4.1 Technological Processes
4.2 Processing Operations

4.1 Technological Processes

The main objective of canning is to obtain a shelf-stable product that can be stored in suitable containers for a considerable length of time (at least two years) without undergoing food spoilage, while retaining desirable nutritional and sensory qualities.

To achieve satisfactory shelf-life the following conditions must be observed:

  1. The contents of the cans must be commercially sterilized.
  2. The inside of the can must be resistant to and damaging effects from the contents and the outside must be resistant to corrosion under reasonable storage conditions.
  3. The can ends must be sealed to prevent ingress of water and/or air or any form of contamination.

4.2 Processing Operations

4.2.1 General

Fish should be chill stored at a temperature between 0 and 2 C, or sorted frozen in freezers at temperatures below -28 C.

When frozen fish is used, it must be thawed before grading and/or dressing.

In the following descriptions of the processing stages for various products, operations which are similar are described in Chapter 4.2.2 "Description of canning tuna in brine", and subsequently only mentioned.

4.2.2 Description of canning tuna in brine

The description is related to canning plant with a capacity of 20 tons whole raw fish (bluefin or yellowfin tuna) per 8 hours. The overall yield is approximately 50-55% which gives approximately 10 000 450 g (1 pound) cans or 20 000 225 g (1/2 pound) cans per 8 hours. See Figure 4 "Layout for tuna cannery".

Simplified flow sheets for canning tuna in brine, tuna flakes with vegetables and tuna. pet food are shown in Figures 5, 6 and 7 respectively.

Sequential processing operations for canning tuna are described as follows:

  1. Frozen tuna is thawed, preferably, by means of running water at a temperature of 10-15 C. Loss during thawing is 0.5-1.0%.

    Holding of frozen tuna for long periods before thawing may lead to oxidation of fat resulting in a yellow to orange discolouration on the surface of the cooked loins. Usually this surface discolouration can be removed when the fish is cleaned.

  1. Longitudinal cuts are common with large sized tuna and the viscera are removed from the fish on board fishing vessels prior to freezing. Bonito and skip jack are frozen with viscera. Once thawed, the tuna is washed and inspected for spoilage. If tuna is not eviscerated on board vessels this must be done in the plant. The splitting and evisceration procedure is the only butchering operation performed on the tuna while it is in the raw condition. All other cleaning is performed after the tuna has been cooked. Loss of weight is approximately 24-27%.

Figure 4 Layout for tuna cannery

Figure 5 Flow sheet for canning tuna in brine

Figure 6 Flow sheet for tuna flakes with vegetables

Figure 7 Flow sheet for tuna pet food

  1. The tuna is given a pre-cook by heating at a temperature in range of 102 to 104 C. This operation is necessary to make it possible to hand pick the light meat from the carcass and also to remove some of the oil from oily fish.

    The fish is placed in baskets which are placed on racks. The racks of butchered fish are rolled into the cookers which are usually of rectangular cross section and made of reinforced steel plate with a door, or doors, at one or both ends. The pre-cooking is a batch type operation.

    Steam is admitted through a steam spreader on the floor of the cooker. Steam vent and drain valves are provided to permit removal of air and condensate. Pre-cooking may also be carried out in boiling brine.

    The pre-cooking time for individual batches varies widely according to the size of tuna. For example, the cooking time may vary from 1 1/2 hours for small tuna to 8 to 10 hours, or more, for larger tuna. Loss of weight is approximately 22-26%.

  1. Tuna is cooled thoroughly to firm the flesh before the manual cleaning operation can be performed. Loss of weight is approximately 3-5%.
  1. After the pre-cooking and cooling operations, tuna is individually cleaned. The head is removed and the fish is skinned and split into halves before removing the tail and backbone. The loins are produced by splitting the halves of the fish along the median line. Red meat is then removed from each loin; the blood and dark meat are scraped away and the loins, edible flakes and waste products are separated; of these portions approximately 15% is flake tuna.
  1. The production of solid packs was formerly a hand-packing operation, but is now carried out by machines. This machine produces a cylinder of tuna loins of uniform density from which can be cut can-zised segments of uniform weight.

