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Studies of spoilage of commercially important tropical fishes under iced storage
Fisheries Research, 7 (1989) 1-9
1
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Studies on Spoilage of Commercially Important Tropical Fishes under Iced Storage P.K. SURENDRAN, JOSE JOSEPH, A.V. SHENOY, P.A. PERIGREEN, K. MAHADEVA IYER and K. GOPAKUMAR
Central Institute of Fisheries Technology, Matsyapuri P. 0., Cochin-682029 (India) (Accepted for publication 24 February 1988)
ABSTRACT Surendran, P.K., Joseph, J., Shenoy, A.V., Perigreen, P.A., Mahadeva Iyer, K. and Gopakumar, K., 1989. Studies on spoilage of commercially important tropical fishes under iced storage. Fish. Res., 7: 1-9. Iced storage of five commercially important species of tropical fish, namely oil sardine (Sardinella longiceps (Valenciennes)), Indian mackerel (RastreUiger kanagurta (Cuvier)), pearl spot (green chromide ) (Etroplus suratensis (Bloch) ), milk fish (Chanos chanos (Forsskal) ) and tilapia (Oreochromis mosambica (Peters)) was studied. Oil sardine and Indian mackerel had an acceptable iced storage shelf life of nearly 1 week and pearl spot, milk fish and tilapia nearly 2 weeks. During storage in ice there was a selection of bacterial types, and by the time incipient spoilage was noticed one or two species of bacteria constituted the bulk of the flora. In all these fishes, the spoilage flora were composed mainly of a single genus of bacteria, namely Pseudomonas.
INTRODUCTION
India has a coast line of 6500 km and her annual fish landings were 2.7 million tonnes in 1984, of which 1.6 million tonnes were from marine sources and the rest from rivers and estuaries (Anon, 1985 ). About 60% of the marine catch is obtained in non-mechanised vessels operated by traditional fishermen, and their catches are landed at various places on the beach. Catches by mechanised vessels, on the other hand, are landed in fisheries harbours, where facilities for icing and proper transportation are available. The fishermen are more interested in prawn and fishes like seer and pomfret, which fetch better prices. Fishes like oil sardine and Indian mackerel are not iced on board at present, and this leads to about 10-15% loss owing to spoilage. They are eventually iced in the landing places, and transported in trucks to hinterlands fbr marketing which takes 6-48 h. At present, the prevalent method of retarding spoilage of fish in India, as well as other tropical countries, is storage in ice. The spoilage of fish when
0165-7836/89/$03.50
© 1989 Elsevier Science Publishers B.V.
stored in ice is mainly caused by psychotrophic bacteria (Shewan, 1977; Santos, 1981; Surendran and Gopakumar, 1981, 1982; Poulter et al., 1985) which are capable of growth and multiplication at low temperatures. Much information is available on the bacterial flora and spoilage characteristics of fishes from cold and temperate waters (Shewan, 1961, 1977; Bramsnaes, 1965; Santos, 1981 ). Information regarding the spoilage characteristics of tropical fishes is rather limited (Surendran and Iyer, 1973, 1976; Disney, 1976; Curran and Disney, 1979; Surendran, 1980; Santos, 1981 ). There is also a great paucity of information on the psychotrophic bacteria involved in spoilage of tropical fishes in iced storage. In the Central Institute of Fisheries Technology, Cochin (India), investigations have been carried out on the iced storage characteristics of many commercially-important Indian food fishes and on the bacterial flora involved in the spoilage of those fishes. This paper presents some of the significant observations. MATERIALSAND METHODS The fish used for these investigations were: (1) two species of seawater fish, oil sardine (Sardinella longiceps (Valenciennes)) and Indian mackerel (Rastrelliger kanagurta (Cuvier)); (2) two species of brackishwater fish, milk fish (Chanos chanos (Forsskal) and pearl spot (green chromide) (Etroplus suratensis (Bloch)); (3) one species of freshwater fish, tilapia (Oreochromis mosambica (Peters)). The fish were procured immediately after catch, washed with seawater or freshwater as appropriate, packed in crushed ice with a fish to ice ratio of 1:1 (weight/weight) and stored in Thermocole (expanded polystyrene) insulated boxes with a drain, and transported to the laboratory for storage. During storage, ice loss (about 25-30% in 2 days) was made up by the addition of fresh ice on alternate days. Sampling of the fish was done at intervals of 2-3 days during the iced storage. Shelf life of fish was evaluated on the basis of: (1) sensory qualities; (2) total aerobic bacterial count; (3) chemical indices, such as total volatile bases and trimethyl amine (for marine fish only). Sensory evaluation of the samples was made in the raw and cooked state by a trained taste panel consisting of 10-12 people. The taste panel was constituted from among the members of the staff of the Institute and they were trained in sensory assessment of fish as described in A.S.T.M. (1968). For organoleptic evaluation of the muscle in the cooked state, the dressed and eviscerated fish meat was cooked in 2% brine for 10 min and presented to the panel members separately. The hedonic system (Hill and Glew, 1973) of scoring was adopted and the statistical mean was accepted as the sensory score.
