Effect of delayed processing on nutrient composition, pH and organoleptic quality of pond raised tilapia (Oreochromis shiranus) stored at ambient temperature
Fish are a nutrient rich food but highly perishable due to its high water activity, protein content, neutral pH and presence of autolytic enzymes. This explains why fresh fish quality deteriorates rapidly if not properly stored after catch such as use of low temperature. The implication is that delayed processing of fresh fish negatively influences consumer taste of fish. Further, fresh fish that are not processed on time lose nutrients due to spoilage. In Malawi, most fresh fish are sold in the open due to lack of storage facilities such as refrigerators – a condition that accelerates spoilage of fish. This study was carried out to assess the effect of delayed processing on nutrient composition, pH level and organoleptic quality of pond raised tilapia - Oreochromis shiranus which is the commonest grown tilapia in Malawi. Thirty freshly pond harvested O. shiranus fish (average weight 100g) were cleaned then deep fried in edible cooking vegetable oil at time intervals of 0, 6 and 12 hours from catch and stored at ambient temperature. A pre-trained sensory evaluation panel consisting six members described changes in the sensory quality of the fresh and fried fish at every sampling time interval. Crude protein, crude fat, ash, moisture content and pH were also determined at each sampling time. Determined crude protein and crude fat decreased while ash content increased with storage time though these were not significantly different from baseline (P>0.05). Moisture content decreased from 0 (fresh) to 6 hours then increased again up to 12 hours of storage. Ash and moisture content correlated positively (0.93 and 0.79% respectively), whilst crude protein, crude fat and pH were negatively correlated (-0.91, -0.05 and -0.97, respectively) to sensory quality demerit scores. pH decreased from 5.94 at 0 hours to 5.92 after 6 hours then increased sharply to 5.98 after 12 hours of ambient fish storage. Consumers started to significantly (P<0.05) dislike the fish after 7 hours and eventually rejected the samples after 12 hours of ambient storage time. The study recommends that for optimum consumer satisfaction and nutritional benefit, fresh O. shiranus should be processed at most 7 hours after harvesting.
Key words: Nutrients, taste, Tilapia, ambient storage