4. CANNING PROCESSES


4.1 Sardine and Sardine-like Fish

4.1.1 Traditional Mediterranean method
4.1.2 Norwegian method

4.2 Tuna and Tuna-like Fish
4.3 Salmon and Salmon-like Fish
4.4 Crustacea

4.4.1 Crab
4.4.2 Shrimp

4. 5 Molluscs

4.5.1 Abalone

4.6 Fish Pastes and Spreads


In this Chapter are summarized the stages in the production for each of the main commercially produced canned fishery items. Where typical retorting schedules are given, they are intended for guidance and should not be adopted without first having their adequacy confirmed through heat penetration trials conducted under commercial operating conditions, or in laboratories equipped to conduct these determinations.

In addition to the requirement for product safety and shelf stability. canned fish are expected to have sensory properties which are characteristic of the species, and the product must be free of objectionable odours, taints or visual defects. Major product compositional and quality requirements are specified in the set of Codex Standards for Fish and Fishery Products (CAC/ VOL. V -Ed. 1.1981 1/), which include specifications for the following canned products:

Salmon, Canned Pacific Codex Standard 3 - 1981
Shrimps or Prawns, Canned Codex Standard 37 - 1981
Tuna and Bonito. Canned in Water or Oil Codex Standard 70 - 1981
Crab meat, Canned Codex Standard 90 - 1981
Sardines and Sardine type Products Canned Codex Standard 94 - 1981
Mackerel and Jack mackerel Canned Codex Standard 119 - 1981

1/ FAO/WHO, 1981. Codex Alimentarius Commission, Codex standards for fish and fishery products. Codex Alimentarius volume 5. Rome, FAO/WHO, Joint FAO/WHO Food Standards Programme, CAC/VOL. V -Ed. 1:135 p. Issued also in French and Spanish

4.1 Sardine and Sardine-like Fish

Sardines are usually canned by one of two methods; the first is inferred to as the traditional Mediterranean method (so named because of its origin, although nowadays similar technology has been adopted elsewhere and is generally described as the "raw pack method" and the second is a method incorporating a hot smoking step, rather than in can pre-cooking. The latter method is commonly practised in Western European countries.

4.1.1 Traditional Mediterranean method

Either fresh or frozen sardines can be used to produce a good quality canned product provided that the preliminary handling conditions have protected the fish from excessive deterioration during transport and storage. The sequence in which preliminary operations are carried out varies from processor to processor, and may reflect such things as the complexities of the line, the speed of production, the degree of automation, the availability of labour and the Source and type of raw materials. One sequence for a Mediterranean style canning line is as follows: the sardines are weighed and washed and then, brined (by immersion in a saturated solution for up to 15 minutes, depending on size and fat content), graded, nobbed and packed. In an alternative sequence nobbing precedes brining so that the order of the pre-treatment operations becomes; weighing, washing, nobbing, brining (in batches, or continuously in screw conveyors, to a final salt content of between 1 and 2%) packing and washing -although a recent modification of this procedure includes direct addition of salt to filled cans, which means the brining step can be eliminated.

Sardines are fed automatically or manually to the nobbing machines in which the heads, viscera and tails are removed. The machines are set to cut the fish to standard lengths, or into cross-out pieces, so that pack uniformity is achieved. Machines are available which complete the traditional nobbing operations and then pack the fish into cans automatically. however in many cases packing remains a manual operation.

Pre-cooking of the sardines in filled cans is carried out in automatic steam cookers. The first stage is a steamer, operating at around 95 C, through which the cans pass while held inverted on perforated conveyors to allow simultaneous entry of the steam and drainage of condensate and oil exuded from the flesh. In some pre-cookers the cans are steamed in the upright position but inverted and drained before passing to the second stage. The final phase of pre-cooking is a drying process taking place at around 130 C. As an alternative pre-cooking method, some canners fry their sardines, but this is generally more expensive.