    Chunk packs are produced from loins which are cut on a moving belt by means of reciprocating cutter blades. The cut loins are then filled into cans by tuna filler machines.

    Flakes and grated tuna, which is produced from broken loins and flakes, are J packed in the same way as chunk packs.

  1. The open cans next pass the line where additives such as salt, vegetables and finally either water or oil are added. Oil should be added slowly over a sufficient stretch of the line to permit its thorough absorbtion by the tuna meat. When oil is not added an equivalent amount of water replaces it. The oil temperature is recommended to be 80 C-90 C.
  1. Small cans may be closed, without a vacuum, and processed directly, whereas larger ones must be vacuum sealed.

    As the pressure in the can increases considerably during heat processing , the vacuum is necessary to minimize the pressure increase in order to reduce the chance of distortion (peaking) and damage to the double seam.

    After processing and cooling the formation of .the vacuum causes the ends of the can to assume a concave profile which is characteristic of vacuum packed and hermetically sealed cans. The vacuum also reduces the residual oxygen content in the can and therefore the extent of internal corrosion.

    In order to form a vacuum, cans are seamed by using either vacuum seamers or an exhaust system.

    When using the exhausting method the lids of the cans are first clipped or clinched on to the body in such a way as to allow free passage of gases and vapours out of the can. The can and contents are then heated by passage through an exhaust box. The lid is seamed to the can immediately it emerges from the exhaust box, so that when the contents cool a vacuum is obtained. Thus the system relies on sealing the can while the contents are hot and allowing product contraction to create the vacuum.

    An alternate method of achieving a vacuum in sealed cans is by using vacuum seamers. These machines close the cans and while so doing draw the air out thus creating a vacuum.

  1. The double seaming method is usually used to seal metal containers. The seam is created in two operations. See Figure 8. "Seaming Operation -Double Seam (CAC/RCP 10-1976) .

The sealing compound renders the seam air tight (hermetic). Around its circumference the double seam consists of five layers of metal -three layers of the can end and two layers of the can body, however at the intersection with the side seam there are seven layers of plate, the extra two being due to side seam overlap.

The seaming operation must be monitored throughout the processing and visual inspections should be carried out at least every 30 minutes (Warne, 1993).

Good manufacturing practice indicates that the. overlap should be at least 45 % of the internal seam lenght to ensure that the seam will function correctly and resist to minor abuse.

Figure 8 Seaming operation -double seam

  1. The sealed cans are transferred by a conveyor through a can washer which cleans the cans in detergent and water before discharging them into retort baskets. The retort baskets are transferred into the retort and the cans sterilized.

Table 7 Examples of retorting temperatures and times for canned tuna

Nominal capacity
of cans


Processing temp.

conditions time

1.8 kg (4 pound)







450 g {l pound)







225 g (l/2 pound )







112 g (l/4 pound)







All canned fish products are sterilized at temperatures above 100 C. Sterilization takes place in retorts, with or without water. Overpressure is between 2-3 kg/cm. Processing conditions shown are suitable for those canneries , operating under conditions of good manufacturing practice. Individual canneries may select different processing times and/or temperatures to suit their manufacturing requirements.

The simplest and most common retorts today are horizontal, or vertical, batch retorts.

The following general description. applies to processing in batch retorts using saturated steam as the heating medium.

After the retort is loaded the door or lid is closed and the seal is checked to confirm that all the lugs are fastened securely. The temperature recorder is checked to ensure that it is working correctly. Following this the vents and bleeders are opened and the drain and overflow are closed {unless the over flow is used for venting).

The retort is now ready for operation during which the following operational procedures should be adopted:

  1. When processing medium sized or larger cans (say greater than 250 g) in retorts using steam it may be necessary to cool the cans under pressure so that the ends do not peak during cooling. Steam may be used to maintain the pressure but compressed air is more usual. The cooling time depends on the processing temperature, the temperature of the water used for cooling, the can size and the nature of the pack (i.e., liquid to solid ratio).
  1. If necessary the cans should be washed before temporary storage, however under no conditions should the processed cans be manually handled while wet.

