{"id":893,"date":"2026-04-25T07:07:04","date_gmt":"2026-04-25T07:07:04","guid":{"rendered":"https:\/\/academicsociety.org\/zoologicalreports\/?p=893"},"modified":"2026-07-31T07:43:29","modified_gmt":"2026-07-31T07:43:29","slug":"effects-of-processing-and-preservation-methods-and-the-nutritional-quality-and-oxidative-stability-of-selected-commercial-fish-species-sold-in-port-hardcourt-nigeria","status":"publish","type":"post","link":"https:\/\/academicsociety.org\/zoologicalreports\/2026\/04\/25\/effects-of-processing-and-preservation-methods-and-the-nutritional-quality-and-oxidative-stability-of-selected-commercial-fish-species-sold-in-port-hardcourt-nigeria\/","title":{"rendered":"Effects of Processing and Preservation Methods and the Nutritional Quality and Oxidative Stability of Selected Commercial Fish species Sold in\u00a0 Port Hardcourt Nigeria"},"content":{"rendered":"\n

INTRODUCTION<\/strong><\/p>\n\n\n\n

Fish is one of the most important sources of animal protein worldwide and contributes substantially to global food and nutritional security. It provides high-quality proteins containing all essential amino acids, long-chain omega-3 polyunsaturated fatty acids (PUFAs), vitamins (A, D and B-complex) and essential minerals such as calcium, phosphorus, selenium and iodine, which are indispensable for normal growth, cognitive development, cardiovascular health and immune function. Increasing awareness of the nutritional and therapeutic benefits of fish has resulted in a continuous rise in global fish consumption, making fisheries and aquaculture indispensable components of sustainable food systems and economic development, particularly in developing countries [4,33]. Consequently, maintaining the nutritional quality and safety of fish products has become a major concern for food scientists, public health professionals and fisheries stakeholders.<\/p>\n\n\n\n

Nigeria is among the largest consumers of fish in Africa, with fish accounting for a substantial proportion of the dietary protein consumed by millions of households. In the Niger Delta region, especially Port Harcourt metropolis, commercially important fish species such as Clarias gariepinus (African catfish), Oreochromis niloticus (Nile tilapia) and Scomber scombrus (Atlantic mackerel) constitute major components of local diets because of their affordability, availability and high nutritional value [3]. These species are marketed in fresh, frozen and smoked forms to meet consumer preferences and extend market availability throughout the year. However, the nutritional composition and overall quality of fish are strongly influenced by the methods employed during processing, preservation and storage [4].<\/p>\n\n\n\n

Fresh fish is highly perishable because of its elevated moisture content, endogenous enzymatic activity, microbial proliferation and rapid lipid oxidation immediately after harvest. These biochemical and microbiological processes result in progressive deterioration characterised by undesirable odours, texture softening, nutrient degradation and reduced shelf life if preservation measures are not promptly applied [35,4]. In tropical countries such as Nigeria, where ambient temperatures favour rapid microbial growth and cold-chain infrastructure remains inadequate, post-harvest losses of fish remain a significant challenge. Consequently, preservation methods including smoking, freezing, chilling and drying are widely employed to prolong shelf life, improve marketability and minimise economic losses [7,20].<\/p>\n\n\n\n

Among these preservation techniques, smoking remains one of the oldest and most extensively practised methods in Nigeria because it simultaneously reduces moisture content, inhibits microbial growth, enhances flavour and improves consumer acceptability. The dehydration associated with smoking increases the relative concentrations of protein, lipid and mineral constituents by reducing the water content of fish tissues, thereby improving apparent nutritional density [2,37]. Similar observations have been reported in Clarias gariepinus, Pseudotolithus species, Chrysichthys nigrodigitatus and several other commercially important fish species [3,10,11]. Nevertheless, exposure to elevated temperatures during smoking may accelerate lipid oxidation and facilitate the formation of undesirable compounds, including polycyclic aromatic hydrocarbons (PAHs), thereby compromising product safety and quality [13,14].<\/p>\n\n\n\n

