EFFECTS OF PROCESSING AND STORAGE ON SOME CHEMICAL CHARACTERISTICS AND LIPID-COMPOSITION OF A GHANAIAN FERMENTED FISH PRODUCT

To evaluate the effectiveness of the traditional method of processing in preservation of the final product, the changes in composition of lipids, free amino acids and organic acids of 'momoni', a Ghanaian fermented fish product, were investigated using Japanese jack mackerel (Trachurus japonicus) which is similar to African jack mackerel (Caranx hippos). The moisture content decreased drastically from 751 to 505 g kg-1 after fermentation for 3 days, and then remained almost constant during subsequent storage. Changes in pH were minimal although a gradual decrease was observed from 6.2 in the raw material to 5.9 after 2 months storage. Salt accumulation due to the salting process increased up to 325 g kg-1 but remained almost unchanged during subsequent storage. Volatile basic nitrogens (VBN) increased to 0.8 g kg-1 on a salt-free dry matter basis, after 2 months storage. Most free amino acids, except histidine which was present in the highest concentration in the samples, increased during processing and subsequent storage. However, no detectable amount of histamine was formed after 2 months storage. Lactic acid, the most prominent organic acid (3.12 g kg-1 salt-free dry matter) in the raw materials, decreased during processing and subsequent storage. Diphosphatidylglycerol, free fatty acids, and lysophosphatidylcholine increased appreciably. Processing resulted in depletion of sphingomyelin, phosphatidylserine and phosphatidylinositol with decreases in triglycerides, cholesterol, phosphatidylethanolamine and phosphatidylcholine. The proportion of the extracted total lipids present as polyunsaturated fatty acids decreased and this was accompanied by increases in the proportions of saturated fatty acids, indicating a high degree of oxidation. Peroxide value and thiobarbituric acid value increased on fermentation, confirming the observed oxidation, but decreased during storage. In conclusion, although oxidation was extensive, following hygienic and good manufacturing practices, this simple method could prevent spoilage taking place in fish if good-quality raw materials are used.