Defrosting
If frozen fish are to be processed, they must first be defrosted. The thawing process involves the elevation of temperature of the fish to 0 °C, at which temperature it may be manually cut or otherwise prepared as necessary.
Butchering
The splitting and evisceration process is normally the only butchering operation performed on the tuna while it is in the raw condition. This operation involves the scaling, evisceration, removal of parts not to be canned, and thorough washing in either salt or fresh water. The scaling may be done by hand with serrated-edge scrapers, the edges of knives or other means devised by the technician.
Salmon is traditionally filled into cans in the raw state. After grading, fish are fed into an iron chink butchering machine for the automatic removal of head, fins and viscera.
Pre-cooking
Sardines, mackerels and tuna are normally pre-cooked prior to sterilization. The object of pre-cooking, particularly in the case of tuna, is to facilitate the removal of flesh from the carcass.
Typically, sardines are packed into quarter club cans, which are placed in square plastic trays. The trays may hold some 4 x 7 cans each. A thin sheet of metal is placed over the tray and the whole inverted and placed on racks on a trolley which is then wheeled into a batch pre-cooker.
Functions of pre-cooking:
• Firm texture of fish
• Reduction in bacterial load
• Inhibition of enzymatic reaction
• Shrinkage for better filling
• Removal of cellular gases
The time for pre-cooking in steam at a temperature of 105 °C is about 30 minutes depending on the size of the fish.
Can filling
Before being canned, the cans and jars are rinsed with water under pressure of steam before being filled.
Filling may be accomplished either by hand or by machine. Can filling in large tuna factories is undertaken by high-speed machinery. The tuna is formed into a tube within the machine and sliced to the correct height and fill weight on presentation to the can. Smaller fish and meat scraps are generally canned as flakes after being pressed into moulds.
Sardines are traditionally filled manually into cans prior to pre-cooking, but automatic machines are also available.
Mackerel are processed in both rectangular and cylindrical cans and the filling operation may take place manually or by suitable machinery according to the size of the operation.
Covering liquid, oil, brine or sauce is added to fish within shallow cans to overflowing. The excess liquid is screened and returned to the filling head.
Fish and the liquid added to the can should be packed tightly to prevent it from shaking around and braking up into undesirable looking pieces, requires the fish to be packed in an orderly fashion.
The weight is checked at this point to prevent excessive filling that could lead to the foodstuff not being processed appropriately, swelling of the tin or even bursting of the seams.
The can may then be closed using steam flow to remove air and provide a vacuum within the can. Then the can is sealed. The sealed cans are cleaned to eliminate any remarks of filter and they are prepared for sterilization.
Heat Processing
Fish products and other low-acid foods are usually processed in steam at temperatures of 115° to 120°F. This operation is usually carried out in heavy metal airtight receptacles (retort) that will stand internal pressures of 20 and 30 pounds or more to the square inch.
Can coding
Coding may be applied to the ends of cans by use of embossing machines, or more frequently nowadays, by the use of ink jet machines.
Processing of canned fish
