A solar dryer with dimensions 93cm x 63cm x 37cm was designed and
constructed with metal drum. It has detachable stand and removable cover constructed
with perspex. The solar dryer was evaluated with three fresh water fish species:
Gymnarchus niloticus, Heterotis nllotrcus and Clariar gariepinus, These were also
dried conventionally on exposed rack Organoleptic assessment was conducted for the
The highest mean temperature of 68.3O C at 14 00 bur was attained inside the dryer.
At h s time the ambient temperature was 34.4 + 3.5′ C, while the radiation was 742.2
f 105 w&?. The highest radiation measured was 859.6 w/M2 at 1300 hour, when
the lowest humidity measurement was 34.6 f 2.9 %. The lowest mean temperature
recorded in the dryer was 33″ C at 0900 h when ambient was 25:4′ C and radiation
was 317.4 wIM2. The quality of the solar dryer dried product was found to be
superior to the open sun-dried ones. Bacterial load was less (40 orgslg) in solar dried
than the open sun dried fish product (1,820 orgdg). The organoleptic assessment test
showed that solar dried fish were judged better than sun dried fish. It took three days
for the fish to be completely dried in solar dryer compared with open sun dried, which
took seven days to be
TABLE OF CONTENTS
List of figures
List of tables
List of plates
Table of contents
CHAPTER ONE: INTRODUCTION AND LI11EMTP”URE REVIEW
1.1 Literature review
1.1.2 Fish spoilage
188.8.131.52 Microbiological spoilage
184.108.40.206 Chemical spoilage
1.1.2..3 Autolytic spoilage
1.1.3 Common methods used in fish preservation
220.127.116.11 Conventional sun drying
18.104.22.168 Artificial dryers
1.1 3.6 So!ar drying
22.214.171.124 Classification of solar dryers
126.96.36.199 Methods of fish preservation in the tropics
188.8.131.52 Factors that affect the handling of fish for drying
. . .
184.108.40.206 Factors that influence the rate of drying
1.1.4 Means of drying.
1.1.5 Solar radiation
1.1 s.1 Sol ar collectors
220.127.116.11 Conventional open sun drying
CHAPTER TWO: MATERIALS AND METHODS
Construction of fish solar dryer
Fish procurement & preparation
Dehydration of fish species
Microbiological Analysis of Sun and Solar Dried products
Determination of total viable count (TVC)
Proximate Analysis of product
Determination of moisture content
Protein content determination
Crude fibre content
Determinalion of acid value or Free faity acids (FFA)
Determination of peroxide value
Determination of Thiobarbituric Acid Number or value (TBA)
Water Activity (a W) Determination
2.6.10 Organoleptic Assessment.
2.6.1 1 Statistical Analysis
CHAPTER THREE: RESULTS
3.1 Construction of Fish Solar Dryer
3.2 Microbiological Analysis
3.3 Details of measurements of operating conditions during the
drying period of sun and solar fish products.
3.4 Proximate Analysis
3.5 Thiobarbituric Acid Number or value (TBA)
3.6 Peroxide value
Free fatty acids (FFA)
Water Activity (a W)
INTRODUCTION AND LITERATURE REVIEW
Fish provides a good source of readily digested high quality animal protein
together with a high concentration of vitamins A and D, a significant source of
phosphorous and iron, as well as high concentrations of calcium and phosphorous in
the bones, (Choo and Williams, 2003). It is also a good source of selenium, coenzyme
Q10 and taurine (Thilsted and Roos, 1999). Marine fish has a high
concentration of iodine, and those fiom cold temperate seas contain high levels of
Omega-3 fatty acids such as eicosapentaenoic acid (EPA), docosahexaenoic acid
(DHA) (Table I). The benefits of omega3 fatty acids are widely documented.
Reports fiom various sources noted that fish oils significantly lower blood pressure,
protect against blood vessel constriction, thrombosis and heart arrhythmia. Increased
consumption of finfish reduces the risk of sudden death fiom heart attacks, improves
symptoms of rheumatoid arthritis, decreases the risk of bowel cancer, and reduces
insulin resistance in skeletal muscles. DHA supplements promote brain cell and
synapse growth, and improve disposition. Recent findings showed that consuming
two or more servings of fish with high omega-3 fatty acids may lower the risk of agerelated
muscular degeneration, which may cause blindness or vision impairment
(Choo and Williams, 2003).