Freezing represents another widely adopted preservation strategy because it substantially slows microbial proliferation, enzymatic reactions and biochemical deterioration by maintaining fish at sub-zero temperatures. Proper freezing preserves the sensory characteristics and nutritional quality of fish for extended periods and is generally considered superior to traditional preservation methods in retaining the original composition of fish muscle [7]<\/strong>. However, prolonged frozen storage may still induce biochemical alterations resulting from protein denaturation, lipid oxidation and cellular damage caused by ice crystal formation. Previous investigations have reported gradual reductions in nutritional quality and oxidative stability during frozen storage of freshwater and marine fish species [1,20,23].<\/p>\n\n\n\n

The nutritional composition of fish is influenced not only by preservation method but also by species-specific biological characteristics. Differences in lipid content, moisture distribution, muscle structure and fatty acid composition contribute significantly to variations in protein, fat, mineral and carbohydrate concentrations following processing. Several investigators have demonstrated significant alterations in proximate composition after smoking, freezing or cooking in rainbow trout, lais fish, Mediterranean mussels, crayfish and hilsa shad [9,2]16,17,18,3. These findings collectively indicate that processing conditions and intrinsic biological characteristics jointly determine the nutritional value of processed fish products.<\/p>\n\n\n\n

Lipid oxidation represents one of the principal mechanisms responsible for quality deterioration in processed fish because fish lipids contain large proportions of highly unsaturated fatty acids that readily undergo oxidative degradation during processing and storage. Oxidation initially produces lipid hydroperoxides, commonly quantified using peroxide value, before progressing to secondary oxidation products such as malondialdehyde, which is measured by the thiobarbituric acid reactive substances (TBARS) assay [12,35]. Acid value and free fatty acid concentration further provide important information regarding hydrolytic rancidity and lipid deterioration during storage [36,35]. Collectively, these oxidative stability indices are widely recognised as reliable indicators of the freshness, storage stability and overall quality of fish products.<\/p>\n\n\n\n

Numerous studies have evaluated the influence of preservation techniques on fish quality. Smoking has consistently been associated with reduced moisture content and increased protein, fat and mineral concentrations due to dehydration, whereas freezing generally preserves the original nutritional characteristics of fish more effectively than thermal processing [10,21,39]. Similarly, investigations conducted in Cameroon, Sudan and Iran have demonstrated that processing methods significantly influence the proximate composition, fatty acid profile and oxidative stability of commercially important fish species [1,20,16,38]<\/strong>. These findings suggest that preservation technique is a major determinant of nutritional quality irrespective of geographical location.<\/p>\n\n\n\n

Recent investigations conducted within the Niger Delta have further highlighted concerns regarding the nutritional quality and safety of processed fish. Variations in drying methods have been shown to significantly influence the proximate composition of Oreochromis niloticus, while smoked products may contain elevated concentrations of polycyclic aromatic hydrocarbons, thereby presenting a nutritional\u2013toxicological trade-off [13,14]. Furthermore, petroleum hydrocarbon contamination, heavy metal pollution and deteriorating aquatic ecosystems have been reported to adversely affect fish quality, aquatic biodiversity and public health within the Niger Delta [21,25\u201331]. These environmental challenges underscore the need for continuous evaluation of commercially available fish products to ensure consumer safety and nutritional adequacy.<\/p>\n\n\n\n

Although numerous studies have independently examined the effects of smoking, freezing or drying on fish quality, comparative information regarding the simultaneous influence of these preservation methods on both proximate composition and oxidative stability of commercially available Clarias gariepinus, Oreochromis niloticus and Scomber scombrus marketed in Port Harcourt remains limited. In addition, little information is available concerning species-specific responses to preservation under prevailing commercial marketing conditions in the Niger Delta. Addressing this knowledge gap is essential for improving fish processing practices, guiding consumer choices and supporting evidence-based food quality regulations. Therefore, the present study comparatively evaluated the effects of fresh, smoked and frozen preservation methods on the nutritional quality and oxidative stability of selected commercially important fish species sold in Port Harcourt, Nigeria. <\/p>\n\n\n\n

