A batch type direct passive solar dryer was designed and fabricated to dry salted tilapia fish fillets. The dryer has a capacity a capacity of 1500g/batch. The tilapia fish species were obtained from Domita Farms Nig. Ltd,.Uyo, Akwa Ibom State and transported immediately to Nsukka in a cold storage container. The salted fish fillet was prepared to conform to thin layer drying specification of 3.0mm-5.0mm thickness before drying. The solar drying experiments were conducted at the Department of Agricultural and Bioresources Engineering, University of Nigeria, Nsukka. Nsukka is located on latitude 6056 N and longitude 7025 E. and 397m above sea level. The work was carried out as a factorial experiment with two factors in two and three levels respectively arranged in a completely randomized design (CRD) with six treatments and ten replicates. The first factor was fish with two levels, descaled fish and fish with intact scale. The second factor was common salt (sodium chloride) (NaCl) having 0%, 10% and 20% concentrations. The samples when removed from refrigerated storage were allowed to equilibrate with the ambient conditions before being prepared for solar drying. The fish were characterized to obtain its initial moisture content (M,% db),length(m), mass(g), crude protein(CP) (%), crude lipids (CL) (%), equilibrium moisture content (EQ) (%db) and ash (%). A total weight of 1200g/batch drawn from refrigerated storage were cleaned, eviscerated, heads removed, washed and filleted longitudinally to expose the backbone in preparation for solar drying. The fish fillets were treated with salt (Nacl) to prevent spoilage by microorganisms and to dehydrate the fish thereby enhancing the drying process. The experimental drying data was fitted to 19 models and the quality of fit evaluated.  A good fit is indicated by a high coefficient of determination (R2), low reduced chi- square (χ2), standard error (Se) and mean relative deviation modulus (P). Four out of the 19 models showed reasonable fit. Out of these four models,  the Wang and Singh model was  fairly good and therefore  was further modified  and ran on a non linear regression program (NLREG) which gave higher R2, and lower χ2, Se and P when compared with the original Wang and Singh model  of the  form MR = M +At +Bt2. A is the initial drying constant; B is the final drying constant, MR is the moisture ratio, M is a constant. A fair quality fit to the experimental data when modified yielded the new model. MR = M+ 0.048 t 0.8 + 0.067 t 0.76. For the new model R2 varied between 0.9779 and 0.9989, Se varied between 0.0014067 and 0.042528, χ2 varied between 1.8495×10-6 and 0.00189 and P varied between 0.0139 and 29.78% respectively. The length of tilapia fish was found to vary between 9cm and 13cm, weight varied between 50g and 100g.The proximate composition of tilapia fish were:  initial moisture content (Mo) varied between 254.43 – 293.13%(db), crude protein (CP) varied between 13.0 and 16.25%, crude lipids (CL) varied between 15.5 and 27.83% and ash between 14.74 and 16.16%, equilibrium moisture content (EQ) varied between 15-20% (wb). Drying kinetics of salted fish had moisture ratio (MR) varying between 0.022 – 1.0 at 10 % salt concentration, 0.0408 – 1.0 at 20% salt concentration. Mo was between10.38 – 573.4 and 18.63 – 564.48 for 10% and 20% respectively. Fish fillets without salt treatment had MR varying between 0.020 – 1.0, Mo varied between 12.82 –564.89.  Physico-chemical quality had the following values  microbial count (MC) varying between 0.6×104 -1.1×104 CFU/g (colony forming unit per gram),Fungal count (FC) varied between 0.7×103 – 4.6×104CFU/g, total volatile basic nitrogen (TVBN) varied between 12.0 –20.7 mg N/100g flesh (milligram nitrogen per 100g flesh). For fish fillets without salt treatment, MC varied between 0.7×104 -1.3×104,FC varied between 0.9×103 – 2.8×103,TVBN was between 29 -50.TVBN for control was above 30.0. Specific solar radiation available varied between 32.53 -37.70MJ/m2.



