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Science and Technology Journal of Food, Agriculture & Environment Vol.4 (2) : 95-97. 2006

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Effects of different sources of heat on the quality of smoked fish
B. A. Oyewole*, B. J. Agun and K. F. Omotayo
The Federal Polytechnic, Department of Agricultural Engineering, P.M.B. 5351, Ado-Ekiti, Ekiti State, Nigeria. *e-mail: tundeoyewole2006@yahoo.co.uk

Received 18 November 2005, accepted 18 March 2006.

Abstract
Selected average good sizes of four species of fish, viz tilapia, mackeral, titus and clarias, were subjected to four different sources of heat treatment for smoking, viz sawdust, charcoal, rice bran and firewood, for 48 hours. Physical examination, quality assessment and microbial count under scale 10-6 were carried out to determine which of the four sources of heat gives the best result. An arithmetic average of physical examination of weight and sizes showed that sawdust had 15-36%, charcoal 30-52%, rice bran 10-27% and firewood 33-54% reduction in weight for all species. Charcoal and firewood sources of heat treatment yielded the best quality in terms of colour, taste, texture and flavour. Also, microbial count showed that use of charcoal and firewood caused the lowest level of microbial count of 4x106 while use of sawdust and rice bran caused microbial counts of 10x106 and 20x106 respectively. Furthermore, use of charcoal and firewood gave the lowest weight reduction, highest quality and lowest microbial count of all the four sources of heat. Key words: Heat, smoking, fish, quality, effects.

Introduction Increasing awareness that a good quality food reduces the incidence of food induced-health problem, the emphasis is mainly on making safe food. There is real danger of eating an improperly processed food as it may cause illness 7. Scromboid fish poisoning due to the formation of histamine resulting from microbiological decay have been established 8. Fish is a valuable source of high quality protein and in the case of oily fish, fats of considerable nutritional importance 3 . Nutritionally, fish proteins are highly digestible and at least as good as red meat with respect to content of essential amino acids 8. The fat of fish is an excellent source of vitamins A and D. In Nigeria, fish is an important source of animal protein in the diets of the low and middle level income groups 1. The upper class people take pleasure in fresh and dried fish in the form of pepper soup and other delicacies. There is, however, a short supply in fish, a major source of animal protein in the diets of average Nigerians. Of the estimated 1,139,833 metric tones demanded by population based on the population of 95 millions of people, only 280,307 metric tones were produced leaving a deficit of 859,526 metric tones 2. The deficit in the supply was largely due to post-harvest losses owing to inadequate processing and handling methods adopted. Birewar 4 categorized the post-harvest losses into economic and nutritive losses. Barrie et al. 3 linked most losses in the developing countries (Nigeria inclusive) to traditional method of fish processing under basic conditions on beaches and river banks with little or no attention given to hygiene or good handling practices, resulting in great variation in quality and high losses, particularly due to infestation after the fish has been dried or smoked. To improve the quality, they suggested among other things the use of the same type of wood for smoking. Because of the great tendency of fish to spoil, a number of methods of
Journal of Food, Agriculture & Environment, Vol.4 (2), April 2006

preservation have been developed over the years. The most basic methods are smoking and salting with subsequent drying. This is effective, but such preserved fish are not accepted in all cultures. Other society find such preserved fish highly desirable. To assist in the development of the appropriate method of smoking fish that would lead into smoked fish of desirable quality acceptable to all cultures, investigation of the effects of different sources of heat for smoking on fish presently in existence is essential thereby increasing the fish output. Materials and Methods Materials: The fishes used in the experiment were obtained from central market and from the Ekiti State Water Corporation located in Ado-Ekiti. The fishes were obtained in the state of desirable quality. Mackerel and titus were obtained from the market while tilapia and clarias were harvested freshly on demand at the water corporation dam. The fishes were kept in the state of freshness till the time of experiment. The rice bran used in the smoking was obtained from the rice processing center at Ado-Ekiti. The firewood and sawdust were obtained from the sawmill located in Ado-Ekiti. The firewood type was of the rough scrab pieces of wood from iroko tree. The sawdust was from the previously sawed wood of many varieties of different species of wood whose origin could not be easily determined. The charcoal of wood type that could not be determined was also obtained from charcoal seller in the city of Ado-Ekiti. Other materials such as salt, knife, distilled water, conical flasks, test tubes, pipettes, Petri dishes, foil sheets, oven, incubator, weighing balance, test-tube rack, colony counter, autoclave, thermometer and reagents were sourced in the Departments of Agricultural Engineering and Food Science Technology.

