Fish Products and Byproducts Technology

Unit 3 - Salting and Drying

3.4. Microbial spoilage in salted fish
What are all the Microbial Spoilage can happen in salted and dried fish?
Most of the micro-organisms normally associated with fish spoilage, for example, Pseudomonas spp., are halophobic and will not grow in salt concentrations exceeding 5%. There are, however, certain organisms that may be both common and pathogenic, and which are halotolerant, growing in a 10, or even 20% salt environment. Staphlyococcus aureoous is a highly significant example.
The most important spoilage micro-organisms are the halophiles which actually require salt for growth and will not grow unless 10% salt is present. These bacteria, which are responsible for pink spoilage, so called because of the colour of their colonies and consequent appearance of the cured fish, include Halobacterium salinaria, H. cutirubum, Sarcina morrhuae and S. litoralis. They are aerobic and usually not found in pickled fish where only limited oxygen access is possible through the brine. They are also thermophilic with an optimum growth temperature of about 42^oC and minimum growth temperature of 5^oC.
The first sign of pink spoilage is a delicate pink sheen on the surface of the fish in wet stack or during pining. This can be easily rubbed off without damaging the fish. Treatment with formaldehyde or sulphur dioxide vapours, or dipping the fish in a solution of sodium metabisulphite prevents recurrence, although maintenance of the ambient temperature below 10oC is likely to prevent initial germination and growth. Cases of food poisoning said to have been caused by the consumption of pink spoiled fish have probably, in fact, been due to the growth of exotoxin producing Staphylococcus aureus. The latter will commence growth at water activities slightly higher than those required for growth of pink bacteria. Pink bacteria themselves have been proved to be non-toxic and non-pathogenic.
The water activity of slat fish after drying is too low to support bacterial growth but, should temperature and humidity conditions become suitable, certain osmophilic moulds can grow. Dun spoilage derives its name from the brown surface discoloration caused by the growth of moulds of the Wallemia genus. They are able to grow in salt concentrations between 5 and 26% although they are not specific to sodium chloride and can grow on osmotic equivalent concentrations of potassium chloride, ammonium chloride, glycerol of glucose. Hence they are obligate osmophiles rather than halophiles. Other conditions for growth are: (I) temperature 10-37^oC (optimum 25^oC); (ii) pH 4.0-8.0 (optimum 6.0 to 7.0); and (iii) optimum relative humidity 75%.
Unlike the pink bacteria, dun moulds do not decompose the flesh but make the surface unsightly and, consequently, the product less saleable. They can be brushed off the surfaces but growth will rapidly recur if dry, cool conditions are not maintained. Old and rotting wood harbour such moulds, therefore wood should be avoided in dried fish stores or kept well covered. The causative agents of both pink and dun spoilage abound in solar curing salts, so the maintenance of low temperatures and humidity are the essential means of combating such spoilage during production and storage. Unfortunately, much of the market for dry salted fish is in areas where as hot, humid climate predominates, therefore the prevention of moisture ingress is an essential feature of any packaging used. The use of plastic bags is unsuitable because any temperature fall in the surrounding ambient would cause condensation. Dipping cured fish in vegetable oil approximately halves the rate of moisture uptake from a humid environment. This might be sufficient to delay microbiological spoilage beyond the required storage life.