2.0 MATERIALS AND METHODS<\/strong><\/h2>\n\n\n\n

2.1 Study Area<\/strong><\/h3>\n\n\n\n

The study was conducted in Port Harcourt, Rivers State, Nigeria. Port Harcourt is a major urban and commercial centre in the Niger Delta region, where fish constitutes an important source of animal protein and is marketed in fresh, smoked and frozen forms. Fish samples were obtained from selected major markets within Port Harcourt metropolis. The markets were selected based on the availability and commercial importance of the fish species investigated.<\/p>\n\n\n\n

2.2 Research Design<\/strong><\/h3>\n\n\n\n

The study adopted a comparative experimental design to evaluate the effects of preservation methods on the nutritional quality and oxidative stability of selected commercial fish species. Three commonly consumed fish species, namely African catfish (Clarias gariepinus<\/em>), Nile tilapia (Oreochromis niloticus<\/em>) and Atlantic mackerel (Scomber scombrus<\/em>), were investigated under three preservation conditions: fresh, smoked and frozen. The experimental design therefore comprised three fish species and three preservation methods, resulting in nine sample groups. All laboratory analyses were conducted in triplicate to improve analytical reliability and permit statistical evaluation of the results.<\/p>\n\n\n\n

2.3 Sample Collection<\/strong><\/h3>\n\n\n\n

Fresh, smoked and frozen samples of C. gariepinus<\/em>, O. niloticus<\/em> and S. scombrus<\/em> were purchased from selected fish vendors within Port Harcourt metropolis. Three independent samples representing each fish species and preservation method were obtained, giving a total of 27 individual fish samples.<\/p>\n\n\n\n

The samples were collected using clean polyethylene bags, appropriately labelled and transported in insulated containers to the laboratory for preparation and analysis. Fresh and frozen samples were maintained under chilled conditions during transportation to minimise deterioration before laboratory analysis.<\/p>\n\n\n\n

The samples were coded according to fish species and preservation method. Clarias gariepinus<\/em> samples were designated CF, O. niloticus<\/em> samples were designated TF, while S. scombrus<\/em> samples were designated MF. The numerical designations 01, 02 and 03 represented fresh, smoked and frozen samples, respectively.<\/p>\n\n\n\n

2.4 Sample Preparation<\/strong><\/h3>\n\n\n\n

Upon arrival at the laboratory, the fish samples were thoroughly rinsed with distilled water to remove adhering foreign materials. The samples were dissected using clean stainless-steel instruments, and representative portions of edible muscle tissues were collected.<\/p>\n\n\n\n

The muscle tissues from replicate samples belonging to the same experimental group were separately homogenised using a laboratory homogeniser. Portions intended for proximate analysis were stored in clean, appropriately labelled sample containers, while samples intended for lipid oxidation analysis were maintained under refrigerated conditions and analysed as soon as practicable to minimise further oxidative deterioration.<\/p>\n\n\n\n

All laboratory glassware and equipment used during sample preparation and analysis were properly cleaned and rinsed with distilled water before use.<\/p>\n\n\n\n

2.5 Determination of Proximate Composition<\/h3>\n\n\n\n

The proximate composition of the fish samples was determined using standard analytical procedures. The parameters investigated included moisture content, crude protein, crude fat, ash and carbohydrate.<\/p>\n\n\n\n

2.5.1 Determination of Moisture Content<\/strong><\/h3>\n\n\n\n

Moisture content was determined using the oven-drying method. Approximately 5 g of homogenised fish sample was weighed into a previously dried and weighed moisture dish. The sample was dried in a hot-air oven at 105\u00b0C until a constant weight was obtained. The sample was subsequently cooled in a desiccator and reweighed.<\/p>\n\n\n\n

Moisture content was calculated as:<\/p>\n\n\n\n

Moisture content (%) = [(W\u2082 \u2212 W\u2083)\/(W\u2082 \u2212 W\u2081)] \u00d7 100<\/p>\n\n\n\n

