Introduction
What is minced fish?
How a bone separator works
The advantages of mincing
The disadvantages of mincing
Preparation of fish for mincing
Operation of the separator
Keeping quality of mince
Handling and storing mince
Sensory properties of mince
Utilization of mince
This note refers principally to the mince produced when fish flesh is separated from skin and bone in a bone separator. The advantages and disadvantages of mincing are discussed, and advice is given on the preparation of fish for mincing, use of the bone separator, and the handling and storage of mince. The quality of mince is compared with that of other forms of fish, and there is some comment on how mince can be fully utilized.
The most common way of separating edible flesh from waste is by filleting, but a greater amount of flesh can be recovered in the form of a coarse mince by putting either the unfilleted fish, or the waste left after filleting, through a bone separator.
Bone separators working on different principles are available commercially, but the kind most widely used for fish is of comparatively simple design. Fish, or pieces of fish, are fed from a hopper to pass between a moving rubber belt and the outside of a revolving perforated drum of stainless steel. The flesh is forced through the perforations into the drum from where it is expelled as a coarse mince by a fixed screw. Skin and bone are retained on the outside of the drum and removed continuously by a scraper blade. The drum perforations are commonly 5 mm in diameter, but drums with smaller or larger holes are available, which produce mince of different texture. Yield can be increased by increasing the tension on the belt, at the expense of some increase in the degree of fragmentation of the flesh, and in the amounts of bone, pieces of skin and black belly wall lining.
The total yield of flesh of low bone content is higher than with filleting alone; up to twice as much can be recovered by separating flesh directly from headless gutted fish. When the fish are first filleted, an additional 8-12 per cent flesh can be separated from the filleting waste.
Some people do not like fatty fish such as herring and mackerel partly because of the large numbers of small bones remaining in the fillets; mince made from these fish is relatively free from bones and might therefore be more widely acceptable. Flesh from underexploited species such as blue whiting, which are difficult to fillet economically because of their small size or awkward shape, can readily be removed in a bone separator.
Mincing offers an opportunity to exercise greater control over flavour, appearance and keeping quality by the incorporation of additives. Rancidity in fatty fish, for example, can be controlled more easily in minced flesh by intimate mixing with permitted antioxidants, or minces of different fat content can be mixed together to give a more desirable result. Mince can be moulded into different shapes, and lends itself to continuous production methods.
When fish flesh is minced the texture, flavour and sometimes colour are also changed; hence minced fish, and the products derived from it, have at present only limited outlets. Small amounts are used in fish cakes and in less expensive grades of fish finger, and some is used to fill voids in frozen laminated blocks of fillets from which portions and fingers are cut. The present market for mince is small compared with the amount of mince that could be made available from all suitable species.
Mince spoils faster than fillets of the same material, mainly because the structure of the flesh is destroyed during separation, and extra care has to be taken to maintain good quality; in particular, the fish used for making mince has to be initially of very high quality, and processing has to be completed quickly.
Whole white fish such as cod and haddock should be gutted and headed. In addition the section of backbone immediately above the belly cavity should be removed; otherwise blood along the backbone causes discoloration and spoilage of the mince, and large pieces of bone tend to damage the rubber belt of the separator. Fillet trimmings can be fed directly into the separator. Whole fatty fish should be gutted and headed, and then split to make them easier to pass through the separator. The skin should be removed from soft-skinned species before putting them through the separator; otherwise too many fragments of skin will pass through with the mince.
AH fish or trimmings being prepared for separation should be washed to remove adhering debris and drained before being fed into the machine.
Mixed species should not be put through the separator unless it is known that there is no adverse interaction; otherwise there is a risk that enzyme activity may result in chemical changes that give an unacceptable texture.
The separator should be cleaned and cooled before use and at intervals during use; feeding flake ice through the machine while running, followed by washing with a high pressure hose, is a simple and effective cleaning and cooling procedure. If the machine becomes clogged during operation, it should be stripped down, a fairly simple task, and the parts washed thoroughly with a hose.