FA0 (1993) recorded the production of approximately 270,000 tons of fish in
Nigeria while the world’s total catch was approximately 97 million tons. ‘
Tabk 1: Functions of various nutrients available from
fish in human nutrition
Vitamin A Required for growth and differentiation of epithelial, nervous and
bone tissues,-11-Cis retinal is a constitu&t of rhodopsin and
other light pigments.
1, 25-Dihydroxy – vitamin D3 is a major hormonal regulator of
bone mineral (calcium and phosphorous) metabolism.
Constituent of bones, teeth, ATP, phosphorylated metabolic
intermediates nucleic acids.
Heme enzymes (hemoglobin, cytochromes, etc).
Involved in transport and metabolism of thyroid hormones.
Constituent of bones, teeth, regulation of nerve, muscle hnction.
Plays a major role in enzyme systems (glutathione peroxidase)
that control the accumulation of free radicals in the body.
Cwenyme Q 10 Functions as an antioxidant at the sub-cellular level.
Taurioe An amino acid which plays a role in the formation and excretion
of bile salts, which are the breakdown products of cholesterol.
EPA Essential for structural integrity of mitochondria1 membrane,
(Eicosapentaenoic involved in prostaglandin leukotriene formation.
DHA Essential nutrient in the brain and retina for optimal neuronal
(Docosahexaenoic functioning and visual Performance.
Source: Choo and Williams (2003)
FA0 (2002) estimated the total world production of fish fiom 1998 to 2002 to
be 90 million tonnes of fish caught globally.
A study conducted by the World-Fish Centre and the International Food
Policy Research Institute (IFPRI) showed that fish is the primary source of animal
protein (Choo and Williams, 2003). Food and Agricultural organization (FAO, 1981)
has estimated that post- harvest losses of fish reached 35 percent, nearly 25 million
tons, of the world’s fish catch. It is also estimated that in some developing countries
post-harvest losses of fish exceed those of any other commodity often surpassing 50
% percent of the landed catch. The losses are highest in the country whose
populations have the lowest protein intake (BOSTLD, 1988). For fishermen and
farmers, one way of increasing their income is to enhance the quality of their fish
through proper post-harvest handling procedures to ensure freshness and food safety.
Artisanal fishermen do not utilize the proper procedures to handle fish and do not
have the proper facilities to store their catch resulting in quality degradation.
In Nigeria sources of fish include coastal and brackish waters, rivers, lakes and
Fish spoilage is due to three main factors; activity of micro-organisms
(bacteria, moulds and yeast), chemical deterioration due to enzymatic activity (breakdown
of oils and fats i.e. rancidity), attack by insects (blowfly and beetle infestations)
and vermin (UNIFEM, 1988; Ogunja ef al., 1992).
Post-harvest losses of fish occur during the numerous steps from catch to
market or delay between catch and distribution. These contribute toward fish spoilage
(UNIFEM, 1988; Ogunja, ef al., 1992).
For example in Nigeria, Lokoja located at the confluence of Rivers, Benue and
Niger is an important f ~ trhad ing center in Nigeria. Fish is transported from up river
towns such as Yelwa, Patesi and Jebba on River Niger or Yola, Ibi and Makurdi on
River Benue to Lokoja. Although some fish may be sold fresh, the long distances
between the tishing centers and consuming market make this impossible,
necessitating processing to prevent spoilage.
Large quantities of smoked and sun dried fish are transported from Lake Chad area of
Northern Nigeria to market centers in Onitsha, Enugu, Jos and Lagos (UNIFEM,
1988; Ogunja, et al., 1992). Three basic traditional methods are used in the fish
processing in Lake Chad and Kainji.
