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Image Embedded Canned tuna IS NOT "raw". It is cooked.
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Published: 17 years ago
This is a reply to # 784,572

Canned tuna IS NOT "raw". It is cooked.

Canned tuna production details.


5.2 Machines for Canning Tuna

5.2.1 Pre-cookers

The most common pre-cookers are live-steam cookers. fitted with condensate drains. vents and safety valves. The pre-cookers operate on a batch system. with doors at each end (so that fish may be rolled in and out on a flow-through basis). The fish are loaded into galvanized iron baskets. and the baskets are placed on racks which are rolled into the cookers for steaming.

Other preparatory stages taking place before filling are completed manually. and in many canneries. filling is also a manual operation. There are. however. fully automatic filling machines suitable for packing tuna in all pack styles in round and oval cans.

Figure 28 Side elevation of continuous flash cooker for pre-cooking sardines; (diagram Courtesy of Trio Mask in Industry A/S)

5.2.2 Filling machines

Machines are available for filling chunk and grated (shredded) tuna which operate at speeds of between 80 and 350 cpm with cans ranging from 112 to 445 9 (approx.). There are a number of manufacturers with various operating procedures, but one manufacturer (Carruthers Equipment Co., USA) has several machines for automatic tuna filling. In one machine (the Pack-Former) fish is discharged into filler bowls from where it is transferred into a series of piston pockets positioned around the circumference of the machine. As the filling heads complete a revolution, the fish is compressed into a cylindrically shaped slug in the pocket, in which form it is pushed out the bottom of the piston and is trimmed to the correct length, so that the weight of the pack in each can is controlled. The fish is then fed into the can which has been located below. A machine (a Carruthers Nu-Pak) operates on a similar principle, at speeds ranging from 200 to 600 cpm, with 225 g cans (and smaller).

Solid style tuna loins are packed fully automatically by a machine (a Carruthers Pak.-Shaper) which handles cans ranging in size from 112 g to 1.8 kg (approx.) at speeds from 30 to 130 cpm. The machines are fed with solid loins which are transferred to a forming hoop in which the flesh is molded into the desired shape and then cut off cleanly to produce segments of the required length (and therefore weight).

Equipment used for the remainder of the tuna canning process is described in the following section.

5.3 General Fish Processing Machinery

5.3.1 Brining machines

Brining machines are sometimes coupled with washing machines, so that the two operations occur simultaneously. In continuous applications, the machine is usually a rotating perforated drum partially immersed in a brine bath and through which the fish pass at a predetermined rate. In less sophisticated operations. brining can be a batch process in which the fish are loaded into perforated drums which rotate and, because of the tumbling action, gently transport the fish through the salt solution. Whether using automatic, semi-automatic or batch equipment, it is important that, the salt concentration be maintained at the desired level -this means that periodically the effects of gradual dilution must be monitored and salt added. The material used for construction of the equipment must resist the corrosive effects of the salt.

5.3.2 Exhaust boxes

The exhaust box is used to heat the contents of cans, so that they may be sealed hot, thus ensuring that, after cooling, a vacuum has formed in the container. Exhausting also drives entrapped air from the pack. Exhaust boxes may i take many shapes and forms, depending on the requirements of the cannery; basically they consist of a tunnel through which the open and filled cans pass while being exposed to atmospheric steam. They require a feed and a discharge mechanism, and a conveying system for transporting the cans from one end to the other. Recent models are frequently constructed with stainless steel, however many canneries still find painted mild steel systems adequate.

5.3.3 Sealing machines

When selecting can sealing machines. fish canners must consider the following factors:

  • the size and shape of the container,
  • the anticipated speed and volume of production,
  • the level of skill required to maintain the machine in good working order.
  • the cost and availability of spare parts, and
  • the ease of "changeover" when the machine settings have to be converted to accommodate cans of more than one shape and/or size.

In order to cater for the diverse requirements of fish canners, there is a wide range of machines from which manufacturers can choose a model to suit their operations. Since many sealing machines have features in common, the following is confined to a general description of the major categories which are readily available.

The simplest of machines are required by those packers who run their lines at speeds of from 8 to 25 cpm using hand operated or semi-automatic single-head equipment with motorized drives. For those with a low output (i.e., < 20 cpm), hand operated models are ideal -as with seasonal production or in those plants which are required to prepare test packs.

Single head seaming machines may be fitted with steam-flow closing or mechanical evacuation apparatus as a replacement for, or as an adjunct to, hot filling or exhausting. When mechanical vacuum closing is required the operator places the container (with the can end sitting in place on top of the can) in a chamber, which is then closed and evacuated by opening a line leading to a vacuum pump. When the desired vacuum is obtained in the chamber, the sealing operation is initiated by depressing a foot pedal which lifts the can up to the chuck on the sealing head and into position for double seam rolling. The first and second action rollers are sequentially brought into action while the can is rotated by the spinning seaming head. At the completion of the seaming operation the sealing chamber is opened to the atmosphere and the hermetically sealed container is removed. Machines of the type described can frequently have the facility for steam flow closing, in which case steam is injected across the headspace of the container (while it is positioned in the sealing chamber) immediately prior to double seaming.

Fully automatic in-line single-head steam flow closing machines which operate in the range of 70-90 cpm are available; while for canneries operating at higher speeds there is a variety of multiple-head machines from which to choose. Of the latter, three, four and six spindle machines are common and can be selected to cover seaming speeds of from 200 to 600 cpm, depending on can sizes and production capacity.

