Microbiology and Spoilage Trail in Nile Perch (Lates niloticus), Lake ...

as well as indices of fish spoilage. Spoilage is also recognized when there is a significant
accumulation of sulphur-containing compounds such as methyl mercaptan, dimethyl
sulphide and H2S produced by Pseudomonas putrefaciens. In 1970 Walker P., Cann D., and
Shewan J. M., reported the bacteria spoilage association in the order of: the dominant flora
at the first stage included mixture of Acinetobacter- Moraxella and Pseudomonas; then
Acinetobacter-Moraxella, Flavobacteria and Pseudomonas and finally during the
exponential phase the Pseudomonas became the dominant genus with partly Acinetobacter
and Moraxella.
Studies by Gram et al., (1986); Ravn Jorgensen et al., (1988); and Gram, (1990) in
temperate and warm fish indicated significance difference in keeping qualities in ice.
Temperate water-fish and fish products when stored in melting ice spoiled after 9-10days
and seldom after 2-3 weeks (14-21days), while tropical water fishes generally spoiled after
4 weeks (28 days) of ice storage (Nile perch). According to Gram et al, (1986,1990)
temperate water fishes when stored at ambient temperature were rejected after one day and
warm water fish - Nile perch stored at 20 0 C- 30 0 C spoiled after 11-17 hours,. Fresh fish
caught from warm tropical waters stored in melting ice was found to cause decrease in
bacterial counts and extend the bacterial lag phase from 5 to 7 days before exponential
phase started. However in the fourth week of storage at the point of rejection the total
counts usually reach 10 6 - 10 8 cfu/g; of which 10 6 – 10 7 cfu/g represent counts for H2S
producing bacteria on spoiled and rejected Nile perch, (Gram et al., 1990).
2.5 FACTORS CONTRIBUTING ON FISH SPOILAGE
Factors contributing to spoilage of food (fish) include insect’s infestations and rodents,
parasites; and activities of bacteria, yeast and molds in food that may render food to
become undesirable. Exposure of food products to the light may cause degradation of
proteins and fats to produce off flavours and off odours, (Doyle, 2007, Gram et al., 2002).
According to Huss, (1995) review, the storage or shelf life of fish and fish products may be
influenced by fish species, fishing grounds, fishing season, pH and hygienic handling.
Large, lean, thick skin, high pH, and flat fish species keep longer than small, fatty, thin
skin, low pH and round fish species. Bruising/damaging fish during handling causes rapid
spoilage. The wide range of micro flora in fish is reflected by its environmental habitats
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