Unit 2. Chemical composition of fish

Exercise 1. a) Read and translate international words without a dictionary:

nature, regulation, balance, minor, combination, complex, seasonal, extreme, to extract, variation, plankton, sum, indicator.

b) Check if you remember the meaning of these words:

fish flesh, the fullest use, spawning time, combination of amino acids, odour, freshwater, ammonia, intake, to affect.

c) Read and translate the following word combinations:

the edible part of the fish, constituents of fish products, one of the most valuable sources, a basic food resource, the average water content, the mineral and vitamin content, extractive compounds, time of storage, greater quantities, freshwater fish, indicate the storage time, a starved herring, feeding heavily.

d) Give all the possible synonyms to the following words:

individual, tightly, to produce, amount, to utilize, to estimate, to add, minor, fatty, wide, correct, lipid, to rise, steam.

e) Give the antonyms to the following words:

in practice, personal, valuable, to suffer, to decrease, available, extreme, minor, fatty, correct, natural, to rise.

Exercise 2. Read and memorize the words from the text below:

nutritionist, n	диетолог; диетврач
constituent, n	компонент(а), составная часть
contribution, n	взнос, вклад
comply, v	исполнять, удовлетворять
lean, adj	тощий
muscle, n	мускул, мышца
flesh, n	мясо (рыбы)
fish meal	рыбная мука
specimen, n	образец, образчик; экземпляр
bind, v	вязать; связывать
retain, v	вмещать
dissolve, v	растворять
substance, n	вещество
drip, v	капать, падать каплями; стекать
approach, v, n	подходить, приближаться; приближение
estimate, v	оценивать
reduction, n	снижение, понижение, сокращение
suit, v	подходить
fluctuation, n	колебание
utilize, v	использовать
volatile, n	летучее вещество
supplement, n, v	добавление, дополнение дополнять, пополнять
provide, v	доставлять; обеспечивать
maintenance, n	поддержание; сохранение
carbohydrate, n	углевод
promote, v	выдвигать; продвигать
abundance, n	изобилие; избыток

Exercise 3. Read and translate the text.

Chemical composition of fish

a) Why chemical composition is important to know

The processor, the nutritionist, the cook and the consumer all have a direct interest in the composition of fish. The processor needs to know the nature of the raw material before he can apply correctly the techniques of chilling, freezing, smoking or canning. The nutritionist wants to know what contribution fish can make to the diet and to health and the cook must know for example whether a fish is normally lean or fatty in order to prepare it for the table. The consumer is interested not only in whether a particular fish tastes good, which is a matter of opinion, but also whether it is nutritious.

While the consumer is interested mainly in the edible part of the fish, that is the flesh or muscle, the fish meal manufacturer is concerned with the composition of the whole fish, and the processor of fish oils wants to know what is in the liver. Measurement of constituents of fish products is sometimes necessary to meet specifications or to comply with regulations. For example, the fish content of fish cakes or the oil content of fish meal may need to be known in order to meet certain commercial or legal requirements. Fish is one of the most valuable sources of high grade protein available to man in this hungry world, and knowledge of its composition is essential if the fullest use is to be made of it.

b) The principal components of fish muscle

Water

The main constituent of fish flesh is water, which usually accounts for about 80 per cent of the weight of a fresh white fish fillet. Whereas the average water content of the flesh of fatty fish is about 70 per cent, individual specimens of certain species may at times be found with water content anywhere between the extremes of 30 and 90 per cent.

The water in fresh fish muscle is tightly bound to the proteins in the structure in such a way that it cannot readily be expelled even under high pressure. After prolonged chilled or frozen storage, however, the proteins are less able to retain all the water, and some of it, containing dissolved substances, is lost as drip. Frozen fish that are stored at too high a temperature, for example, will produce a large amount of drip and consequently quality will suffer. In the living fish, the water content usually increases and the protein content decreases as spawning time approaches; thus it is possible, with cod for example, to estimate the condition of the fish by measuring the water content of the muscle.

In cod, the water content of the muscle is slightly higher at the tail than at the head; this slight but consistent increase from head to tail is balanced by a slight reduction in protein content.

Protein

The amount of protein in fish muscle is usually somewhere between 15 and 20 per cent, but values lower than 15 per cent or as high as 28 per cent are occasionally met in some species.

All proteins, including those from fish, are chains of chemical units linked together to make one long molecule. These units, of which there are about twenty types, are called amino acids, and certain of them are essential in the human diet for the maintenance of good health. Furthermore, if a diet is to be fully and economically utilized, amino acids must not only be present but must also occur in the correct proportions. Two essential amino acids called lysine and methionine are generally found in high concentrations in fish proteins, in contrast to cereal proteins for example. Thus fish and cereal protein can supplement each other in the diet. Fish protein provides a good combination of amino acids which is highly suited to man's nutritional requirements and compares favourably with that provided by meat, milk and eggs.

