Fish Microbiology and Quality Assurance

Unit 9 - Assurance and management quality of fish

9.1.3. Biochemical / chemical methods
Biochemical or chemical methods are useful in determining the composition of raw material and products and also in detecting the deteriorations in the products.
Determining chemical composition
Chemical composition involves determining the proximate composition of fish and fishery products by employing standard protocols. These include measurement of moisture, protein, fat, ash or mineral content, and acid content in marinated fishes. Determination of metal contents such as mercury, lead, cadmium, zinc, etc., and chlorinated hydrocarbons, radioactive isotopes, colouring matters, additives and preservatives are done by analytical methods that involve complex procedures.

Detecting product deterioration 
The extent of spoilage and oxidative rancidity in the chilled fish can be measured by chemical and biochemical methods by measuring the complex series of changes in flesh constituents brought about by autolytic enzymes and putrefactive microorganisms.
The spoilage microorganisms such as Pseudomonas, Alteromonas, Flavobacterium, Acinetobacter, Aeromonas and Moraxella in tropical fish cause deterioration by their ability to produce H₂S, reduce trimethylamine oxide (TMAO) to trimethylamine (TMA) and convert urea to ammonia. Members of Pseudomonas also produce many volatile sulphur compounds. Thus, the degree of spoilage can be indicated by quantitative measurement of these compounds. Generally, fish flesh show visible signs of spoilage when bacterial load rises to above 107 organisms/ g.

Chemical indicators of quality
In chemical assessment of quality, the various products of spoilage in fish muscle are quantitatively determined and correlated with sensory characteristics.
Several chemical compounds produced in fish muscle by autolytic enzymes, putrefactive microorganisms or by chemical reactions like lipid oxidation can be used as indicators of spoilage. During spoilage, these compounds gradually accumulate in the flesh and hence their determination provides a measure of the progress of spoilage.
The compounds useful as quality indices are
1. Volatile bases
Basic nitrogenous compounds such as ammonia, trimethylamine (TMA), dimethylamine (DMA) etc.
2. Nucleotides
Degradation products from adenosine triphosphate (ATP) eg. inosine monophosphate (IMP), hypoxanthine (Hx), etc.
3. Lipid oxidation
Peroxides, hydroperoxides, aldehydes, etc.