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close this book Fish handling, preservation and processing in the tropics: Part 2 no. G145
View the document Summaries
View the document Acknowledgements
View the document Introduction
View the document Salting of fish: salt
View the document Salting of fish: methods
View the document Drying of fish: basic principles
View the document Drying of fish: methods
View the document Smoking of fish
View the document Marinades
View the document Fermented fish products: a review
View the document Boiled fish products
View the document Fish canning: theory and practice
View the document Freeze drying
View the document Irradiation
View the document Miscellaneous products: crustaceans
View the document Miscellaneous aquatic products used as food
View the document Food by-products
View the document Non-food by-products
View the document New and delicatessen products
View the document Fish meal
View the document Fish silage
View the document Chemical and physical methods of quality assessment
View the document Organoleptic (sensory) measurement of spoilage
View the document Microbiology of spoilage
View the document Microbiology of fish spoilage
View the document Public health microbiology
View the document International standards for fisheries products
View the document Large-scale fish landing facilities
View the document Small-scale landing facilities: design and operation
View the document Retail sale facilities
View the document Fisheries extension services: their role in rural development
View the document Training in the field
View the document Appendix

Microbiology of fish spoilage

Post-mortem bacterial growth

Methods of controlling spoilage

Tests for assessing microbial 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:

Organism

Minimum pH

Maximum pH

Escherichia cold

4.4

9.0

Salmonella typhi

4.5

8.0

Streptococcus lactis

4.3 - 4.8

-

Lactobacillus spp.

3.8 - 4.4

7.2

Moulds

1.5 - 2.0

11.0

Yeasts

2.5

8.0 - 8.5

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:

Equilibrium relative humidity = 100 aw

Here is a table showing the minimum aw at which different groups of microorganisms can grow:

Organisms

aw

Most spoilage bacteria

0.91

Most spoilage yeasts

0.88

Most spoilage moulds

0.80

Halophilic bacteria

0.75

Xerophilic moulds

0.65

Osmophilic moulds

0.60

 

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:

Per cent salt w/v

aw

0.9

0.995

1.7

0.99

3.5

0.98

7.0

0.96

10.0

0.94

13.0

0.92

16.0

0.90

19.0

0.88

22.0

0.86

 

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.

 

Tests for assessing microbial spoilage

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.