Fat

Taking all species into account, the fat content of fish can vary very much more widely than the water, protein or mineral content. Whilst the ratio of the highest to the lowest value of protein or water content encountered is not more than three to one, the ratio between highest and lowest fat values is more than 300 to one.

The term fat is used for simplicity, although the less familiar term lipid is more correct, since it includes fats, oils and waxes as well as more complex, naturally-occurring compounds of fatty acids.

There is usually considerable seasonal variation in the fat content of fatty fish; for example a starved herring may have as little as ½ per cent fat, whereas one that has been feeding heavily to replenish tissue may have a fat content of over 20 per cent. Sardines, sprats and mackerel also exhibit this seasonal variation in fat content. As the fat content rises, so the water content falls, and vice versa; the sum of water and fat in a fatty fish is fairly constant at about 80 per cent. Although protein content falls very slightly when the fat content falls, it nevertheless remains fairly constant, somewhere between 15 and 18 per cent.

The fat is not always uniformly distributed throughout the flesh of a fatty fish. For example in Pacific salmon there may be nearly twice as much fat in muscle from around the head as there is in the tail muscle.

In white fish of the cod family, the fat content of the muscle is always low, usually below 1 per cent, and seasonal fluctuations in fat content are noticeable mainly in the liver, where the bulk of the fat is stored.

c) The minor components of fish muscle

Carbohydrates

The amount of carbohydrate in white fish muscle is generally too small to be of any significance in the diet; hence no values are given in the tables. In white fish the amount is usually less than 1 per cent, but in the dark muscle of some fatty species it may occasionally be up to 2 per cent. Some mollusks, however, contain up to 5 per cent of the carbohydrate glycogen.

Minerals and Vitamins

These include a range of substances widely different in character that must be present in the diet, even if only in minute quantities, not only to promote good health but also to maintain life itself.

Although fish is very unlikely to be the only source of an essential mineral in the diet, fish does provide a well balanced supply of minerals in a readily usable form. The table of mineral constituents of fish muscle gives values averaged from a large number of species and is intended to serve only as a rough guide. It would be impracticable in this short note, and of limited value, to give a detailed analysis for individual species.

Composition tables for fish often include a value for total ash. Since ash consists largely of a number of different minerals, and the total rarely exceeds 1–2 per cent of the edible portion, this figure has also been omitted, except from the table of fish products.

Vitamins can be divided into two groups, those that are soluble in fat, such as vitamins A, D, E and K, and those that are soluble in water, such as vitamins B and C. All the vitamins necessary for good health in humans and domestic animals are present to some extent in fish, but the amounts vary widely from species to species, and throughout the year.

The vitamin content of individual fish of the same species, and even of different parts of the same fish, can also vary considerably. Often the parts of a fish not normally eaten, such as the liver and the gut, contain much greater quantities of oil-soluble vitamins than the flesh; the livers of cod and halibut for example contain almost all of the vitamins A and D present in those species. In contrast, the same two vitamins in eels, for example, are present mainly in the flesh.

Water-soluble vitamins in fish, although present in the skin, the liver and gut, are more uniformly distributed, and the flesh usually contains more than half the total amount present in the fish. The roe, when present, is also a good source of these vitamins.

In general the vitamin content of white fish muscle is similar to that of lean meat and, with the exception of vitamin C, can usually make a significant contribution to the total vitamin intake of man and domestic animals.

The mineral and vitamin content of fish is not markedly affected by careful processing or by preservation, provided storage is not very prolonged.

Extractives

These substances are so called because they can easily be extracted from fish flesh by water or water-based solutions. Unlike the proteins, substances in this group have comparatively small molecules; the most important extractives in fish include sugars, free amino acids, that is free in the sense that they are not bound in the protein structure, and nitrogenous bases, which are substances chemically related to ammonia. While many of these extractives contribute generally to the flavour of fish, some of them, known as volatiles, contribute directly to the flavour and odour characteristic of particular species; as the name suggests, volatiles are given off from the fish as vapours. Most of the extractives are present at very low concentrations but, because of their marked flavour or odour, are nonetheless important to the consumer. Detailed analyses of these substances have not been given because of the large variation existing both between and within species. An additional complication is the way in which the concentrations of these compounds change during storage and spoilage.

