Review ARticle Journal of Agricultural and Marine Sciences 2021, 26(2): 1–9 DOI: 10.24200/jams.vol26iss2pp1-9 Received 25 Oct 2020 Accepted 22 March 2021 A Review on Solar Drying of Fish Nasreen S. Al-Mahruqi and Abdulrahim M. Al-Ismaili* Abdulrahim M. Al-Ismaili*( ) abdrahim@squ.edu.om; ams.ismaili@ gmail.com, Department of Soils, Water and Agricultural Engineering, Sultan Qaboos University, Oman Introduction Oman is considered one of the largest fish pro-ducers and consumers in the Gulf Cooperation Council (FAO, 2015). Fish is a very popular food due to its high protein and nutrients content. Howev- er, the due to the high moisture content in fish (Tiwari et al., 2009), spoilage is a critical issue facing fish pro- ducers. Because of the high spoilage rate of fish (Jain and Pathare, 2007), Ghaly et al. (2010) found that 30% of fish is lost every year. Therefore, several techniques have been practiced to preserve fish in order to increase their shelf life and to maintain their texture, flavour and nutritional value (Ghaly et al., 2010). Smoking, drying, chilling, brining and freezing are among the common- مراجعة على التجفيف الشمسي لألمساك نسرين س. احملروقية ، عبدالرحيم م. اإلمساعيلي * Abstract. Oman is one of the major fish producers in this region. Fish is highly perishable, therefore different pres- ervation techniques, such as smoking, drying, chilling, brining and freezing are being used. Solar drying is one of the most popular technique due to its simplicity and low cost as compared to other techniques. This study aimed to review the different types of solar drying techniques and highlighted the quality measures of solar dried fish. Solar drying tech- niques can be divided into three types: open-sun drying, direct and indirect solar drying. The open-sun drying is the most adoptable method because it is the cheapest preservation technique. However, this technique has several draw- backs, such as the uncertainty of weather, large implementation area, required time, poor drying rate, high labor costs, possible attack by insects, microorganism and birds, and contaminated with dust and foreign materials. Solar dryers, on the other hand, overcome the most of drawbacks associated with open-sun drying. They have shorter drying time and higher drying rate, and at the same time these enhance the physical properties of dried fish. For better understanding of the drying processes many regression models were used and the exponential model was found to be the best fitted mod- el describing the drying behavior. Fish possesses good nutritional value due to higher amount of proteins, lipids and ash contents. For higher shelf life, fish has to meet certain characteristics with respect to pH, water activity, microbial load, total volatile base nitrogen (TVB-N), trimethylamine nitrogen (TMA-N) and enzymatic autolysis. For a very good qual- ity, the pH must be ranged from 6.0-6.9 and the water activity must be lower than 0.6. The TVB-N and TMA-N are the indicators of spoilage and their upper acceptable limits are 10-15 mg/100g and 35-40 mg/100g, respectively. Total plate count (TPC) and total fungal count (TFC) are two attributes used to assess the microbiological quality of fish products. The autolysis changes in the fish lead to spoilage as a result of the production of biogenic amines and microbial growth. Studying the health aspect of dried fish is very important for the human body to obtain a greater proportion of proteins and important substances away from the harmful chemicals that may appear in traditional draying technique. Keywords: Solar drying techniques, fish, quality امللخص:عمــان هــي واحــدة مــن أكــر منتجــي األمســاك يف هــذه املنطقــة، وألن األمســاك قابلــة للتلــف بدرجــة كبــرة فإنــه يتــم اســتخدام تقنيــات خمتلفــة حلفظهــا؛ مثــل التدخــن والتجفيــف والتريــد والتخمــر والتجميــد، ويعــد التجفيــف الشمســي مــن أكثــر التقنيــات شــيوًعا نظــرًا لبســاطته وتكلفتــه املنخفضــة مقارنــة ابلتقنيــات األخــرى. ولقــد هدفــت هــذه الدراســة إىل مراجعــة األنــواع املختلفــة لتقنيــات التجفيــف الشمســي وإبــراز مقاييــس اجلــودة لألمســاك اجملففــة مشســيا. وميكــن تقســيم تقنيــات التجفيــف الشمســي إىل ثالثــة أنــواع: التجفيــف الشمســي املفتــوح، والتجفيــف الشمســي املباشــر وغــر املباشــر داخــل اجملففــات الشمســية، ويعتــر التجفيــف حتــت أشــعة الشــمس املفتوحــة هــو األســلوب األكثــر اســتخداًما ألنــه أرخــص تقنيــة للحفــظ، ولكــن هــذه التقنيــة هلــا العديــد مــن العيــوب؛ مثــل تقلبــات الطقــس، واملســاحة الكبــرة الــي حتتاجهــا، وطــول الوقــت املطلــوب للتجفيــف، وبــطء معــدل التجفيــف، وتكاليــف العمالــة املرتفعــة، وتعرضهــا للحشــرات والكائنــات احليــة الدقيقــة والطيــور، واحتماليــة التلــوث ابلغبــار واملــواد اخلارجيــة. ولقــد وجــد أن اجملففــات الشمســية تتغلــب علــى معظــم العوائــق املرتبطــة ابلتجفيــف حتــت أشــعة الشــمس املفتوحــة وذلــك ألن اجملففــات الشمســية حتتــاج لوقــت جتفيــف أقصــر وهلــا معــدل جتفيــف أعلــى، ويف نفــس الوقــت فإهنــا حتســن مــن اخلصائــص الفيزايئيــة لألمســاك اجملففــة، ولفهــم عمليــات التجفيــف بشــكل أفضــل قــام الباحثــون