162
ROBINA B. SCHOLES AND J. M. SHEWAN
During these phases marked changes occur not only in the numbers
but also in the types of the bacteria present and also in the sensory
characteristics of the fish. In such a complex dynamic system it is
obviously not always easy to disentangle the course of events but in
general it may be said that, for the gadoids such as cod and haddock,
the more slowly growing Gram-positive groups are overgrown by the
Pseudomoms types, so that by the tenth to the twelfth day, the latter
oonstitute about SO-SO% of the flora. The Fhvobaeterium-Cytophaga
group frequently appears to make a transient increase about the 8th to
12th day, just at the point where significant changes occur in the
chemical and organoleptic aspects of spoilage. By the time inedibility
has been reached, usually between the 14th to 16th day, 90% of the
flora consists of Pseudomonas spp. The figures given in Table V showing
the changes in the flora during the spoilage of cod stored in ice under
carefully controlled laboratory conditions, have been confirmed by
direct observation of the flora of cod landed from distant trawlers at
Hull (Spencer, 1961), in which the period of stowage in ice could vary
from 5 to 16 days (Table VI). It is interesting to note that the proportion of mesophiles, i.e. organisms with an optimum growth temperature
of about 37"C, are much higher in market fish than in the fresh fish or
fish from the laboratory experiment. This is almost certainly due to
contacts with decks after catching and to human handling.
Another interesting feature of the flora of spoiling fish is that most
of the Pseudomonas species appear to belong to a group of acid forming
types closely allied to Pseudomonas fraqi. This organism, present only
in a few per cent in the newly caught fish, accounts for from 20 to 50%
of the Pseudomow spp. in fish 12 to 15 days in ice, Moreover, it is
known that this species can produce the fruity spoilage odours 80
characteristic of gadoids in their later stages of spoilage (Castell el al.,
1069; Castell and Greenough, 1959). The implication of the Pseudomoms group in fish spoilage has also been shown by the action of pure
cultures on sterile fish muscle (Shewan and Jones, 1957).
Finally, it should be added that the changes in the flora just
given would not necessarily apply to other species such as herring and
mackerel or to the elasmobranchs such as dogfish, skates and rays.
This is because the nature and amounts of muscle extractives, the first
compounds to suffer bacterial attack during spoilage, differ widely
from fish to fish. Differences within the gadoid group are much smaller
than those between this group and herring, mackerel and tunny, on the
one hand and the elasmobranchs on the other (Shewan et al., 1952;
Shewan, 1953 ; Shewan, 1955). Thus, the elasmobranchs are characterized by the large amount of urea, amounting to about 1% in rays and
ROBINA B. SCHOLES AND J. M. SHEWAN
During these phases marked changes occur not only in the numbers
but also in the types of the bacteria present and also in the sensory
characteristics of the fish. In such a complex dynamic system it is
obviously not always easy to disentangle the course of events but in
general it may be said that, for the gadoids such as cod and haddock,
the more slowly growing Gram-positive groups are overgrown by the
Pseudomoms types, so that by the tenth to the twelfth day, the latter
oonstitute about SO-SO% of the flora. The Fhvobaeterium-Cytophaga
group frequently appears to make a transient increase about the 8th to
12th day, just at the point where significant changes occur in the
chemical and organoleptic aspects of spoilage. By the time inedibility
has been reached, usually between the 14th to 16th day, 90% of the
flora consists of Pseudomonas spp. The figures given in Table V showing
the changes in the flora during the spoilage of cod stored in ice under
carefully controlled laboratory conditions, have been confirmed by
direct observation of the flora of cod landed from distant trawlers at
Hull (Spencer, 1961), in which the period of stowage in ice could vary
from 5 to 16 days (Table VI). It is interesting to note that the proportion of mesophiles, i.e. organisms with an optimum growth temperature
of about 37"C, are much higher in market fish than in the fresh fish or
fish from the laboratory experiment. This is almost certainly due to
contacts with decks after catching and to human handling.
Another interesting feature of the flora of spoiling fish is that most
of the Pseudomonas species appear to belong to a group of acid forming
types closely allied to Pseudomonas fraqi. This organism, present only
in a few per cent in the newly caught fish, accounts for from 20 to 50%
of the Pseudomow spp. in fish 12 to 15 days in ice, Moreover, it is
known that this species can produce the fruity spoilage odours 80
characteristic of gadoids in their later stages of spoilage (Castell el al.,
1069; Castell and Greenough, 1959). The implication of the Pseudomoms group in fish spoilage has also been shown by the action of pure
cultures on sterile fish muscle (Shewan and Jones, 1957).
Finally, it should be added that the changes in the flora just
given would not necessarily apply to other species such as herring and
mackerel or to the elasmobranchs such as dogfish, skates and rays.
This is because the nature and amounts of muscle extractives, the first
compounds to suffer bacterial attack during spoilage, differ widely
from fish to fish. Differences within the gadoid group are much smaller
than those between this group and herring, mackerel and tunny, on the
one hand and the elasmobranchs on the other (Shewan et al., 1952;
Shewan, 1953 ; Shewan, 1955). Thus, the elasmobranchs are characterized by the large amount of urea, amounting to about 1% in rays and