Miscellaneous products: crustaceans
The crustaceans comprise a large group of animals all of which
have an external skeleton composed of chitin; in some species, the skeleton is
largely calcified. The crustaceans used for food are almost all from the order
Decopoda, which means literally ten-legged, and include the prawns or shrimps;
crabs; true lobsters; rock lobsters or crawfish and crayfish.
The group is more primitive than the true fish since they lack
the backbone of vertebrates. However, like the true fish, they obtain oxygen by
means of gills and, if these are kept moist, the larger animals can be kept
alive for quite long periods out of water; all are easily kept alive in well
oxygenated water.
Feeding habits vary but many feed on other animals and some,
e.g., crabs, have large claws which are used for foraging or capture of prey.
Others feed on detritus or dead plant or animal matter. The food is ground up in
the gastric mill and then passes through the intestine. In many species, the
faecal waste is passed out as pellets. Some species, e.g., the penaeid prawns
grow rapidly and reach maturity in less than a year; others, such as rock
lobsters (crawfish), grow more slowly and reach maturity at the age of 4 or 5
years.
The parts which are eaten include the muscles (flesh), which
provide the white meat; the liver, which largely forms the brown meat; and the
eggs (roe). The gills are always thrown away in the larger species.
All types of crustaceans may be sold whole, sometimes alive,
other times freshly killed or cooked. All may be used to make frozen or canned
products; some are dried after cooking; others are used to make fermented
products. The shells are usually discarded but may be used to make a meal for
animal feeding or chitosan. Meal made from whole small shrimp or the heads of
larger animals is used for fish feeding, especially salmonids, e.g., rainbow
trout, to produce the pink colour in the flesh which is desired by consumers.
Crabs
Most species are cannibalistic and scavengers. Most of them are
quite easily kept alive out of water (for example mangrove crabs); others can be
kept alive only in water, so live boxes must be used. All species spoil rapidly
once dead, so they should be kept alive if possible until they are landed. Where
this is impossible, they should be chilled but in this case they must be sold
and eaten very soon after capture. In Europe, crabs are invariably boiled
immediately before sale. They are killed first to prevent the claws and legs
dropping off and then boiled in water containing 2 - 3 per cent salt for 20 - 30
minutes depending on size. Boilers should be fitted with a thermometer and a
timer. After cooking, the animals should be cooled to set the meat.
Cooked crabs are sold either whole, or 'picked', i.e., the meat
is removed. The yield is variable and can be up to 30 per cent of the total
weight but only a third of this is white meat. Yield depends on the species, the
size of the animal and the season. For example, animals which have just spawned
yield very little meat.
It is possible to use machines for meat removal. These normally
operate using the principle of centrifugal force to remove the flesh. The yield
is then rather low, however, and the machines are expensive to buy.
Freezing crabs
Crabs or their meat should always be cooked prior to freezing.
If crabs are frozen raw, the meat becomes very watery and the yield is very low.
Crabs can be frozen whole in air blast freezers; alternatively, the meat can be
frozen in waxed cartons or consumer packs, or in blocks for caterers and
manufacturers. The blocks must be glazed to prevent drying and then wrapped.
Freezing must be rapid so that the temperature is reduced from 0 to - 5°C
in less than 2 hours and the material must be kept in the freezer until the
warmest part is at - 20°C. Storage should be at - 30°C; at this
temperature, whole crabs can be kept for 6 months and meat for 4 months. At -
23°C, whole crabs can be kept for 3 months and meat for only 2 months.
Canning crabs
It is normally only the white meat that is canned. The meat is
washed and dipped in a weak acid to prevent the blue discoloration which
otherwise occurs. The dip may be made using 2 oz of glacial (28 per cent) acetic
acid to one gallon of water, or 70 parts of 9 per cent salt brine and 30 parts
of 1 per cent citric acid. The meat is then packed in parchment paper lined cans
or in cans which have a special lacquer lining.
Prawns (shrimps)
Prawns have usually been feeding actively when caught and the
organs in the head contain large quantities of very active enzymes. Bacterial
counts on tropical prawns are often high (104 to 10(6)) and dead prawns spoil
rapidly unless chilled properly. Even when well chilled immediately after
catching, prawns start to lose their delicate flavour after 2 - 4 days. In 5 - 8
days, black spot or melanosis usually occurs in tropical species. This is due to
the production of the black pigment melanin by the action of the enzyme
tyrosinase on tyrosine. In itself, black spot is harmless but it spoils the
appearance of the prawn and indicates that spoilage has started.
Good handling practice in boats used for chilling prawns
1. The trip length should not exceed 5 days after catching
starts.
2. The trawling time should be short in order to prevent
crushing in the net.
3. The net should be towed at the surface for a short time to
wash off any mud and clean the catch; this should not be done for too long
because the surface water is relatively warm.
4. On hauling the net on board, the prawns should be sorted from
the by-catch at once. They should be protected from the sun and wind and handled
carefully.
5. After sorting, they should be washed carefully in clean sea
water.