ابســتخدام العديــد مــن منــاذج االحنــدار، ووجــد أن النمــوذج األســي هــو أفضــل منــوذج مالئــم يصــف ســلوك التجفيــف. ومــن انحيــة أخــرى؛ فــإن األمســاك متتلــك قيمــة غذائيــة جيــدة بســبب احتوائهــا علــى كميــات كبــرة مــن الروتينــات والدهــون وحمتــوايت الرمــاد، وللحصــول علــى عمــر ختزيــي أعلــى فإنــه جيــب أن تفــي األمســاك خبصائــص معينــة فيمــا يتعلــق بدرجــة احلموضــة والنشــاط املائــي واحلمــل امليكــرويب وإمجــايل النيرتوجــن األساســي املتطايــر )TVB-N(، والنيرتوجــن ثالثــي ميثيــل األمــن )TMA-N( واالحنــالل الــذايت األنزميــي، وللحصــول علــى جــودة جيــدة جــًدا جيــب أن يــرتاوح الرقــم اهليدروجيــي بــن 6.0-6.9 وأن يكــون نشــاط املــاء أقــل مــن 0.6. ويعــد TVB-N و TMA-N مهــا مؤشــرا التلــف وحدودمهــا القصــوى املقبولــة هــي 10-15 جمــم/100 جــم و 35-40 جمــم/100 جــم علــى التــوايل، وأمــا عــن إمجــايل عــدد الصفائــح )TPC( وإمجــايل عــدد الفطــرايت )TFC( فهمــا مستــان تســتخدمان لتقييــم اجلــودة امليكروبيولوجيــة ملنتجــات األمساك. وتؤدي تغرات التحلل الذايت يف األمساك إىل تلفها نتيجة إنتاج األمينات احليوية والنمو امليكرويب. وتعتر دراسة اجلانب الصحي لألمساك اجملففة أمرًا مهًما جــًدا لصحــة اإلنســان مــن أجــل احلصــول علــى نســبة كبــرة مــن الروتينــات واملــواد املهمــة بعيــًدا عــن املــواد الكيميائيــة الضــارة الــي قــد تظهــر يف تقنيــة التجفيــف التقليديــة. الكلمات املفتاحية: طرق التجفيف التقليدية، األمساك، اجلودة. 2 SQU Journal of Agricultural and Marine Sciences, 2021, Volume 26, Issue 2 A Review on Solar Drying of Fish ly-practiced techniques for fish preservation (Ghaly et al., 2010). However, fish drying is the most popular technique (Jain and Pathare, 2007; Sahu et al., 2016), which is achieved using different approaches such as solar, electrical, spray and mechanical drying meth- ods (Prakash and Kumar, 2014; Singh et al., 2017). Due to the increase in the prices of fossil fuels, solar dry- ing became the most widely-used drying method as it uses a renewable source of energy and does not rely on fossil fuel (Bala and Janjai, 2009; Prakash and Ku- mar, 2014). The objectives of this article were to re- view the different types of solar drying techniques and to highlight the quality measures of solar dried fish. Solar Drying Techniques Solar energy is the most abundant source of energy on the earth and consequently, it is used in many pro- cesses. Solar drying is a traditional drying process that harnesses solar energy to speed up the drying process. There are three types of solar drying techniques, which are open-sun drying, direct and indirect solar drying (Sahu et al., 2016). The most popular technique is drying by the direct exposure to sun which is considered the cheapest technique as it does not demand a significant infrastructure and operational costs (Jain and Pathare, 2007) (Figure 1). When the product is exposed to sun, its temperature increases due to heat absorption, which in turn leads to moisture reduction, i.e. drying. In the direct solar drying technique, the product is placed inside a structure that is covered with a trans- parent material, such as plastic and glass (Figure 2). This kind of structure traps the solar heat (greenhouse effect) as it allows shortwave radiation to pass and cap- tures the longwave radiation (Singh et al., 2017), i.e. causing higher air temperatures and thus, faster drying (Sahu et al., 2016). In this technique, drying rate can be additionally increased by the use of fans to bring dry- er air over the products and this technique is known as “forced convective” direct drying (Singh et al., 2017). In the indirect drying techniques (Figure 3), products are not exposed to direct sun light but instead, ambient air is heated by solar radiation via a solar collector and then the hot air flows over the products. The air is circu- lated in this technique either by natural convection (i.e. indirect dryer under passive mode) or by the use of ex- haust fans. In the former, ambient air enters to the solar collector through a lower vent and then the hot air leaves to the drying chamber through a higher vent, i.e. ther- mosyphic effect (Prakash and Kumar, 2014). In the latter, exhaust fans located at the outlet vent are used to move the air stream and this technique is known as forced convection indirect solar drying (Sahu et al., 2016). The forced convection dryer is more suitable for products with high moisture content while natural convection is suitable for low moisture products (Sahu et al., 2016; Singh et al., 2017). Prakash and Kumar (2014) found that the convective mass transfer coefficient in the forced convection mode was double than that in the natural convection mode. As illustrated in Figures 2 and 3, the structure of direct and indirect solar dryers is similar to the structure of greenhouses, which may explain the rea- son why these dryers are also called “greenhouse dryers”. The efficiency of solar dryers is enhanced by the in- tegration with photovoltaic (PV) systems. The use of PV-integrated solar dryers is very practical if forced con- vection is to be implemented in the areas where electric- ity is not available or not affordable (Sahu et al., 2016). In terms of solar drying performance, the greenhouse dry- er is considered the best alternative to sun drying. Bala and Janjai (2009) reported that PV-integrated green- house solar dryers can reduce the drying time by almost 50%. The shape of the greenhouse is another factor af- fecting the drying process. It was reported by Charters et al. (2017) that the dome shape (hemispherical) can offer the maximum utilization of solar radiation and the even-span shape is utilized for proper air mixing. Figure 1. Solar drying by direct sun exposure for (a) Red chili pepper (Walters and Jha, 2016), and (b) Fish (Ochieng et al., 2015). 