Regular lubrication is essential; the machines are fitted with grease nipples, and a suitable edible grease should be injected through these every few hours when the machine is in daily use.
Fish can be fed from the hopper into the separator in a random manner, but performance can be improved by putting fish in with the skin side against the belt and the cut surface of the flesh against the drums; skins are removed cleanly, the separator is less likely to clog, and the mince is less likely to contain scales and pieces of skin.
Mince made in a hygienic manner has the same initial quality as the raw material from which it was made; when mince is made from fresh white fish, fillets yield the best mince, followed by fillet trimmings, frames or skeletons from which the fillets have been cut, and backbones only, in that descending order of quality. Mince made from stale fish is poor, no matter what parts of the fish are used.
Mince made from fillets cut from 4-day-old iced cod has an unacceptably high bacterial count after 24 hours storage at 5-10°C. Mince made from fillets cut from 12-day-old cod has a high initial count which changes little during 24 hours storage. Hence mince should be made only from fresh raw material, handled quickly and hygienically, and then frozen.
Mince should be frozen as soon as it is made, or incorporated in products and then frozen within 4-5 hours of manufacture; products should be kept chilled while awaiting freezing.
Mince can be frozen in blocks in cartons in a horizontal plate freezer in the same manner as for laminated fillet blocks.
The storage life of frozen mince made from good quality cod and haddock flesh is at least 6 months at - 30°C, or 3 months at - 20°C, without any significant loss of quality, but some fish, notably hake and Alaska pollack, have been found to have a shorter cold store life. Minces of all species that include a proportion of active tissue from the backbone and gut will also have a reduced cold storage life. Mince washed with water keeps better than unwashed mince, and mince incorporating additives such as sucrose or sorbitol keeps better than mince without additives. Minces made from fatty fish require protection against oxidation in cold storage.
The appearance and texture of mince are different from those of fillets because the flesh is fragmented, but limited tests have shown that the consumer is not unduly deterred by the unfamiliar form of mince when presented in fish fingers provided it is made from fresh raw material; indeed in tasting tests children showed an apparent preference for fish fingers made from mince rather than fillet.
There is some loss of the sweet flavour of fresh fish during mincing and, more seriously, a slight 'cardboardy' flavour can sometimes be detected, which is more usually associated with the cold storage of whole fish. There may also be a slight increase in firmness and dryness.
When blood-rich tissue from beneath the backbone is included among the raw material, the resulting mince will be red; the mince turns brown when cooked, and has an unacceptable and sometimes extremely objectionable metallic flavour.
Bone separators do not remove all pieces of bone; the table shows typical average bone contents of minces made from different parts of fish, using a 5 mm drum.
|
Species |
Part offish |
Weight of bone mg/kg of mince |
|
Cod |
Fillets |
84 |
|
Trimmings |
272 |
|
|
Frames |
1736 |
|
|
Backbones |
4050 |
|
|
Herring |
Split fish |
2610 |
Existing uses are in the manufacture of fish cakes, as a filler in laminated fillet blocks with up to 15 per cent mince, and for the manufacture of cheaper fish fingers and portions.
It is possible to make highly acceptable cook-freeze dishes from mince. Fatty fish mince can be used for a range of flavoured fish finger products by adding a suitable sauce, for example tomato, curry, mustard or cheese. Mince is also suitable for making speciality products like Gefüllte Fish. Mince made experimentally from blue whiting has been found suitable for making surimi and kamaboko, two important Japanese fish products; a valuable UK export trade could possibly emerge to meet this market.
Good quality frozen mince in small blocks could find a retail market for use from home freezers as an ingredient of homemade dishes.
Mince lends itself to considerable modification of texture, flavour and appearance, and to the addition of stabilizers and additives; given imaginative development and marketing there may be good prospects of introducing a wide range of commercial food products from minced fish.
