CHEMICAL COMPOSITION OF FISH
Fish is chemically made up of water, fat protein, minerals, vitamins, extractives, and contaminant.
WATER: Water is the major component of all species and types of fish. It is typically in the range
70-80% of the fresh weight, although some deep water Spp. may contain in excess of 90%. There
are seasonal variations, a slight increase occurring when the fish is starving (rise of 1-2% in
demersal sp. but may be as much as 17% in pelagic sp.). The water content may also rise slightly
when fish are stored in melting ice or refrigerated seawater. In simple terms, it may be said that
the high water content is responsible for the perishability of fish.
FAT (OIL): The fat of true fish is found mainly in the dark muscle just below the skin and in the
lateral line area. Some species store fat extensively in their liver, e.g. Cod and halibut, others in
their head, e.g., halibut, salmon, snappers, and mullets or in the peritoneum, e.g. eels. Many bony
fish, sharks, and rays have a slightly higher fat content in the belly flap rather than the thicker
muscle of the back. Fat in the muscle consists mainly of triglycerides, i.e. molecules produced
from the combination of one molecule of glycerol and 3 mols. of fatty acids.
Structure of a Typical Triglyceride
CH2
O
CO
R1
CH
O
CO
R2
CH2
O
CO
R3
R₁, R₂, R3 are typically unsaturated fat acid radicals with between 14 and 22 carbon atoms.
In fish oil/fats, the fatty acids usually contain an even number of carbon atoms in a long straight
chain configuration and then included one or more double bonds. All fish oil is highly perishable
and very susceptible to rancidity because of the high content of double bonds. The exact nature of
fatty acids in fish seem to be determined partly by what the fish feeds upon.
Also, the triglyceride rich tissues contain minor components such as the fat-soluble vitamins (A,
D and E), fat-soluble pigments (carotenoid, heamoglobin and myoglobin) and Cholesterol. The fat
content of fish changes markedly during the year, often falling to a minimum when fish are
migrating and/or spawning and rising rapidly once feeding recommences. In general, high fat
content correlates with high eating quality. (See Table below)
Seasonal Variation in fat & water contents of Mackerel, herring and oil Sardine
Month
Mackerel
(Scomber scombrus)
Herring
(Clupea harengus)
Oil Sardine
(Sardinella longiceps)
January
% water
57
% fat
24
% water
-
% water
67
April
70
09
72
08
77
03
June
74
07
67
13
78
02
July
74
08
59
20
78
03
October
66
14
66
15
66
13
December 57
22
-
-
67
13
% fat
-
PROTEIN: Proteins are polymers of amino acids.
The 3 types found in fish are;
i.
myofibrillar or Contractile proteins (65-75%),
ii.
Sarcoplasmic or enzymic proteins (20-30%) and
iii.
Stroma or Connective tissue proteins (1 - 32%).
% fat
13
The last value being much lower than for meat or poultry. Protein is found in the muscle (16-20%),
in the skin (approximately 16 %) and in the organs.
The protein figures given in most publications are the crude proteins and these in some instances
may be a substantial over estimate of the true protein content. Protein is determined normally by
measuring the nitrogen content of the foodstuff since protein is usually the major nitrogen Containing Component. The Nitrogen value is usually converted to protein content by multiplying
by a factor of 100/16 = 6.25, because pure proteins usually contain 16% nitrogen. When this
procedure is used nitrogen in other compounds is also measured and calculated as though it were
protein. In most cases, the non - protein- nitrogen (NPN) is approximately 10% of the protein
nitrogen (PN) and crude protein is a fair estimate of true protein. However, in cartilaginous fish
and crustaceans, the inclusion of NPN in the Calculation may lead to substantial over-estimation.
The protein in fish is of high nutritional values, comparing favourably with other foods of animal
origin and generally superior to proteins of plant origin.
CARBOHYDRATES: Fish contain far less CHO than foods of plant origin. The small amounts
that are present can be ignored so far as nutritive value is concerned. The major CHO in fish muscle
is known as glycogen or animal starch. A typical muscle in a live fish or crustacean may contain
between 0.1- 1.0 %. Molluscs have a higher content 1 -17 %. This quantity can vary seasonally
and in any case declines during the stress and struggle associated with capture and “post mortem".
The products include small amounts of glucose, sugar phosphates, pyruvic acid and slightly larger
amounts of lactic acid in most Sp. Some molluscs do not produce lactic acid but instead a mixture
of alanine, Succinic acid and Octopine.
