Fish constitutes one of the most important and widely consumed sources of animal protein globally, particularly in developing countries where it provides a cheap, accessible, and nutritionally balanced dietary option. It is rich in high-quality protein, essential amino acids, polyunsaturated fatty acids (PUFAs), vitamins, and minerals that are crucial for human health and development [1]. According to Tacon [2], fish contributes significantly to food security and nutritional well-being, especially in rural and coastal communities where it often represents the primary source of animal protein. Beyond its basic nutritional role, fish is increasingly recognized for its bioactive compounds, including phenolics, flavonoids, peptides, and trace minerals that exhibit antioxidant, anti-inflammatory, and disease-preventive properties [3]. These bioactive components play a critical role in reducing oxidative stress in humans, thereby lowering the risk of chronic diseases such as cardiovascular disorders, diabetes, and certain cancers [4]. The antioxidant potential of fish is largely attributed to endogenous enzymes such as superoxide dismutase (SOD), catalase, and glutathione-related systems, as well as exogenous compounds obtained through diet and environment [5].

Despite its nutritional and functional importance, fish is highly perishable due to its high moisture content, neutral pH, and enzymatic and microbial activities [6]. This necessitates processing and preservation to extend shelf life, improve safety, and enhance distribution. Common fish processing techniques include smoking, oven drying, frying, salting, and freezing. These methods, however, are known to induce physicochemical and biochemical changes that may affect the nutritional quality and functional properties of fish [7]. Thermal processing, in particular, has been reported to significantly alter the structure and concentration of bioactive compounds [8]. For instance, Liu et al. [9] noted that heat treatments can lead to oxidation of lipids, degradation of vitamins, and loss of thermolabile antioxidants. Similarly, Cheng et al. [10] emphasized that processing methods strongly influence protein denaturation and lipid oxidation, which directly affect the nutritional quality of fish products.

Smoking and drying have traditionally been used as low-cost preservation techniques in many developing countries. These methods reduce moisture content and microbial load, thereby extending shelf life. However, they may also introduce smoke-derived phenolic compounds, which can either enhance or reduce antioxidant activity depending on processing conditions [11]. In contrast, frying exposes fish to high temperatures and oxygen, often resulting in significant lipid peroxidation and nutrient losses, as reported by FAO [12]. Coptodon guineensis (commonly known as Guinean tilapia) is an important brackish water fish species widely consumed in West Africa. Despite its nutritional relevance, limited information exists on how different processing methods affect its bioactive compounds and antioxidant properties. Understanding these effects is essential for optimizing processing techniques that preserve both nutritional and functional qualities. Therefore, this study investigates the effects of oven drying, smoking, and frying on the bioactive compounds and antioxidant properties of Coptodon guineensis, with the aim of providing scientific evidence for improved processing and consumption practices.

Study Area

Fresh samples of Coptodon guineensis were obtained from Choba market, Choba, in the Obio-Akpor Local Government Area of Rivers State, Nigeria. The area is located in the Niger Delta region, characterized by tropical climate conditions with high humidity and temperature, which support active fish trade and aquaculture activities.

Sample Collection and Preparation

A total of freshly harvested Coptodon guineensis specimens of uniform size and weight were purchased from fish vendors. The fish were transported in an ice-packed insulated container to the laboratory to minimize post-harvest spoilage. Upon arrival, the samples were thoroughly washed with clean potable water to remove debris, slime, and surface contaminants. The fish were then divided into four experimental groups: fresh (control), oven-dried, smoked, and fried samples.

Processing Methods

Fresh Fish

For fresh fish, fresh samples were analyzed immediately without any processing to serve as the baseline for comparison.

Oven Drying

Fish samples were placed in a hot-air oven at 60–65°C for 24–36 hours until constant weight was achieved, indicating complete moisture reduction.

Smoking

Fish were smoked using a traditional smoking kiln at a temperature range of 70–90°C for 6–8 hours until properly dried and evenly smoked.

Frying

Fish samples were deep-fried in refined vegetable oil at 170–180°C for 5–10 minutes until golden brown and fully cooked. Excess oil was drained using absorbent paper.

