Comparative study of physical, chemical, microbiological and sensory aspects of some sun dried fishes in Bangladesh

An investigation was carried out to compare the physical, chemical, microbiological and sensory properties of three freshwater dried fish products of Wallago attu Bloch & Schneider, 1801 (Siluriformes: Siluridae), Channa striatus (Bloch, 1793) (Perciformes: Channidae) and Glossogobius giuris (F. Hamilton, 1822) (Perciformes: Gobiidae). The traditionally sun dried fishes were collected from Kawran Bazar Fish Market, Dhaka, Bangladesh. Results showed that the water reconstitution properties varied among the dried fishes where maximum rehydration was observed in the C. striatus at room temperature, 40 oC and 60 oC. Moisture content of W. attu, C. striatus and G. giuris were 22.70% ± 0.45%, 18.75% ± 0.58% and 21.93% ± 0.54%, respectively; the protein content were 61.85% ± 0.99%, 66.44% ± 1.02% and 62.83% ± 0.87%, respectively; the lipid content were 6.21% ± 0.93%, 6.81% ± 0.72% and 5.98% ± 0.55%, respectively, and the ash content were 6.79% ± 1.11%, 6.49% ± 1.29% and 7.83% ± 0.98%, respectively. Peroxide value, acid value, pH, TVB-N value and aerobic plate count were found to be highest in W. attu followed by G. giuris and C. striatus. Results of this study revealed that traditionally sun dried fish products were acceptable quality in terms of physico-chemical, microbiological and sensory aspects.


Introduction
Fishes play an important roles in human diets as good sources of animal protein that also provide other important elements necessary for the maintenance of healthy bodies (Ravichandran et al., 2012). Fish is an extremely perishable food item and quality losses occur very rapidly after harvest (Dewi et al., 2011;Musa et al., 2010;Khan and Khan, 2001) and require preservation. Several methods are followed Braz. J. Biol. Sci., 2017, v. 4, No. 8, p. 323-331. over the world for preserving fish to extend its shelf life including drying, salting and smoking. Fish drying is an important method of fish preservation for most traditional fishing communities in tropical countries like Bangladesh (Balachandran, 2001).
Dried fish (Shutki in Bengali) is one of the popular food items in Bangladesh. It preserves the quality for an extended time and offers several advantages such as insignificant alterations and minimum deterioration in the product. About 20% of the artesian catch is sun dried and consumed in the domestic market . The significant amount of dried fishes (approx. 622 metric ton) were exported from Bangladesh and earned 3.13 million USD as foreign currency (DoF, 2011).
Recently, Bangladeshi dried fish and fishery products are decreasing tendency of export market demand for the use of low quality raw fish for drying, traditional drying practices, unhygienic and improper sanitation facilities and random use of unauthorized chemicals and insecticides at the different stages of drying process of fish (Hasan et al., 2016).
Sun drying is one of the most important low-cost fish preservation methods not only in Bangladesh but also in many areas in the world. Dried fish is an important source of protein and other essential nutrients for the maintenance of a healthy body (Arannilewa et al., 2005). It is relished by many people of coastal, central and North-eastern districts of Bangladesh. Nutritional quality of dried fishery product remains intact -sometimes retains higher quality standards compared to fresh fish (as per unit weight). Moreover, the special flavor is highly relished by different ethnic people. The main principle of fish drying is to stop the progress of muscle enzymatic activity and microbial growth by reducing the water activity of fish (Humayun, 1995).
However, it is suggested that the quality of the traditionally sun dried fishes found in the local market are not satisfactory for human consumption (Reza et al., 2005). One of the major problems associated with the traditional sun-drying of fish is an infestation of the products by the blow fly and beetle larvae. Other problems markedly evident with dried fish are the contamination during different stages of handling and processing and the indiscriminate use of various types of pesticides (Nowsad, 2005). Therefore, the present study was aimed to compare the quality of three sun dried fish species such as Boal Wallago attu Bloch & Schneider, 1801 (Siluriformes: Siluridae), Shol Channa striatus (Bloch, 1793) (Perciformes: Channidae) and Bele Glossogobius giuris (F. Hamilton, 1822) (Perciformes: Gobiidae) by sensory, physical, chemical and microbiological analysis.

Methods and materials
Collection of samples Sun-dried fishes such as W. attu (Bengali-Boal), C. striatus (Bengali-Shol) and G. giuris (Bengali-Bele) were collected from Kawran Bazar Fish Market, Dhaka. Dried fish samples were brought by tightly packing in polyethylene bags and stored at room temperature (24-29 ºC) until further analysis. The experimental work of this study was carried out at the laboratory of Department of Fisheries Technology, Bangabandhu Sheikh Mujibur Rahman Agricultural University, Gazipur, Bangladesh.

Physical characteristics
Physical characteristics such as color, odor, texture and insect infestation of the traditionally sun-dried fishes were examined by sensory test on the basis of the method described by Roy (2013) with the help of five member panels of experts.

Proximate composition analysis
The compositions of moisture, protein, crude lipid and ash content of dried fish were analyzed in triplicate according to the standard procedure given in Association of Official Analytical Chemists (AOAC, 1995).