4.2.3 Description of the processing operations of an automatic canning line for skipjack

The descriptions are related to a plant with a capacity of 10 tons whole fish per 8 hours. The overall yield is 50-55% and approximately 9 000-10 000 450 g (1 pound) cans are produced per 8 hours. See Figure 9 "Layout por skipjack cannery". The weight of the fish to be processed is approximately 2 to 5 kg per fish.

  1. From the chill room the whole fresh skipjack is brought to a bulk elevator which transports the fish to a gutting machine which consists of a semi-automatic machine in which the fish is placed with its belly upermost. A rotating knife opens the fish and removes the viscera.

    The fish is conveyed to a rotating nylon brush under which it is cleaned by water spray nozzles.

    After evisceration the tuna is conveyed to an inspection table where the final cleaning is completed manually; or where those fish, too large for the gutting machine, can be butchered. At the discharge of the gutting machine the offal is directed to a rotating filter drum for separations of water; the retained offal, is pneumatically pumped to containers.

Figure 9 Layout for skipjack cannery

  1. Inspectors examine the fish, while it passes on the conveyor, to ensure that is has been satisfactorily cleaned.
  1. Fish are automatically size-graded to assist in feed to the rotor cooker and regulation of cooking time.
  1. Rotory cookers are frequently built with counter flow and discharge belts, and a capacity of approximately 1.3 tons per hour with variable cooking times of up to 2 hours.

    The cooker is divided into 16 cells into which the fish is fed. The bottom of the cooker is separated by a heavy mesh plate, and under this are placed heat exchangers for indirect steam heating.

    After cooking the fish are transported on a continuous belt to the counter-flow cooler (a stainless vessel with freshwater supply) and then on to the cleaning stations.

  1. Typical cleaning arrangements consist of:

The operations are as follows:

  1. The loins then pass to an automatic scale for continuous weighing which makes it possible to monitor production yields. After weighing the loins pass to an I accumulation station prior to passing to a pack-shaper which packs the loins into cans.

The filled cans pass to an oil/brine filling machine before the vacuum seaming machine.

Description of the subsequent operations and their effects on yield are similar to those described for canning tuna in brine.

4.2.4 Description of canning sardines in oil

The method for canning sardines in oil is often called the raw packaging method which contrasts with the method in which the sardines are thermally treated before packing into cans.

The processing plant related to the description has a capacity of 15 tons raw fish per 8 hours. The overall yields is approximately 50% of the weight of raw fish. See Figure 10 "Layout for sardines in oil, cannery" which shows a plant of this capacity.

Figure 10 Layout for sardines in oil cannery

One ton of raw sardines is required to produce 5 200 (5 263) 1/4 club cans, packed into 52 cases of 100 cans each or 5 800 (5 882) 1/4 dingley cans packed into 58 cases of 100 cans each.

Figure 11 shows "Flow sheet for canning sardines in oil".

The operations after the thawing of the frozen fish or after transporting the fresh fish from the chill store are as follows:

  1. The sardines are size-graded by an automatic grading machine which selects sardines to suit the size of cans used in the plant.

    The number of sardines per can permitted for various can types is given in Table 8

Table 8. Number of sardines per can permitted* for various can types

Type of can

Number of sardines



1/4 dingley 1 layer

6 sardines

12 sardines


6 sardines

12 sardines

1/16 oblong 2 layers

8 sardines

14 sardines

1/4 club 1 layer

4 sardines

8 sardines

* Permitted by ?

The smallest size of sardines allowed for canning (in Norway) is 9.5 cm.

  1. After grading, the sardines are discharged onto a conveyor for transportation to the nobbing machine where the head and guts of the fish are removed. On a typical nobbing machine, the fish are placed on continuous belts with one fish in each compartment. The belt feeds the fish to a cutting wheel which cuts the head, and together with the guts, draws it away from the body with rollers. If necessary the tails are cut in the same operation.

The packers examine the fish to ensure complete removal of guts. Loss of weight during nobbing is approximately 21%.

Nowadays there are also nobbing machines with greater capacity, served by four or more operators, which can complete following operations:

The automatic nobbing/packing machine has a capacity of 35-80 cans per minute and requires approximately 50% of the manpower required by a traditional canning line (Perovic, 1983).