Total bacterial count of the fish muscle (with skin) was determined by the FDA (1978) methods. Muscle with skin was cut from both sides of 3-4 fishes and minced aseptically. Ten grams of the minced meat was used for sampling. Physiological saline (0.85% NaC1) was used as the diluent. Seawater agar (Surendran and Iyer, 1971) was used for plating marine fish. Brackishwater and freshwater fish muscle was plated using tryptone-glucose beef extract agar (TGA). The plates were incubated at 28 +_2 °C (room temperature, RT) for 48 h for mesophilic bacterial count and at 1 _+1 ~C for 14 days for psychotrophic bacterial count. For qualitative studies of the bacterial flora from the fish, cultures were isolated from the poured plates after enumeration, as reported by Surendran and Gopakumar (1981). Cultures were morphologically and biochemically characterised and identified, as described by Buchanan and Gibbons (1974) and Lee and Pfeifer (1975). Trimethyl amine nitrogen (TMAN) and total volatile base nitrogen {TVB N ) were determined by the Conway microdiffusion method (Conway, 1962). RESULTS
The length and weight range and the fat content of the 5 species of fish studied are given in Table 1. Based on organoleptic qualities, both oil sardine and Indian mackerel became unacceptable after 5-6 days of storage in ice (Fig. 1). This was mainly caused by the development of rancid taste in their muscle, even though the aerobic bacterial counts at RT were still less than 1 million g-~ muscle (Fig. 2). Even though T M A N values were found to increase more or less steadily TABLE1 List of fish used and their size characteristics Fish
Length and weight range
F a t content (wet basis) (%)
Oil sardine
11-18 cm ( 12-42 g) 15-22 cm (80-110 g) 16-25 cm (400-900 g) 30-40 cm (600-900 g) 25-35 cm (500-900 g)
5 -12
( Sardinella longiceps) Indian mackerel
( RastreUiger kanagurta ) Pearl spot (green chromide)
( Etroplus suratensis ) Milk fish
(Chanos chanos ) Tilapia
(Oreochromis mosambica )
3
10
2 - 3 1.5-3 2 -3.5
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6 Doys in ice
Days in ice
Fig. 1. Changes in the average overall sensory scores of oil sardine ( o - - o ) and Indian mackerel ( m - - m ) during iced storage. Arrow mark shows limit of acceptability. Fig. 2. Changes in aerobic bacterial counts (TPC) at 28 + 2 ° C and 1 _+1 ° C, trimethyl amine (TMA) and total volatile bases (TVB), respectively, of oil sardine ( o - - o , × - - × , A - - Z ~ and E 3 - - r 7 ) and Indian mackerel ( 0 - - 0 , ×-- X, •--• and , - - B ) during iced storage.
¢o
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6
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L 20
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~
20
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Fig. 3. Changes in the average overall sensory scores and total aerobic bacterial counts ( T P C ) of Etroplus suratensis during iced storage: o - - o sensory score: × - - × T P C at 28_+ 2 ° C, zk--Z~ T P C at 1 _-21 ° C. Arrow mark shows the limit of acceptability. Fig. 4. Changes in the average overall sensory scores and total aerobic bacterial counts ( T P C ) at 28 _+2 ° C, respectively, of Chanos chanos ( o - - o and A - - A ) and tilapia ( 0 - - 0 and • - - • ) during iced storage. Arrow mark shows the limit of acceptability.