Determination of lipid oxidation
To analyze the degree of lipid oxidation of dried fish, lipids was extracted and then peroxide value and the acid value was determined.

Extraction of lipid
Total lipid was extracted from dried fish samples with a solvent combination of chloroform: methanol: distilled water according to the method of Bligh and Dyer (1959) with slight modification making those final ratios 10:5:3, v/v/v. Then the extracted lipid was dissolved in chloroform and stored at -20 ºC until further analysis.

Determination of peroxide value
Peroxide value was determined according to the standard procedure given in AOAC (1995). Lipid (0.5 g) was poured in a conical flask and mixed with 30 mL of acetic acid and chloroform solution (ratio 3:2). Then 0.5 mL of saturated potassium iodide was added to it. The mixture was kept in the dark for about 10 min, and then 30 mL of distilled water and 0.5 mL of freshly prepared 1% starch were added. After shaking, the samples were titrated with 0.01 N sodium thiosulfate. The peroxide value was expressed as meq/kg of lipid.

Determination of acid value
Acid value was analyzed according to the method of AOAC (1995). Lipid (0.5 g) was dissolved in a mixture of 100 mL of ethanol and diethyl ether (1:1; v/v) and titrated with 0.01 N potassium hydroxide. Phenolphthalein was used as an indicator. The results were expressed as mg KOH/g lipid.

Determination of pH
About 10 g of sample was weighed in a beaker using electronic balance. Then the sample was homogenized using tissue homogenizer with 10 volumes of distilled water and the pH was measured by a pH meter.

Determination of water reconstitution behavior
About 8-10 g of sample from individual dried fish was taken and weighted by analytical balance and then immerged into water at room temperature (24-29 ºC), 40 °C and 60 °C. The dried fish sample was soaked into water for 60 min. Any loose muscle that attached to sample was removed before dipping into water. The dried fish sample was removed from the water at every 15 minutes interval and the surface water was removed with blotting paper and reweighted each sample by analytical balance. The percentage of water uptake in rehydrated fish samples was calculated as follows: Water reconstitution (%) = (Wr -Wi) / Wi × 100 Where, Wi = Initial weight of the dried fish Wr = Weight of dried fish after water absorption Determination of TVB-N Total volatile base nitrogen (TVB-N) was estimated according to the method of Antonacopoulos and Vyncke (1989).

Determination of microbial load
In order to determine the microbial load of the sun-dried samples, aerobic plate count was done by spread plate count method. Physiological saline (0.85% NaCl) and plate count agar of commercial preparations were prepared in the laboratory as per the method is given in Cowan and Steel's Manual for the Identification of Medical Bacteria (Barraw and Feltheam, 1993). Plate count agar (Hi media, India) was used in this experiment.

Statistical analysis
For each measurement, analyses were repeated 3 to 5 times, data were pooled, and the mean and standard deviation were determined. Statistical analysis was performed using Microsoft Office Excel 2007.

Water reconstitution of dried fish products
The water reconstitution of dried fish products are presented in Figure 1. Generally, water holding capacity of dried fish products are increased with the increase of water temperature and soaking time. In this study, W. attu was shown to hold 37.07% moisture after 60 min at room temperature, whereas C. striatus and G. giuris dried products were shown to have 39.01% and 36.30%, respectively. Similar trend was also observed when dried fish were soaked at 40 ºC and 60 ºC for 60 min. Overall, C. striatus dried fish products were rehydrated more rapidly than G. giuris and W. attu dried fish products. And the rehydration ability of dried fish products was depended on elevated soaking temperature and extended period of time. It has been reported that there was a positive relationship between rehydration ability and physical properties of dried fish products (Reza et al., 2005). This might be due to the fact that increased temperature of water opens the internal structure of fish muscle which maximizes the scope of rapid rehydration (Brennan et al., 1990;Tunde-Akintunde, 2008). Moreover, rehydration ability of dried fish products depends on the variation of species besides time and temperature (Nurullah, 2005; Tunde-Akintunde, 2008).

pH value of sun dried fishes
The pH values of three sun dried fish products are shown in Figure 2. The pH values were ranged from 6.67 to 6.96. The highest pH value was found in W. attu while the lowest was observed in C. striatus. The lowest pH values of dried products may enhance microbial inhibition and contribute to extend the shelf life of dried fish by inhibiting the activity of the endogenous proteases. On the other hand, an increase in pH indicates the loss of quality in fishes (Farid et al., 2014).