1. 1 General Background of Research

Fish is one of the most diverse groups of animals known to man with more than 20,500 species in existence (Lagler et al.1977) as referenced by Eyo (2001).These species consist of  teleost or the bony fishes (20,000 species) and the elasmobranchii or cartilaginous fishes (500 species).  The bony fishes are of two types, the pelagic and demersals. The pelagic are the shoal moving fatty fishes such as sardines, mackerel, bonga, herring, tuna etc which move at the surface of the water bodies. The demersals dwell at the bottom of the water bodies with less than five percent fat in their tissues. Examples of demersals are cod, haddock and croakers amongst others.

 Fish is highly perishable especially in the tropical temperature and needs to be preserved for long term storage. Soon after harvest; the fish dies and begins to spoil. In a healthy fish, all the complex biochemical reactions are balanced and the fish flesh is sterile. After death however, irreversible changes begin to occur resulting in fish spoilage. The resultant effect is the decomposition of the fish (Akinola et al. 2006). A lot of factors are responsible for fish spoilage. Some of these factors are the fish health status, the presence of parasites, bruises, and wounds on the skin and mode by which fish was captured. The caught fish quality depends on the handling and preservation, the fish received from the hands of fishers after capture (Akinneye et al. 2007).The handling and preservation practice after capture affects the degree of the spoilage.  Wrong fishing method does not only cause mechanical damage to the fish but also creates stress and the conditions which accelerates fish deterioration after death (Okonta and Ekelemu, 2005). The less developed countries capture 50%   of the world’s  harvest and a large proportion of this is consumed internally (FAO,1999) as referenced by Eyo (2001) .In some Asian countries more than 50%   of the animal protein intake comes from fish whereas in Africa the proportion  is 17.5%  (Willman et al.1998). Here in Nigeria fish contributes about 40% of animal protein intake (Olatunde,1989) as referenced by Eyo (2001). The implication of this fact is that any shortfall in fish availability will affect the animal protein intake of people in tropical countries.

1.1.1 Importance of fish

The uses of fish are so enormous in practice such that it is enumerated as follows.

Use of fish as food

Fish has long been recognised as a healthy food in many countries of the World. Fish is one of the most abundant animal protein foods available in the tropics. It is also rich in minerals and essential fatty acids, vitamins A and D and rich in iodine. Above all man can digest it easily. In Southern part of the World, fish is an essential part of their nutrition often providing most, if not all their animal protein.  In the North too, fish is considered a healthy food choice and is therefore in demand. Appropriate processing of fish enables maximal use of raw material and production of value added products which is obviously the basis of processing profitability. The processing of fresh water fish like other food raw materials should ensure best possible quality; provide proper form of semi-processed final products. It should apply the most appropriate processing method and reduce wastes to the barest minimum

Fish for controlling diseases: Diseases like malaria, yellow fever and other dreadful diseases that are spread through mosquitoes can be controlled. Larvivorous fish eat larva of mosquito. The important larvivorous fish are Gambusia, Panchax, Haplochitus, Trichogaster, etc

Aesthetic value of fish: A large number of fishes are cultured in aquarium for their beauty and graceful movements. The important aquarium fish are Macropodus, Trichogaster, Carassinus (gold fish) and Pterophyllum (angel fish)

Fishery bye-products:

(a) Fish oil: The by-products obtained during the drying process include fish oil, which when taken, can help regulate cholesterol in the body. It can be extracted from the liver of the sharks, sawfishes, skates and rays and has medicinal value. These mainly include cod liver oil and shark liver oil .There is a wide variety of delicious dishes that can be prepared from dry fish.  Fish meal is a commercial product made from both whole fish and the bones and offal from processed fish. It can be used as protein concentrate animal feed. It is a brown powder or cake obtained by pressing the whole fish or fish trimmings to remove the fish oil. Fish emulsion is a special kind of coat that is produced as a by-product. Example is the fertilizer emulsion that is produced from the fluid remains of fish processed for fish oil and meal industrially. The emulsion is anti corrosive and has wide industrial usages as paints and coating.