.
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with bacterial count, TVBN values were not found to follow a similar pattern (Fig. 2 ). Such unsteady changes in the values of TVBN have been observed in the case of the other species of fish. Hence, only sensory qualities and bacterial count could be taken as useful indices of shelf life during iced storage of fish. Pearl spot retained an acceptable taste for 12-14 days in ice. By this time, the total count of the psychotrophic bacteria reached 1 million, even though the total aerobic counts at 28 + 2 ° C were well above 10 million g- 1 muscle (Fig. 3). For milk fish, another brackishwater species, held in iced storage the trends in the changes of sensory scores and total aerobic bacterial counts were similar to those of pearl spot. Milk fish had a shelf life of 14 days in iced storage (Fig. 4). Iced storage shelf life of tilapia, based on sensory qualities, was 12-13 days, but in this case the increase in bacterial counts of the muscle was very slow compared with the other fishes (Fig. 4). During iced storage, there was a selection of bacterial types. The native flora of oil sardine and Indian mackerel underwent significant changes. As the days of storage in ice increased the percentage of Pseudomonas spp. increased progressively. After 21 days of iced storage, when the fish were completely spoiled, 75-81% of the flora consisted of Pseudomonas alone (Table 2). Almost similar changes were noticed in the bacterial flora of pearl spot, milk fish and tilapia during iced storage; but even though Pseudomonas formed the major group at the time of spoilage, its share was comparatively less (Table 2). DISCUSSION There are wide variations in the reports of the length of acceptable iced storage life of fish. For fish like cod, haddock, whiting, hake and red fish 8-15 days have been reported (F.A.O., 1968). In the case of herring (Clupea harengus) 4-6 days have been obtained and for mackerel (Scomber scombrus) 48 days (Bramsnaes, 1965; Stroud, 1972; Smith et al., 1980; Santos, 1981). The oil sardine and Indian mackerel had an acceptable shelf life of almost 1 week in ice, as do the cold water pelagic species like herring and mackerel. The small size of these fish and their relatively fatty nature, might be the reason for speedier spoilage during iced storage. The longer shelf life reported for certain marine fishes like pomfret, seer and perch in ice by Velankar and Kamasastri (1956) was based merely on TMA values, which are not reliable as an index of shelf life. However, Poulter et al. (1978) found that some tropical fishes from the South China Sea, chub mackerel (Rastrelliger brachysoma) and fusilier (Caesion cuning) had an acceptable shelf life of 2-5 weeks and grouper (Plectropoma meculatus ) 4 weeks. For pearl spot, milk fish and tilapia, which are lean fishes, an acceptable
shelf life of nearly 2 weeks could be obtained. Nair et al. (1971) found that the freshwater carp, mrigal, had a shelf life of nearly 5 weeks in ice. However, Bandyopadhyay et al. (1985) recorded an iced storage life of 13-17 days only for mrigal (Cirrhinus mrigala). Joseph et al. (1980) reported only 12-14 days of iced storage life for milk fish (Chanos chanos). Some earlier workers observed that tropical fish had a much longer shelf life in ice than cold-water fish (Disney et al., 1974; Disney, 1976; Shewan, 1977). This is because the naturally-occurring bacterial flora of tropical fish contain only a low proportion of psychophilic bacteria and the large drop in temperature during icing, had a more pronounced effect on mesotrophic bacteria (Disney et al., 1974; Poulter et al., 1985). But a considerable proportion of the bacterial strains from tropical fish easily adapted to growth at lower temperature (Table 2 ). They are also biochemically more active (Karthiyani and Iyer, 1971; Surendran, 1980). Hence, the presence of a large number of bacteria capable of adapting to growth at lower temperature and their biochemically more active nature, contribute to the speedier spoilage of tropical fish, during iced storage. The pattern of change in the flora of the tropical fishes during iced storage (Table 2 ) appears to be similar to those of the fish of temperate waters. In the initial flora, MoraxeUa, Acinetobacter and Vibrio groups accounted for 58-76% in the marine fishes and 25-45% in the brackish- and freshwater fishes. Pseudomonas formed 10% in the flora of Indian mackerel and 20% of oil sardine; but in pearl spot, tilapia and milk fish, the Pseudomonas accounted for 20-25% of their flora. As the days of storage progressed, Pseudomonas spp. emerged as the dominant group. They accounted for 75-81% of the flora in the marine fishes and 61-70% in the brackish and freshwater fishes at the time of spoilage. As observed by Shewan (1977), irrespective of the initial flora of fish, Pseudomonas and Alteromonas groups emerged as the predominant genera during spoilage of fish in ice. ACKNOWLEDGEMENTS
The authors express their sincere thanks to Shri. M.R.Nair, Director, Central Insitute of Fisheries Technology, Cochin-682029 for the consent, encouragement and facilities provided for the work.