EXTRACTIVES: The term extractives is applied to small (i.e. low molecular mass), water
soluble components, some of which are also mentioned in other sections. These substances rarely
exceed some 2% of the fresh wt. and many individual compound are present. The ratios of the
individual compounds differ markedly with species and such variations are considered to be
largely responsible for differences in flavour. Compounds found in the extractives include: free
amino acids; Small peptides; Glucose and Sugar phosphates; Trimethylamine oxide and
trimethylamine; Nucleotides and nucleotide degradation products e.g. (Adenosin triphosphate,
Inosine monophosphate and hypoxanthine; Creatine and urea; Pyruvic and Lactic acids. Since
these compounds are water-soluble, they are easily lost by leaching when fish (especially prawns)
are stored in ice or refrigerated seawater. Inevitably, losses cause changes in flavour and these will
generally be undesirable.
VITAMINS: Vitamins may be sub divided into two groups depending on weather they are soluble
in Water or fat. In general, fish compare favourably with other foods of animal origin in terms of
the Vitamin content. Fatty fish are usually very valuable sources of the fat-soluble vitamins (A, D
and E). Some fish livers are so rich in these vitamins that they may be recovered as a by-product;
such Livers may be toxic because of these very high vitamin contents if eaten in large quantity.
Fish flesh and fish gonads are useful source of many of the water - soluble vitamin particularly
thiamin and Cobalamin (B₁2). The latter being found only in foods of animal origin. It should be
noted that fish do not supply significant amounts of ascorbic acid (Vit. C).
MINERALS: Total mineral, or ash, content of most species falls in the range of 1 to 2%. A wide
number of mineral are present in fish flash, usually in a readily available form. The calcium content
of shell and bone is not normally available to the body; however, canning does solubilize it and
thus makes available some of the calcium which is present in the bone.
Typical Values for Minerals in Fish
Element
Content mg/100g
Sodium
30 – 134
Potassium
19 – 502
Calcium
19 – 881
Magnesium
4 – 452
Phosphorus
68 – 550
Iron
1 – 5.6
Chlorine
3 – 761
Iodine
0 – 2.73
CONTAMINANTS: In general terms any compound found in the water in which fish swim or in
containers or stores where fish are kept may pass into the flesh. Such substances may come from
the feed and cause an off- flavour, in which case little can be done to avoid such tainting. Of more
concern is the pick-up of undesirable Components arising from pollution. Examples that have
received particular attention include methyl mercury, radioactive elements and petroleum
hydrocarbons.
APROACHES TO PRODUCT EVALUATION /ASSESSMENT
OF LOSSES IN CURED FISH
Loss assessment should form an initial phase of the programme concerned with achieving
improvements and a reduction of losses. It can be used to enable the specific problems of existing
methods to be identified, measured and evaluated in an objective way. Loss assessment may also
be employed in extension programme as a means of demonstrating the worth of improvements.
PROCEDURE FOR EVALUATING LOSSES.
The first pre-requisite is to define the objective for the study, which may include one or all of the
following aspects:
1.
To determine the amount of wet fish which is lost at the curing stage and fails to become
processed into a satisfactory cured product
2.
To determine the amount of cured fish which is produced but lost before reaching the
marketing system
3.
To determine the physical deterioration in cured fish between the curing stages and
purchase by the consumer and to assess the financial consequences.
4.
To consider consumer preferences for fish and to evaluate nutritional values of available
cured fish.
As a first practical step it is necessary to obtain full and appropriate background information about
the system in which cured fish is moving. This will be in relation to the:
(a) Geographical locations concerned with the industry and their environmental and climatic
features.
(b) Handling and transport Systems
(c) Ownership changes and variations in unit value of products
(d) Time scale involved and seasonal nature of operations
(e) Estimates of total amounts handled
(f) The parameters set for acceptability (or quality and grade, if such exist).
Much of this information will be obtained through observation and discussion with those close to
the trade, and by consulting reports and records and the completion of data required may, however,
nесеssitates planning and executing a survey of the field situation.
There are three (3) basic elements pertinent to the consideration of losses of cured fish:
1. Determination of the type of losses
2. Measurement of the amount and extent of loss
3. Evaluation of the importance of the loss
The causes of losses should be readily apparent but quantitative evaluation presents some
problems.