The lack of adequate methods to preserve the fish on board results in heavy
losses due to physical disintegration of fish. This may be brought about by improper
handling techniques, inadequate packaging materials that offer little protection and
poor processing techniques. Now that fish resources are frequently over-exploited
there is increasing emphasis on up-grading post-harvest techrlologies (UNIFEM,
1988). Ogunja et al. (1992) noted thal inadequate handling facilities and delay
between catch and distribution contribute toward fish spoilage. These factors together
with the activity of micro-organisms (bacterial, moulds and yeast), chemical
deterioration due to enzymatic activity (breakdown of oils and fats i.e., rancidity)
attack by insect (blowfly and beetle infestations) and vermin can be reduced to the
barest minimum if adequate preservation and handling techniques are used (UNIFEM,
The high ambient temperature in the tropics cause fish to start to decay a
few hours of being caught, unless fish is preserved or processed in some way to retard
Traditional methods used for preserving fish for centuries are
smoking, salting and drying. Salting or drying is used on its own or in conjunction
with each other. Dried salted products are still very popular in parts of Africa
Reducing the moisture content of fresh fish by drying to around 25% will stop
bacterial growth and reduce autolytic activity, but the moisture. content must be
reduced to 15% to prevent mould growth. Salt aids the removal of water out of the
fish by osmosis and also retards bacterial action.
There has been a great deal of interest in the area of solar drying. The
development of a variety of solar dryers as improved methods of drying fish in
developing countries is indeed welcomed. This will greatly increase dlying rates
and produce lower moisture content in the final products with improvement in fish
quality. A wide variety of designs for solar dryers have been developed. Some are
built with inexpensive and readily available materials e.g. plastic iilm, bamboo,
discarded oil drums, mud and thin metal sheeting. In solar drying, solar energy is
used as either the sole source of the required heat or as a supplemental or forced
convention. olar dryers evaluated in the field include solar tent dryer, solar cabinet
dryer, solar dome dryer, solar with separate collector and drying chamber.
The drum solar dryer has been evaluated with fish species and found to be
working effectively and efficiently. If it is introduced to fish farmers it will help to
reduce the problems of traditional techniques of preserving fish to some extent.
Aims and Objectives
The aims and objectives of this study are as follows:
1, to develop a solar dryer which can reduce fish losses and increase profitability,
2. to assess the solar drying processes, rate of drying and product quality, and
3. to compare the traditional open-sun drying of fish with the experimental solar
dryer, using fresh water fish species that are of economic importance e.g. CInrias
gariepinus . Heterotis niloticus, Gymnarchus niloticus..
1 Likwature Review
As the world populations have grown, so has the demand for food, especially
food rich in protein. F~shesh ave a great significance in the life of mankind, being the
most important source of protein and providing certain other useful products.
Fish is an excellent source of protein, lipids, vitamins and mineral nutrients
people need for ti good diet. Fish flesh contains water (60-84%), protein (1 5-24%), fat
(0.1 -22.0%), and mineral usually1 -2%. The proportions of the constituents are
species specific and the main variations between species cue in fat contents.
Fatty fish are usually very valuable sources of fat soluble vitamins (A, D, E
and K). The minerals present in fish muscle include potassium, sulphur, and chloride.
Trace minerals like iron, copper, iodine, bromine, rnangmese and other elements are
also found in fish muscle. Minerals are substances widely distributed in minute
quantities. They not only promote good health, but also maintain life itself (Thilsted
and Roos, 1999; Huss, 1994).
Fish protein contains all the essential amino acids comparable to milk, eggs,
and meat. It rank high because of ease with which proteins and fat contained in the
fish are digested.
Fish and other sea foods may be spoiled by autolysis, oxidation of oils or
bacterial activity or by combination of these.
1.1.2 Fish spoilage
Fish tissue is characteristic in being rich in protein and non-protein-nitrogen
(e.g. amino acids, timethylamine oxide (TMAO), Creatinine), but low in
carbohydrate resulting in a high post mortem pH (>6.0). Pelagic, fatty fishes have a
high content of lipids consisting mainly of triglycerides with long-chain fatty acids
which are highly unsaturated. Also the phospholipids are highly unsaturated and
these have important consequences for spoilage process under aerobic storage
Obvious signs of spoilage are;
detection of off- odours and off- flavours
– slime formation
– gas production
– Changes in texture
The development of these spoilage conditions in fish and fish products is due to a
combination of microbiological, chemical and autolytic phenomena (Huss, 1994).