REFERENCES Anon, 1985. Statistics of marine products. Marine Products Export Development Authority, Cochin-16, India. 263 pp. A.S.T.M., 1968. Manual of Sensory Testing Methods. American Society for Testing and Materials, Philadelphia, PA 19103. A.S.T.M. Spec. Publ., no 434, 77 pp. Bandyopadhyay, J.K., Chattopadhyay, A.K. and Bhattacharyya, S.K., 1985. Studies on the ice
storage characteristics of commercially important fresh water fishes. In: Harvest and Post Harvest Technology of Fish. Society of Fisheries Technologists India, Cochin-29, 744 pp. (pp. 381-385 ). Bramsnaes, F., 1965. Handling of fresh fish. In: G. Borgstrom (Editor), Fish as Food. Vol. IV. Academic Press, New York, 518 pp. (pp. 5-63). Buchanan, R.E. and Gibbons, N.E., 1974. Bergey's manual of determinative bacteriology. 8th edn. The Williams and Wilkins Company, Baltimore, 1268 pp. Conway, E.J., 1962. Microdiffusion analysis and volumetric error. 5th edn. Parch Croskey and Sockwood and Son Limited, London, 467 pp. Curran, C.A. and Disney, J.G., 1979. The iced storage life of tropical fish. Paper presented at the Indo-Pacific Fisheries Commission Working Party on Fish Technology, Jakarta, Indonesia, 19 pp. Disney, J.G., 1976. The spoilage of fish in tropics. Proc. First Annu. Trop. Subtrop. Fish. Technol. Conf. of the Americas. Texas A & M University, College Station, Texas, 686 pp. (pp. 23-39). Disney, J.G., Cole, R.C. and Jones, N.R., 1974. Considerations in the use of tropical fish products. In: R. Kreuzer (Editor), Fishery Products. Fishing News (Books) Ltd., Surrey, 462 pp. (pp. 329-337). F.A.O., 1968. Ice in Fisheries. F.A.O. Fish Rep. no. 59, 68 pp. F.D.A., 1978. Bacteriological Analytical Manual, Food and Drug Administration of the U.S. Association of Official Analytical Chemists (Publ.), Washington D.C. Hill, M.A. and Glew, G., 1973. Organoleptic assessment of products frozen by two methods, liquid nitrogen and blast freezing. J. Food Technol., 8: 205-210. Joseph J., Perigreen, P.A., George, C. and Govindan, T.K., 1980. Iced and frozen storage characteristics of cultured Chanos chanos (Forskal). Fish. Technol., 17: 21-25. Karthiyani, T.C. and Mahadeva Iyer, K., 1971. Temperature influence on the biochemical characteristics of microflora of fresh sardine (SardineUa longiceps). Fish. Technol., 8: 100-101. Lee, J.S. and Pfeifer, D.K., 1975. Microbial characteristics of dunginess crab (Cancer magister). Appl. Microbiol., 30: 72-78. Nair, R.B., Tharamani, P.K. and Lahiry, N.L., 1971. Studies on chilled storage of fresh water fish I. Changes occurring during iced storage. J. Food. Sci. Technol., 8: 53-56. Poulter, R.G., Nicolaider, I. and Hector, D.H., 1978. Quality changes in fish from the South China Sea. I. Iced storage of chub mackerel, grouper and fusilier. Proc. Indo-Pacific Fish. Comm., 18: 169-185. Poulter, R.G., Curran, C.A., Rowlands, B. and Disney, J.G., 1985. Comparison of biochemistry and bacteriology of tropical and temperate water fish during preservation and processing. In: Harvest and Post harvest Technology of Fish. Society of Fisheries Technologists (India) Cochin. 