SURVEY AND SAMPLING METHODS
The practicalities of organizing a survey will depend so much upon local conditions and objectives
that guidelines can be laid upon only in a general way. Since it is not practical to examine the
whole country, care must be taken in the selection of those factors which are to be studied. The
survey may be one sector, one route, one type of cured fish entering one market. For example, if
there are two species of fish (large and small) cured in two ways (salted and smoked), transported
in two ways (road and rail) to two markets (near and far), this may indicate the need for a minimum
of 16 sets data. Sampling of fish of variable size and weight present problems, consequently for
measurement, it may be best to make a comparison using a similar fish as the basis and it is easier
to work with fleshy rather than bony species. Individual fish or whole bales may be tagged with a
label as a means of Identification and followed through the system.
INDIRECT MEASUREMENT OF LOSSES
Measurements of pest density, and even the physical separation and weighing of frass resulting
from insect infestation have been suggested as being suitable approaches to the measurement of
Dermestes Spp. activities in dried fish (Moes, 1980). Both will relate in some way to loss levels
but neither can be regarded as reliable measurement of losses. Pest density has been found to be
subject to wide fluctuation even within the same sample of fish and as both the adults and larvae
are able to freely migrate. The marked migration of Dermestes Spp. due to daily temperature
fluctuations have also been noted earlier, so unless extreme care is taken pest densities will depend
to a very large extent on the time of day and on what part of the heap or sacks of fish the sample
was taken from.
DIRECT MEASUREMENTS OF LOSSES.
Losses should be viewed as a loss of weight and nutrient (potentially available to the consumer)
and as a loss of income (to a trader whose products decrease in quantity and / or quality).
Measuring the losses as directly as possible has very advantages in that margins of error will be
minimized than using any sophisticated indirect method. Direct weight measurement can be
perform quickly, easily and non-destructively with a simple spring balance or scales and so a
change in weight is the recommended parameter whereby losses can be quantified.
If cured fish was a uniform product of fixed composition, then weight losses could be directly
related to losses of nutrients. However, this is not relatively the case, as cured fish consist of several
components categorized as;
a). Edible flesh and nutrients, mainly protein but including fat
b). Moisture content
c). Non-edible bony materials
d). Salt (if added).
During processing and storage these components exhibit different characteristics and overall
weight changes are a sum of all the changes in the different components. Example is moisture
content.
QUALITY LOSSES
Low quality as perceived by a consumer may be shown by a correspondingly low price, although
due to the workings of a market, price will be relative rather than absolute evaluation. If both good
and bad products are available in sufficient quantities. It may mean that only the good ones will
sell; the inferior ones will not necessarily become less expensive. Assessments of quality must be
done in the market place by an experienced assessor familiar with local preferences. The opinions
of those affected or responsible for quality should be sought. The factors of importance are
Appearance; Colour; Smell; Evidence of mouldiness and decay, Consistency and texture of flesh;
Proportion of bone and skin to muscles tissue; Size of pieces and their relative frequency, Presence
of insect infestation.
FINANCIAL LOSSES
Losses in quantity and quality have financial consequences for all sections of cured fish trade.
Hence the prospect of an increased income resulting from loss reduction will be useful tool in
putting improvements into commercial practice.
CONSTRAINTS TO LOSS ASSESSMENT
These may include the following
1. Limitation of the study to a particular season, geographical area, trade route or market
sector.
2. Limitation due to the size of budget committed, number and qualification of available staff,
adequacy of project transport and analytical facilities and time constraints.
3. The degree of cooperation forth coming and the access to information about the operations
of commercial sector.
CHEMICAL AND PHYSICAL METHODS OF QUALITY ASSESSMENT
Over the years, many different methods have been developed and investigated in an attempt to
find the most suitable index for use in quality control testing. The ideal method should have the
following attributes
A. It should be non-destructive
B. The equipment should be cheap to purchase and maintain
C. The equipment should be robust, portable and simple to operate and suited to minimum
facility “field” operations.
D. It should produce consistent results on standard material. It is pertinent to state that few
methods even approach this ideal.
PRECAUTIONS APPLICABLE TO ANY QUALITY ASSESSMENT OPERATION
1. Ensure that your sample of fish from the bulk truly represents the bulk.
2. Ensure that, when a part of fish is analyzed, it is always the same part.
3. Ensure that a rigidly standardized method is employed. The result obtained frequently
depends upon the analytical method employed
4. Never attempt to compare directly, results obtained by different methods.
5. In export operations, levels of acceptance and rejection must correspond to the importer's
legislation and/or specifications.
6. In local operations, levels of acceptance and rejection should be related to local
requirements