Cans containing pre-cooked fish pass to a liquid filling station where one of either brine, water, edible oil, sauce or marinade is added manually or automatically. For those products which have not been brined, the salt is added in solid form prior to the addition of the liquid medium, or it can be blended with the liquid. The cans are then transferred to can sealing machines for double seaming with pre-coded can ends. While adding the liquid, there is usually some overfill which can be recovered. When the outside of the cans are contaminated with oil and/or fish remnants they should be washed in water and detergent before they pass to the retort for sterilization. It is preferable that can washing be completed prior to, rather than after, sterilization so that the risks of post-process contamination are reduced. if post-process washing cannot be avoided, it is essential that it be carried out in hot water of sound microbiological quality and preferably in which there has been included a surface active agent to assist can drying -at all times extreme care should be taken to ensure that processed cans are not manually handled while still wet. Sterilization is usually in batch retorts in which the heating medium is either pure saturated steam, hot water or recirculated hot water which is pumped over the cans. Because of the large surface area, and therefore flexibility. of the traditional club, dingley and hansa style sardine cans the ends are prone to distort as a result of the internal pressure generated during processing. This can be compensated for by processing the cans under an over-pressure; but more importantly cans should be pressure-cooled, at least until the internal can temperatures have fallen. Retorting temperatures and times are selected to suit the desired textural properties and the target Fo value of the process. As a o general guide 1/4 club and dingley cans are processed for between 45 and 60 min at around 115.6 C; although some processors choose to process at 112 C, because they find the bone softening at these temperatures to be preferable to that achieved by a shorter process at a higher temperature.

After cooling the cans are dried in air, packed in individual cartons and then into master cartons. Before release the finished product is held to ripen in order to develop the characteristic flavour and textural properties.

4.1.2 Norwegian method

The major difference between the Norwegian and Mediterranean methods of canned sardine manufacture is that with the former the fish are not eviscerated and are usually hot smoked, whereas the Mediterranean method includes evisceration and pre-cooking. With the Norwegian method evisceration is unnecessary because the catch is held alive for at least 48 hours in nets prior to landing (the holding process is known as thronging) during which time the fish digest their feed and thereby minimize the enzymic activity which if left unchecked would lead to belly-burst, while the smoking process replaces flash pre-cooking. After thronging the fish are transported fresh to the factory for immediate use or for frozen storage. The following description is typical for the traditional Norwegian method of canned sardine manufacture.

If frozen raw material is used the blocks are thawed under running fresh water or sea water. When thawed with sea water. it is often not necessary to brine the fish; however when using fresh water. or when fresh fish are used. the sardines are flumed in a brine solution which washes them, removes scales and enables the fish to absorb from 1 to 2% salt.

The fish are automatically size graded and passed into threading machines where they are fed through a series of parallel' plastic pipes out of which they emerge. one fish at a time. in rows. Metal rods (spits) are threaded through the eyes of the sardines. a row at a time. The spits are hung on frames and the frames are then stacked on trolleys and transported to smoking ovens.

The drying and smoking process takes place in the smoking oven. the temperature of which is set to suit the size and fat content of the fish; typical inlet temperature is between 40 and 60 C, while normal outlet temperature is between 120 and 140 C. The total drying and smoking process takes approximately one hour. The hot air for drying is derived from steam heated heat exchangers, while the smoke is generated by burning oak or other hardwood chips. The removal of moisture in the smoking oven prevents the release of excess water during retorting. and the addition of smoke gives the sardines their characteristic flavour. In some instances canners use artificially flavoured oil to impart the "smoked" taste. however. when this technique is used there must be an accompanying declaration on the label; in these circumstances the other stages of the process are similar to those described in the Mediterranean style production line.

In the Norwegian hot smoking process the fish are smoked, while hanging on the spits, and then passed to rotating knives where their heads are removed with a cut directly under the gillbone. The bodies fall into trays below and are transferred to the filling floor for hand packing into cans. Filled cans are automatically conveyed to an in-line oil filler from which they then pass to a can double seamer where coded ends are applied. Sealed cans are transferred, via a flaming channel, to retort baskets sitting immersed in a tank containing hot water and detergent in which contaminating oil is washed from the outside surface of the containers.