3Review Article Al-Mahruqi and Al-Ismaili Advantages and Disadvantages of Solar Drying Solar radiation is an abundant, eco-friendly and inex- haustible source of energy (Ghaly et al., 2010). Hence, it represents the cheapest way to preserve food (Jain and Pathare, 2007). Nevertheless, there are many dis- advantage related to open-sun drying method, such as the uncertainties of weather, requirement of large im- plementation area, time-consuming, poor drying rate, high labour costs, attacking by insects, microorganism and birds, and mixing with dust and foreign materials (Jain and Pathare, 2007; Al Rawahi et al., 2013; Martunis, 2013; Sontakke and Salve, 2015). Mansur et al. (2013) found that, during open-sun drying, some fish samples contained a large amount of debris from the poor quality underlying material and the dried samples have under- gone surplus drying or inappropriate drying and han- dling. However, some of these limitations, e.g. time-con- suming, insect attacks and dust contamination, can be eliminated by the use of greenhouse dryers (Charters et al., 2017; Sontakke and Salve, 2015). Solar Fish Drying In many coastal areas, fish is considered a main source of food and income (Belwal et al. 2015). However, fish products are highly perishable and thus, have a short shelf life (Bala and Mondol, 2001). Drying is the most widely-used preservation technique used to overcome the spoilage problem. Because of their high market value and availability, several fish species such as ribbon fish, golden anchovies, croker, prawns, paplet and surmai are dried using solar drying techniques (Sengar et al., 2009). In Oman, due to the high fish production and con- sumption, traditional sun drying techniques are widely practiced to dry a variety of fish types such as sardine, anchovy and jake mackerel (Al Bulushi et al., 2013; Al Rawahi et al., 2013). Sardine fish alone represents 80% of the total Oman small fish catching and the annual amount of dried sardine is 23000 tons, which is mainly used as a livestock feed (Al-Jufaili and Al-Jahwari, 2011; Basunia et al., 2011). Open-sun drying of fish is an old method used worldwide to dry a variety of fish types. In Oman, as an example, sardine fish (Figure 4) are tra- ditionally dried by dispersing the fish on sandy beaches for 7 days in low ambient temperatures and 4-5 days in high temperatures (Basunia et al., 2011). This method, although widely-accepted, is associated with a substan- tial loss of almost 30-40% of the total dried amount due to a variety of reasons that include rain, wind, dust and contamination and consumption by animals, birds and rats (Al Rawahi et al., 2013). Other solar drying techniques such as greenhouse dryers are implemented to overcome the drawbacks of open-sun drying. For instance, Sengar et al. (2009) used a low cost passive (natural convection) multi-shelf greenhouse solar dryer (Figure 5) to dry prawns. The fish dried inside the solar dryer were found better than the fish dried in open-sun in terms of drying time, texture and color. In a similar study, silver jewfish was dried by a natural convection solar tunnel drier in Bangladesh (Bala and Janjai, 2009). The solar dried products had better quality with a significant reduction in drying time as compared with open-sun drying. In another study, a low cost passive solar tunnel dryer (Figure 6) was de- veloped in Oman to dry sardine, skater bream and jake mackerel (Al Rawahi et al., 2013). On the other hand, an active (forced convection) greenhouse solar dryer was used in Indonesia for drying anchovy fish (Figure 7) (Martunis, 2013). In a similar study, jewfish was dried in a hybrid solar drying system and it was found that the drying time took only 8 h to reduce the moisture content from 64 to 10% (Singh et al., 2017). All of the abovemen- tioned dryers managed to reduce the drying time with increased water loss. Several studies focused on the calculation of drying time, drying rate and the final moisture content of the Figure 2. Direct solar dryer (Singh et al., 2017). Figure 3. Indirect forced convection solar dryer; 1: Vent , 2: Exhaust fan , 3: Heating chamber, 4: Drying chamber and 5: Glass cover with fibre (Singh et al., 2017). 