compared with around 5.5 in mammalian muscle. Fish which have a
lower pH such as halibut (approx. 5.6) tend to have better keeping
qualities. Secondly, the absence of carbohydrate means that bacteria
present on the fish will immediately resort to using the soluble pool of
readily assimilated nitrogenous materials, producing off-odours and
flavours far sooner. This can be less pronounced in fish produced by
intensive aquaculture since they are normally fed to satiation which
increases glycogen levels in the liver and muscle.
The composition of the non-protein nitrogen fraction differs signifi-
cantly from that in meat (Table 5.7). Trimethylamine oxide (TMAO)
occurs in appreciable quantities in marine fish as part of the osmoreg-
ulatory system. TMAO is used as a terminal electron acceptor by non-
fermentative bacteria such asShewanella putrefaciensand this allows
them to grow under microaerophilic and anaerobic conditions. The
product of this reduction is trimethylamine which is an important
component in the characteristic odour of fish (Figure 5.6.) TMAO also
contributes to a relatively high redox potential in the flesh since theEhof
the TMAO/TMA couple isþ19 mV.
Elasmobranchs such as dogfish and shark contain high levels of urea.
Bacterial urease activity in the flesh can produce ammonia very rapidly
giving the product a pungent odour. Not only does this render the flesh
itself uneatable but it can also taint the flesh of other fish stored nearby.
It is for this reason that in many areas fishermen will discard all but the
fins of shark when they catch them.
Shellfish such as lobster have a particularly large pool of nitrogenous
extractives and are even more prone to rapid spoilage; a factor which
accounts for the common practice of keeping them alive until immedi-
ately prior to consumption.
Table 5.7 Nitrogen-containing extractives in fish
Cod Herring Dogfish LobsterTotal nitrogenous extractives gkg^112123055
Free amino acids 7 30 10 300
(mM l^1 )
TMAO 5 3 10 2
(mM l^1 )
Urea 0 0 33 0
(mM l^1 )
Creatine 3 3 2 0
(mM l^1 )
Betaine 0 0 2 1
(mM l^1 )
Anserine 1 0 0 0
Adapted from D.M. Gibson, ‘Microbial spoilage of foods in Micro-organisms in Action: Concepts
and Applications in Microbial Ecology’. J. M.Lynch and J. E.Hobbie, (eds.) Blackwell, Oxford,
1988
Chapter 5 143