18.104.22.168 Mbwblologiwl pUage
Initial loss of quality of fish (non preserved) lean or non fatty fish species,
chilled or not chilled, is caused by autolytic changes while spoilage is mainly due to
the action of bacteria Fish caught in tropical areas, may cany a slightly higher load
of grarn-positive organisms and enteric bacteria. The specific spoilage organisms are
producers of the metabolites responsible for the off-odours and off-flavours associated
Shewanella putrefadens is a typical specific spoilage organism for the aerobic chill
spoilage of many fish fiom temperate waters and produces Trimethylamine (TMA),
hydrogen sulphide (H2 S) and other volatile sulphides which give rise to the fishy,
sulphidy cabbage like ofi-odours and flavours. Similar metabolites are formed by
Vibrionaceae and Enterobacteriaceae during spoilage at higher temperatures. During
storage in modified atmosphere (COz-containing), a psychrophilic photobacterium
producing large amounts of TMA is one of the major spoilage bacteria. Some fresh
water fish and many fish from tropical water are during iced, aerobic storage
characterized by P,mrdomoncrs types of spoilage which is dacribed as fruity
sulphydryl and sickening. Several volatile sulphides (e.g methyl mercaptan (CH3SH)
and dimethylsulphide ((CH3hS), ketones, esters and aldehydes but not hydrogen
sulphide are produced by Pseudomonas as are several ketones, esters and aldehydes.
For fresh, non-preserved fish, the putrefaction or spoilage proceeds very rapidly once
the load of specific spoilage organisms exceeds approximately lo7 CFUIg.
Microbiological activity is also the cause of spoilage of many preserved fish products
stored at temperatures > O°C. In most cases the specific spoilage bacteria are not
known. The addition of small amounts of salt and acids, as in lightly preserved fish
products, changes the dominating microflora to consist mainly of Grarn-positive
bacterial species (Lactic acid bacteria, Brochotrix) and some of these may act as
specific spoilage organisms under certain conditiora
Strongly preserved fish products such as salt cured or fermented products spoil due to
the action of certain micro-organisms. The dominating flora on these products is gram
positive, halophlic or halotolerant micrococci, yeasts, spore formers, lactic acid
bacteria and moulds. A number of specific spoilage organism are known such as the
extremely halophilic , anaerobic gram negative rods and halophilic yeasts identified as
specific spoilage organisms by causing off-odours and-flavours (sulphidy ,fruity in
wet salted herring. An extreme halophile spoilage bacteria cause a condition known as
‘pink’. These bacteria (Halococcus and Halobacterium) cause pink discoloration of
salt , brines and salted fish as well as off odours and flavours normally associated with
spoilage (hydrogen sulphide and indole) (Huss, 1994; Clucas ,1982)
1,1.2.2 Chemkal spoilage
The most important chemical spoilage processes are changes taking place in
the lipid fiaction of the fish. Oxidation processes, autoxidation, is a reaction involving
only oxygen and unsaturated lipid. At first step leads to formation of hydroperoxides,
wich are tasteless but can cause brown and yellow discoloration of the fish tissue. The
degradation of hydroperoxides gives rise to formation of aldehydes and ketones.
These compounds have a strong rancid flavour. Oxidation may be initiated and
accelerated by heat, light (especially W light) and several organic and inorganic
substances (e.g. Cu and Fe). Also a number of antioxidants with the opposite effect
are known (alphatocopherol, ascorbic acid, citric acid and carotenoids).
1J.2.3 Aa~tot~& piIrrge
Autolytic spoilage or autolytic changes are responsible for early quality loss in
fresh fish but contribute very little to the spoilage of chilled fish and fish products.
An exception from this statement is the rapid development of off- odours and
discolourations due to action of gut enzymes in certain ungutted fish. In frozen fish
the autolytic changes are of great importance. One example is the reduction of
trimethylamine oxide (TMAO), which in chilled fish is a bacterial process with the
formation of trimethylamine (TMA). In frozen fish, however, bacterial action is
inhibited and TMAO is broken down by autolytic enzymes to dimethylamine (DMA)
and formaldehyde (FA). ‘fie effect of the FA formed in frozen fish is increased
denaturation of fish tissue, changes in texture and loss of water binding capacity.