4 SQU Journal of Agricultural and Marine Sciences, 2021, Volume 26, Issue 2 A Review on Solar Drying of Fish dried fish products. For example, the drying time to reduce the initial moisture content of prawns from 75 to 16.4% in open-sun drying and to 16.5% in a low-cost passive solar dryer was 11 and 8 h, respectively (Sengar et al., 2009). Similarly, Abraha et al. (2017) found that open-sun drying of anchovy took 5 days and inside a solar tent drier, it took only 3 days. Using a natural con- vective solar drier, it was found that the drier was able to reduce the initial moisture content of bayad fish flakes from 78.67% to a final moisture content of 11.41% (Ba- biker et al., 2014). Mustapha et al. (2014) studied five dif- ferent solar driers and found that black stone-inserted glass drier showed the fastest drying rate in comparison to the plastic drier, mosquito net drier, glass drier and aluminum drier for drying African catfish and Nile tila- pia. Martunis (2013) reported that the drying rate of fish using a force convection greenhouse was 3.29% per hour in a total drying time of 11 h while the same amount of fish took 2 days to dry in the sun. Using a parabolic-dish solar collector (Figure 8), Solomon et al. (2016) found that salted prawn and unsalted prawn required 8 h and 15 h to dry, respectively which highlights the effect of salting on the drying time. In all drying processes, the drying time increased with increasing humidity in the ambient air (Bala and Janjai, 2009). In general, the drying rate starts high at the beginning of the drying process and decreases with the reduction in moisture content (Jain and Pathare, 2007). The solar drying technique has a great effect on the physical properties of dried fish. Under open-sun drying, (Mansur et al., 2013) compared three physical character- istics, namely, color, odor and texture, of three types of fish; labeo, channa and wallago attu. The results showed that labeo and channa fish had better quality than walla- go fish which developed bitter taste, rancid odor and soft and fibrous texture. However, all three fish had brown color as compared to the fresh fish which have cream color. In a similar study the channa fish showed good physical quality as compared with wallago and glosso- gobius fish (Majumdar et al., 2017). Islam et al. (2012) reported differences in the physical properties of dried mola fish in a solar tunnel. The final color of the dried fish was ranging from white to light brown and the tex- ture was firm and flexible with good odor. The overall quality of the dried mola fish was excellent as compared with the traditionally dried mola fish, where the color was brown and the texture was soft with off-odor. In general, Mustapha et al. (2014) found that the physical parameters of open-sun dried fish was the least accept- able among the fish dried in five other solar driers. Abraha et al. (2017) reported that the physical prop- erties, such as color, flavor, appearance, texture and odor was superior for fish dried in a solar tent drier to fish dried in open-sun. For instance, anchovy fish dried in a greenhouse dryer undergone no color change during the drying process (Martunis, 2013). Sengar et al. (2009) Figure 4. Traditional open-sun drying of sardine, (a) Sar- dine fish dispersed on a sandy beach and (b) Animals eating from the dried sardine (Al-Jufaili and Al-Jahwari, 2011). Figure 5. Low cost passive solar dryer (Sengar et al., 2009). 5Review Article Al-Mahruqi and Al-Ismaili found that the color and texture of salted fish inside a low cost dryer were better as compared with unsalted fish, but the dried fish in open-sun was the least accepted one. In a similar study, the addition of salts in the dried fish affected positively the physical (organoleptic) prop- erties, such as aroma, taste, texture and general accept- ability. Solomon et al. (2016) observed good taste and total acceptability of salted fish were superior to those of unsalted fish when both were dried in a solar drier. Therefore, direct and indirect solar drying techniques provide better quality fish than open-sun drying and salt- ed fish gave better physical properties than unsalted fish. For better understanding of the behavior of drying processes, regression models were extensively used. To optimize the drying process, accurate simulation-mod- els can be to predict the performance of the product to be dried (Belessiotis and Delyannis, 2011). Non-linear regression models were used to best-fit experimental drying curves with the closest matching models. The drying rate could be either constant (fixed) or falling (de- creasing), however the falling rate prevails (Toujani et al., 2013) because in most biological products, the constant rate does not exist (Bellagha et al., 2002). In the drying process, the moisture is removed rapidly at the beginning then it decreases slowly as the drying progresses. This is because at the beginning moisture evaporates from the surface of the fish and then moisture moves by diffu- sion inside the fish material toward the surface (Toujani et al., 2013). This behavior is described using different regression models such as exponential, logarithmic, diffusion-approximate, two-term and many others. Hubackova et al. (2014) found that the