the relationship between the pressure in glass jars and that in the retort when processing with a counterbalanced system while in Figure 23 can be seen the pressure relationship that would arise if glass jars were processed in a standard (i.e. , non-counterbalanced) system. Use of excessive overpressure with large diameter caps can cause panelling, and for this reason it is advisable to gradually reduce the air pressure in the retort during cooling.

It is important that the water level In the retorts be maintained above the top layer of containers throughout the process. Should the level fall, so that jars become exposed to the air/vapour cushion in the top of the retort, there is a serious risk that they will receive an inadequate thermal process. In order to prevent this, sight glasses should be installed to indicate that the water level is held at not less than 10 cm above the top layer of jars.

Figure 22 Counter balanced retorting system for processing glass containers in water; with air overpressure retort pressure (P1) exceeds pressure in container (P2) an closure remains in place

Figure 23 Standard retorting system; when pressure in the retort (P1) due to steam alone is less than pressure in the glass container (P2) the closure is displaced

After closing the retort. air and steam are introduced through the steam spreaders. During the come-up time the air supply should be at a higher level than it is during processing. Once processing temperature is reached the air supply is cut back. however. at all times it must be sufficient to maintain water circulation and a uniform temperature distribution. as well as the desired overpressure. In horizontal retorts it is necessary to include a recirculating pump to achieve adequate heat distribution throughout the entire heating phase and to provide uniform cooling. Failure to reduce the air supply during the processing stage will cause unnecessary vibration.

Once process time has elapsed the steam is turned off and chlorinated cooling water is introduced. Air pressure is maintained until the product has cooled sufficiently for a vacuum to be drawn in the container, after which it is gradually reduced as cooling proceeds.

3.7 Post-process Handling

Delivery of the thermal process schedule must be strictly controlled to avoid under-processing spoilage; however, no matter how severe the process, product safety will be compromised if there is post-process leaker spoilage. There are several contributory factors leading to post-process leaker spoilage; these include the following:

It is considered that even when can seam attributes comply with GMP guidelines for double seam formation, there is a shall number of cans which "breathe" or leak after seaming. Some estimates put this figure as high as 1% of all cans sealed. The generally sound record of the fish canning industry suggests that, if this estimate is correct, only a fraction of those cans which leak ever spoil; this implies that either "micro-leakage" does not (necessarily) result in contamination, or not all contaminants are able to grow in the environment in the can. While this may be reassuring. there are no grounds for complacency. In 1978 and 1982 post-process leaker contamination by C botulinum type E was held responsible for the death of three people who contracted botulism after eating commercially canned salmon.

3.7.1 Chlorination and cooling water quality

As product temperatures fall during cooling, there is a corresponding fall in the internal pressures in caps; and when the product temperature falls below the fill temperature a vacuum forms. This means that the pressure differential across the ends of cans undergoing the final stages of pressure cooling, will favour the entry of cooling water into those cans in which there are seal imperfections. It is prudent, therefore. to accept the possibility of there being micro-leakage through the double seams of some cans (or glass closure seals, or the seals on laminated pouches) and that when this occurs cooling water will mix with sterile product. On the few occasions that post-process leaker contamination does occur, it is important that the cooling water be of sound microbiological quality, for otherwise there is an unacceptably high probability of spoilage. It is because of the risks of post-process leaker spoilage that fish canners use sanitizing agents to control contamination levels in retort cooling water. Of those available, the most widely used are elemental chlorine and chlorine based compounds, however, other sanitizing agents include elemental iodine, iodine compounds and iodophors (a combination of iodine and a solubilizing compound which aids the controlled release of free iodine into the cooling water).

It cannot be assumed that sanitizers will be totally effective in eliminating contamination by viable vegetative bacteria and their spores; rather it is better to regard their action as being one which reduces the probability of survival to acceptable levels. Chlorine, for example is most effective against vegetative bacteria, less so against Clostridium spores and least of all against Bacillus spores. This is why the most likely contaminants in chlorinated cooling water are expected to be spores belonging to the genus Bacillus.