The cans are processed in counterbalanced retorts in which an overpressure is necessary to prevent deformation (caused by the high internal pressure generated in the cans during the thermal process) of the relatively large and flexible ends. Typically for 1/4 dingley cans the total pressure in the system is approximately 122 kPa (18 psig). This means that when processing at 112 C, the pressure due to the steam vapour pressure will be around 52 kPa (7.5 psig), while that due to the air overpressure will be approximately 73 kPa (10.5 psig). Retorting temperatures and times vary, but generally for 1/4 dingley cans, the process is for 60 min at 112 C which is sufficient to deliver a target Fo of > 6 min. The retorts use conventional steam heating or, alternatively, the heating medium can be water which is pre-heated in overhead vessels and then dropped into the retorts below, where it is brought to operating temperature with steam under pressure.

After sterilization, cans are dried in hot air and passed to automatic or manual case-packing for packing into cartons. As with Mediterranean style sardines, it is necessary to hold the finished product to allow it to "ripen" and develop fully its characteristic sensory properties.

4.2 Tuna and Tuna-like Fish

There are several styles of canned tuna described in Codex Standard No 70 (referred to at the beginning of this Chapter); however, apart from minor handling differences arising from variation in the size of the species and the pieces, the relative proportions of light and dark meats, and the styles of liquid fillings, the stages in the canning processes are substantially the same.

The pre-treatment stages include thawing in running water, heading and evisceration of the smaller species (which are usually frozen whole). Larger fishes are headed and gutted on board prior to freezing. Once thawing is complete, or when fresh chilled fish are used, the fish are cut into vertical pieces, or horizontally into loins, and washed and placed on metal trays which are transferred on racks into the atmospheric steam pre-cookers. Pre-cooking is carried out in steam at between 100 and 105 C for as little as one hour for small species, or over eight hours for large specimens. The temperature and time combinations of pre-cooking are often regarded by canners as being critical to their overall yields; generally, the common aim is to raise backbone temperatures to between 60 and 85 C, after which the portions are removed from the cooker and allowed to air cool, often overnight. In climates where ambient temperatures are around 30 C, or more, it may be necessary to assist cooling by placing the fish into chilled storage, so that the flesh will not be held for too long in conditions favourable to contamination or microbial activity. Cooling can also be achieved by water spray in order to hasten the process. After cooling the flesh firms, which makes the subsequent cleaning and picking operations easier for the operators.

If not already done so, the head, tail and fins are removed; the skin is scraped from the flesh surface and the white and dark meat portions picked from the frames and segregated. The edible portions are selected for solid, chunk, flake or grated (shredded) style packs and then transferred to filling areas. In many of those countries where labour costs are relatively low, packing is a manual operation; however, there are machines which perform these tasks fully automatically for all styles of packs.

The filled cans are transferred to brine or oil fillers, or in some cases they first have dry salt added, after which the water, oil or sauce is added. Cans then pass to the can seaming machine where they are closed under vacuum by coded can ends attached in a double seamer. The hermetically sealed cans are manually or automatically loaded into the retort baskets of manually operated batch retorts, or they may be directly conveyed into crateless retorts or hydrostatic retorts for sterilization.

The temperatures and times selected for retorting depend on the container size, the pack weight, filling temperature and the pack style. Generally, while operating under GMP conditions it is sufficient to process to Fo values of around 10 to 15 min; however this there is evidence that some canners select unnecessarily severe conditions which deliver Fo values in excess of 30 min. Apart from being wasteful of time and energy, such severe processes adversely affect the sensory characteristics of the product. As a guide to selecting temperatures and times for processing, a summary of the conditions used commercially for a variety of can sizes is shown in Table 7.

Table 7 Typical retorting conditions for tuna processed at 115.6 and 121.1 C in a variety of can sizes

Can dimensions

Retorting time

Diam.
(mm)

Height
(mm)

115.6 C
(min)

121.1 C
(min)

66

40

65

40

84

46.5

75

55

99

68.5

100

85

154

118.5

230

190

After thermal processing cans are cooled (preferably under pressure, although some manufacturers pressure cool only their larger cans because they are the ones most likely to peak), dried in air or with the assistance of air blowers, and held in "bright-stacks" prior to labelling and packing, or labelled and packed directly off the line.