appropriate models describing the natural convection solar drying kinetics of five species of fish were as follows; loga- rithmic model for climbing perch and Nile tilapia fish; diffusion-approximate model for swamp eel and walk- ing catfish; and two-term model for channa fish. In an- other study, the drying kinetics of sardine muscle were well-fitted using two models, namely, exponential and page (Djendoub et al., 2009). Figure 9 illustrates the ex- ponential drying curve of tilapia fish (Kituu et al., 2010). On the other hand, a logarithmic regression model was used to describe the drying rate of prawn and chelwa fish under open-sun drying (Jain and Pathare, 2007). In electric oven-drying, the most appropriate mod- el for all fish species is the Modified Page1 because of the uniform drying conditions except for channa fish where the exponential model showed the best-fit. All mentioned models can help in predicting the dry- ing kinetics of fish using different drying techniques. Health aspects of solar-dried fish The quality of dried fish has to meet certain character- istics with respect to protein, lipid and ash content. Re- cent studied proved that dried fish with high amount of protein, ash and fat content have very good nutritional value (Siddique et al., 2012). Proteins control the metab- olism in the nerves and bones and control blood sugar level by producing peptides (Jónsson et al., 2007). The protein content in dried fish is higher compared to fresh fish, which is a desirable characteristic. The protein is an important factor that helps in the quality assessment of dried fish (Hazarika et al., 2016). Lipids and ash are two important sources of energy for the body. It was found that the amount of lipids is affected by the dry- ing technique where it was reported that lipid content of open sun dried fish is less than that in fish dried with other drying methods (Anh et al., 2015). Commonly, ash content of dried fish is higher than that of fresh fish. However, it could be less in the dried fish because the ash content comes from inorganic matter, i.e. a residue that remains after drying, where water content and or- ganic matter are reduced (Oladipo and Bankole, 2013). When the values of protein, lipids and ash content were analyzed for 10 fish species dried under open sun, they ranged 28.63-53.84, 4.42-16.52 and 8.96-30.30 g/100 g, Figure 6. Fish drier diagram (Al Rawahi et al., 2013). Figure 7. Force convection greenhouse solar dryer (Mar- tunis, 2013). 6 SQU Journal of Agricultural and Marine Sciences, 2021, Volume 26, Issue 2 A Review on Solar Drying of Fish respectively (Hazarika et al., 2016). In a similar study, Islam et al. (2013) reported that the protein, lipids and ash content for 4 fish types, namely, Amblypharyngodon mola (Mola), Puntius spp. (Punti), Channa punctata (Taki) and Glossogobius giuris (Bele), dried under open- sun amounted to 32.02-41.38 g/100 g (Channa puncta- tus had the highest value), 3.21-14.03 g/100 g (Channa punctatus had the lowest value) and 20.14-24.40 g/100 g. Sultana et al. (2008) compared the protein, lipids and ash content of fresh silver jew fish, Bombay and duck ribbon fish with the dried samples in a solar tunnel dry- er. The protein content of fresh fish ranged from 65.90- 71.60 g/100 g, lipids ranged from 13.42 to 21.30 g/100 g and ash content from 11.27 to 12.44 g/100 g where- as after drying they ranged 71.9-80.52, 8.05-19.18 and 9.47-10.24 g/100 g, respectively. Similarly, Babiker et al. (2014) found that the protein, fat and ash content of dried bayad fish flakes using a natural convective solar drier were 69.19, 0.23 and 30.68 g/100g of product, re- spectively while, the fresh fish contained 92.51, 0.22 and 7.27 g/100 g of product, respectively. Across the developing countries, dried fish is con- sidered a widespread delicacy. Therefore, extending the shelf life of dried fish is necessary for long time storage. To achieve this, dried fish has to meet certain charac- teristics such as pH, water activity, microbial load, total volatile base nitrogen (TVB-N), trimethylamine nitro- gen (TMA-N) and enzymatic autolysis which differ due to the fluctuations in processing conditions (Al Bulushi et al., 2013). Lower pH values of dried products offer more enhancement in microbial inhibition and conse- quently, increase the shelf life of dried fish by preventing endogenous proteases activities (Majumdar et al., 2017). Dried fish under a pH range of 6.0-6.9 are considered to be of very good quality (Kakati, 2017). This implies that the loss of fish quality can result from the increase in pH. In literature, Puntius sophore, Setipinna phasa, Amblypharyngodon mola, Pseudeutropius atherinoi- des, Pseudambassis ranga and Corica soborna fish were dried under open-sun drying where the pH was 6.2-6.6 (Kakati.B, 2017). In another study, Noemacheilus beav- ani, Chanda ranga, Barilius tileo, Amphipnous cuchia, Anabas testudineus, Amblypharyngodon mola, Channa punctatus, Tor putitora, Puntius chola and Conta elon- gate fish were also dried under open-sun drying where most of these fish