Fish proteins are less stable than mammalian protein. As with meat,
extensive proteolysis does not become apparent until the product is
already well spoiled, but limited protein degradation may improve
bacterial access to the nutrient pool of extractives.
The speed with which a product spoils is also related to the initial
microbial load on the product: the higher the count the sooner spoilage
occurs. Since fish from cold waters will have a larger proportion of
psychrotrophs among their natural microflora, this can shorten the chill
shelf-life appreciably.
Spoilage of chilled fish is due principally to the activity of psychrotro-
phic Gram-negative rods also encountered in meat spoilage, particularly
Shewanella putrefaciensandPseudomonasspp. The uniquely objection-
able smell of decomposing fish is the result of a cocktail of chemicals,
many of which also occur in spoiling meat. Sulfurous notes are provided
by hydrogen sulfide, methyl mercaptan and dimethyl sulfide and esters
contribute the ‘fruity’ component of the odour. A number of other
amines in addition to TMA are produced by bacterial catabolism of
amino acids. Skatole, a particularly unpleasant example produced by the
degradation of tryptophan, also contributes to the smell of human
faeces. The level of volatile bases in fish flesh has provided an index of
spoilage, although this and other chemical indices used are often poor
substitutes for the trained nose and eyes.
Figure 5.7 illustrates some of the different products made from fish,
most of which are discussed elsewhere in terms of the general processing
technologies used. One interesting aspect that relates to some of the
discussion above will be discussed here. The combination of a near neutral
pH and availability of TMAO as an alternative electron acceptor means
that vacuum and modified-atmosphere packing of fish does not produce
the same dramatic extension of keeping quality seen with meat. Typically
the shelf-life extension of vacuum and modified-atmosphere-packed
cod will vary from less than 3 days to about 2 weeks. Shewanella
Figure 5.6 Reduction of trimethylamine oxide
144 Microbiology of Primary Food Commodities