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Smoking: The smokings of the fish were conducted strictly in following orders: The fish were thoroughly clean with water. 8-cut pieces of fish from fish species sprinkled with salt were used for four sources of heat treatments, with 2–cut pieces per a heat source. The procedure above was repeated on another set of unsprinkled pieces of fish. (Not sprinkled with salt). Each heat source was prepared. Fire was set on each heat source and the intensity of burning of each source were carefully observed and regulated where necessary. The fish were laid on wire–mesh on the top of the prepared smokers. The fish were regularly turned to encourage even drying and smoking.The fish were subjected to four hours heat treatment per day for two days. Microbial count determination: The glass wares were washed and sterilized. The culture media was prepared. A 10.4 g plate count agar (PC A) was weighed and diluted in 200 ml of distilled water. 9.7 g of eosin methylene blue agar (EMB) was also weighed into 200 ml distilled water (Food Science Technology Model, Federal Polytechnic Ado-Ekiti). The culture media was sterilized in autoclave at a temperature of 121°C for 15 minutes. A serial dilution of fish samples was prepared with a 10 g of fish sample in 90 ml of distilled water. 9 ml of distilled water was put into ten sterilized test tubes on test-tube rack. The test tubes were serial labelled viz 10-1,10-2, 10-3 …10-n. From 90 ml distilled water, 1 ml was withdrawn with the aid of a sterile pipette into the first test tube labelled 10-1. From the test tubes labelled 10-1, a 1 ml sample content therein was taken and placed into test tube labelled 10-2. From the test tube labelled 10-2, a 1 ml sample of the content was removed with the aid of a pipette and placed into the third test tube, labelled 10-3. The procedure was repeated till all the test tubes on the testtube rack were fully treated. 1 ml from the least diluted series above (i.e. 10-10) was withdrawn with the aid of another pipette and was placed in a sterile Petri dish. The prepared molten agar was poured into Petri dishes and allowed to solidify. The solidified agar was incubated at a temperature of 37oC for 48 hours by inverting the Petri dishes. The plates were observed after incubation for colonial growth. The number of colonies that developed on the agar surface was counted with a colony counter. Results and Discussion All species of fishes smoked using charcoal or firewood gave the best result in terms of microbial count and other physical attributes such as taste, texture and flavor (Table 1 and 2 ). However, charcoal yielded the best-smoked fish in term of quality. Also, it was observed that the fish treated with salt had the best quality attributes compared with the unsalted fish. The microbial count was low in all fish treated with salt. The exception, however, was clarias fish which yielded higher microbial count in salted than unsalted. Many reasons could have accounted for the quality performance displayed by tilapia, titus and mackerel fishes smoked on wood and charcoal. The role of salt was two-fold: Firstly, it rapidly removed moisture from the flesh to a point at which microbial growth was retarded. Secondly, many spoilage organisms cannot grow in salt concentration above 6%. The water content of fresh fish is around 80%. When the water content is reduced to 25%, bacterial growth and spoilage cease. Lowering the water content to 15% will, in addition, stop the growth of moulds. By removing the moisture, salt retarded the action of enzymes, most importantly those that cause rancidity.
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The wood and charcoal burns hotter than sawdust and rice bran. The hot smoking involved the temperature that was high enough to cook the fish. The flame temperature, which determines the quality of heat available for combustion and establishes the maximum attainable thermal efficiency of converting the useful heat into work, is higher in hardwood than in sawdust and rice bran. The wood, especially hardwood, has a high heating value (HHV) average between 17-24 MJ/kg 5. The charcoal has a higher fuel value than ordinary wood, both on a volumetric and weight basis, (about) 18.5x106 J/kg 6. The enormous heat value coupled with thermal efficiency and right flame temperature of wood dried the fish, thereby reducing the moisture content to a level that will prevent the growth of microorganism. This is seen from the percentage of water expelled. The percentage of moisture expelled from all species of fishes was higher in charcoal and wood compared with the percentage of water expelled in the same species of fish using sawdust and rice bran. Smoke contains tar-based phenolic compounds that inhibit microbial growth. The smoke phenolics also have antioxidant properties that delayed the development of rancidity in oily fish. For the fish smoked using rice bran and sawdust, it was observed that the quality of the fish was not as good as those smoked with charcoal and firewood. The microbial count in fish smoked using rice bran and sawdust was exceptionally high. The texture, taste and aroma were very poor when compared to those smoked using charcoal and firewood. The reasons for the disparity observed in fish smoked using rice bran and sawdust could have been as follows. Rice bran and sawdust have low flame temperature, hence the quantity of heat and the maximum attainable thermal efficiency of rice bran and sawdust are very low compared with what obtained in charcoal and firewood. The heating value of the rice bran and sawdust were equally very low. The heat needed to provide the right temperatures that are high enough to cook the fish was very low. More importantly, it is the heat from the fire that dries the fish, reducing the moisture content to a level that will prevent growth of microorganism. The rice bran and sawdust could not generate enough heat to expel the moisture from the fish. Thus, the percentage of the moisture expelled (i.e. lost in weight) in fish smoked using rice bran and sawdust was very low compared with that obtained in the fish smoked using charcoal and firewood. For tilapia fish, the loss in moisture (i.e. loss in weight) was 22.2% when sawdust was used, but the corresponding moisture losses (loss in weight) on charcoal and firewood were 31 and 33.3% respectively. Also, the moisture loss (loss in weight) in tilapiafish when smoked on rice bran was 22.2%. For all the species of fish, moisture expelled (lost in weight) was very low in sawdust and rice bran when compared with charcoal and firewood. Sawdust is a combination of dusts from many varieties of wood with different heating property. Barrie et al. 3 suggested the use of the same type of wood for better result. This could be the reason for the unsatisfactory result obtained for using sawdust. The sawdust produces profuse and dense smoke that tends to form soot. This explained the reason for the observed bitter taste, coarse texture and unpleasant flavour in fish smoked on sawdust. Also, Barrie et al. 3 object to the use of resinous wood. It is more probable that the unpleasant flavour noticed in the fish smoked on sawdust emerged from these resinous woods.