Figure 11 Flow sheet for canning sardines in oil

  1. After nobbing the sardines pass through a washing process to remove blood and surface slime. Washing machines of many types may be used. Washing should be with potable water or sea water of similar quality. (For certain species with hard and inedible scales, such as pilchard, special washing machines suitable for removing scales are used).
  1. The sardines are then transported, usually by a conveyor, to a brining machine. The brining can either be a batch or a continuous operation. The speed of the process and the concentration of the brine can be controlled. The fish are loaded into the brining unit in one end where they are immersed in concentrated salt solution, and then screwed through the unit and simultaneously brined. The fish are then discharged at the opposite end of the unit.

    Salt is absorbed by the flesh; a salt content of about 1-2% of fish weight, when in boxes is acceptable. The brining machine is equipped with a filter to screen particles from the brine.

    In addition to giving the product the desired salt content, brining has other beneficial effects. The process brightens the appearance of the fish by removing remaining slime and also toughens the skin; when unbrined fish are canned much of the skin adheres to the can.

    After brining, the fish are allowed to drain before being transported to the packing table.

    The brine must be prepared from salt which does not contain large quantities of magnesium chloride, a common contaminant in unrefined salt. The brine should be replaced regularly as otherwise it becomes a source of contamination.

  1. After the brining process the sardines are packed (manually or automatically) into pre-washed cans.

    A manually operated packing line is equipped with conveyor belts, the speed of which can be altered in order to ensure a smooth supply of sardines and cans to the packers. The filled cans and waste are removed by the conveyor after packing.

    In common sardine lines the filled cans pass over a weight control unit to a can aligner and then to a can pusher, which automatically feeds the conveyor transferring them to the pre-cooker.

    Change of yield during the packing process depends on the quality of raw material, but on average 5% loss of weight is expected.

  1. A typical flash cooker for sardines consists of 2 sections, one for cooking (in steam at about 95 C) and one for drying (in hot air at about 130 C). The cans with the open end upwards are automatically transferred to crates, which are hanging on crossbars mounted on a continuous chain conveyor running in slings up awards into the cooking section, and into slings downwards in the drying section.

    Between the two sections water is drained from cans as the crates are turned through 360 around a tumbler. The heat in the cooking section is obtained from supply of direct steam while the heat in the drying section is indirectly supplied from a heat exchanger. A fan located on the top of the section circulates the heated air. Weight loss during this operation is approximately 25 %.

  1. From the cooker the cans are automatically transferred onto a conveyor which takes then to the oil dispenser and onto the seaming machine where they are sealed.
  1. Before the retorting process the cans are washed to remove fish residues and oil from the outside of the containers.
  1. Further operations as for canning tuna in brine. See chapter 4.2.2. .

Table 9 Examples of retorting temperatures and time for canning sardines

Can type

Temp. (C)

Retorting time (min)

1/4 club



1/4 dingley



4.2.5 Description of canning pre-smoked sardines in tomato sauce or oil

Most of the operations in this process are similar to those described for canning sardines in oil.

The description is related to a plant capacity of 5 tons of raw fish per 8 hours.

The yield from canning of sardines and other pre-smoked small fishes approximate the yields obtained when canning sardines in oil.

See Figure 12 "Flow sheet for canning pre-smoked small fish in tomato sauce and oil", and Figure 13 "Layout for pre-smoked small fish cannery" which shows a plant of the capacity mentioned above.

The various operations are described below.

  1. Grading is similar to the operation described for canine sardines in oil.
  1. Brining is similar to the operation described for canning sardines in oil.
  1. The fish are rodded, i.e, hung on rods through the eyes. This operation can be done automatically or manually.

    The automatic rodding being a complicated process is only practised in large plants. The rodded fish are placed into frames which are fitted to the smoking racks.

    The frames have usually accommodate for 30 rods, each with 30 fish.

  1. The fish are smoked for approximately 1 hour at temperatures up to l30 C. The main purposes of the smoking is to enhance flavour and to lower the water content of the fish.

    Usually tunnel type smoking kilns are used in which there is a controlled temperature gradient between 30 and 125-130C.