types were within the good-quality pH range and only four of them, viz. Amblypharyngodon mola, Channa punctatus, Tor putitora and Conta elon- gate, were outside the range (Hazarika et al., 2016). The second characteristic is the water activity, which is the ratio of vapor pressure of water in the fish product to the vapor pressure of pure water at the same tempera- ture (Labuza, 1980). The water activity is considered as a criterion and a measure of microorganism develop- ment and toxin release of enzymatic and non-enzymatic browning development. It was found that water activ- ity greatly affects fresh fish endogenous microflora (Al Bulushi et al., 2013). There is a water activity limit for each food product below which the microorganism stop growing (Belessiotis and Delyannis, 2011). Most molds maintains their growth at a minimum water activity of 0.7 (Oparaku et al., 2017) and many bacteria sustains the growth at 0.9, while some biogenic amines, such as E. cloacae, need water activity of 0.48 (Al Bulushi et al., 2013). Majumdar et al. (2017) found that when moisture content of dried fish is less than 15%, no microbe can grow. This is because with the increase in water absorp- tion from the surronding, the water activity increases, causing an increase in microbial growth and reduction of shelf life of dried products. Further reduction in the water activity limits the spoilage and microbial growth. The microbial quality of fish products is affected by the drying method and the proper handling anox- ic conditions (hygienic conditions) (Al Bulushi et al., 2013; Oparaku et al., 2017). Developing countries pro- duce low microbiological quality products which could be loaded with Staphylococcus aureus, Clostridium spp. and fecal Streptococcus spp. pathogens (Al Bu- lushi et al., 2013). To assess the microbiological qual- ity of fish products, Total Plate Count (TPC), which uses plate count agar, and Total Fungal Count (TFC), Figure 8. Parabolic-dish solar collector (Solomon et al., 2016). Figure 9. Exponential drying curve of tilapia fish (Kituu et al., 2010). 7Review Article Al-Mahruqi and Al-Ismaili which uses potato dextrose agar by APHA, are used as microbial parameters. Immaculate et al. (2012) ob- served that TPC and TFC were high in open-sun dried sardine but absent in the sardine dried in a solar drier. They also reported the presence of E. coli pathogens in open-sun samples, however other pathogens such as Salmonella and vibrio were absent in both methods. Total volatile base nitrogen (TVB-N) and trime- thylamine nitrogen (TMA-N) are signs (indicators) of spoilage which are correlatated strongly with the bacte- rial activity, endogenous enzymes and thus, the rate of spoilage (Kakati, 2017). They can be determined from trichloroacetic acid removal by micro diffusion from the sea product (fish). The recommended acceptability upper limit of these indicators for human consumption is 10-15 mg/100g for TMA-N and 35-40 mg/100g of TVB-N in dried fish (Immaculate et al., 2013). The qual- ity of dried fish with respect to TVB-N can be classified into very high quality at TVB-N value of 25 mg/100g or less, good quality at 26-30 mg/100g, limit of acceptabili- ty at 30-35 mg/100g and spoilt quality above 35 mg/100g (Jinadasa, 2014). However, in previous studies, the ac- ceptablity limit for TVB-N was considered to be 100 mg/100g (Connell, 1980), which is much higher than the recent classifications. Accordingly, Hossain et al. (2017) found that TMA-N and TVB-N values of open-sun dried Silver Pomfret and open-sun dried Perch fish were within the human acceptablity levels. The TMA-N and TVB-N values for Perch dried-fish were 8.21±0.12 and 46.97±1.00 mg/100g, respectively and for Silver Pom- fret, the values were 9.41±0.37 and 85.68±1.60 mg/100g, respectively. In another study, the TVB-N value were found to be acceptable for human consumption for mola fish dried in a solar tunnel dryer (15.68 mg/100g) and in open-sun (20.36 mg/100g) (Islam et al., 2012). Similarly, Sultana et al. (2008) repoted that in a solar tunnel dryer, the TVB-N values of dried ribbon fish, silver jew fish and Bombay duck were ranging from 15.46 to 19.21 mg/100g. The last cause of fish spoilage is the autolytic chang- es (Oparaku et al., 2017) which denotes the biological and chemical changes occurring because of quali- ty losses at early stage of fresh fish (Ghaly et al., 2010; Oparaku et al., 2017). Major fish molecules undergo en- zymatic breakdown and as a result of autolysis of fish muscle proteins peptides and free amino acids can be produced which cause fish meat spoilage as a result of production of biogenic amines and microbial growth (Ghaly et al., 2010). For instance, open sun dried an- chovy fish was reported to contain biogenic amines that can cause scombroid-poisoning (Al Bulushi et al., 2013). The scombroid poisoning can also be caused by the increase in the level of histamine in the fish prod- uct so it is also called histamine fish poisoning (Hun- gerford, 2010). Bulushi et al. (2009) found that biogenic amines could be involved in the