4.3 Salmon and Salmon-like Fish

Fresh or frozen salmon are transported from storage and graded before passing to the iron chink machine for dressing, an operation which automatically removes the head and tail, splits the belly, and removes the viscera and fins. After butchering, the fish are transferred to the sliming table for the final removal of flesh and blood remnants and for washing. The fish are cut to size for automatic or manual filling into pre-washed cans. Filled cans have salt added and then pass, via a check weighing machine, to a can seamer for vacuum sealing (using coded can ends). Between filling and seaming, cans pass to a "patching" station where operators check for traces of skin, bones or meat lying across the flange of the can; while making adjustments to pack fill weights if required.

Sealed cans are washed, packed into baskets and then loaded into retorts for processing. There should be no more than one hour's delay between container filling and the commencement of the thermal process as longer delays may lead to pre-process (incipient) spoilage. Cooled dry cans are either bright-stacked or labelled directly and then transferred to warehouses for storage.

Retorting conditions will vary depending on can dimensions and pack weights, however, the processing conditions summarized in Table 8 can be used as a guide.

Table 8 Typical retorting conditions for salmon processed at 115.6 and 121.1 C in a variety of can sizes

Can dimensions

Retorting time

Diam.
(mm)

Height
(mm)

115.6 C
(min)

121.1 C
(min)

74

35

50

35

84

46.5

70

55

74

118.5

100

85

154

109.5

230

200

4.4 Crustacea

4.4.1 Crab

The preliminary stages of cooking and picking of crab meat are simple operations, often suited to small scale operators working in relatively unsophisticated conditions, but who are close to the supply of the raw material. It is important that the crabs should be handled under conditions which limit the opportunity for degradative enzymic action leading to the deterioration of their fresh flavour. This means that, ideally, the crustacea should be held wet and cool as soon as they are caught; and that preferably they should be cooked alive, or as soon as possible after death. Cooking can be either in boiling water, or in retorts using steam under pressure. If the cooked meat is not to be butchered and picked immediately, it should be refrigerated or iced.

After cooking, the crabs are washed to remove sand and the scum that adheres to the shell after cooking. The claws and legs are removed from the body which is then eviscerated. It is important that the butchering operation be carried out separately from picking, and that at all times the opportunities for cross contamination are avoided by implementation of hygienic handling practices. Shell particles are removed and then the legs, claws and washed bodies are held ready for picking.

During the picking operation fragments of meat are drawn from the claws, legs and body either by hand or in some cases using automatic equipment. In the latter cased, the shell is crushed and the flesh separated by brine flotation which allows the shells to sink and the meat to float. A disadvantage of mechanical picking is that the flesh tends to become shredded and the structure of lumps is damaged. Pickers separate the flesh from the shell, and segregate it into grades with the best prices being paid for flesh in which the structure of the piece is retained. If there is a delay prior to canning. the picked flesh is stored under refrigeration so as to avoid contamination. because even though the meat has been cooked, it is nevertheless vulnerable to microbial deterioration, particularly in warm climates.

On receipt at the cannery (if the picked flesh has come from elsewhere), or prior to further treatment (if the crabs have been picked on site), the meat should be inspected for uniformity of grade, acceptable odour (off-odours indicating deterioration), satisfactory colour and the absence of shell and other contaminants. Once passed by inspectors, the flesh may be blanched in boiling water- this operation is optional, and is more usual when pre-cooking operations have not totally cooked the flesh. Blanching firms the flesh and protects yields, which otherwise are found to be depressed if pre-cooking is incomplete. Some manufacturers include metabisulphite in the blanching brine, and/or soak the crab prior to pre-cooking -both treatments are used to prevent discoloration. After blanching. the hot meat is cooled in potable water, or in air.

As detailed in the Codex Standard, various pack styles are available. The flesh is packed in lacquered cans to prevent sulphur staining and in some cases parchment is also used. The filled cans are topped up with brine containing 2-3% NaCl, and in some circumstances a 0.1-0.5% citric acid solution to prevent discoloration. Cans are then vacuum sealed and retorted. Typical processing conditions are shown in Table 9.

Table 9 Typical retorting conditions for crab processed at 115.6 and 121.1 C in a variety of can sizes

Can dimensions

Retorting time

Diam.
(mm)

Height
(mm)

115.6 C
(min)

121.1 C
(min)

84

46.5

50

35

84

63.5

60

45

After processing and cooling in chlorinated water, cans are either bright stacked, or labelled and then transferred to the warehouse for storage.