744 pp. (pp. 407-413). Santos Lima Dos, C.A.M., 1981. The storage of tropical fish in ice--a review. Trop. Sci., 23: 97127. Shewan, J.M., 1961. The microbiology of sea water fish. In: G. Borgstrom (Editor), Fish as Food, Vol. I. Academic Press, New York and London, 725 pp. (pp. 487-560). Shewan, J.M., 1977. The bacteriology of fresh and spoiling fish and the biochemical changes induced by bacterial action. In: Proc. Conf. Handl. Process Market. Trop. Fish. Tropical Products Institute, London, 511 pp. (pp. 51-66). Smith, J.G.M., Hardy, R. and Young, K.W., 1980. A seasonal study of the storage characteristics of mackerel stored at chill and ambient temperatures. In: J.J. Connel and Torry Research Station staff (Editors), Advances in fish science and technology. Fishing News (Books) Ltd., Surrey, 512 pp. (pp. 229-303). Stroud, G.D., 1972. The herring Torry Research Station, Aberdeen. Torry Advisory Note, no. 57, 17 pp. Surendran, P.K., 1980. Chemical preservatives in relation to control of microbial changes in fishery products. Ph.D. thesis, University of Kerala, Trivandrum, India.
Surendran, P.K. and Gopakumar, K., 1981. Selection of microbial flora in CTC-treated oil sardine (Sardinella longiceps ) Indian mackerel (RastreUiger kanagurta ) and prawn (Metapenaeus dobsoni) held in iced storage. Fish. Technol., 18: 133-141. Surendran, P.K. and Gopakumar, K., 1982. Bacterial flora of EDTA treated oil sardine (SardineUa longiceps ), Indian mackerel (RastreUiger kanagurta ) and prawn (Metapenaeus dobsoni) in ice storage. Fish. Technol., 19: 33-39. Surendran, P.K. and Iyer, K.M., 1971. Use of antibiotics in the preservation of prawn. Fish. Technol., 8: 55-60. Surendran, P.K. and Iyer, K.M., 1973. Experimental preservation of sardines (SardineUa longiceps) using chlortetracycline. Fish. Technol., 10:110-115. Surendran, P.K. and Iyer, K.M., 1976. Bacterial flora of fresh and iced Indian mackerel (Rastrelliger kanagurta) and its response to chlortetracycline (CTC) treatment. Fish. Technol., 13: 139-145. Velankar, N.K. and Kamasastri, P.V., 1956. The bacterial flora, trimethyl amine and total volatile nitrogen of fish muscle at 0 °C (in ice). Indian J. Fish., 3: 269-289.
1
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Studies on Spoilage of Commercially Important Tropical Fishes under Iced Storage P.K. SURENDRAN, JOSE JOSEPH, A.V. SHENOY, P.A. PERIGREEN, K. MAHADEVA IYER and K. GOPAKUMAR
Central Institute of Fisheries Technology, Matsyapuri P. 0., Cochin-682029 (India) (Accepted for publication 24 February 1988)
ABSTRACT Surendran, P.K., Joseph, J., Shenoy, A.V., Perigreen, P.A., Mahadeva Iyer, K. and Gopakumar, K., 1989. Studies on spoilage of commercially important tropical fishes under iced storage. Fish. Res., 7: 1-9. Iced storage of five commercially important species of tropical fish, namely oil sardine (Sardinella longiceps (Valenciennes)), Indian mackerel (RastreUiger kanagurta (Cuvier)), pearl spot (green chromide ) (Etroplus suratensis (Bloch) ), milk fish (Chanos chanos (Forsskal) ) and tilapia (Oreochromis mosambica (Peters)) was studied. Oil sardine and Indian mackerel had an acceptable iced storage shelf life of nearly 1 week and pearl spot, milk fish and tilapia nearly 2 weeks. During storage in ice there was a selection of bacterial types, and by the time incipient spoilage was noticed one or two species of bacteria constituted the bulk of the flora. In all these fishes, the spoilage flora were composed mainly of a single genus of bacteria, namely Pseudomonas.
