Effect of Smoking Duration on the Microbiological Quality of Cold-smoked Atlantic Cod, Gadus morhua (Linnaeus, 1758)
Cold-smoked Atlantic cod, Gadus morhua that floods the smoked fish market of the South-Western region of Nigeria are not microbiologically shelf-stable; hence, the need for a study on varying smoking durations in order to produce smoked fish with longer shelf life; safe for consumption. This study evaluated the effect of varying smoking durations of 6, 6.5, 7 and 7.5 h on the microbiological quality and percentage moisture content of cold-smoked Atlantic Cod. The fish samples were cold-smoked using the traditional smoking drum. Four batches of the smoked fish were stored in metal baskets at ambient temperature (25-28°C) for 12 days. Each batch was assessed for; Total Plate Count (TPC), Total Coliform Count (TCC), Mould Count (MC), Staphylococcus aureus (SA) count and presence or absence of Escherichia coli. Analyses of the smoked fish samples were carried out at the initial stage (day 0) and subsequently every alternate days. The result obtained showed significant variations (p<0.05) for all the microbial counts of the four smoked fish samples. The best microbiologically stable cold-smoked samples were that smoked at the longest duration of 7.5 h which recorded the least mean TPC range of 1.50x103, 0.0 MPN g-1 (TCC), 1.32x103 (MC) and 3.0x103 CFU g-1 (SA). The moisture content ranged from 15.2 to 25.3%. All the samples tested negative to Escherichia coli and met the specified microbiological limit. It can be concluded that the cold-smoking Gadus morhua at 7.5 h will preserve the fish more and make it safe for consumers.
Received: June 20, 2012;
Accepted: August 17, 2012;
Published: October 19, 2012
In Nigeria, fish is cooked fresh and eaten, preserved or processed. Therefore,
the need to improve the quality and shelf-life of fish products cannot be over-emphasised
(Bolorunduro et al., 2005). Smoking is carried
out in fishermen camps in chambers of traditional kilns made of clay, cement
blocks, or drums. The traditionally smoked fish is typical of the Lake Chad
region of Nigeria (Abobarin, 2000). According to Shimang
(1990), in the 1970s, most of the fishes caught in the Lake Chad were
smoked before the advent of drought. Similarly, out of the 100 million tonnes
estimated worlds fish production in 1989, fifteen percent were cured and
one third of the cured fish were smoked (Da Silva, 2002).
Technically, the duration of smoking determines the moisture content of fish
product. Also, the moisture (or water activity) of a smoked fish has been found
to determine the rate of microbial growth and invariably the shelf life or keeping
qualities of the fish. Microbes e.g., L. monocytogenes has been found
in crabmeat samples and smoked fish samples (Gram, 2001);
they are ubiquitous in nature and able to grow at low temperatures and in high
salt concentration up to 10% (Da Silva, 2002). Hence,
preservation by drying is effected by lowering the water activity of the fish
to a level which micro-organisms can no longer grow (Eyo,
2001). In cold smoking, the temperature of the smoke does not exceed 30°C
(Eyo, 2001). Cold smoking is an uncommon preservation
technique in Nigeria, due to the need for a supporting or alternative method
of preservation (Waterman, 1976). Also, study has revealed
that cold-smoked fish products do not last long (Moses, 1983).
According to Oyelese (2006), cold-smoked samples had
the highest moisture content in comparison with moisture content of hot-smoked
and oven dried tilapia samples. This is because the cold-smoked fish is not
fully dehydrated. However, Eyo (1981) compared cold-smoked
and hot-smoked fish and observed that cold-smoked fish possess higher nutritive
value than hot-smoked fish which became cooked in the process. Therefore, the
objective of this study is to establish the most effective smoking duration
on the microbiological quality of the cold-smoked Atlantic Cod, Gadus morhua.
MATERIALS AND METHODS
Sample site and study duration: This research was carried out around Bells University, Ota, Ogun State, Nigeria. The drum-type smoking kilns were used for the study. Furthermore, the study was carried out within 12 consecutive days in June, 2010.
Preparation of fish samples: In this study, 20 kg carton of Atlantic Cod (Gadus morhua) with each of the pieces ranging between 150-200 g was purchased from a reputable frozen foods dealer in Ota and taken to an artisanal fish smoking site located at Bells Junction, Ota, Ogun State.