Some canned crab tends to discolour and form a blue/black or grey/black pigment. This reaction has been discussed by Howgate (1984), who pointed out that the mechanism is not clearly understood, partially because the phenomenum appears to be species related. Howgate summarizes three current explanations for this discoloration, which have given rise to three different solutions to the problem:

  1. The blueing is due to the presence in the flesh of copper; the solution is to include in the brine a metal chelating agent such as citric acid, or ethylene diamine tetraacetic acid ( EDTA ).
  2. The grey discoloration results from a variation of the well known Maillard browning reaction which occurs between sugars and amino acids at high temperatures; a partial solution is to lower retort temperatures and increase processing time (e.g., process at. 115.6C rather than at 121.1C), and/or to include sulphur dioxide in the brine.
  3. The discoloration is the result of melanin formation, derived from an enzymically related oxidation of tryosine; the solution is to expose the flesh to a sodium metabisulphite treatment as a dip, or include it as an additive in the blanch water.

While processors may find any or all of these solutions acceptable as a means of controlling or eliminating discoloration, they must first assure themselves that regulators in importing countries permit the inclusion of the additives which overcome the problem.

4.4.2 Shrimp

Raw shrimp should be received refrigerated, or well iced, to limit enzymic and microbial action which, if unchecked, leads to lost of quality. Some canneries receive their shrimp peeled and cooked, however, even under these conditions the need for adequate cool storage cannot be neglected. The shrimp are inspected on receipt at the cannery, and then washed to remove adhering dirt and ice. Peeling can be either manual or by machine. In an example of the latter, the shrimp are size graded, and the head and shell removed by a combination of gentle pressure and a controlled rolling action. The final pre-treatment is to devein the shrimp after which they are washed and reinspected.

Cleaned and shelled shrimp are pre-cooked in hot brine, or steam. The choice of salt concentration and cooking time varies from processor to processor. Generally, salt concentration will range from 3 to 13% NaCl, and pre-cooking time will range from 2 to 10 min; the conditions chosen will be affected by shrimp size, the temperature of the solution and whether the shrimp are to be for a wet pack (i.e., packed with brine) or for a dry pack. Steam pre-cooking is usually carried out at around 95 to 100 C for 8 to 10 min, depending on shrimp size. The shrimp are then cooled, dried, inspected and size graded, prior to hand packing in cans which have been lacquered to resist the formation of unsightly black sulphide stains. Filled cans are topped up with hot brine (to which some processors add citric acid to reduce discoloration) and then sealed. Vacuum in dry pack cans is achieved by either exhausting the cans prior to sealing, or, alternatively, by sealing the cans in a mechanical vacuum closing machine.

Retorting temperatures and times vary according to pack dimensions and style (i.e., dry or wet pack), however, shown in Table 10 are guidelines for a variety of packs.

The difference in the processing times required for wet and dry packs in the same sized containers, arises because of the convection currents in the brine pack increasing the rate of heat transfer to the SHP of these containers.

At the completion of the thermal process, the cans are cooled with chlorinated water and removed from the retorts for either bright stacking or direct labelling and packing after which they are transferred to the warehouse for storage.

Table 10 Typical retorting conditions for dry pack and wet pack shrimp processed at 115.6 and 121.1 C in a variety of can sizes

Can dimensions

Retorting time

Diam.
(mm)

Height
(mm)

Wet pack

Dry pack

115.6 C
(min)

121.1 C
(min)

115.6 C
(min)

121.1 C
(min)

66

101.5

26

14

80

60

84

63.5

27

15

75

50

84

101.5

27

15

75

55

4.5 Molluscs

4.5.1 Abalone

The best quality canned abalone is manufactured from the fresh product, although some canners may use frozen stocks, however, when they do, their yields are decreased and the texture of the finished product tends to be too soft. Fresh abalone is received chilled and is then shucked by hand before the meat is transferred to washing tanks for the removal of the pigment from around the lip. The cleaning is achieved by immersing the abalone in warm (35 - 40 C) water for approximately 30 min, during which time the flesh is gently abraded by rotating the tank holding the brine or by stirring the brine with paddles. Some canners use proteolytic enzymes to assist removal of. the pigment, in which case it is necessary to arrest enzymic activity by dipping the molluscs in a solution of hydrogen peroxide. Cleaning is completed by gently scrubbing the flesh with nail brushes or abrasive pads.