    The air passes through a heat exchanger while smoke added. The smoke is produced by a smoke generator using sawdust from hard woods (oak or similar). Loss during smoking is approximately 25%.

  1. After smoking the bodies of the fish are separated from the heads with an automatic cutting machine. The heads are then removed from the rods with an automatic rod stripping machine. Loss of weight during the deheading operation is approximately 10-12%.
  1. The packing, the filling of sauce or oil and the double seaming operations are similar to those described for canning sardines in oil.

    When preparing tomato sauce, puree of good quality must be used and to this is added olive oil or fish oil, etc.

    Typical mixtures comprise one part oil and two parts tomato puree (20% concentration).

  1. All subsequent operations are similar to those described for canning sardines in oil.

4.2.6 Description of canning fish paste products

The description is related to a fish paste canning plant with a capacity of l0 tons whole raw fish per 8 hours. The production is based on either deheaded, gutted fish with an average weight of 1.5 kg or smaller industrial fish. See Figure 14 "Layout for fish paste products cannery" which shows a plant of the capacity mentioned above.

Overall yield when manufacturing canned fish paste products depends on species involved, however, approximately 45-65% of raw fish weight could be used for planning purposes.

The output from a plant producing meat balls is approximately 25 000/850 ml cans per 8 hours; alternatively 8 000/850 ml cans of fish cakes or 15 000/850 ml cans of meat balls could be produced.

Figure 12 Flow sheet for canning pre-smoked small fish in tomato sauce and oil

The flow sheet for canning fish paste products is shown in Figure 15.

The various processing operations are summarised as follows.

The raw material has to go through a grinding/chopping and mincing process.

  1. The fish are washed, descaled when necessary, eviscerated and deheaded.
  1. According to skin thickness and size, the fish will have to be split into halves from head to tail along the backbone.
  1. The split and opened fish are washed and cleaned.
  1. The washed split fish is separated in a meat-bone separator. According to the type of fish one will get an acceptable minced product, and offal, bones and skin.
  1. If the mince is unacceptable, (i.e. ,discoloured because of blood etc. ), it may be necessary to have it washed and then dewatered by means of a screwpress
  1. The minced fish is mixed in a bowl chopper or mincer with the other ingredients. it is important to add the salt first and to allow the mincer to work for a few minutes before adding the remaining ingredients to the mixture.

    This will improve the binding properties of the fish. See Figure 16 "Processing line for fish meat".

Figure 13 Layout for pre-smoked small fish cannery.

Figure 14 Layout for fish paste products cannery.

Figure 15 Flow sheet for canning fish paste product.

  1. Starch, spices and, if necessary, onions, fat, etc., are added.
  1. Liquid milk is added and according to the type of mincer, it will take 10-12 min for the materials to reach a soft and formable conditions.
  1. The fish balls can be formed in machines, by hand with spoon, or by hands only, and dropped into a 1 1/2% solutions of salt water at approximately 90 C.
  1. After cooking in the salt solution for approximately 5-8 min the fish balls are filled into cans.
  1. Hot brine or fish bouillon is added.
  1. After seaming the product goes immediately to retorting.

Figure 16 Processing line for fish meat

Figure 17 Layout for salmon cannery

Several alternatives for retorting temperatures and times are shown in Table 10.

Table 10 Examples of retorting temperatures and times for canned fish paste products


Initial product temp. (C)



temp. (C)


temp. (C)


Fish meat balls in brine/bouillon  
850 ml (99 x 119 mm) cans  













425 ml (99 x 63 mm) cans  













Fish cakes in brine/bouillon  
850 ml (99 x 119 mm) cans  







450 ml (99 x 63 mm), cans          







Further descriptions are as for canning tuna in brine.

4.2.7 Description of canning salmon

The description is related to a processing plant with a capacity of 8 tons of raw fish per 8 hours; this capacity has been based on using whole fish weighing 4 kg each. See Figure 17 "layout for salmon cannery".

The overall yield when canning salmon is between 55 to 60 %, depending on the size of the fish. With a yield of 55%, 1 ton of raw salmon will give 3 400
(3 437) 225 g (1/2 pound) cans.