When fish is stored after capture, the amount of some of the extractives present will change with time; thus measurement of the amount can often indicate the storage time and hence indirectly the quality. Extractive compounds whose concentration in fish varies directly with time of storage have long been studied since they may provide indicators of the quality of fish.

d) Factors affecting the composition of fish

The composition of a particular species often appears to vary from one fishing ground to another, and from season to season, but the basic causes of change in composition are usually variation in the amount and quality of food that the fish eats and the amount of movement it makes. For example, fish usually stop feeding before they spawn, and draw on their reserves of fat and protein. Again, when fish are overcrowded, there may not be enough food to go round; intake will be low and composition will change accordingly. Reduction in a basic food resource, plankton for example, can affect the whole food chain. An example of how abundance of food supply can markedly change the composition of a species is shown by the sheepshead, an American freshwater fish: when taken from certain small lakes that were overstocked, the sheepshead had an average fat content of 1 per cent, compared with 6–10 per cent for those taken from rivers or lakes where food was plentiful.

Exercise 4. Find the Russian equivalents for the following:

commercial or legal requirements, to be tightly bound to the proteins, variation in the amount and quality, to be overstocked, to be plentiful, to vary from one fishing ground to another, extractive compounds, to change during storage, to be stored after capture, detailed analyses, measurement of the amount, to estimate the condition of the fish, uniformly distributed throughout the flesh, to be overcrowded, reserves of fat and protein, substances chemically related to, the total vitamin intake, characteristic of particular species, by careful processing.

Exercise 5. Find the English equivalents for the following:

запасы жира и белка, изменения в составе, быть важным для потребителя, важный вклад, сравнительно маленькие молекулы, азотная основа, основной компонент, содержание углеводов, витамины и минералы, жирорастворимые витамины, треска, палтус, белая рыба, жирная рыба, пресноводная рыба, морская рыба, хорошо сбалансированный, различия в количестве и качестве, фосфор, кальций, йод, аминокислота.

Exercise 6. Complete the sentences using information from the text:

1. Furthermore, if a diet is to be fully and economically utilized,…

2. When fish is stored after capture…

3. The vitamin content of individual fish of the same species,…

4. Most of the extractives are present at…

5. In general the vitamin content of white fish muscle is…

6. Vitamins can be divided into two groups,…

7. In white fish of the cod family, the fat content of the muscle…

Exercise 7. Fill in the blanks with prepositions for, of, to, in:

1. Two essential amino acids called lysine and methionine are generally found … high concentrations … fish proteins.

2. Reduction … a basic food resource, plankton … example, can affect the whole food chain.

3. The vitamin content … different parts … the same fish, can also vary considerably.

4. These extractives known as volatiles, contribute directly … the flavour and odour characteristic.

5. Free amino acids are not bound … the protein structure.

Exercise 8. Say if the sentences are true or false using the information from the text:

1. If spawning period is to be fully and economically utilized, amino acids must not only be present but must also occur in the correct proportions.

2. Unlike the proteins, extractives have comparatively large molecules.

3. The mineral and vitamin content of fish is greatly affected by careful processing or by preservation.

4. Measurement of the extractives amount can often reduce the storage time and hence indirectly the quality.

5. The term lipid is more correct, since it includes fats, oils and waxes as well as more complex, naturally-occurring compounds of fatty acids.

Exercise 9. Answer the following questions:

1. Why is it important to know chemical composition of fish for consumer?

2. Why is it important to know chemical composition of fish for producer?

3. How and when does it change?

4. What factors influence the chemical composition of fish?

5. What are the principal chemical components of fish?

6. What is the main constituent of fish flesh?

7. How does water and protein amount changes in spawning time?

8. What is the average amount of protein in fish muscle?

9. What two main amino acids are found in fish proteins?

10. What vitamins do fish contain?

11. What two groups are vitamins divided into?

12. What minerals are found in fish?

13. What other chemical components can be found in fish?

Exercise 10. Speak about a) factors affecting the composition of fish;

b) the principal components of fish muscle;

c) the minor components of fish muscle.

Exercise 11. Make the report about chemical composition of one of the fish inhabited in the waters of our region.

Exercise 12. Translate from Russian into English:

1. Минеральные вещества, содержащиеся в рыбе, помогают построению костей, мышц, нервной системы.

2. В рыбе содержатся различные химические вещества, в основном белки, жиры, витамины и различные минеральные вещества.

3. Их присутствие в рыбе определяется наличием минеральных веществ в окружающей среде и в питании рыб.

4. Содержание жира – один из критериев оценки ценности рыбы.

5. Молекулярный химический состав помогает определить качество и свежесть рыбы.

6. Большая часть экстрактивных веществ представлена в рыбе в очень малых количествах.

7. В печени трески и палтуса содержится практически все количество витаминов А и D, находящихся в рыбе.