Machines for sealing glass containers generally do not operate at the speeds of can closing equipment, however, they can be fitted for steam-flow closing or mechanical evacuation. Fully automatic steam flow closing machines are available to apply caps at around 400 to 500 cpm (depending on container size), while semi- automatic machines can be operated at around 15 cpm. As with cans, vacuums in glass jars may be also obtained by hot filling, or by addition of hot brine, or by exhausting.

Laminated packaging materials are sealed by the fusion of the two facing layers of the innermost ply. The material is heated while clamped between jaws of the sealing machine for sufficient time for the two layers (usually polyethylene or polypropylene) to fuse and form an hermetic seal. One of the greatest difficulties faced by users of laminated packaging materials is that of ensuring effective seal formation. Under all circumstances the sealing surface must be clean and free of particulate matter, which can present difficulties when packing fish products, as it is not always possible to prevent flakes of flesh from contaminating the sealing surfaces. The solution to the problem is to clean the seal area before passing the package to the sealing machine, however, this further retards what is in many cases an already slow sealing operation.

5.3.4 Retorting systems

For a detailed description of recommended retorts and retort fittings reference should be made to the following publications:

  • FAO/WHO. 1977. Codex Alimentarius Commission, Recommended international code of practice for canned fish. Rome, FAO/WHO, Joint FAO/WHO Food Standards Programme, CAC/RCP 10-1976: 42 p. Issued also in French and Arabic
  • FAO/WHO. 1983. Codex Alimentarius Commission. Recommended international code of practice for low-acid and acidified low-acid canned foods. Rome, FAO/WHO. Joint FAO/WHO Food Standards Programme, CAC/RCP 23-1979: 50 p.

The main types of retorts used in the manufacture of low-acid canned foods include the following:

  1. Batch retorts heated with saturated steam. These may be either vertical or horizontal and are by far the most common retorts used by fish canners. Simplified drawings of these types of retorts are shown in Figures 29 and 30; in Figure 31 is shown a less frequently used batch system for processing cans in saturated steam. The latter system is referred to as a crates. Brief descriptions of these systems are found in sections 3.6.1, 5.3.5 and 5.3.6.
  1. Batch retorts heated with water under pressure. These retorts are vertical or horizontal and are most frequently used for processing glass containers which cannot be processed in pure steam because of the risks of thermal shock breakage. They are also widely used for sterilization of products packed in aluminium cans with score-line easy open ends. Simplified drawings of these types of retorts are shown in Figures 32 and 33; operational guidelines are given in section 3.6.2 and features of the system are described in section 5.3.7.

Figure 29 Controls and fittings for a vertical batch retort for processing in saturated steam and pressure cooling

Figure 30 Controls and fittings for a horizontal batch retort for processing 1n saturated steam and pressure cooling (For code to symbols see Figure 29)

Figure 31 Crateless retort -operating sequence (Courtesy of FMC Corporation)

  1. Continuous retorts (other than hydrostatic retorts). Containers are passed through a mechanical inlet port into a pressurized chamber containing steam where they are processed before passing through an outlet port and. depending on the make of the retort. into either another pressurized shell. or an open water reservoir. for cooling. The motion of the cans through the retort causes some forced agitation which aids the rate of heat transfer to the SHP of the container.
  1. Hydrostatic retorts. A simplified drawing of this type of retort is shown in Figure 34 and the system is described in section 5.3.8.
  1. Retorts heated by a mixture of steam and air. The containers are processed under pressure in a system which relies on forced circulation (by a fan or a blower) for the continuous mixing of the steam with the air. Inadequate mixing can result in the formation of cold spots which could lead to under-processing spoilage. As with water filled retorts. this system is suitable for retortable pouches which require a counterbalancing overpressure to prevent their rupture.

Code to symbols:

A Water line
B Steam line
C Temperature control
D Overflow line
El Drain line
E2 Screens
F Check valves
J Petcocks
L Steam spreader
M Temperature control probe
N Reference thermometer
O Water spreader
P Safety valve
Q Vent
R Pressure gauge
T Pressure control
U Air line
V To pressure control instrument
W To temperature control instrument
X Wing nuts
Y1 Crate support
Y2 Crate guides
Z1 Constant flow orifice used during come-up
Z2 Constant flow orifice valve used during cook

Figure 32 Vertical retort for processing glass containers

There is a comparatively rarely used retorting system whereby sterilization is achieved by directly heating cans with flames from gas burners positioned underneath containers which spin past on guide rails. This system is suitable for packs which contain a high proportion of liquid, thus permitting rapid transfer of heat by convection, but it is not used commercially in fish canning operations.

The most frequently used style of retort found in commercial fish canneries today, is the static batch system for processing cans in saturated steam. A description of the fittings for these retorts is given in the following section; however, many of the other retorting systems referred to above are similar with respect to fittings and methods of operation. The most significant difference between static retorts and continuous systems, is that the latter must have container transfer mechanisms to regulate the movement of cans at a predetermined rate through the heating and cooling sections.

Code to symbols:

A Water line
B Steam line
C Temperature control
D Overflow line .
E Drain line
I El Screens
F Check valves
G Line from hot water storage
H Suction line and manifold
I Circulating pump
J Petcocks
K Recirculating line
L Steam spreader
M Temperature control probe
N Reference thermometer
O Water spreader
P Safety valve
Q Vent
R Pressure gauge
S Inlet air control
T Pressure control
U Air lines
V To pressure control instrument
W To temperature control instrument
Z Constant flow orifice valve

Figure 33 Horizontal retort for processing glass containers

Figure 34 Hydrostatic retort (Courtesy of Churchill Livingstone)


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