Miscellaneous aquatic products used as food

In this session we will consider some aquatic resources that are used as food but are not 'fish' in the strict sense of the word. Many fisheries enterprises and government departments dealing in fisheries take other products such as these under their umbrella, and so it is important that you should know something about them.

Frog legs

Frog legs are a popular commodity in many European countries, Japan and North America, and much of the production comes from tropical areas of the world including the sub-continent of India, Mexico, Cuba and the Far East, e.g., Indonesia. The most common products are frozen frog legs. There are a number of different species used, most of which belong to the genus Rana.

Processing for freezing, as recommended by FAO, is as follows:

1. Use live frogs only.

2. Place live frogs in a 10 per cent solution of salt (NaCl) containing 250 ppm chlorine for 15 minutes. This treatment partially paralyses and anaesthetises the animal.

3. Cut hind legs at the abdomen, not more than 2.5 cm above the waist so as not to disrupt the gut contents.

4. Wash the legs in chlorinated running water.

5. To reduce Salmonella, hold the legs in chilled (with ice) chlorinated water (500 ppm chlorine) for 2 minutes.

6. Skin the legs and clip the feet as soon as possible.

7. Wash again in iced, running water containing 20 ppm chlorine for 20 minutes to facilitate bleeding.

8. Trim excess pieces of skin, guts etc. and examine for defects such as blood spots, parasites etc.

9. Wash again in chlorinated (500 ppm) chilled water.

10. Finally, wash in 4 - 5 changes of chilled chlorinated (20 ppm) water.

11. Grade into different sizes.

12. During packaging, take care not to contaminate the product. Pack the legs in individual polyethylene bags or film and secure with a rubber band. Treat the wrappers with 20 ppm chlorinated water before use.

13. Freeze the packs into blocks and store at a low temperature (- 40°C).

We can see that the above regime involves multiple washing and treatment with chlorinated water. This is necessary because of the high incidence of Salmonella (a pathogenic bacterium) in frogs. Salmonella is present only in the intestine and on the skin of healthy frogs and has no deleterious effect on the living frog; however, once the frog is cut open, contamination of the end product can easily occur unless scrupulous cleanliness and strict separation of raw material and end product are exercised. The end product should comply with the following bacteriological standards according to FAO:

1 Total viable count at 37°C	Max 5 000 000/9
2 Escherichia coli	Max 1019
3 Coagulase positive staphylococcus	Max 100/g
4 Salmonella or Arizona	Zero/25 g sample

Many exporting countries run into difficulties with health regulations in the importing countries because of high incidence of Salmonella in frog legs.

Frog legs and crustaceans such as shrimp and prawns should not be processed in the same working area.

Molluscs

Many different molluscs are used as food throughout the world. They can be processed in a great variety of ways including smoking, drying, canning, freezing, or eaten fresh or even alive. The same basic principles which apply to the processing of other more conventional fishery products are used in the processing of most molluscs.

With gastropods such as conch and trochus, the large head/foot is usually eaten fresh or sometimes marinated with vinegar. The muscle is particularly tough and requires tenderisation by beating with a mallet before cooking if eaten fresh. The process of marination also helps to tenderise the flesh.

Squid and octopus can be prepared fresh and frozen for use in a variety of dishes and are particularly popular in countries of southern Europe. They can also be dried. Squid is prepared for drying by splitting the ventral side of the body and carefully removing the ink sack and internal shell. The inside is scraped and thoroughly washed before sun drying. Drying can take 10 days and a translucent product is formed.

Many bivalves such as mussels and oysters are marinated or brined and bottled or canned. Alternatively, they can be lightly smoked and canned in vegetable oil. In many countries, bivalves such as oysters are dried and smoked to preserve them for local markets. These form a useful supplement to otherwise low-protein diets.

Sea cucumbers or beche-de-mer

Sea cucumbers (also known as beche-de-mer, sea slugs or trepang) are holothurians which occur, usually, on coral reefs in many tropical areas. They vary in size and colour from species to species. Their market value depends primarily on the species concerned, the most valuable being the teat fish (Microthele nobilis), but also depends largely on the size of the specimen (the larger the better), appearance, odour, colour and moisture content.

The basic processing consists of removing the viscera from the animals and cleaning the gut cavity; boiling for up to 1 - 1½ hours; a second cleaning to remove the remains of the guts usually by making a longitudinal cut on the top of the animal, then drying either by sun drying, if climatic conditions allow, or by smoking.

The main market for beche-de-mer is amongst the Chinese community of the Far East. Most marketing is through agents in Hong Kong and Singapore.

Fish roes

True caviar is made from the roe of the female sturgeon but an inferior caviar may be made from the eggs of a number of fish such as salmon and cod. A number of tropical species could yield roes which are suitable as a caviar substitute. The roes are removed from freshly killed fish and rubbed gently through a sieve to remove the membrane. The eggs are mixed with salt (4 - 1 0 per cent by weight), stirred and left for 10 - 15 minutes; they are then drained, bottled and stored at chill temperatures or pasteurised.

Some female roes (e.g., from mullet, shad or Spanish mackerel) are salted and dried in the round. The roes may be dry salted or brined (10 per cent salt by weight) and the salting time varies from 10 to 15 hours. After draining, the roes are sun dried for 5--10 days. The roes may also be smoked to dry them. The shelf life of the dried products depends on the extent of their drying but it can be extended by coating them with a 50:50 mixture of beeswax and paraffin wax. Some female roes can be lightly smoked to give them a characteristic flavour. Soft or male roes from, for instance, herring find a market in Britain when canned.

Turtles

Turtles and turtle products have formed the basis of a sizeable industry until recently. However, worries of over-exploitation and dwindling stocks of wild turtles have caused a recent decline.

The most important turtle used for food is the green turtle (Chelonia mydas) which has been used for the production of high value turtle soup in the canned form. For soup manufacture, the calipee and calipash are primarily used. The red meat of all turtles is also eaten. The leathery turtle (Dermochelys coriacea) is chiefly exploited for its oil. The Hawksbill turtle (Eretmochelys imbricata) is exploited chiefly for its shell as an ornament.

The eggs of all turtles are eaten in many parts of the world. Rearing of green turtles particularly has been practised in various parts to try to overcome conservation problems. These projects have had some degree of success. Many countries, however, now have very strict conservation measures in force to help conserve wild stock whilst others are restricting the imports of turtle products.