Other enzymatic re;rcbiolr; such as formation of free fatty acids are also believed to
greatly influence the semory quality of frozen fish (Huss, 1994).
All proteinaceous foods spoil sooner or later, but a number of measures can be
taken to reduce spoilage rate. Greatest effect can be obtained by control of storage
Chemical spoilage or development of rancidity can be prevented by rapid catch
handling on board and storage of products under anoxic conditions (vaccum packed or
modified atmosphere packed).
Moulds can cause losses of fish due to discolouration or viable growth. They
may also produce mycotoxins which can have a variety of harmful effect, products
with water activity as low as 0.60, although 0.70 is the minimum as water activity
which sustains the growth of most spoilage moulds.
Deterioration may also be due to the pre-rigor mortis state sf the fish when
caught, especially when there is struggling before the fish dies. The onset and
duration of rigor mortis which contribute immensely to fish spoilage depend upon or
number of factors such as the glycogen-ATP relationship of the fish when caught, the
pH reaching acidic values when after death, the hydrolyzing activity of some enzymes
and the splitting proteins.
1.13 Common methods used in fish preservation
In many African countries including Nigeria, many food preservation method
used includes salting, smoking and drying. In most cases, smoking and drying are
combined with sal ting.
1.1 3.1 Salting
The use of salt to preserve fish is an ancient practice. Addition, of salt reduces
the amount of free water in the fish thus reduces both bacterial and enzymatic
activities. Addition of salt also favours the “osmotic dehydration of the fish flesh.
Fish preserved with salt can retain up to 30-40% of water and yet have a storage life
of six months at ambient temperature, if properly stored. The salt uptake of fish
depends on its fat content, thickness and its freshness. Fatty fish tend to have a lower
salt up- take than lean fish. Stale fish have been observed to have a higher salt uptake
than Fresh fish. Higher concentration of salt favours osmotic dehydration of fish and,
also results in a satisfactory cured product with a good storage life. When the relative
humidity of the atmosphere rises to over 75%, salt tend to absorb moisture from the
atmospheric air and may bmme moist. Salting reduces the susceptibilities of the
drying fish to insect infestation (UNIFEM, 1988).
Smoking is a traditional preservation technique that is used to prepare fish
products with long storage lives. Smoke contains substances that kill bacteria, thus
helping to preserve the product. The heat also dries the fish. OAen fish are salted
before they are smoked.
Eyo (1983) reported that smoking is the commonest and oldest method of fish
preservation in the country. Traditional smoking techniques vary widely. At its
simplest level fish may be placed in a pit containing smouldering grasses or wood,
and so cooking and favouring of fish which is usually charred and has a short storage
life. Alternatively, the fish may be laid on racks contained in an oil drum, or mud
oven, or hung on bamboo sticks in the smoke of fire.
Some smokzrs used in other parts of West hkim consists of racks raised on
poles or racks placed on top of a rectangular mud or flattened oil drum base with
openings for the fire. However, they have many disadvantages which include:
1. Constant attention is required to control the fire and turn the fish. This may
involve working throughout the night
2. The operation is both a health hazard and fire hazard.
There is little or no control over the temperature of the fire and the density
of the smoke produced.
4. The open construction leaves the fish susceptible to climatic conditions and
5. The fish product is of poor quality due to burning and charring on the outside.
Drying involves the removal of moisture from fish flesh. The rate of drying
depends on several factors, including the following:-
1. The air temperature
2. The speed of the air
3. The surface area exposed to drying
4. The relative humidity of the air.
At initial stages of drying called constant rate period the fish flesh has a large quantity
of free water and during this period an increase in the air temperature, speed of the air
and surface area exposed to drying brings about an increase in the rate of drying. A
low relative humidity favours drying. As the surface water is removed during drying,
water from the interior of the fish gradually comes to the surface. As the “free water”
content of the fish flesh decreases gradually, certain textural changes take place in the
fish flesh. As a result of both physical and chemical changes, which take place in the
flesh, the process of drying slows down. This second stage is known as falling rate
period and is reached when the water contents of the fish reaches a very low level.