Where:<\/p>\n\n\n\n

W\u2081 = weight of empty moisture dish<\/p>\n\n\n\n

W\u2082 = weight of moisture dish + sample before drying<\/p>\n\n\n\n

W\u2083 = weight of moisture dish + sample after drying<\/p>\n\n\n\n

2.5.2 Determination of Crude Protein<\/strong><\/h3>\n\n\n\n

Crude protein content was determined using the Kjeldahl method. A known weight of homogenised sample was digested with concentrated sulphuric acid in the presence of a suitable catalyst until a clear digest was obtained.<\/p>\n\n\n\n

Following digestion, the sample was made alkaline using sodium hydroxide solution and distilled. The liberated ammonia was collected in boric acid solution and titrated against standard hydrochloric acid.<\/p>\n\n\n\n

The percentage nitrogen content was calculated, and crude protein was estimated using the conventional nitrogen-to-protein conversion factor of 6.25.<\/p>\n\n\n\n

Crude protein (%) = Nitrogen (%) \u00d7 6.25<\/p>\n\n\n\n

2.5.3 Determination of Crude Fat<\/strong><\/h3>\n\n\n\n

Crude fat was determined using Soxhlet extraction. Approximately 5 g of dried homogenised fish sample was placed in an extraction thimble and extracted using petroleum ether.<\/p>\n\n\n\n

Following extraction, the solvent was evaporated, and the flask containing the extracted lipid was dried, cooled in a desiccator and weighed.<\/p>\n\n\n\n

Crude fat was calculated as:<\/p>\n\n\n\n

Crude fat (%) = (Weight of extracted fat\/Weight of sample) \u00d7 100<\/p>\n\n\n\n

2.5.4 Determination of Ash Content<\/strong><\/h3>\n\n\n\n

Ash content was determined by incineration of a known weight of fish sample in a muffle furnace at approximately 550\u00b0C until a light grey or white ash of constant weight was obtained.<\/p>\n\n\n\n

The crucible containing the ash was cooled in a desiccator and weighed.<\/p>\n\n\n\n

Ash content was calculated as:<\/p>\n\n\n\n

Ash (%) = (Weight of ash\/Weight of sample) \u00d7 100<\/p>\n\n\n\n

2.5.5 Determination of Carbohydrate Content<\/strong><\/h3>\n\n\n\n

Carbohydrate content was estimated by difference using the relationship:<\/p>\n\n\n\n

Carbohydrate (%) = 100 \u2212 [Moisture (%) + Crude protein (%) + Crude fat (%) + Ash (%)]<\/p>\n\n\n\n

2.6 Assessment of Oxidative Stability<\/strong><\/h3>\n\n\n\n

The oxidative stability of the fish samples was evaluated using peroxide value, acid value, free fatty acid content and thiobarbituric acid reactive substances (TBARS). These parameters were selected to assess primary lipid oxidation, hydrolytic deterioration and secondary lipid oxidation.<\/p>\n\n\n\n

2.6.1 Determination of Peroxide Value<\/strong><\/h3>\n\n\n\n

Peroxide value was determined using the iodometric titration method. Extracted lipid from each fish sample was dissolved in an appropriate organic solvent mixture, followed by the addition of saturated potassium iodide solution.<\/p>\n\n\n\n

The mixture was allowed to react under controlled conditions, after which distilled water and starch indicator were added. The liberated iodine was titrated against standardised sodium thiosulphate solution.<\/p>\n\n\n\n

Peroxide value was calculated as:<\/p>\n\n\n\n

PV (meq O\u2082\/kg) = [(S \u2212 B) \u00d7 N \u00d7 1000]\/W<\/p>\n\n\n\n

Where:<\/p>\n\n\n\n

S = volume of sodium thiosulphate used for sample titration (mL)<\/p>\n\n\n\n

B = volume of sodium thiosulphate used for blank titration (mL)<\/p>\n\n\n\n

N = normality of sodium thiosulphate<\/p>\n\n\n\n

W = weight of lipid sample (g)<\/p>\n\n\n\n

2.6.2 Determination of Acid Value<\/strong><\/h3>\n\n\n\n

Acid value was determined by dissolving a known weight of extracted lipid in neutralised ethanol and titrating the resulting solution against standard potassium hydroxide solution using phenolphthalein as an indicator.<\/p>\n\n\n\n