Good quality canned abalone has a creamy/yellow colour, however under some circumstances, not always clearly understood, there is a blue surface discoloration. It believed that the mechanism for this action is related to the formation of a metallic complex, which explains why it can be controlled by the addition of chelating agents such as citric acid and/or EDTA; it can also be controlled by the addition of metabisulphite. These additives may be included at a number of stages in the pre-treatment; such as in the cleaning brine, in the blanch water, or in a dip. They may be added to the canning liquor, provided that the country in which the product is to be sold does not prohibit their inclusion.

After cleaning, the abalone are trimmed (to remove the viscera and gonads) and then blanched for 5 min at 70C, before packing into lacquered 74 x 118.5 mm cans. Most manufacturers pack three to four whole abalone per can (individual abalone weights can range from approximately 90 to 180 g), and make up to minimum pack weight (i.e. , usually 50% of net weight) with portions. The cans are topped up with brine containing approximately 2% NaCl and vacuum sealed with coded can ends. If hot brine is used, it may not be necessary to vacuum close the cans, however, under these circumstances it is important that the cans be pressure cooled in the retort.

During thermal processing there is a textural inversion associated with the softening of the pedal sole and the toughening of the myofibrillar proteins at the base of the adductor muscle. There is also a weight loss, which can account for reductions in yield of between 12 and 30% of fill-in weight under extreme conditions (e.g., with a severe thermal process combined with the use of stale or frozen stock). Given the high selling price of this commodity, manufacturers are therefore keen to avoid overprocessing, without compromising the safety of the product. This means that despite the unavoidable weight losses caused by retorting, canners must still be sure that their minimum target Fo values are>2.8 min.

Choice of thermal processing conditions depend upon total pack weight and the size and weight of the individual abalone, however, in Table 11 are shown typical process conditions suitable for abalone packed in 74 x 118.5 mm cans.

Table 11 Typical process conditions for abalone packed in 74 x 118.5 mm cans

No. of abalone per can

Maximum individual abalone weight(g)

Process time at retort temperature

110 C (min)

113 C (min)

115.6 C (min)

118 C (min)

121.1 C (min)

2

180

93

73

62

55

49

3

20

88

68

59

52

46

4

90

76

57

47

41

35

At the completion of the thermal process the cans are cooled in chlorinated water, removed from the retort and either bright stacked, or labelled directly, then transferred to the warehouse for storage.

4.6 Fish Pastes and Spreads

Fish pastes and spreads are manufactured from by-products or from under-utilized species, for instance those fish which are generally too small for other purposes. They usually require mincing and/or blending with other ingredients such as salt. sauces, spices, fat, emulsifying agents and thickening agents. Often they are sufficiently viscous to be filled as liquids or pastes in automatic filling machines as well as in manually operated devices. In some cases a blended mixture is formed into fish balls or fish cakes which are hand filled and topped up with brine or sauce. The scope for development of these items is large, and they have particular appeal as a relatively cheap, yet nutritious fishery product.

Generally, the products are packed into lacquered cans or glass jars, and sealed with coded ends and caps, respectively. Processing of cans is in conventional retorts, whereas glass jars, retort trays and cylindrical shaped flexible laminate packs require thermal processing in counterbalanced retorts. Some canners produce fish paste and seafood spreads in retortable aluminium trays and pouches. Because of the flat profile of these packages, heat penetration to the SHP of the container is rapid (relatively), which means that heat sensitive products (e.g., scallop and lobster pastes) can be processed without excessive loss of flavour and colour.

Processing temperatures and times depend upon the dimensions of the container and the nature of heat transfer to the SHP of the pack -for pastes this is largely by conduction, while for fish balls in brine it is by a mixture of convection and conduction. Post-process handling procedures are similar to those for other heat sterilized fishery products.