INTRODUCTION
India has a coast line of 6500 km and her annual fish landings were 2.7 million tonnes in 1984, of which 1.6 million tonnes were from marine sources and the rest from rivers and estuaries (Anon, 1985 ). About 60% of the marine catch is obtained in non-mechanised vessels operated by traditional fishermen, and their catches are landed at various places on the beach. Catches by mechanised vessels, on the other hand, are landed in fisheries harbours, where facilities for icing and proper transportation are available. The fishermen are more interested in prawn and fishes like seer and pomfret, which fetch better prices. Fishes like oil sardine and Indian mackerel are not iced on board at present, and this leads to about 10-15% loss owing to spoilage. They are eventually iced in the landing places, and transported in trucks to hinterlands fbr marketing which takes 6-48 h. At present, the prevalent method of retarding spoilage of fish in India, as well as other tropical countries, is storage in ice. The spoilage of fish when
0165-7836/89/$03.50
© 1989 Elsevier Science Publishers B.V.
stored in ice is mainly caused by psychotrophic bacteria (Shewan, 1977; Santos, 1981; Surendran and Gopakumar, 1981, 1982; Poulter et al., 1985) which are capable of growth and multiplication at low temperatures. Much information is available on the bacterial flora and spoilage characteristics of fishes from cold and temperate waters (Shewan, 1961, 1977; Bramsnaes, 1965; Santos, 1981 ). Information regarding the spoilage characteristics of tropical fishes is rather limited (Surendran and Iyer, 1973, 1976; Disney, 1976; Curran and Disney, 1979; Surendran, 1980; Santos, 1981 ). There is also a great paucity of information on the psychotrophic bacteria involved in spoilage of tropical fishes in iced storage. In the Central Institute of Fisheries Technology, Cochin (India), investigations have been carried out on the iced storage characteristics of many commercially-important Indian food fishes and on the bacterial flora involved in the spoilage of those fishes. This paper presents some of the significant observations. MATERIALSAND METHODS The fish used for these investigations were: (1) two species of seawater fish, oil sardine (Sardinella longiceps (Valenciennes)) and Indian mackerel (Rastrelliger kanagurta (Cuvier)); (2) two species of brackishwater fish, milk fish (Chanos chanos (Forsskal) and pearl spot (green chromide) (Etroplus suratensis (Bloch)); (3) one species of freshwater fish, tilapia (Oreochromis mosambica (Peters)). The fish were procured immediately after catch, washed with seawater or freshwater as appropriate, packed in crushed ice with a fish to ice ratio of 1:1 (weight/weight) and stored in Thermocole (expanded polystyrene) insulated boxes with a drain, and transported to the laboratory for storage. During storage, ice loss (about 25-30% in 2 days) was made up by the addition of fresh ice on alternate days. Sampling of the fish was done at intervals of 2-3 days during the iced storage. Shelf life of fish was evaluated on the basis of: (1) sensory qualities; (2) total aerobic bacterial count; (3) chemical indices, such as total volatile bases and trimethyl amine (for marine fish only). Sensory evaluation of the samples was made in the raw and cooked state by a trained taste panel consisting of 10-12 people. The taste panel was constituted from among the members of the staff of the Institute and they were trained in sensory assessment of fish as described in A.S.T.M. (1968). For organoleptic evaluation of the muscle in the cooked state, the dressed and eviscerated fish meat was cooked in 2% brine for 10 min and presented to the panel members separately. The hedonic system (Hill and Glew, 1973) of scoring was adopted and the statistical mean was accepted as the sensory score.
