Effects of Boiling and Frying on the Proximate Composition of Four Fish Species Sold in Ilese, Ogun State, Nigeria

Processing changes the composition, nutrient density and storage characteristics of fish, yet locally generated evidence is limited for fish products sold in Ilese, Ogun State. This study compared the proximate composition of raw, boiled, and boiled–fried fish. Twelve market samples representing four species hake/Panla (Merluccius sp.), African catfish (Clarias gariepinus), mackerel (Scomber scombrus) and croaker (Pseudotolithus sp.) were analysed. Each species was represented in three processing states. Moisture was determined by oven drying at 105 °C, crude protein by the Kjeldahl procedure, crude fat by Soxhlet extraction with petroleum ether, and ash by incineration at 550 °C, following AOAC procedures. Group results were expressed as mean ± standard deviation. Raw fish contained 74.95 ± 2.15% moisture, 18.35 ± 1.04% protein, 3.50 ± 1.20% fat and 1.20 ± 0.18% ash. Boiled fish contained 61.25 ± 2.03%, 23.50 ± 1.19%, 4.85 ± 1.35% and 2.30 ± 0.26%, respectively. Boiled–fried fish contained 16.00 ± 1.47% moisture, 45.85 ± 2.19% protein, 14.85 ± 1.35% fat and 4.50 ± 0.26% ash. Relative to raw fish, boiling reduced moisture by 18.3%, whereas boiling followed by frying reduced it by 78.7%. The corresponding wet-weight increases in protein, fat and ash after boiling–frying were 149.9%, 324.3% and 275.0%. Exploratory Welch tests indicated clear differences between raw and boiled–fried samples for all four parameters; raw and boiled samples did not differ significantly in fat (p = 0.193). The apparent increases in protein and ash primarily reflect water loss, while the marked fat increase after frying additionally reflects oil uptake. Boiling therefore produced a lower-fat product than boiling followed by frying. Larger, replicated studies that control processing time, temperature and oil are required before market-wide inference