Biology Reference
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as amino acids. These amino acids are used directly by the bacteria for
their growth. Shrimps generally contain large amounts of arginine and
glycine and small amounts of cysteine and methionine (Chinivasagam et
al., 1998).
Shelf life is variable and depends on the nature of the initial fl ora and the
spoilage bacteria, often unknown, that may develop during storage. It can
be relatively short: 5 to 8 d for ice-packed Penaeus merguiensis (Gonçalves
et al., 2003); 6 d at 0°C for Parapenaeus longirostris (Mendes et al., 2005).
Other authors have found longer shelf lives for tropical shrimps: 13 to 16 d
when ice-packed (Cann, 1974; Jayaweera and Subasinghe, 1988; Shamshad
et al., 1990); 10 to 17 d when ice-packed or over 20 d when stored in liquid
ice for different species of the genus Penaeus found in Australian waters
(Chinivasagam et al., 1996).
The major cause of early shrimp spoilage is melanosis, which is the
formation of black spots on the cephalothorax. This process is biochemical
and is not due to bacterial activity, but rather an enzyme complex called
polyphenol oxidase. The benzoquinones thus produced interact with
amines, amino acids and oxygen to form the melanin responsible for this
black colouration.
Other organoleptic changes are generated by the action of bacteria.
Chinivasagam et al. (1996) have shown that when tropical Australian
shrimps are ice-packed in an oxygen-rich environment, the growth of
Pseudomonas fragi increases, whereas using liquid ice where oxygen is limited
leads to the development of Sh. putrefaciens . The sensory characteristics of
contaminated products are very different depending on the predominance
of one or the other of these bacteria. To confi rm these observations,
Chinivasagam et al. (1998) have demonstrated experimentally that strains
of Ps. fragi inoculated into shrimp juice produce odours of fruit and onion,
whereas strains of Sh. putrefaciens give off a sulphurous odour. Moreover,
these two bacteria display different profi les of volatile compounds.
Packaging fresh shrimps (Parapenaeus longirostris) in a modifi ed
atmosphere combining 40-45% CO 2 with 30 or 5% O 2, respectively, delays
microbial growth and the production of total volatile nitrogen bases (TVBN)
and TMA compared to air-packed or iced stored shrimp, especially at the
end of storage (López-Caballero et al., 2002). Amine production and the
low level of H 2 S-producing microorganisms and enterobacteria suggest
that Ph. phosphoreum could be involved in the spoilage of shrimps both in
air and MAP. The production of biogenic amines (tyramine, putrescine,
cadaverine, agmatine) during storage is higher for atmospheres with
modifi ed O 2 concentrations than for air. Gonçalves et al. (2003) have shown
that, for the same species of shrimp and using the same gas compositions,
shelf life can be prolonged at least by 2 d (9 d instead of 4 to 7 d with
the ice storage). This type of packaging can also extend the shelf life of
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