Smoking procedure and storage: Following standard processing steps, 68 pieces of the frozen fish were sorted out for cold smoking at temperature around 30°C. They were thawed, sorted into four batches of 17 pieces per batch and rinsed in clean water. Each piece of fish in the batches were folded into a round shape and held in place with a sharp stick. The smoking was done on a drum-type smoking kiln with a single smoking rack which was fabricated from a 44 gallon drum. The batches of folded Gadus morhua were arranged on the smoking rack and were subsequently subjected to different smoking durations of 6, 6.5, 7 and 7.5 h at temperature ranging between 30 35°C. The temperature was controlled by regulating the burning of the fuelwood. Smoking temperature was monitored at intervals by checking the heat that gets to the fish at the rack and dipping a thermometer into the flesh of the fish. After smoking, the fish samples A-D were cooled and separately stored in four labelled metal baskets each containing 17 pieces of the cold-smoked fish. The samples of cold-smoked Gadus morhua, were coded based on the different smoking durations they were subjected to. The cold-smoked fish were then taken to the Microbiology Laboratory of Bells University of Technology, Ota for analyses. The samples of the smoked fish were stored in metal baskets on the shelf at an ambient temperature for 12 days.
Analyses of cold-smoked fish samples: Analyses of the smoked fish samples
were carried out at the initial stage (day 0) and subsequently every alternate
days (i.e., days 2, 4, 6, 8, 10 and 12); making up seven testing days. The methods
of microbiological analysis described by Lyne (1976),
were adopted for the analysis of samples of the cold-smoked Gadus morhua.
The parameters determined were Total Plate Count (TPC), Total Coliform Count
(TCC), Mould count, Staphylococcus aureus count and Escherichia coli.
Percentage moisture content was also determined by AOAC (1990)
Statistical analysis: Data obtained from the different smoking durations against the storage days were subjected to Analysis of Variance (ANOVA) at 5% level of significance (p<0.05), while Duncans multiple range test was used to determine significant differences between the means.
RESULTS AND DISCUSSION
The microbial load (TPC, Staph., mould and TCC) and moisture content
of the frozen fish sample was higher than that of the smoked fish (Table
1). For example, TPC reduced from 4.3x104 in frozen cod to an
average of 1.50x103±0.05 CFU g-1 in fish smoked
at 7.5 h in day 0. Similar reduction patterns were observed for the values of
Staph. aureus, mould, TCC and percentage moisture content of the smoked
Atlantic cod. Also, the values of the microbial loads of the four batches of
Gadus morhua cold-smoked at different durations and expressed in colony
forming per gram (CFU g-1) were significantly different (p<0.05).
The microbial load and moisture content of the smoked fish increased with increased
days of storage. In addition, for all the parameters assessed, the values reduced
with increased smoking duration (Table 2). The initial 2 days
of analyses recorded very low microbial loads in all the four smoked samples,
A-D. Sample D which was smoked for the longest duration of 7.5 h had the lowest
TPC of 1.50x103 CFU g-1 at day 0 which rose to 2.0x105
CFU g-1 at day 12. This shows the usefulness of TPC in measuring
the effectiveness of time profile and heat treatment as processing procedures
(FNB/NRC, 1985). In addition to this, the phenolic fraction
of wood smoke has been said to possess the highest inhibiting ability on bacteria.
The dual effects of heat treatment and phenolic fraction of wood smoke as displayed
in sample D are made evident in the enumerated TPC values of the smoked fish
samples. This result is in agreement with the findings of Ikeme
and Gugnani (1988), who, in a comparative assessment of the effect on varying
periods (i.e., 4, 5, 5.5 and 6.5 h) of smoking on the acceptability and storage
stability of mackerel, reported that the samples of mackerel smoked for the
longest period of 6.5 h (lowest moisture content), were the most stable. The
TPC of the frozen sample of Gadus morhua was recorded to be 4.3x104
Total coliform counts are particularly useful as indicators of contamination
when they occur in small numbers. Their occurrence in large numbers indicates
mishandling such as temperature abuse (Mossel, 1967;
Silliker and Gabis, 1976). The differences in the values
of TCC of the four cold-smoked samples were significant (p<0.05). At day
0, Gadus morhua cold-smoked at 6, 6.5, 7 and 7.5 h had TCC values of
3.0, 1.0, 1.0 and 0.0 MPN g-1, respectively. Subsequently, by the
12th day, these have increased to 17.0, 17.0, 14.0 and 12.0 MPN g-1.