Journal of Food, Agriculture & Environment, Vol.4 (2), April 2006

Table 1. Weight loss of fish smoked using different heat sources and microbial count of salted and nonsalted samples.
Fish species Weight, kg Before After smoking smoking 18 25 20 28 12.5 16.5 14.0 13.5 Weight loss

kg
5.5 8.5 6.0 14.5

%

No. of colonies EMB plate PCA plate Salted Unsalted Salted Unsalted 1 x 106 4 x 106 2 x 106 -1 x 106 3 x 106 -4 x 106 9 x 106 1 x 107 1 x 106 4 x 106 1 x 107 1.8 x 107 8 x 106 5 x 106 3 x 106 2 x 106 2 x 106 4 x 106 1 x 106 2 x 106 -1 x 106 6 x 106 2 x 106 1 x 106 4 x 106 1.6 x 107 5 x 106 3 x 106 4 x 106 4 x 106 3 x 106 1 x 106 3 x 106 4 x 106 1 x 106 --1 x 106 4 x 106 5 x 105 4 x 106 6 x 106 1 x 107 8 x 106 6 x 106 7 x 106

Tilapia Mackerel Titus Clarias Tilapia Mackerel Titus Clarias Tilapia Mackerel Titus Clarias Tilapia Mackerel Titus Clarias

Charcoal 31.0 1 x 106 34.0 30.0 52.0 3 x 106 2 x 106 3 x 106

18 25 20 28

12.0 15.0 12.0 13.0

6.0 10.0 6.0 15.0

Firewood 33.0 1 x 106 40.0 33.0 54.0 1 x 106 -1 x 106

18 25 20 28

14.0 18.5 17.0 18.0

4.0 6.5 3.0 10.0

Sawdust 22.2 6 x 106 26.0 15.0 36.0 4 x 106 1 x 106 3 x 106

18 25 20 28

15.0 22.0 18.0 20.5

3.0 3.0 2.0 7.5

Rice bran 17.0 1 x 107 12.0 10.0 26.8 7 x 106 5 x 106 4 x 106

Table 2. Observed quality characteristics of the fish smoked using sawdust, charcoal, rice bran and firewood.
Colour Taste Texture Flavour Sawdust Light brown, good Sweet Smooth Unpleasant Charcoal Brownish, good Sweet Smooth Pleasant Rice bran Dark Bitter Coarse Unpleasant Firewood Brownish, good Sweet Smooth Pleasant

Conclusions and Recommendations Firewood and charcoal yielded the best-smoked fish. The sawdust and rice bran did not yield good quality fish. Smoking of fish should be done using firewood as fuel. The use of the same type of firewood should be strictly adhered to in order to obtain a high quality of smoked fish. Also, resinous woods that give rise to unpleasant flavour should be discouraged. Salting treatment should in addition be carried out before smoking. The role of salt has been enunciated in the discussion. Constant turning of fish and regulating of the smoke density and temperature should be adequately taken care of during smoking. This ensures good quality of smoked fish. The design of an appropriate and affordable smoker that meets the needs of local fishermen and that would enable them operate it with ease devoid of sophisticated/complex technology should be pursued.

References
Akande, G. R 1997. Technical and Economic Evaluation of Two Smoke Dry Kilns. A Consultancy Report Commissioned by CSU Ibadan. 2 Ames, G. R 1992. Traditional and Modern Technologies for Increased Food and Supply from Inland Fisheries in Africa. Conf. Proc. Symposium on Post-harvest Fish Technology, Cairo, Egypt, Oct. 20— 22, 1990. FAO/CIFA Technical paper 19, Pp. 11. 3 Barrie, A., Fellow, P., Gedi, L., Lubin, H., Oti-Boateng, P. and Zulu, R. 2003. Opportunities in Food Processing; Setting up and Running Small Meat or Fish Processing Enterprises. CTA Publication, Wageningen, Netherlands. 4 Birewar, B. R. 1996. Development of improved on-farm grain drying facility in Nigeria. AMA 27(1):51–53. 5 David, A.T, Amadeo, J. R. and William, D.K. 1981. Wood Combustion; Principles, Processes and Economics. Academic Press, New York. 6 Desh, H.E. 1977. Timber; Its Structure and Properties. 5 Th Edition.Macmillan Press Ltd, London. 7 Fellow, P. 1997. Traditional Foods; Processing for Profit. CTA Publication, Wageningen, Netherlands. 8 Norman, N. and Joseph, H.H. 1996. Food Science. 5th Edition. CBS Publication, Daryaganj, New Delhi.
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