(b) Fish manure: The fish waste after the extraction of oil, is used as fertilizers,

(c) Fish glue:  It is a sticky product, obtained from the skin of the cod and is used as gum.

(d) Isinglass: It is a gelatinous substance, obtained from the air bladder of perches, Indian Salmons and cat fish used in the preparation of special cement and in the clarification of wine and beer

(e) Shagreen: The skin of sharks and rays, which has pointed and sharp placoid scales are used in polishing the wood and other materials. It is also used for covering the jewellery boxes and swords,

(f) Leather: A highly durable type of leather is prepared from the skin of sharks and rays,

(g) Artificial pearls: The silvery boney scales of cypnmids (a type of fish) are used in the manufacture of artificial pearls especially in France.

h)Fish meal : Many fishes are dressed and cooked. Then they are dried. Fishes are made into fine powder. This is called fish meal. It is used by weak and convalescent people.

i) Fertilizers: The ‘fish waste’ materials have more calcium, protein, phosphorous and other substances. These materials are ground into powder. It is used as fertilizer to Grape gardens, Coffee and Tea plants.

Fish harvesting, handling and distribution provide livelihood for millions of people as well as providing foreign exchange earning to many countries. (Al-Jufaili and Opera, 2006).

 j) Employment: Development of fishing industry generates more employment opportu­nities.

k) Source of Income: The fishing industry has brought a lot of income to the farmers in particular and the country in general. Now we can talk about “Blue Revolution” (fish production) on the same lines as ‘Green Revolution’ (for producing enough food for all).

1.1.2 Importance of fish drying Drying basically dehydrates or removes the moisture from the fish. This action inhibits the growth of bacteria, mold, and yeast. It also slows down the enzyme action without deactivating them .This factor ensures that fish does not spoil easily and hence makes drying an effective fish preservation technique. Since drying removes water from the fish, the weight of the fish is also reduced. This makes the fish lighter and also shrinks in size.

1.1.3 Importance of thin layer drying and pretreatment of fish with salt.

Thin layer drying is a process of drying of individual particles or grains of material which are fully exposed to the drying air. In thin layer drying, the product thickness is sufficiently small such that the air flow characteristics, humidity and temperature is considered uniform at a given section parallel to the product. The process is often divided into two periods of drying: (1) the constant drying rate period and (2) the falling drying rate period. Radiation, conduction or convection provides the energy needed for drying.

It has been observed that fish is a highly perishable food material especially when solar drying is considered. Since equilibrium moisture content is likely to be achieved in solar drying after several hours as compared to drying with firewood, natural gas or electricity, pre-treatment is often recommended. The pre-treatment with salt helps to preserve the quality of fish until equilibrium moisture content is achieved. The pre-treatment   is able to accomplish this task by decreasing water activity in the fish, inhibits the growth of micro organisms by drawing out water from their tissues through osmosis.

1.1.4 Importance of modeling

            Modeling is a process of providing a representation of the construction and working of some system of interest. A model is often simpler than the system it represents. The main purpose of a model is to enable the analyst to predict the effect of changes to the system. A model should be a close approximation to the real system and incorporate most of the salient features.  It should be simple and easy to experiment. Simulation practitioners recommend increasing the complexity of a model iteratively. Simulation is a tool to evaluate the performance of existing or proposed model under different configurations of interest over long periods of real time. A simulation of a system is the operation of a model of the system. Model can be reconfigured and experimented with, usually this is impossible, too expensive or impractical to do in the real system it represents.