The rate of the drying at this stage depends on the nature of the fish, for example,
fatty or lean and the thickness. It should be noted that if fish are dried too rapidly a
hard impermeable outer crust would form which will prevent the passage of any more
moisture. This phenomenon is known as ‘Case hardening’. When it happens, the
surface of the fish flesh looks well dried but the centre of the fish will still be moist
and could spoil. A fish, which has been damaged in this way, will be hard on the
outside, but may feel soft or spongy internally when pressed. Optimum drying
temperature tbr temperate salt-water species can be as low as 27 ‘c, whereas tropical
species can generally withstand higher temperature and can be dried using air at 45-50
OC (UMFEM, 1988, ILO, 1976).
1.1 3A Conwniionol sun drying
Natural drying methods use the combined action of the sun and wind without
the use of any equipment. All the fish surfaces must be exposed so as to dry quickly
before spoil. Traditionally, in the tropics fishermen spread the f sh on the ground, on
rocks, or on beaches to dry in the sun. Some use mats or reeds laid on the ground to
prevent contamination of the fish by dirt, mud and sand. Drying fish in this way has
many disadvantages. Sun drying has many limihtions even when racks are used. It is
Long periods of sunshine without rain are required; the quality of sun dried fish is
likely to be low due to slow drying, insect damage, and contamination lkom air borne
dust. Uniform product is difficult to obtain (ILO, 1986).
1.135 Miflcial drym
UNIFEM (1988) also reported of the development of artificial dryers to
alleviate the problems of sun drying during the rainy season in developing countries
when the traditional drying technique is impossible to carry out. Dryers which are
fuelled by ago wastes e.g. rice husk, have been tested. Such designs also have the
potential of &ording more control over temperatures, airflow and thus over the
drying process. However, the high capital and maintenance cost of these artificial
dryers compared with traditional techniques, and the additional level of skills and
training required for their operation may make them unsuitable or difficult for smallscale
fish processors to afford.
1.13.6 !Mar drying
The use of solar has been investigated as an alternative to traditional sun
drying. Solar dryers employ some means of collecting or concentrating solar
radiation with the resdt that elevated temperature and, in turn, lower relative
humidities are achieved for drying. When using solar drying:-
1. the drying rate can be mcreased;
2. lower moisture content can be attained;
3. product quality is higher;
4. the dryers are less susceptible to variations in weather.
5. ahhough drying is obviously slower during inclement weather, they do provide
shelter from the rain.
6. The high internal temperature discourages the entry of pests into the dryers and
can be lethal to any which do enter.
1.13.7 Classi@utkm of solar dryem
Solar dryers can be classified using the following characteristics:-
1. Whether or not the drying commodity is exposed directly to insolation. Solar
dryers can be termed either direct or indirect (Lwand, 1966). Direct are those in
which the commodity is exposed to sun, and indirect dryers are those in which the
crop is placed in an enclosed drying chamber and thereby shielded from insolation.
2. The means of air flow through the dryer. There are two possible types of air flow,
natural convection and forced convection.
3. The temperature of the air circulated to the drying chamber.
The air entering the drying chamber of a solar dryer can either be at the ambient
temperature or at some higher temperature, the elevation in temperature of the air
being achieved by its passage through a solar collector prior to the drying chamber.
1.1 3.8 Mdkods offlPk p m m l o n in the lropics
This includes maintaining the fish at low temperature. If fish is well chilled
with sufficient ice, they may be in an edible form for up to three weeks, depending on
the size. In some places ice may not readily be availzble to the small processor, in
which case fish can be kept cool by other means. These include;
Keeping the fish in the coolest spot available. such as the shade;
Placing damp sack over the fish. As the water evaporates from the cloth it
helps to lower the temperatures of the fish. The sacking must be kept wet and
the fish must be well ventilated.
Mixing the fish with wet grass or water weeds in an open sided container so
that the water can evaporate and cool the fish. (Burgs, 1980), (UNIFEM,
These measures are just for a few hours unless ice can be used.