Proximate and Bioactive Compound Analysis

Moisture content was determined using the oven-drying method as described by the Association of Official Analytical Chemists [13]. Total phenolics were quantified using the Folin–Ciocalteu method and expressed as mg gallic acid equivalent (mg AE/g). Flavonoids were determined using an aluminium chloride colorimetric assay and expressed as mg quercetin equivalent (mg QE/g). Mineral contents (zinc, iron, and selenium) were analyzed using Atomic Absorption Spectrophotometry (AAS) after acid digestion of samples.

Statistical Analysis

Data obtained were expressed as mean ± standard deviation (SD). Statistical analysis was performed using one-way analysis of variance (ANOVA) to determine significant differences among treatments. Where significant differences existed (p < 0.05), means were separated using Duncan’s Multiple Range Test (DMRT).

The effects of different processing methods (oven drying, smoking, and frying) on the bioactive compounds and antioxidant properties of Coptodon guineensis are presented in Tables 1 and 2.

Bioactive Compounds

Processing methods exerted significant (p < 0.05) effects on all evaluated bioactive parameters. The moisture content of fresh fish was markedly high (72.45 ± 1.12%) compared to processed samples, with frying resulting in the lowest moisture level (8.75 ± 0.70%). Oven-dried and smoked samples exhibited intermediate moisture contents of 12.30 ± 0.85% and 15.60 ± 0.92%, respectively, indicating effective water removal during thermal processing. Total phenolic content was highest in fresh fish (2.85 ± 0.15 mg AE/g), although it was statistically comparable to smoked samples (2.65 ± 0.12 mg AE/g). A significant reduction was observed in oven-dried (2.40 ± 0.10 mg AE/g) and fried samples (1.95 ± 0.08 mg AE/g), with frying exhibiting the greatest loss. Similarly, flavonoid content followed the same trend, with fresh fish recording the highest value (1.75 ± 0.09 mg QE/g), while fried samples had the lowest (1.20 ± 0.05 mg QE/g). In contrast, mineral elements such as zinc and iron showed relatively higher concentrations in processed samples compared to fresh fish. Zinc content was significantly higher (p < 0.05) in oven-dried (0.042 ± 0.003 mg/g) and smoked samples (0.040 ± 0.002 mg/g) than in fresh fish (0.035 ± 0.002 mg/g), while fried fish showed intermediate values. A similar trend was observed for iron, with oven-dried samples recording the highest concentration (0.035 ± 0.003 mg/g), followed by smoked and fried samples, and the lowest value in fresh fish (0.028 ± 0.002 mg/g). Conversely, selenium content decreased significantly with processing, with fresh fish having the highest value (0.0065 ± 0.0004 mg/g) and fried samples the lowest (0.0049 ± 0.0002 mg/g).

Antioxidant Properties

The antioxidant properties of Coptodon guineensis were also significantly influenced (p < 0.05) by processing methods. Fresh fish exhibited the highest DPPH radical scavenging activity (68.20 ± 2.10%), indicating superior antioxidant potential. This activity declined significantly in oven-dried (60.45 ± 1.85%) and smoked samples (63.10 ± 1.95%), with the lowest activity observed in fried fish (52.75 ± 1.70%). Superoxide dismutase (SOD) activity followed a similar pattern, with fresh samples recording the highest activity (12.50 ± 0.60 U/mg protein). Processing led to a significant reduction, particularly in fried fish (8.75 ± 0.45 U/mg protein), suggesting thermal degradation of endogenous antioxidant enzymes. Lipid peroxidation, as measured by TBARS (mg MDA/kg), showed an opposite trend. Fresh fish recorded the lowest value (0.85 ± 0.05 mg MDA/kg), while processed samples exhibited significantly higher levels, with fried fish showing the highest lipid oxidation (1.80 ± 0.08 mg MDA/kg). Oven-dried and smoked samples had moderate but significantly elevated TBARS values compared to fresh fish. Total antioxidant capacity (TAC) was highest in fresh fish (3.20 ± 0.15 mg AAE/g) and decreased significantly with processing. Smoked samples retained slightly higher TAC (2.90 ± 0.14 mg AAE/g) compared to oven-dried samples (2.75 ± 0.12 mg AAE/g), while fried fish recorded the lowest value (2.30 ± 0.10 mg AAE/g).