formation of nitrosa- mines at a high significant level when biogenic amines are present at a high concentration in fish products. It was reported, in the same study, that impure salts en- hance the formation of nitrosamine whereas, pure salts (sodium chloride) inhibit nitrosamine formation. Conclusion A review on solar drying of fish was done, several points can be concluded from this the study: (i) Open-sun draying is the most popular preservation technique of food products, (ii) The drawbacks of open-sun drying can be overcome by implementing other solar drying techniques such as greenhouse tunnel dryers, (iii) Solar dryers have shorter drying time and higher drying rate, and the physical properties of dried fish are enhanced, (iv) Many regression models were used for better under- standing of the drying processes, (v) Fish have very good nutritional value due to their high amount of proteins, lipids and ash contents, and (vi) Some characteristic, such as pH, water activity, microbial load, total vola- tile base nitrogen (TVB-N), trimethylamine nitrogen (TMA-N) and enzymatic autolysis analyzes, are used to assess the quality of dried fish to achieve longer shelf life. References Abraha B, Samuel M, Mohammud A, Habte-Tsion HM, Admassu H, Al-Hajj, NQM. (2017). A comparative study on quality of dried Anchovy (Stelophorus het- erolobus) using open sun rack and solar tent drying methods. Turkish Journal of Fisheries and Aquatic Sciences 17(6): 1107-1115. C-5: Al-Jufaili S, Al-Jahwari OS. (2011). The Omani coastal traditional sardine fishery 1994-2007: A review. Jour- nal of Agricultural and Marine Sciences 16: 1-12. Al Bulushi IM, Guizani N, Dykes GA. (2013). Effect of ambient storage on the microbial characteristics of traditional dried anchovies (Encrasicholina puncti- fer). African Journal of Microbiology Research 7(28): 3575-3581. Al Rawahi ZNA, Munusami A, Kaithari DK. (2013). Per- formance analysis of solar drying system for marine product of Oman. International Journal of Students’ Research in Technology & Management 1(6): 610-613. Anh NTN, Nhi NT, Van Hoa N. (2015). Effect of differ- ent drying methods on total lipid and fatty acid pro- files of dried Artemia francis-cana biomass. Can Tho University Journal of Science 1: 1-9. Babiker AMO, Ismail IA, Osman OE, Salih ZA. (2014). Effect of solar drying using a natural convective so- lar drier on bacterial load and chemical composition of bayad (Bagrus bayad) fish flakes. International Journal of Multidisciplinary and Current research 2: 2321-3124. Bala B, Janjai S. (2009). Solar drying of fruits, vegetables, spices, medicinal plants and fish: Developments and Potentials. International Solar Food Processing Con- 8 SQU Journal of Agricultural and Marine Sciences, 2021, Volume 26, Issue 2 A Review on Solar Drying of Fish ference, Indore, India, January 14-16. Bala B, Mondol M. (2001). Experimental investigation on solar drying of fish using solar tunnel dryer. Dry- ing Technology 19(2): 427-436. Basunia MA, Al-Handali HH, Al-Balushi MI, Rahman MS, Mahgoub O. (2011). Drying of fish sardines in Oman using solar tunnel dryers. Journal of Agricul- tural Science and Technology B 1: 108-114. Belessiotis V, Delyannis E. (2011). Solar drying. Solar Energy 85(8): 1665-1691. Bellagha S, Amami E, Farhat A, Kechaou N. (2002). Dry- ing kinetics and characteristic drying curve of lightly salted sardine (Sardinella aurita). Drying Technology 20(7): 1527-1538. Belwal R, Belwal S, Al Jabri O. (2015). The fisheries of Oman: A situation analysis. Marine Policy 61: 237-248. Bulushi IA, Poole S, Deeth HC, Dykes GA. (2009). Bio- genic amines in fish: roles in intoxication, spoilage, and nitrosamine formation—a review. Critical Re- views in Food Science and Nutrition 49(4): 369-377. Charters W, Macdonald R, Kaye D, Xiaoren S. (2017). Passive greenhouse type solar dryers and their devel- opment. International Energy Journal 11(2): 51-60. Connell JJ. (1980). Control of fish quality II, Vol. 12, Fish- ing News Books Ltd.: 7-129. Djendoubi N, Boudhrioua N, Bonazzi C, Kechaou N. (2009). Drying of sardine muscles: Experimental and mathematical investigations. Food and Bioproducts Processing 87(2): 115-123. FAO. (2015). Fishery and Aquaculture Country Profiles. Oman. http://www.fao.org (accessed 7 March 2018). Ghaly AE, Dave D, Budge S, Brooks M. (2010). Fish spoilage mechanisms and preservation techniques. American Journal of Applied Sciences 7(7): 859. Hazarika P, Ullah N, Handique PJ. (2016). Assessment of biochemical quality of ten selected dried fish prod- ucts of North East India. Assessment 3(3): 183-186. Hossain M, Jamil M, Mia M, Uddin M, Mansur M. (2017). Studies on the proximate composition, qual- ity and heavy metal concentration of two sun-dried marine fish (sun-dried Silver Pomfret and sun-dried Perch) of Cox’s Bazar District of Bangladesh. Jour- nal of Environmental Science and Natural Resources 10(1): 25-32. Hubackova A, Kucerova I, Chrun R, Chaloupkova P, Ba- nout J. (2014). Development of solar drying model for selected Cambodian fish species. The Scientific World Journal 