Advantages of Simulation Modeling: When simulation modeling is properly applied to a system under study, the following advantages are obtained.

i) A better understanding of the system by developing a mathematical model of a system of interest and observing the systems operation in detail over long periods of time.

ii) Test the hypotheses about the system for feasibility.

iii) Compress time to observe certain phenomena over long periods or expand time to observe a complex phenomenon in detail.

iv) Study the effect of certain informational, organizational, environmental and policy changes on the operation of a system by altering the systems model. 

v) Experiment with new or unknown situations about which only weak information is available.

vi)  Identify the “driving” variable –one that performance measures or most sensitive to and the inter –relationship among them.

vii) Identify bottlenecks in the flow of entities (materials, people, etc) or information.

viii) Use multiple performance matrices for analyzing system configurations

ix) Employ systems approach to problem solving.

x) Develop well designed and robust systems and reduce system development time.

1.2 Statement of Problem

In the past, various researches have been conducted on fish drying using various methods. These include the use of conventional fossil fuel energy sources, solar and a combination of them for drying fish. In most cases it makes the final cost of the fish prohibitory and unaffordable whereas in other cases spoilage may result. Due to this high cost coupled with unavailable / epileptic power supply of the energy during drying some farmers resorted to sun drying .This method too has their attendant shortcoming. Some of the problems of sun drying include attack by rodents and insects; success also depends on the weather condition which is beyond the control of the fish farmer. Solar dryer therefore becomes a suitable alternative for farmers who could not afford to dry their products using conventional energy sources with its attendant benefits.  Having known that tilapia fish can be dried in thin layer in a  solar  dryer effectively and having found that no thin layer drying model exist for tilapia fish fillets, this research was conceived to bridge the gap in knowledge.

1.3 Justification

        The purpose of modelling is to allow engineers to choose the most appropriate method of drying for a given product as well as to choose suitable operating conditions. Moreover, full scale experimentation for different products and system configurations is sometimes costly and not possible; hence the prediction of drying kinetics of specific crops under various conditions is very useful in the design and optimization of dryers. The results of the research will provide a procedure for selecting optimum solar drying conditions that would guarantee the retention of desirable qualities of tilapia fish after drying. The work will enable accurate prediction of drying rates and drying times thereby help a processor map out a schedule for drying a known or anticipated quantity of tilapia fish. The results from the research will provide invaluable industrial information, which have hitherto been unavailable, for energy efficient and high –quality drying of tilapia fish. The results from the research will provide information on the equilibrium moisture content of tilapia fish at prevailing temperature and relative humidity which are useful in designing efficient solar dryers. From the standpoint of processing, preservation and storage of tilapia fish with high efficiency and economy, the research is justifiable

1.4       Research Objectives

The major objective of this work is to identify a thin layer drying model that best suits the solar drying of salted fish fillets in a direct   passive solar dryer. Specificobjectivesinclude:

  1.  To design, fabricate and test  run a direct passive solar dryer.
  2.  To  select, and evaluate a thin layer solar dryer model for salted tilapia  fish fillets
  3.  To characterize the tilapia fish used in the study to obtain the initial moisture content, length, mass, crude protein and lipids, ash, and equilibrium moisture content.
  4.  To investigate  the drying kinetics of  the salted  fish fillets  in the direct   passive  solar dryer using the   identified thin layer drying model
  5.  To establish the physico- chemical qualities of the dried  salted fish  fillets
  6. To evaluate the specific solar radiation available, optimum angle of inclination of the 

 dryer possible at any location using the program codes written in MATLAB.

1.5  Scope of the Research The Tilapia fish (Oreochromis Niloticus ) is an important fish species  reared in most aquacultural  systems in various parts of the world. It is highly valued due to its low fat content, slim size and high quality protein. In Nigeria it is also abundantly harvested in the wild as well as being reared. Losses are often encountered due to poor or absence of storage systems. In view of the inherent advantages as listed, it was chosen for this research. The work will be limited to the drying kinetics encountered in direct passive solar drying of eviscerated salted tilapia fish fillets.