Table 1: Effect of Different Processing Methods on Bioactive Compounds of Coptodon guineensis (Mean ± SD).

Means within the same row with different superscripts are significantly different (p < 0.05).

Table 2: Effect of Different Processing Methods on Antioxidant Properties of Coptodon guineensis (Mean ± SD).

Means within the same row with different superscripts are significantly different (p < 0.05).

The present findings demonstrate that processing methods significantly influence the bioactive composition and antioxidant properties of Coptodon guineensis, with clear variations observed across oven drying, smoking, and frying. These trends are largely consistent with previous studies on fish and other animal-derived foods subjected to thermal processing [14, 15, 16]. The substantial reduction in moisture content observed in processed samples, particularly in fried fish, aligned with the findings of Chukwu [17], who reported that traditional processing methods such as smoking and drying significantly reduce water activity, thereby enhancing shelf stability. The extremely low moisture content in fried samples can be attributed to high-temperature oil-mediated dehydration, which promotes rapid water loss. The apparent increase in zinc and iron concentrations in processed samples corroborates earlier reports by Daramola et al. [18], who observed that mineral concentrations in processed fish often increase due to moisture loss (concentration effect) rather than actual mineral enrichment. Similar observations were made by Adeyeye, et al. [19], who reported higher mineral density in heat-processed fish products. However, the reduction in selenium content with processing, especially frying, agrees with the findings of Akinwumi [20], who noted that certain trace elements are heat-labile and susceptible to oxidative degradation during high-temperature processing.

The decline in total phenolics and flavonoids observed across processing methods, particularly frying, is consistent with studies by Qui et al. [21], who demonstrated that thermal processing significantly reduces phenolic content due to oxidation and thermal degradation. Similarly, Shen et al. [22] reported significant losses of flavonoids in heat-treated Russian sturgeon fish, attributing this to structural breakdown and leaching into processing media such as oil.

Interestingly, the relatively higher retention of phenolics in smoked samples compared to oven-dried and fried samples may be linked to the deposition of phenolic compounds from smoke, as reported by Chen et al. [23]. These smoke-derived phenolics may contribute to the preservation of antioxidant capacity in smoked fish.

The observed reduction in DPPH scavenging activity and total antioxidant capacity following processing is in agreement with the findings of Hayes et al. [24], who reported that heat treatment disrupts antioxidant compounds and reduces free radical scavenging potential. Similarly, Tekle et al. [25] documented a decline in antioxidant activity in smoked and fried fish, attributing this to degradation of bioactive compounds. The decrease in superoxide dismutase (SOD) activity observed in processed samples, particularly in fried fish, supports the findings of Karami et al. [26], who reported that endogenous antioxidant enzymes are highly sensitive to heat and become denatured during thermal processing. This loss of enzymatic activity further contributes to reduced oxidative stability in processed fish.

The significant increase in TBARS values in processed samples, especially fried fish, indicates enhanced lipid peroxidation and oxidative deterioration. This finding is consistent with the work of Ahmed et al. [27], who reported that high-temperature processing accelerates lipid oxidation, particularly in fatty fish species. Frying, in particular, promotes oxidative reactions due to exposure to oxygen, high temperatures, and reused oils, as also noted by Pilon et al. [28]. The relatively lower TBARS values in smoked samples compared to fried fish may be attributed to the antioxidant properties of smoke constituents, which have been shown to inhibit lipid oxidation [29]. This explains the comparatively better oxidative stability observed in smoked C. guineensis.

This study clearly demonstrates that processing methods significantly influence the bioactive composition and antioxidant properties of Coptodon guineensis. Fresh samples consistently exhibited superior levels of phenolic compounds, flavonoids, antioxidant activity (DPPH, SOD, and total antioxidant capacity), and lower lipid peroxidation, indicating better nutritional and functional quality in the unprocessed state. Thermal processing—particularly frying—resulted in substantial degradation of heat-sensitive bioactive compounds and antioxidant enzymes, alongside a marked increase in lipid oxidation (TBARS). Although oven drying and smoking also caused reductions in antioxidant constituents, their effects were less severe compared to frying. Notably, smoking demonstrated a relatively better retention of phenolic compounds and antioxidant capacity, likely due to the protective and antioxidant effects of smoke constituents. Conversely, mineral elements such as zinc and iron appeared more concentrated in processed samples, primarily due to moisture loss rather than actual nutrient enhancement. However, selenium showed a declining trend with increased processing intensity, suggesting susceptibility to thermal degradation. Overall, while processing improves shelf life and safety through moisture reduction, it compromises important nutritional and functional attributes of C. guineensis, with frying having the most detrimental impact. Among the methods studied, smoking provides a more balanced approach by preserving a greater proportion of bioactive compounds and antioxidant properties.

