Microbiology of fish spoilage
Post-mortem bacterial growth
As soon as a fish dies, a series of changes starts to take place
which is collectively known as spoilage. The degradation of the tissue is
brought about both by indigenous fish enzymes and by micro-organisms which are
present on the surface of the skin, on the gills and in the intestines. The
chemical and autolytic changes that take place are the subject of another
lecture and will not be dealt with here.
Where do bacteria come from?
It is a fact that newly caught healthy fish have sterile tissues
and that bacteria can only be found on the skin, gills and in the intestines.
Whilst still alive, the fish and the bacteria exist in a state of equilibrium
and it is only after death that the bacteria can invade the tissues and spoil
the fish. Invasion of the muscle from the gut is, of course, made easier by the
autolysis brought about by the gut enzymes. The numbers of bacteria in the gut
are highest when the fish has recently been feeding. It is reasonable to assume
that fish caught in polluted waters will be more heavily contaminated than fish
from clean areas, and the literature on the subject bears this out. The
situation changes when we consider the fate of the fish once it is caught: it
will be contaminated to some extent by all the materials with which it comes
into contact, e.g., ice, fish boxes, the boat itself and even the crew.
What do they do?
The bacteria grow using the fish as a food source and producing
various waste products which accumulate and produce off-odours and bad flavours.
It is well known that trimethylamine oxide can be reduced by bacteria to give
trimethylamine which imparts an off-odour to the fish. Other bacterial
by-products are ammonia and hydrogen sulphide, both of which have objectionable
smells. In the quest for nutrients, bacteria make use of the simplest compounds
first and intact proteins may only be used when they have been broken down by
autolytic enzymes.
The result of the activities of bacteria, coupled with the
autolytic changes, is fish which are organoleptically, and often visually,
spoiled. In some cases, the production of bacterial waste products may be such
that fish become inedible before the tissue is visibly damaged.
Methods of controlling spoilage
The lecture on the fundamentals of microbiology has provided us
with some hints as to the methods to be used for controlling spoilage. The
technology of preservation will be dealt with elsewhere and we will only by
considering how the techniques affect the bacteria themselves. Even under ideal
conditions, fish will only keep for a defined period with the possible exception
of canned fish; the purpose of preservation is to make the storage life of the
product suitably long whilst not adding too much to the selling price.
You will recall that we have discussed generation times of
bacteria in the first lecture on microbiology. An increase in the generation
time will mean that the time scale of the typical growth curve will be
lengthened. This can be achieved in a number of ways, the first of which is to
lower the temperature of the environment. It is of little consequence to the
cells how the cooling is effected: it is the temperature which is important
assuming that there are no other factors at work such as dehydration (often
found in refrigerators).
The lowering of the temperature means that the enzymes in the
cell cannot function at their optima and, since the metabolism of the whole cell
relies on enzymes, the cells are slow to grow and divide. The effectiveness of a
particular temperature in preserving a food will depend on a number of factors
as follows:
(a) What proportion of the flora is psychrotrophic (i.e., able
to grow at low temperatures).
(b) The growth rate of the organisms at the
given temperature.
(c) The previous treatment given to the food.
The last factor requires a little explanation. If a food is
heated to a temperature which is sufficient to kill all vegetative cells but not
the resistant spores of Bacillus spp. or Clostridium spp., then storage in the
refrigerator can be quite lengthy since the spores will not germinate unless the
temperature rises into the normal mesophilic range. It is difficult to be
precise about this since the conditions, as well as the particular organism,
will influence the temperature at which germination takes place.
If the temperature is taken to below freezing, the situation is
a little different. In most cases, growth is completely stopped and the change
in state of the water may well kill a large proportion of the cells. Death can
be attributed to many factors including mechanical damage, dehydration,
concentration in cellular solution, cold shock and metabolic injury. The fact
that the last mentioned of these factors does occur is demonstrated by the
exacting nutritional needs of cells recovered from frozen foods.
For some time, it has been known that fish from tropical waters
can be stored for longer in ice than fish from temperate waters. This is due, in
some measure, to the tropical flora being unable to adjust rapidly to the large
drop in temperature upon the addition of ice: a drop of the order of 30°C.
In temperate waters, the corresponding drop may be only a few degrees
Centigrade.
The growth of bacteria can also be arrested by shifting the pH
of the environment so that the cells' enzymes are again not able to function at
their optima. This is what occurs in pickles and marinades. Many spoilage
organisms find the low pH so hostile that they die during storage. Spoilage of
pickled foods is usually the result of mould growth, the mycelium being visible
on the surface of the liquid. The growth of mould may bring about a rise in the
pH of the pickle and this will enable yeasts and perhaps specialised bacteria to
grow. The following table shows the pH minima and maxima of a few common
organisms:
It will be obvious that since the pH of fish tissue will be 5.6
or more, almost any micro-organism can grow on and spoil it. Some bacteria,
particularly Lactobacillus spp., have the ability to reduce the pH to a level
where the normal spoilage flora is inhibited, the usual mechanism being the
production of lactic acid from the carbohydrate in the substrate or food. Many
of the traditional fermented foods of South East Asia owe their long shelf life
to such a mechanism as this. Unfortunately, there is not much information on the
exact nature of these products or the organisms which are responsible for the
preservative effect.
All the reactions which take place in the cell require an
aqueous environment for their proper functioning. Thus, reducing the amount of
available water in the foodstuff will bring about a slowing, or complete
cessation, of bacterial growth. Water content is usually recorded as percentage
moisture but, in bacterial terms, it is the free water which is important.
Microbiologists measure water content as water activity (aw), which is derived
from the following formula:
Here is a table showing the minimum aw at which different groups
of microorganisms can grow:
The water activity of a food can be lowered by removal of water
or the addition of a solute which makes the water no longer available to the
cells. Sodium chloride is such a solute; the aw obtained for different
concentrations of salt are given in the table below:
It is obvious that although a 22 per cent salt solution is too
salty for the average palate, it still does not give complete control of
spoilage organisms, especially moulds and halophilic bacteria. In order to
provide the best protection to the food, it is usual to remove some of the water
and add salt. The removal of water can be by the direct application of heat but
a more interesting technique is the smoking of foods.
All the operations involved in these tests must be carried out
in such a way that there is no contamination of the sample by the technician;
this is known as aseptic technique. The sample is weighed out into a sterile
bottle from which it is transferred to a blender and homogenised with 450
cm³ of sterile diluent. The resulting suspension is returned to the bottle.
One cm³ of this suspension is pipetted into each of two petri dishes and 1
cm³ is transferred to a bottle containing 9 cm³ of diluent. Two more
petri dishes are inoculated in a similar fashion from this bottle; this process
is repeated until a suitable dilution has been reached. About 20 cm³ of
nutrient agar, cooled to 45°C, is poured into each dish and after mixing
with the sample the agar is allowed to set. The prepared plates are placed in an
incubator for a defined period at a set temperature. After incubation, the
plates are examined; those for the dilution which has between 30 and 300
colonies growing are counted. By multiplying by the dilution factor, the actual
count for the sample can be
calculated.