Chlorine may be added as gaseous chlorine (Cl2} which hydrolyses to form hydrochloric acid (HCl) and hypochlorous acid (HOCl, the agent which is responsible for the destruction of vegetative bacteria and spores). Hypochlorites may also be used for chlorination of cooling water, the most usual forms being as liquid sodium hypochlorite (NaOCl) or solid calcium hypochlorite (Ca[OCl]2). Irrespective of which form of chlorine is used. it is important to allow for the reactions that take place with inorganic and organic impurities in the water. When chlorine is added to commercial quality water, it first combines with these impurities (e.g.. minerals and nitrogen containing organic compounds) to form chloro-derivatives which lack the germicidal properties of free chlorine. As the dose is increased these are oxidized, at which point the chlorine demand of the water is said to be satisfied and the "break-point" reached. The chlorine residual remaining after break-point chlorination is called the "total residual chlorine". Total residual chlorine comprises the chloramines and chloro-nitrogen compounds ( i.e. the "combined residual chlorine" which exists below the break point) plus "free available chlorine" (i.e. the free chlorine or loosely combined chloro-nitrogen compounds which exist above the break-point). Once the break-point has been reached the addition of more chlorine will lead to a proportional increase in the free available chlorine.

At the normal pH of cooling water free available chlorine is a more effective bactericide than combined residual chlorine. It is usual to dose cooling water so that free available chlorine remains detectable after a contact time of 20 min. Excessive chlorination of cannery cooling waters is wasteful and it also should be avoided because chlorine is corrosive to some metals. The lethal effect of chlorination increases at low pH (at levels where undissociated hypochlorous acid predominates), at high temperature and with high levels of free available chlorine. There are practical constraints as to how low the pH can be, given that normal cannery cooling water is in the pH range of 6.5 - 8.5. Another constraint is that at high temperatures chlorine loses solubility and is driven off; elevated temperatures also make rapid cooling of cans difficult. High levels of organic matter increase chlorine demand, and, like inorganic impurities, they also protect bacterial contaminants.

Under GMP conditions it is sufficient to maintain residual free available chlorine levels of 2-4 mg/L after a 20 min contact time in order to be confident of holding total aerobic counts at less than 100 organisms/mL of cooling water. Free available chlorine should be still detectable in the cooling water at the completion of the cooling cycle. At all times records of free available chlorine levels should be maintained to provide confirmation that cooling water chlorination procedures were adequate.

3.7.2 Post-process hygiene and sanitation

It is known that when conveyors and can handling equipment down the line from the retort are unclean. they harbour high numbers of contaminants which can contribute to the incidence of post-process leaker spoilage. These basic hygiene problems can often be compounded because when cans pass from the retorts they are still warm, and this means that the plastisol lining compound in their ends will not have had sufficient time to "set up" and form a seal that is resilient to impact and deformation. Also at this stage the vacuum in the can will have partially developed, so that contaminants on and around the double seam are liable to be drawn into the container should the seal leak, even momentarily. Because of this, it is important to clean and regularly sanitize all those surfaces which come into contact with containers.

Conveyor guide rails, twist conveyors, transfer plates, elevators, push bars and accumulation tables should all be made of impervious materials which can be cleaned easily, thoroughly and regularly. In order that the containers are dry during post-process handling, it is good practice to include in the line, close to where the cans are unloaded from the retort, air blow-driers (or similar equipment). These systems are preferable to the inappropriate plastic curtains which are all too frequently installed to drag over the surface of cans as they are conveyed underneath. The longer the cans remain wet, the greater the opportunity for post-process leaker contamination. For this reason containers should be dried as quickly as possible, so that exposure to wet post-retorting conveying and handling equipment is at a minimum. In line with GMP guidelines conveyors or equipment surfaces should be effectively cleaned every 24 h, as well as being disinfected during production, if they are wet while in use. Container drying may be accelerated by dipping the retort crates containing the cans into hot water containing a wetting agent. It is sufficient to submerge the crates for approximately 15 sec, and after they are removed from the bath they should be tilted to allow any adhering fluid to drain from the surface of the cans. If this procedure is adopted it is important that the dip tank be held at > 80 C and that the water be changed regularly to avoid microbial build-up. Use of porous labeller pads and drive belts is discouraged, as these materials can provide an excellent environment for the accumulation and multiplication of microbial contaminants, particularly when they are wet, dirty and irregularly cleaned and sanitized.

