Testing for rancidity in fresh and decomposing Tilapia fish
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In order to test for spoilage in fish tissues, rancidity indicating substances were quantified using the TBA-MDA assay. The reaction between thiobarbituric acid (TBA) and thiobarbituric acid reactive substances (TBARS) to form a pink product is an effective way of measuring the extent of lipid peroxidation in fish tissues. A quantitative TBA procedure for measurement of MDA was used to standardise the method and to establish a standard calibration curve. By this method, the malondialdehyde content in fresh and decomposing tilapia fish tissues from Bwaise fish market was measured, and the degree of oxidative rancidity expressed in grams of malonaldehyde per 1000 g of sample. One gram of the fish tissue was homogenised in 9.0 ml of 1.15% KCl to prepare tissue homogenate. Aliquots of 0.1ml of the homogenate was mixed with sodium dodecyl sulphate (SDS), followed by acetic acid buffer (pH 3.5 = ideal for the reaction), and aqueous solution of TBA and then heated at 95° C for an hour. The pink pigment formed by the reaction was extracted with n-butanol-pyridine mixture and the concentration of MDA estimated by absorbance at 532 nm. Fresh tissues from the market contained 0.012 g of MDA per kg of fish tissue on average and fish tissues frozen for 6 days contained 0.016 g of MDA per kg. Whole fish tissues deliberately exposed to spoilage contained 0.02 g of MDA per kg after the 6 days. The results of this study show that there is continuous spoilage of fish tissues as indicated by persistent levels of MDA. Malondialdehyde was observed in the refrigerated tissues as well as in the un-preserved fish tissues.