Total bacterial count of the fish muscle (with skin) was determined by the FDA (1978) methods. Muscle with skin was cut from both sides of 3-4 fishes and minced aseptically. Ten grams of the minced meat was used for sampling. Physiological saline (0.85% NaC1) was used as the diluent. Seawater agar (Surendran and Iyer, 1971) was used for plating marine fish. Brackishwater and freshwater fish muscle was plated using tryptone-glucose beef extract agar (TGA). The plates were incubated at 28 +_2 °C (room temperature, RT) for 48 h for mesophilic bacterial count and at 1 _+1 ~C for 14 days for psychotrophic bacterial count. For qualitative studies of the bacterial flora from the fish, cultures were isolated from the poured plates after enumeration, as reported by Surendran and Gopakumar (1981). Cultures were morphologically and biochemically characterised and identified, as described by Buchanan and Gibbons (1974) and Lee and Pfeifer (1975). Trimethyl amine nitrogen (TMAN) and total volatile base nitrogen {TVB N ) were determined by the Conway microdiffusion method (Conway, 1962). RESULTS
The length and weight range and the fat content of the 5 species of fish studied are given in Table 1. Based on organoleptic qualities, both oil sardine and Indian mackerel became unacceptable after 5-6 days of storage in ice (Fig. 1). This was mainly caused by the development of rancid taste in their muscle, even though the aerobic bacterial counts at RT were still less than 1 million g-~ muscle (Fig. 2). Even though T M A N values were found to increase more or less steadily TABLE1 List of fish used and their size characteristics Fish
Length and weight range
F a t content (wet basis) (%)
Oil sardine
11-18 cm ( 12-42 g) 15-22 cm (80-110 g) 16-25 cm (400-900 g) 30-40 cm (600-900 g) 25-35 cm (500-900 g)
5 -12
( Sardinella longiceps) Indian mackerel
( RastreUiger kanagurta ) Pearl spot (green chromide)
( Etroplus suratensis ) Milk fish
(Chanos chanos ) Tilapia
(Oreochromis mosambica )
3
10
2 - 3 1.5-3 2 -3.5
,c[
20
0)
o u
81"--
E o m
/ /
E
~ /
~ t ~/
t2
== =
8
4
O Z E
o t..
0>=
z
o
I
2
I
~
I
~
8
6 Doys in ice
Days in ice
Fig. 1. Changes in the average overall sensory scores of oil sardine ( o - - o ) and Indian mackerel ( m - - m ) during iced storage. Arrow mark shows limit of acceptability. Fig. 2. Changes in aerobic bacterial counts (TPC) at 28 + 2 ° C and 1 _+1 ° C, trimethyl amine (TMA) and total volatile bases (TVB), respectively, of oil sardine ( o - - o , × - - × , A - - Z ~ and E 3 - - r 7 ) and Indian mackerel ( 0 - - 0 , ×-- X, •--• and , - - B ) during iced storage.
¢o
1,o
IO P o
(
e o
=6
6~
6
o~k
/
~.~_
I
o
3
,
~
2Z
2 I 4
L 8 Days
I
I 16
12 in
ice
L 20
0
4
8
12
16
~
20
Days in ice
Fig. 3. Changes in the average overall sensory scores and total aerobic bacterial counts ( T P C ) of Etroplus suratensis during iced storage: o - - o sensory score: × - - × T P C at 28_+ 2 ° C, zk--Z~ T P C at 1 _-21 ° C. Arrow mark shows the limit of acceptability. Fig. 4. Changes in the average overall sensory scores and total aerobic bacterial counts ( T P C ) at 28 _+2 ° C, respectively, of Chanos chanos ( o - - o and A - - A ) and tilapia ( 0 - - 0 and • - - • ) during iced storage. Arrow mark shows the limit of acceptability.
.