Recommendations

• Smoking should be preferred over frying and excessive oven drying, as it better preserves bioactive compounds and antioxidant capacity while still ensuring adequate shelf life.
• Processing parameters such as temperature, duration, and exposure to oxygen should be carefully controlled to minimize nutrient losses and oxidative damage. Lower-temperature drying techniques or improved smoking technologies (e.g., controlled kilns) are recommended.
• Frying should be minimized, especially in nutritional interventions, due to its significant role in degrading antioxidants and increasing lipid peroxidation. Where frying is unavoidable, the use of fresh, stable oils and shorter frying times should be encouraged.
• The use of natural antioxidants (e.g., plant extracts, spices) during processing may help mitigate oxidative losses and improve the stability of processed fish products.
• Public awareness campaigns should emphasize the nutritional differences between fresh and processed fish, guiding consumers toward healthier processing choices.

1. The State of World Fisheries and Aquaculture 2020. Sustainability in action. Rome. 2020; 1-244.
2. Tacon AG. Trends in global aquaculture and aquafeed production: 2000–2017. Rev Fish Sci Aquac. 2020; 28: 43-56.
3. Wu J, Lin X, Cui C, Li J. Comparative Analysis of Kokumi Effects in Ile-Leu, Ile-Ile, Leu-Leu, and Leu-Ile: Sensory, Simulation, and Structural Insights. Food Chem. 2025; 474: 143004.
4. Ferraro V, Carvalho AP, Piccirillo C, Santos MM, Castro PM, Pintado ME. Extraction of high added value biological compounds from sardine, sardine-type fish and mackerel canning residues—A review. Mater Sci Eng C. 2013; 33: 3111-3120.
5. Rehman A, Tong Q, Jafari SM, Assadpour E, Shehzad Q, Aadil RM, et al. Carotenoid-loaded nanocarriers: a comprehensive review. Adv Colloid Interface Sci. 2020; 275: 102048.
6. Hu D, Xu Y, Yu D, Xia W, Jiang Q. The Impacts of Salt with Chinese Liquor on the Inhibition of Microbial Spoilage and Quality Attributes of Grass Carp (Ctenopharyngodon idellus) Fillets Stored at 4 ?C. J Food Process Preserv. 2020; 44: e14817.
7. Li D, Zhang J, Song S, Feng L, Luo Y. Influence of Heat Processing on the Volatile Organic Compounds and Microbial Diversity of Salted and Vacuum-Packaged Silver Carp (Hypophthalmichthys molitrix) Fillets during Storage. Food Microbiol. 2018; 72: 73-81.
8. Asefa BG. Effect of Temperature Fluctuation on Quality of Frozen Atlantic Salmon (Salmo salar) Fillet. J Food Technol. 2021; 19: 43-47.
9. Liu W, Luo X, Huang Y, Zhao M, Liu T, Wang J, et al. Influence of Cooking Techniques on Food Quality, Digestibility, and Health Risks Regarding Lipid Oxidation. Food Res Int. 2023; 167: 112685.
10. Cheng AC, Liang HY, Balantyne R, Liu CH. Purification and identification of hydrolyzed peptides from Lates calcarifer byproducts and their anti-inflammatory function in lipopolysaccharide-induced RAW 264.7 macrophage cells. 2024.
11. Jung Y, Oh S, Kim D, Lee S, Lee H-J, Shin D-J, et al. Effect of Cinnamon Powder on Quality Attributes and Off-Flavor in Fried Chicken Drumsticks Made from Long-Term Thawed Korean Native Chicken. Poult Sci. 2024; 103: 103583.
12. Food and Agricultural Organization of the United Nations (FAO). Cultured Aquatic Species Information Programme: Tilapia guineensis. Food and Agriculture Organization of the United Nations. 2021.
13. Official Methods of Analysis of the Association of Official Analytical Chemists. 20th Edition, AOAC Inc., Washington DC. 2016.