2014. vol. 2014.10.1155 Hungerford JM. (2010). Scombroid poisoning: a review. Toxicon 56(2): 231-243. Immaculate J, Sinduja P, Jamila P. (2012). Biochemical and microbial qualities of Sardinella fimbriata sun dried in different methods. International Food Re- search Journal 19(4): 1699-1703. Immaculate K, Sinduja P, Velammal A, Patterson J. (2013). Quality and shelf life status of salted and sun dried fishes of Tuticorin fishing villages in different seasons. International Food Research Journal 20(4): 1855-1863. Islam M, Hossain M, Mian S. (2012). Nutritive value of dried and heat processed mola fish (Amblypharyn- godon mola) products. International Journal of Nat- ural Sciences 2(2): 43-48. Islam MT, Ahmed S, Sultana MA, Tumpa A, Flowra FA. (2013). Nutritional and food quality assessment of dried fishes in Singra upazila under Natore district of Ban- gladesh. Trends in Fisheries Research 2(1): 2319-4758. Jain D, Pathare PB. (2007). Study the drying kinetics of open sun drying of fish. Journal of Food Engineering 78(4): 1315-1319. Jinadasa B. (2014). Determination of quality of ma- rine fishes based on total volatile base nitrogen test (TVB-N). Nature and Science 12(5): 106-111. Jónsson Á, Finnbogadóttir GA, Þorkelsson G, Mag- nússon H, Reykdal Ó, Arason S. (2007). Dried fish as health food. Matis-Food Research. Innovation & Safety Report 5: 1-22. Kakati BK, Sharma P, Goswami UC. (2017). Quality evaluation of dried fish products commerce in As- sam, india. International Journal of Advanced Bio- logical Research 7(3): 465-469. Kituu GM, Shitanda D, Kanali C, Mailutha J, Njoroge C, Wainaina J, Silayo V. (2010). Thin layer drying model for simulating the drying of Tilapia fish (Oreochro- mis niloticus) in a solar tunnel dryer. Journal of Food Engineering 98(3): 325-331. Labuza TP. (1980). The effect of water activity on reac- tion kinetics of food deterioration. Food Technology 34(4): 36-41. Majumdar BC, Afrin F, Rasul M, Khan M, Shah A. (2017). Comparative study of physical, chemical, mi- crobiological and sensory aspects of some sun dried fishes in Bangladesh. Brazilian Journal of Biological Sciences 4(8): 323-331. Mansur MA, Rahman S, Khan MNA, Reza MS, Uga S. (2013). Study on the quality and safety aspect of three sun-dried fish. African Journal of Agricultural Re- search 8(41): 5149-5155. Martunis M. (2013). Performance of a forced-convec- tion greenhouse dryer for fish drying. Rona Teknik Pertanian 6(1): 426-430. 9Review Article Al-Mahruqi and Al-Ismaili Mustapha MK, Ajibola TB, Salako AF, Ademola SK. (2014). Solar drying and organoleptic characteristics of two tropical African fish species using improved low-cost solar driers. Food Science & Nutrition 2(3): 244-250. Ochieng OB, Oduor OPM, Nyale MM. (2015). Biochem- ical and nutritional quality of dried sardines using raised open solar rack dryers off Kenyan coast. Jour- nal of Food Resource Science 4(2): 33-42. Oladipo I, Bankole S. (2013). Nutritional and microbial quality of fresh and dried Clarias gariepinus and Oreo- chromis niloticus. International Journal of Applied Microbiology and Biotechnology Research 1: 1-6. Oparaku NF, Mgbenka BO, Eyo JE. (2017). Proximate and organoleptic characteristics of sun and solar dried fish. Animal Research International 7(2): 1169-1175. Prakash O, Kumar A. (2014). Solar greenhouse drying: A review. Renewable and Sustainable Energy Reviews 29: 905-910. Sahu TK, Jaiswal V, Singh AK. (2016). A review on solar drying techniques and solar greenhouse dryer. IOSR Journal of Mechanical and Civil Engineering 13: 31-37. Sengar S, Khandetod Y, Mohod A. (2009). Low cost solar dryer for fish. African Journal of Environmental Sci- ence and Technology 3(9). Siddique M, Mojumder P, Zamal H. (2012). Proximate composition of three commercially available marine dry fishes (Harpodon nehereus, Johnius dussumieri and Lepturacanthus savala). American Journal of Food Technology 7(7): 429-436. Singh P, Shrivastava V, Kumar A. (2017). Recent develop- ments in greenhouse solar drying: A review. Renew- able and Sustainable Energy Reviews 82: 3250-3262. Solomon S, Okomoda V, Egwumah A. (2016). Design and performance of a pioneering solar collector us- ing parabolic dish for fish processing. Jordan Journal of Agricultural Sciences 12(2): 581-590. Sontakke MS, Salve SP. (2015). Solar drying technolo- gies: A review. International Journal of Engineering Science 4: 29-35. Sultana S, Hossain M, Shikha F, Islam M, Kamal M. (2008). Quality assessment of rotating and solar tun- nel dried marine fish product. Bangladesh Journal of Fisheries Research 12(1): 121-128. Tiwari G, Das T, Chen C, Barnwal P. (2009). Energy and exergy analyses of greenhouse fish drying. Interna- tional journal of exergy 6(5): 620-636. Toujani M, Hassini L, Azzouz S, Belghith A. (2013). Ex- perimental study and mathematical modeling of sil- verside fish convective drying. Journal of Food Pro- cessing and Preservation 37(5): 930-938. Walters SA, Jha AK. (2016). Sustaining chili pepper production in Afghanistan through better irrigation practices and management. Agriculture 6(4): 62-71.