6. In all the traditional fisheries in which very high quality
material is produced, the prawns are headed at sea, that is, the heads are
removed from the bodies and only the tails are stored. This is because (a)
heading removes a major source of enzymes; (b) in some places, it is known that
the bacterial load on the prawn is heavier in the head; (c) both bulk and weight
are reduced so that there is less material to chill.
7. The material should be cooled quickly. This requires that
plenty of ice in very small pieces should be used, the prawns being stowed in
shallow layers. Where boxes are used, it is important that these should not be
over-filled so that the prawns are crushed when one box is stood on another. The
boxes should be labelled as to species, and each day's catch should be kept
separate. The boxes should be stowed in insulated holds. It is important that
only clean ice should be used.
8. If chilled or refrigerated sea water is used, the water must
be at ice temperature.
Good practice in boats used for freezing prawns
1. Follow points 1 - 6 above.
2. Quick freezing must be practised. It is better to freeze in
blocks rather than to individually quick freeze (IQF) the prawns; this causes
less damage, there is less drying and less storage space is needed.
3. The prawns should be placed in a cold store at - 30°C.
Control of black spot in prawns
Some control is possible by using solutions of sodium
metabisulphite or ascorbic acid. Control is effected either by dipping in a 1.25
per cent solution of sodium metabisulphate for 1 minute before icing or by
dipping in 1 per cent ascorbic acid. In either case, it is most important that
the dip should be kept at the right strength. When using metabisulphite, it is
also important that the dipping time should be kept to 1 minute; shorter times
are ineffective and longer times produce the discoloration.
Unloading and handling prawns ashore
Chilled material
1. The material should be unloaded as quickly as possible.
2. The material must not be washed in dock water to remove ice
nor must it be left lying around on the ground.
3. The material must be kept chilled.
Preparation for freezing
The details of suitable plant construction will be discussed in
another lecture because many of the details are similar for prawns, fin fish and
other material. Ideally, a plant should process only prawns or other
crustaceans. In factories where other material such as fin fish or squid must be
processed, there should be a separate line. Where frogs must be processed within
the same building, this should be carried out in a separate room because in most
places frogs carry a very high bacterial load. Tables and implements used for
work on frogs should only be used for other material after very thorough
sterilisation.
1. All work surfaces should be of smooth, impervious, non-toxic
material which is corrosion-resistant.
2. Unfrozen material must be kept chilled in clean containers
using plenty of small pieces of ice. Material which has been frozen at sea
should be kept frozen until it is required for thawing.
3. A typical sequence for either chilled or thawed material
would be:
4. Absolute cleanliness is essential at all stages. All surfaces
should be cleaned frequently. The water used must be clean and fit to drink. It
may be chlorinated but the free chlorine level should be kept low so that
flavour and colour are not impaired. A level of 10 ppm should be regarded as the
maximum.
5. Personnel should be properly dressed. They must wash their
hands on entering the plant, using soap and hot water and drying their hands
only on paper towels which cannot be re-used. Where people are to work on wet
processes, there is no need for them to dry their hands but they must wash off
the soap. The plant should not have hand-operated taps. Foot taps are ideal.
Workers should also wash their hands or gloves at intervals and during
processing. They must wash their hands whenever they have been to the
toilet.
Drying prawns
While very small prawns are sometimes dried without boiling, the
larger ones are almost invariably boiled in brine before drying. Local
preferences as to saltiness vary but often a brine of about 5 per cent is used.
A typical process would be as follows:
1. The cooking time should be controlled; about 3 - 5 minutes at
100°C is sufficient for all but the largest prawns. The boiling time should
be timed only from when the brine has come up to boiling point. Some processors
use stronger brine and cook for longer in order to remove as much water as
possible but this produces a slightly inferior over-salted product.
2. Cool rapidly by spreading in a thin layer.
3. Air-dry on a raised surface. Dried prawns are a relatively
valuable material and, in some places, it is profitable to use hot air driers to
produce better quality material than would be obtained by slow air drying when
the humidity is high. Some degree of fermentation is, however, liked by some
consumers.
In order to achieve a top quality product, these must arrive in
the freezing plant while still alive. Although it was shown some years ago that,
as with many other crustaceans, the best results were obtained by cooking rock
lobsters before freezing, almost all importers insist on buying frozen raw
material. This enables the purchaser to use a variety of different cooking
methods.
1. Remove tails from live animals and grade for size.
2. Remove the hind gut.
3. Wash.
4. Freeze - this is usually done in a blast freezer. The
material should be quick frozen as for other fish products. The anterior end of
the tail is usually wrapped in a small piece of plastic sheet which may be
secured with a rubber band.
5. Pack the material and then place in cold storage.
As with crabs most species are quite easily kept alive out of
water, and all spoil rapidly once dead. They should be kept alive until landed
and can be chilled in this condition using ice. In Europe, these are again
marketed live and are normally carried in wooden or cardboard boxes which may
contain insulating materials and ice; lobsters can remain alive for up to 36
hours, depending on the conditions prevailing. When eaten fresh they are boiled
whole immediately prior to consumption.
Lobsters can be frozen and either the whole animal, which has
just died, or the tails are used. They are also canned: the meat is cooked in
its own juice or in jelly, mayonnaise or cream
sauce.