The processing operations are described below. See Figure 18 "Flow sheet for canning salmon"

  1. The fish are transported to a sorting table and sorted by species.
  1. The salmon are eviscerated with an iron chink machine which removes the head, fins and viscera from the salmon (see Figure 19). The loss of weight is approximately 35-40 %
  1. The eviscerated, deheaded fish is next conveyed to the sliming table. Sliming consists of removing fins, viscera and blood, etc. not removed by the buchering machine. During the sliming operation the fish is throughly washed. The loss of weight is approximately 3-5 %.

Figure 18. Flow sheet for canning salmon.

  1. From the sliming table the cleaned fish is conveyed to a fish cutter fitted with revolving sknifes which cut the fish into slices. The slices are then .fed into an automatic filling machine. The loss of weight is approximately 0.5-1.0 %.
  1. The filling machine fills the cans with the salmon after which salt is added.
  1. The filled cans pass through an automatic scale which sorts for under Weight cans.
  1. From the scale the cans pass into a vacuum seaming machine.
  1. The filled and sealed cans are then discharged from the seaming machine through a can washer.
  1. After seaming the product goes immediately to retorting. Retorting conditions of approximately 115 C for 100 min are used for 450 ml cans.
  1. Further descriptions are as for canning tuna in brine.

4.2.8 Description of canning shrimps

Headless, peeled shrimps are graded according to number of shrimps per 100 g of drained product. See Table 11 (Codex Stan 37-1981).


Figure 19 Iron chink for dressing salmon

Table 11 Number of shrimp per 100 g of drained product

Size Designation



Extra large or jumbo

less than 12.3

less than 13.4


12.3 to 17.7 inclusive

19.4 to 19.1 inclusive


more than 19.1 but not more than 31.8

more than 19.1 but not more than 34.6


more than 31.8 but not more than 60

more than 34.6 but not more than 65.3

Tiny or minuscular

more than 60.0

more than 65.3

Figure 20 Layout for shrimp cannery

It is usual to can only those shrimp which fall in the medium to very small size range.

The description is based on a processing plant capacity of 3.6 tons raw shrimp per 8 hours. This corresponds to production of 12 000-14 000 cans, each containing 75 g shrimp, in 8 hours.

The overall yield from canning shrimp is between 25 to 30% of raw weight. See Figure 20 "Layout for shrimp cannery " which shows a plant of the capacity mentioned above. The description is based on using iced raw shrimp as the raw material. The shrimps are usually graded before delivery to the cannery. See Figure 21 "Flow sheet for canning shrimps" .

The operations in the canning process are summarized below:

  1. On arrival at the cannery, ice is removed before washing the shrimps.
  1. The shrimps are throughly washed while being conveyed into the plant.
  1. After washing the shrimps are inspected and foreign debris removed.
  1. The shrimps are then conveyed to a peeling machine, which separates the shells and heads from the body. This operation can also be done manually. Aproximately 46% of the total weight is lost during this process.
  1. After peeling, the shrimps pass through to a waste separator, which removes fragments left after peeling.
  1. The cleaned shrimps are loaded into baskets and placed into a boiling salt solution for cooking. Average salt concentration in the solution is 11-13%. The cooking time varies from 6-10 minutes depending on the size of the shrimps.
  1. After cooking the shrimps are inspected and any broken ones and/or pieces are removed. The shrimps are then dried by fans.
  1. The shrimps are weighed and hand-filled into cans. The weight of the shrimps . should be 64% of the content of the can. Aluminium cans are preferably used for shrimp canning, however if tin plate cans are used, layers of parchment paper should be placed between the shrimps and the can to prevent corrosion. Hot or cold brine is filled into the cans.

Table 12 Example of mixture for brine

Salt 2 kg
Sugar 1 kg
Citric acid 0.9 l
Water 100 l
  1. The cans are automatically closed and transferred to retorts for processing.

Figure 21 Flow sheet for canning shrimp

Table 13 Examples of retorting temperatures and times for canning shrimp


Preheating Conditions

Processing Conditions














  1. Further operations are as for canning tuna in brine