t
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with bacterial count, TVBN values were not found to follow a similar pattern (Fig. 2 ). Such unsteady changes in the values of TVBN have been observed in the case of the other species of fish. Hence, only sensory qualities and bacterial count could be taken as useful indices of shelf life during iced storage of fish. Pearl spot retained an acceptable taste for 12-14 days in ice. By this time, the total count of the psychotrophic bacteria reached 1 million, even though the total aerobic counts at 28 + 2 ° C were well above 10 million g- 1 muscle (Fig. 3). For milk fish, another brackishwater species, held in iced storage the trends in the changes of sensory scores and total aerobic bacterial counts were similar to those of pearl spot. Milk fish had a shelf life of 14 days in iced storage (Fig. 4). Iced storage shelf life of tilapia, based on sensory qualities, was 12-13 days, but in this case the increase in bacterial counts of the muscle was very slow compared with the other fishes (Fig. 4). During iced storage, there was a selection of bacterial types. The native flora of oil sardine and Indian mackerel underwent significant changes. As the days of storage in ice increased the percentage of Pseudomonas spp. increased progressively. After 21 days of iced storage, when the fish were completely spoiled, 75-81% of the flora consisted of Pseudomonas alone (Table 2). Almost similar changes were noticed in the bacterial flora of pearl spot, milk fish and tilapia during iced storage; but even though Pseudomonas formed the major group at the time of spoilage, its share was comparatively less (Table 2). DISCUSSION There are wide variations in the reports of the length of acceptable iced storage life of fish. For fish like cod, haddock, whiting, hake and red fish 8-15 days have been reported (F.A.O., 1968). In the case of herring (Clupea harengus) 4-6 days have been obtained and for mackerel (Scomber scombrus) 48 days (Bramsnaes, 1965; Stroud, 1972; Smith et al., 1980; Santos, 1981). The oil sardine and Indian mackerel had an acceptable shelf life of almost 1 week in ice, as do the cold water pelagic species like herring and mackerel. The small size of these fish and their relatively fatty nature, might be the reason for speedier spoilage during iced storage. The longer shelf life reported for certain marine fishes like pomfret, seer and perch in ice by Velankar and Kamasastri (1956) was based merely on TMA values, which are not reliable as an index of shelf life. However, Poulter et al. (1978) found that some tropical fishes from the South China Sea, chub mackerel (Rastrelliger brachysoma) and fusilier (Caesion cuning) had an acceptable shelf life of 2-5 weeks and grouper (Plectropoma meculatus ) 4 weeks. For pearl spot, milk fish and tilapia, which are lean fishes, an acceptable
shelf life of nearly 2 weeks could be obtained. Nair et al. (1971) found that the freshwater carp, mrigal, had a shelf life of nearly 5 weeks in ice. However, Bandyopadhyay et al. (1985) recorded an iced storage life of 13-17 days only for mrigal (Cirrhinus mrigala). Joseph et al. (1980) reported only 12-14 days of iced storage life for milk fish (Chanos chanos). Some earlier workers observed that tropical fish had a much longer shelf life in ice than cold-water fish (Disney et al., 1974; Disney, 1976; Shewan, 1977). This is because the naturally-occurring bacterial flora of tropical fish contain only a low proportion of psychophilic bacteria and the large drop in temperature during icing, had a more pronounced effect on mesotrophic bacteria (Disney et al., 1974; Poulter et al., 1985). But a considerable proportion of the bacterial strains from tropical fish easily adapted to growth at lower temperature (Table 2 ). They are also biochemically more active (Karthiyani and Iyer, 1971; Surendran, 1980). Hence, the presence of a large number of bacteria capable of adapting to growth at lower temperature and their biochemically more active nature, contribute to the speedier spoilage of tropical fish, during iced storage. The pattern of change in the flora of the tropical fishes during iced storage (Table 2 ) appears to be similar to those of the fish of temperate waters. In the initial flora, MoraxeUa, Acinetobacter and Vibrio groups accounted for 58-76% in the marine fishes and 25-45% in the brackish- and freshwater fishes. Pseudomonas formed 10% in the flora of Indian mackerel and 20% of oil sardine; but in pearl spot, tilapia and milk fish, the Pseudomonas accounted for 20-25% of their flora. As the days of storage progressed, Pseudomonas spp. emerged as the dominant group. They accounted for 75-81% of the flora in the marine fishes and 61-70% in the brackish and freshwater fishes at the time of spoilage. As observed by Shewan (1977), irrespective of the initial flora of fish, Pseudomonas and Alteromonas groups emerged as the predominant genera during spoilage of fish in ice. ACKNOWLEDGEMENTS
The authors express their sincere thanks to Shri. M.R.Nair, Director, Central Insitute of Fisheries Technology, Cochin-682029 for the consent, encouragement and facilities provided for the work.
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