When adhering to GMP guidelines and while implementing adequate post-process hygiene and sanitation procedures, manufacturers should comply with the following guidelines for bacterial counts on container contact surfaces and in the water entrapped in can double seams:

3.8 Final Operations

3.8.1 Container damage during handling and storage

Poor: quality cooling water and/or inadequate hygiene and sanitation will increase the risks of post-process spoilage if containers are subjected to rough handling, particularly when this results in damage to the seal area. While unloading retort baskets extreme care should be taken to avoid mechanical damage to hermetic seals, and, because of the risk of contamination from operators, wet containers should never be unloaded manually. Conveyor systems in which line- pressure prevents easy removal of cans by hand need readjustment or re-design to improve flow. Severe can to can impact leading to damage at the end of twist conveyors is indicative of poor line design which provides an opportunity for contamination, because at the moment of collision the can compound is frequently still warm and soft, and the seams wet.

Not all manufacturers find it appropriate to install semi-automatic or fully automatic equipment and so rely instead on manual handling to complete final operations. While this is often an attractive proposition, as it can be the cheapest. and most versatile mode of operation, it carries with it the heightened risk of post-process cross-contamination from operators and/or their protective clothing when metal cans, glass jars or laminated pouches are mis-handled. Therefore, wherever manual procedures are adopted, manufacturers must be sure that containers are dry and that operators handle them carefully.

Operators must be discouraged from using processed cans, whether packed in cartons or loose in "bright stacks", for other purposes; such as for bench supports, or for seats, or for racks on which to dry wet protective aprons and gloves. The reason for this concern is that in the fish canning plant there are assumed to be food poisoning spoilage organisms which could grow and render the product a threat to public health, if they are able to gain entry into the processed container and contaminate the contents. The potential danger of post-process contamination can be comprehended when it is recalled that the last three botulism outbreaks involving canned fishery products manufactured in the United States (i.e., tuna in 1963 and salmon in 1978 and 1982) are all alleged to have occurred because sterilized containers were contaminated with C. botulinum type E. The 1963 case was believed to be the result of faulty double seam formation in the canner's end; the 1978 outbreak was attributed to seam damage followed by corrosion leading to a small hole in the seaming panel; and the 1982 outbreak was attributed to an indexing fault caused by a malfunction of a can reforming machine. In each incident spoilage through post-process leakage and contamination (by C. botulinum type E, or its spores) was implicated, rather than under-processing spoilage because the microorganism responsible was:

The circumstances surrounding these outbreaks highlight the difficulties faced by all fish canners who, because of the origin of their major raw material, cannot avoid operating under conditions in which contamination by C. botulinum type E must be assumed to be the norm. Although a worst-case scenario such as this is extremely cautious, it confirms the need for extreme care when handling processed containers.

Frequent jamming of conveyors and container handling equipment; indicates a need to re-appraise the machinery, the line design or the speed of operation because the potential risk arising from damage to the hermetic seal are untenable. Problems arising from poor handling are not confined to metal cans they do not fracture like glass, or puncture like retort pouches. However, because they are robust and because it is easy to overlook apparently superficial damage, cans are not always handled with appropriate care. Considering that in some countries post-process leaker contamination has been estimated to account for between 40 and 60% of canned food spoilage it is clear that the problems of post-process container damage ought not be underestimated.

3.8.2 Rate of cooling

Cans should be rapidly cooled to 40 C in retorts, otherwise they may remain at thermophilic incubation temperatures during labelling, packing into cartons, palletizing and storage. When rapid cooling in water is not possible, some manufacturers choose instead to air cool their product; should this option be favoured, care must be taken to ensure that there is unrestricted air circulation around the cans. A further problem associated with inadequate cooling is stackburn which results from the over-cooking that occurs when product is stored while still hot.

3.8.3 Temperature of storage

Selection of storage temperatures for canned produce may be critical for those products containing thermophilic spore-forming survivors. Target Fo values are generally more than sufficient to kill mesophilic spore-forming contaminants provided that raw materials are of reasonable microbiological quality. However, because ambient temperatures in warm climatic zones often encourage the growth and multiplication of thermophiles, processes must be either sufficient to reduce, even these extremely heat resistant bacteria, to a satisfactorily low level (e.g., < 1 in 10), or storage must be at temperatures unfavourable for their growth.

In addition to the concerns about storage temperatures, it is recommended that canned fishery products be stored under conditions which avoid sweating caused by extreme temperature fluctuations, as this phenomenum will encourage external rusting of the containers, particularly in areas of high humidity. These conditions are to be avoided also where containers are packed in retail cartons or outer shipper cartons as these will absorb moisture and may even collapse in the warehouse.