All the samples recorded fairly consistent TCC values on initial 2 days of analyses.
The occurrence of these microorganisms in the four study samples almost throughout
the storage days can then be attributed to contamination from processing utensils,
water, handlers and probably storage materials.
|| Microbial load of the frozen sample of Gadus morhua
|| Microbial and moisture content (%) of cold-smoked Gadus
morhua stored for 12 days
|Values are Mean±SD of triplicate experiments of each
The values of mould count showed that variations of all the four cold-smoked
samples were significant (p<0.05). Mould count of the frozen sample of Gadus
morhua was 1x103 CFU g-1 which was lower than the
values of the four cold-smoked samples. The four cold-smoked study samples A-D
recorded low levels of mould growth in the first 2 days of analysis. Sample
D recorded the lowest values of mould count throughout the storage days, compared
to sample A that recorded the highest mould count. Hence, the effectiveness
of heat treatment and time profile in processing procedure is again established.
According to Olsen (1976), yeast and mould are more
resistant to inhibitory influence of smoke even up to concentration of 60 mg
kg-1. Despite the relatively low percentage moisture content of sample
D, these food borne moulds were able to grow on the low moisture fish samples,
because of their relatively low moisture requirements (AOAC,
1990). The values of Staphylococcus aureus count showed that variations
for the four cold-smoked Gadus morhua samples were significant (p<0.05).
Staphylococcus aureus count of the frozen sample of G. morhua
was 3.7x105 CFU g-1 which was higher than all the values
obtained for the four cold-smoked samples at the initial stage (day 0) of analyses.
Hence, on the subjection of the fish to heat treatment, the values of the S.
aureus count of the raw material dropped. This establishes inhibitory effects
of phenolic fraction of smoke on Staphylococcus aureus. The bactericidal
effect of smoke as established by Olsen (1976) is associated
with the smoke constituents especially the phenols as well as the combined heating
and drying process during smoke curing. Staphylococcus aureus grows poorly
in competition with large numbers of other microorganisms. Small numbers are
to be expected in products handled by humans. Therefore, the presence of large
numbers in any food material indicates possible faulty sanitary or production
practice (ICMSF, 1986). According to Huss
et al. (1995), any handling of fish, and the associated sanitary
practices from the point of harvesting, however, has the potential to contribute
to the microflora on the final product. All the four batches of cold-smoked
Gadus morhua stored for 12 days tested negative to Escherichia coli
throughout the storage days. The resistance of E. coli to adverse physical
and chemical conditions is low which makes E. coli less useful as indicator
organisms in examination of frozen or otherwise preserved fish products. There
was significant difference (p<0.05) between the values of the percentage
moisture contents of the four smoked fish samples. The highest percentage moisture
content of 43.9 was recorded for sample A at day 0 which reduced to 16.0 by
day 12; this can be attributed to the fact that this sample was exposed to the
least duration of smoking (6 h). This agrees with the submission of Eyo
(1981) that the moisture retention of cold-smoked fish product is usually
high and may be in the order of 35-45%. Sample D that recorded the least percentage
moisture content of 25.3 at day 0 was cold-smoked for the longest study duration
of 7.5 h. Despite the consistent decrease in moisture content of the cold-smoked
study samples, the susceptibility to infestation by microorganisms increased
with storage period, such that, the microbial load picked up from day 4 and
remained increasingly high till the last day. This also agrees with of Eyo
(2001) that cold-smoked fish, being not well cooked, has shorter shelf life
and is easily infested by microorganisms such as bacteria and moulds if not
properly stored. The season in which the study was carried out could have contributed
to the decrease in moisture content of the cold-smoked Gadus morhua stored
for 12 days.