Proximate composition of sun dried fish products
Proximate composition of dried W. attu, C. striatus and G. giuris are demonstrated in Table 1. The moisture content of the dried fishes was observed in the range of 18.75%-22.70% with the highest value obtained from W. attu and the lowest value from C. striatus fish. Flowra and Tumpa (2012) reported that the moisture content of five different dried fishes ranged between 12.13% and 18.18%. It is well known that no microbe could grow in dried fish products when moisture content < 15% (Frazier and Westhoff, 1978). The variation of moisture content among these dried fishes may occur due to improper storage, improper drying, unawareness of processors etc. According to Kamruzzaman (1992), when salt is added to the fish before drying, less water needs to be removed to achieve the same effect, and the product with a water content of 35%-45%, depending on amount of salt present, is often dry enough to inhibit the growth of molds and bacteria under most climatic condition. It has been reported that the water activity increases with the water absorption from environment, which enhances the microbial growth and reduces the loss of nutrient and shelf life of dried products (Nowsad, 2005). Sometimes fish processors keep comparatively higher moisture content in the dried fish products to gain more weight for economic benefit (Nowsad, 2003 51.19% (Flowra and Tumpa, 2012). Our result is more or less similar with the previous study conducted by Azam et al. (2003) who found that the protein values ranged from 40.69% to 66.52% in 14 species of dried fishes.
Lipid content was ranged from 5.98% to 6.81% with the highest lipid content in C. striatus and the lowest in G. giuris (Table 1). Flowra and Tumpa (2012) reported that the lipid content of five dried fish species was ranged between 5.38% and 15.86%. Hussain et al. (1992) also reported that 3.7% to 17.8% lipid in 23 sun-dried fish species, which is similar to our study.
Ash content was found in the range of 6.49% to 7.83% with the highest value in G. giuris and the lowest value from C. striatus (Table 1). This is perhaps due to contamination with sand and filth during drying and storage in different marketing chains. Flowra and Tumpa (2012) observed that the ash content of different dried fishes was varied from 10.78% to 15.67%. ¹Each value is expressed as mean ± SD (n = 3). *Percent dry matter basis.

TVB-N value of sun dried fishes
The TVB-N values of different sun dried fish products are shown in Table 2. The TVB-N values were 28.39 mg/100 g, 32.07 mg/100 g and 34.21 mg/100 g in C. striatus, G. giuris and W. attu, respectively. The highest TVB-N value was found in G. giuris while the lowest value was observed in C. striatus. The TVB-N values of some traditionally sun dried fishes were ranging from 28.9-46.32 mg/100g (Sen et al., 1961). The TVB-N values of all the samples were found to be lower than the recommended value (100-200 mg/100 g) of different salted and dried fish products (Connell, 1995).

Lipid oxidation of sun dried fishes
The peroxide value of dried fishes was ranged from 16.27 ± 0.63 meq/kg to 19.12 ± 0.82 meq/kg lipid ( Table 2). The peroxide values observed in this study were within acceptable limits (PV ≤ 20 meq/kg fish lipid) (Connell, 1995). It has been reported that peroxide value of herring (Clupea pallasii) lipids (5.52-11.86 meq/kg) increased significantly during the drying period (Shah et al., 2009). In addition to peroxide value, the acid value was measured to determine the degree of lipid hydrolysis in dried fish products. The acid values of dried fishes were ranged from 15.03 ± 0.77 mg KOH/g to 18.64 ± 0.82 mg KOH/g of lipid in dried fish products. The lower acid values indicate the better quality of a product. Our result is more or less similar to that reported by Shah et al. (2009) who found that the acid value of dried herring fillet ranged between 11.20 and 18.94 mg KOH/g of lipid. An increased amount of acid value of dried fish products might be due to release of free fatty acids by lipid hydrolysis. Lipid peroxidation, corresponding to the oxidative deterioration of polyunsaturated fatty acids in fish muscle, leads to the production of off-flavor and off-odor, there by shortening the shelf-life of food (Ramanathan and Das, 1992

Microbiological load of dried fish products
Aerobic plate count (APC) of dried fish products were ranged from 4.76 × 10 5 to 2.11 × 10 6 CFU/g (Table 3). The highest APC was found in W. attu probably due to the fact that higher moisture content and poor hygienic condition compared with C. striatus and G. giuris. The results obtained from the sun dried products of this study is more or less similar to those of some marine dried products produced in solar tunnel dryers where APC of dried Ribbon fish, Bombay Duck, Big-eye Tuna, Silver Jew fish and Chinese Pomfret were in the range of 3.27 to 4.49 log CFU/g (Reza et al., 2008).

Sensory characteristics of dried fish products
Sensory characteristics of sun dried W. attu, C. striatus and G. giuris are depicted in Table 4. The sensory characteristics such as color, odor, texture, and insect infestation showed that the C. straitus was good in quality, while the W. attu and G. giuris had decreased the quality. W. attu and G. giuris fish lost their color, odor and texture from their original characteristics compared with C. straitus. But all samples were free from insect infestation and broken pieces. Some of the samples contained high quantity of broken pieces, which might be the result of using poor quality raw material, excess drying or improper drying and handling or due to moisture reconstitution (Mansur et al., 2013).

Conclusion
Results of this study demonstrated that C. striatus showed good in sensory quality than W. attu and G. giuris. Among the dried fish products, C. striatus indicates better quality through chemical evaluation such as proximate composition, lipid oxidation (peroxide value and acid value) and TVB-N values. In the case of microbiological analysis, similar trends were also observed in C. striatus. Thus, it can be concluded that C. striatus proved superior in overall quality than W. attu and G. giuris.