1 . 1 3 9 S ~ M N ~
The use of salt to process or preserve fish is an ancient practice. Addition of
salt reduces the amount of fiee water in the fish thus reduces both bacterial and
enzymic activities. A.ddition of salt also favours the ‘osmotic dehydration’ of the fish
flesh. Often the process of salt curing is coupled with sun drying of fish. Fish
preserved with salt can retain up to 3040% of water and yet have a storage life of six
months at ambient temperature, if properly stored. The salt up take of a fish depends
on its fat content, thickness and its freshness. Fatty fish tend to have a lower salt up
take than lean fish. Stale fish have been observed to have a higher salt up take than
fresh fish. Salt tend to absorb water from the atmosphere on exposure when the
relative humidity of the air rises to 75 %. Salting reduces the susceptibility of the
dried fish to insect infestation (BOSTID, 1988).
1.1 3.10 ,rmoking
Smoking is the traditional preservation technique that is used to prepare fish
products with long storage lives. Smoke contains substances that kill bacteria, thus
helping to preserve the product. The heat also dries the fish. Often fish are salted
before they are smoked. In tropical countries, fish are smoked heavily at relatively
high temperatures so that they are cooked.
Some of the chemicals in wood smoke have been shown to have antibacterial
properties and this is used to advantage in processing fish by smoke.
Much study has been done on smoked fish in Nigeria.
Simple processing of fish is not restricted to drying. Other ways of preservation and
processing of fish are fermentation, boiling and fiymg. Fish can also be canned
aseptically in a suitable medium. This is usually heated to destroy autolytic enzyme
and to kill off micro organisms involved in spoilage (UNIFEM, 1988).
1.13.11 Factors thut a f l d the kondlhg ofmh for drying
Size; Small f~shes may be dried whole or the larger, cut to increase surface area.
Small fish may be preserved with the gut content intact while this is almost removed
in larger species.
011 content: Fish with a high content of oil are difficult to convert into good salted
and or dried products since the oil acts as barrier to salt penetration and moisture loss.
Also fish oil oxidizes readily and becomes rancid.
Fish texture: Fish with firm or moderately firm flesh are relatively easy to handle.
They can be cut without falling apart and the dried product can be transported without
breaking up. Fish with son flesh tends to tear when attempts are made to cut them
and the dried products are very fragile and tend to break up during transportation
Solar dryer: Solar dryer is a device which generates heat by absorption of solar
radiation and utilizes the heat to evaporate the moisture fiom the drying material (Fig.
1 and 2)
The difference between solar drying and sun drying is that in solar, the product
for drying is placed in an enclosure as a result heat is generated above the ambient
Fish with while in open sun drying the product is left exposed to the air. The ambient
temperature is used in the drying process which is not high enough.
In solar drying, lower moisture contents can be attained, product quality is
higher, the dryers are less susceptible to variations in weather, and the high internal
temperatures discourage the entry of pests into the dryers and can be lethal to any
Solar dryers are classified into three broad areas as follows: Direct solar dryers,
mixed-mode dryers and indirect solar dryers.
Dirrcct solar dryer, is the most common and simplest examples are the wbinet, tent
and solar dome dryer. The product to be dried is placed in a specially designed
enclosure with a transparent cover. Heat is generated when the products, as well as
the internal surface of the drying chamber, absorb solar radiation.
The heat evaporates moisture from the crops and also expands the air in the enclosure
causing the removal of moisture by the movement of air.
Mixed mode dryers. In this type the product to be dried are heated by solar radiation
incident directly on them as well as hot air from a solar heater.
Indimt solar dryer. Air is heated in a solar collector and then
channeled to the drying chambers and this dries the product.
All these types of dryers may be either natural convection dryers or forced
convection dryers employing a fan or blower (Dmteff er al., 1987; ECN 1988).
Fig 1: Solar cabinet dryer (Doe el al., 1977).
Fig.2: Solar tent dryer (Sachithananthan el a/., 1983).
These dryers usually have a wood or bamboo-framed table covered with black plastic
or glass to produce an enclosed chamber. The surface of the table can be covered with
black plastic or paint to absorb the sun’s heat. With openings at the top and bottom of
the dryer, air will be heated and flow around the fish. Fish exposed to this flow of
heated air will rapidly lose moisture, reducing drying time by as much as half over
open air-drying. Solar dryers have been constructed in Bangladesh, Indonesia,
Rwanda, Philippines, and Papua New Guinea, Burkina Faso, India. (UNIFEM,1988;
Legacy, 2004; ILO 1976; UNDP 2004).