Findings from this study show that fish cold-smoked for 7.5 h with a range
of moisture content level of 25.3 to 15.2% can be microbiologically stable and
safe for consumption even by 12 days of storage. The other three smoking durations,
though not as effective in processing fish products are recommended be supported
with refrigeration storage in other to extend the shelf-stability of the product.
1: AOAC., 1990. Official Methods of Analysis of Association of Analytical Chemistry. 15th Edn., AOAC., Arlington, Virginia, pp: 1298.
2: Abobarin, A., 2000. A diagnostic study of the export of traditional fishery products in Nigeria. National Consultant on Trade in Traditional Fishery Products.
3: Bolorunduro, P.I., A.O.K. Adesehinwa and J.O. Ayanda, 2005. Adoption of improved fish preservation technologies in Northwestern Nigeria. Tropicultura, 23: 117-123.
Direct Link |
4: Da Silva, L.V., 2002. Hazard Analysis Critical Control Point (HACCP), microbial safety and shelf life of smoked blue catfish (Ictalurus furcatus). M.Sc. Thesis, Department of Food Science, Louisiana State University, Louisiana, USA.
5: Eyo, A.A., 1981. The construction and operation of a new mechanical gas smoking kiln (Kainji gas kiln). New Bussa, Nigeria, Kainji Lake Research Institute, (Kainji Lake Research Institute Technical Report Series, 7).
6: Eyo, A.A., 2001. Fish Processing Technology in the Tropics. National Institute for Freshwater Fisheries Research, New Bussa, Nigeria, ISBN-13: 9781770457, Pages: 403.
7: FNB/NRC., 1985. An Evaluation of the Role of Microbiological Criteria for Foods and Food Ingredients (Sub-committee on Microbiological Criteria, Committee on Food Protection). National Academy Press, Washington D.C., USA..
8: Gram, L., 2001. Potential hazard in cold-smoked fish: Listeria monocytogenes. J. Food Sci., 66: S1072-S1081.
9: Huss, H.H., P.K.B. Embarek and V.F. Jeppesen, 1995. Control of biological hazards in cold smoked salmon production. Food Control, 6: 335-340.
10: Ikeme, A.I. and H.C. Gugnani, 1988. Effect of smoking time on product quality of hot-smoked mackerel. In: Proceedings of FAO Expert Consultation on Fish Technology in Africa, Abidjan (Cote d'Ivoire), April 25-28, 1988, FAO, Rome, Italy, pp: 124-130.
11: ICMSF, 1986. Microorganisms in Foods 2: Sampling for Microbiological Analysis: Principles and Specific Applications. 2nd Edn., University of Toronto Press, Canada, ISBN-13: 978-0802056931, Pages: 448.
12: Lyne, P.M., 1976. Microbiological Methods. 4th Edn., Butter Worms, London, pp: 169-195.
13: Moses, B.S., 1983. Introduction to Tropical Fisheries. Ibadan University Press, Nigeria, pp: 94-95.
14: Mossel, D.A.A., 1967. Ecological principles and methodological aspects of the examination of foods and feeds for indicator microorganisms. J. Assoc. Agric. Chem., 50: 91-104.
15: Olsen, C.Z., 1976. Smoke flavour and its bacteriological effect. Proceedings of the Symposium in Advances in Smoking of Foods, September 8-10, 1976, Warsaw, Poland -.
16: Oyelese, O.A., 2006. Quality assessment of cold smoked hot smoked and oven dried Tilapia nilotica under cold storage temperature conditions. J. Fisheries Int., 1: 92-97.
Direct Link |
17: Shimang, G.N., 1990. Post-harvest losses in inland fisheries in Nigeria with emphasis on lake chad and lake Kainji. Proceedings of the Symposium of Post-Harvest Fish Technology, October 21-22, 1990, FAO, Rome, Italy, pp: 78-83.
18: Silliker, J.H. and D.A. Gabis, 1976. ICMSF method studies VII. Indicator tests as substitutes for direct testing of dried foods and feeds for Salmonella. Can. J. Microbiol., 22: 971-974.
19: Waterman, J.J., 1976. Production of dried fish. FAO Fisheries Technical Paper No. 169, Food and Agriculture Organization of the United Nations, Rome, Italy, pp: 39-58.