14. Tian XY, Aheto JH, Huang X, Zheng K, Dai C, Wang C, et al. An Evaluation of Biochemical, Structural and Volatile Changes of Dry-Cured Pork Using a Combined Ion Mobility Spectrometry, Hyperspectral and Confocal Imaging Approach. J Sci Food Agric. 2021; 101: 5972-5983
15. Zhao S, Niu C, Wang Y, Li X, Zheng F, Liu C, et al. Revealing the Contributions of Sunlight-Expose Process and Core-Microbiota Metabolism on Improving the Flavor Profile during Doubanjiang Fermentation. Food Biosci. 2023; 53: 102522.
16. Jiang H, Zhang M, Ye J, Qian M, Li X, Zhao W, et al. HS-SPME-GC-MS and OAV Analyses of Characteristic Volatile Flavour Compounds in Salt-Baked Drumstick. LWT. 2022; 170: 114041.
17. Chukwu O. Influences of drying methods on nutritional properties of tilapia fish (Oreochromis niloticus). World J Agric Sci., 2009; 5: 256-258.
18. Daramola JA, Fasakin EA, Adeparusi EO. Changes in physicochemical and sensory characteristics of smoke-dried fish species stored at ambient temperature. Afr J Food Agric Nutr Dev. 2007; 7.
19. Adeyeye SA, Olanrewaju EF, Abiodun OA. Amino acid, vitamin and mineral profiles of smoked fish as affected by smoking methods and fish types. Journal of Culinary Science & Technology. 2018; 17: 1-14.
20. Akinwumi F. Effects of Smoking and Freezing on the Nutritive Value of African Mud Catfish, Clarias gariepinus Burchell, 1822. J Agric Sci. 2014; 6: 143-149.
21. Qiu D, Duan R, Wang Y, He Y, Li C, Shen X, et al. Effects of Different Drying Temperatures on the Profile and Sources of Flavor in Semi-Dried Golden Pompano (Trachinotus ovatus). Food Chem. 2023; 401: 134112
22. Shen S, Chen Y, Dong X, Liu F, Cai W, Wei J, et al. Changes in Food Quality and Microbial Composition of Russian Sturgeon (Acipenser gueldenstaedti) Fillets Treated with Low Temperature Vacuum Heating Method during Storage at 4 ?C. Food Res Int. 2020; 138: 109665.
23. Chen T, Li C, Huang H, Zhao Y, Xiang H, Wang D, et al. Identification of Key Physicochemical Properties and Volatile Flavor Compounds for the Sensory Formation of Roasted Tilapia. Food Chem. 2024; 460: 140636.
24. Hayes M, Naik A, Mora L, Inarra B, Ibarruri J, Bald C, et al. Generation, characterisation and identification of bioactive peptides from mesopelagic fish protein hydrolysates using in silico and in vitro approaches. Mar Drugs. 2024; 22: 297.
25. Tekle S, Bozkurt F, Akman PK, Sagdic O. Bioactive and functional properties of gelatin peptide fractions obtainedfrom sea bass (Dicentrarchuslabrax) skin. Food Sci Technol. 2022; 42: e60221.
26. Karami Z, Akbari-Adergani B. Bioactive food derived peptides: A review on correlation between structure of bioactive peptides and their functional properties. J Food Sci Technol. 2019; 56: 535-547.
27. Ahmed T, Juhasz A, Bose U, Terefe NS, Colgrave ML. Research trends in production, separation, and identification of bioactive peptides from fungi–A critical review. J Funct Foods. 2024; 119: s106343.
28. Pilon G, Ruzzin J, Rioux LE, Lavigne C, White PJ, Froyland L, et al. Differential effects of various fish proteins in altering body weight, adiposity, inflammatory status, and insulin sensitivity in high-fat–fed rats. Metabolism. 2011; 60: 1122-1130
29. Okeke ES, Okagu IU, Chukwudozie K, Ezike TC, Ezeorba TP. Marine-derived bioactive proteins and peptides: A review of current knowledge on anticancer potentials, clinical trials, and future prospects. Nat Prod Commun. 2024; 19.