Solar radiation is the available energy. The magnitude of the available
radiations depends on the location, time of the year and time of the day (Lwand, 1966;
Components of solar radiation
Solar radiation can therefore be seen to consists of two components; difhse
and beam radiation. Each component has different characteristics which affect solar
drying. Diffuse radiation is the one that cannot be concentrated by means of focusing
Beam radiations comes in a beam directly from the sun. Its presence can be
easily recognized by its ability to cast shadows. The sharper the shadow, the greater
the amount of direct radiation of beam radiatipn, the relative movement of the sun and
earth must also be taken into account in designing solar dryers and collectors
For the purpose of practical work it might be thought that the positioning of
the collector should be horizontal to optimize absorption of diffuse insolation.
However to maximize the effect of beam or directional radiation, the collector surface
should be tilted at right angles to the incidence beam.
Solar collectors are employed to gain useful heat energy from the sun’s
radiation. They are almost invariably used to heat either air or water. For the purpose
of fish drying, simple flat plate air heating collectors can provide the desired
temperature increase. These consist of an absorbing surface which heats up and
warms the ambient air nearest to the surface. Clear cover may be placed above the
absorber to reduce heat loss, and the collector unit may be insulated. Where a
relatively high air flow is required, a fan can be used to blow air through the collector.
However, natural convection systems are widely used and may be more appropriate.
They are therefore two stages in which the energy of the sun’s radiation is
transformed to thermal energy in the drying air. Firstly, the radiation must be
absorbed on the absorber plate. This heat is then transferred to the air by contact
between air and the absorber plate (Lwmd, 1966).
Drying is the removal of water from fish or corps. Normally, the term
‘drying’ implies the removal of water by evaporation but water can be removed by
other methods. For example, the action of salt and application of pressure will remove
water From fish (Desroisier and Rosenstock, 1960). Since water is essential for the
activity of living organisms, its removal will slow down or stop microbiological or
autolytic activity and can still be used as a method of preservation. Where drying has
evolved as a traditional method of preserving fish, the action of the sun and wind is
used to effect evaporation drying. In recent times, the controlled artificial dehydration
of fish has been developed in some industrialized countries so that fish drying can be
carried out regardless of weather conditions (Clucns, 1982). In any process of drying,
the removal of water requires an input of thermal enersy The thermal energy required
to drive off the water can be obtained from a variety of sources, e.g., the sun or the
controlled burning of oil, gas or wood. The thermal energy can also be supplied
directly to the fish tissue by microwave electromagnetic radiation or ultrasonic
heating (Clucas, 1982).
At normal temperature, fish muscle can be considered to be a gel; it remains a
gel until a considerable quantity of water has been removed. During drying
considerable shrinkage takes place, as well as other irreversible changes and dried fish
will not reconsistute to the original conditions (Clucas, 1982). During air drying,
water is removed from the surface of the fish and water moves fiom the deeper layers
to the surface. Drying takes place in two distinct phases. In the first phase, while the
surface of the fish is wet, the rate of drying depends on the condition (velocity and
relative humidity, e.t.c) of the air around the fish (FAO, 1988). When the surface of
the solid remains saturated with liquid water by virtue of the fact that movement of
water within the solid to the surface takes place at a rate as great as the rate of
evaporation from the surface, the rate of drying will remain constant; this phase is
called the constant r&3 period. Once all the surface moisture has been carried away,
the second phase of drying begins and this depends on the rate at which moisture can
be brought to the surface of the fish. As the concentration of moisture in the fish falls,
the rate of movement of moisture in the surface is reduced and the drying rate
becomes slower. This phase is called the “falling rate period.” If the material is dried
rapidly it will result in case hardening that is the outside of the product is dried while
the inside is still moist, if not discovered in time will cause the product to spoil.
1.1 3.12 Factors that influence the mte of drying
Factors that influence the rate of drying are as follows: the surface area of the
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