Acid value was calculated as:<\/p>\n\n\n\n

Acid value (mg KOH\/g) = (V \u00d7 N \u00d7 56.1)\/W<\/p>\n\n\n\n

Where:<\/p>\n\n\n\n

V = volume of potassium hydroxide used for titration (mL)<\/p>\n\n\n\n

N = normality of potassium hydroxide<\/p>\n\n\n\n

56.1 = molecular weight of potassium hydroxide<\/p>\n\n\n\n

W = weight of lipid sample (g)<\/p>\n\n\n\n

2.6.3 Determination of Free Fatty Acid Content<\/strong><\/h3>\n\n\n\n

Free fatty acid content was determined by titration of the extracted lipid against standard potassium hydroxide solution. The percentage free fatty acid was expressed as oleic acid and calculated as:<\/p>\n\n\n\n

FFA (%) = (V \u00d7 N \u00d7 28.2)\/W<\/p>\n\n\n\n

Where:<\/p>\n\n\n\n

V = volume of potassium hydroxide used for titration (mL)<\/p>\n\n\n\n

N = normality of potassium hydroxide<\/p>\n\n\n\n

W = weight of lipid sample (g)<\/p>\n\n\n\n

2.6.4 Determination of Thiobarbituric Acid Reactive Substances<\/strong><\/h3>\n\n\n\n

Thiobarbituric acid reactive substances were determined to assess secondary lipid oxidation. A known weight of homogenised fish sample was mixed with an appropriate extraction solution and homogenised thoroughly.<\/p>\n\n\n\n

The extract was reacted with thiobarbituric acid reagent and heated in a water bath under controlled conditions to facilitate colour development. Following cooling, the absorbance of the resulting solution was measured using a UV-visible spectrophotometer at approximately 532 nm against a reagent blank.<\/p>\n\n\n\n

TBARS concentration was determined from an appropriate calibration curve and expressed as milligrams of malondialdehyde per kilogram of fish sample (mg MDA\/kg).<\/p>\n\n\n\n

2.7 Quality Assurance and Quality Control<\/strong><\/h3>\n\n\n\n

Appropriate quality assurance and quality control procedures were implemented throughout sample collection, preparation and laboratory analysis. All samples were appropriately labelled to prevent misidentification and cross-contamination.<\/p>\n\n\n\n

Analytical-grade reagents were used throughout the study. Glassware was thoroughly cleaned and rinsed with distilled water before use. Equipment was calibrated according to the manufacturers\u2019 specifications before analysis.<\/p>\n\n\n\n

Reagent blanks were included where appropriate to assess potential contamination and analytical interference. All analyses were conducted in triplicate, and results were expressed as mean \u00b1 standard deviation.<\/p>\n\n\n\n

2.8 Statistical Analysis<\/strong><\/h3>\n\n\n\n

The experimental data were subjected to statistical analysis using appropriate statistical software. Results were expressed as mean \u00b1 standard deviation.<\/p>\n\n\n\n

A two-way analysis of variance (ANOVA) was performed to determine the individual and interactive effects of fish species and preservation method on proximate composition and oxidative stability parameters.<\/p>\n\n\n\n

Where statistically significant differences were observed, Tukey\u2019s Honestly Significant Difference (HSD) post hoc test was used for multiple mean comparisons. Differences were considered statistically significant at p < 0.05.<\/p>\n\n\n\n

3.0 RESULTS<\/strong><\/p>\n\n\n\n

3.1 Proximate Composition of Fresh, Smoked and Frozen Fish Samples<\/strong><\/p>\n\n\n\n

The proximate composition of the selected fish species is presented in Table 1. Moisture content ranged from 43.62 \u00b1 0.45% in smoked S. scombrus<\/em> to 76.18 \u00b1 0.35% in fresh O. niloticus<\/em>. Fresh samples generally recorded the highest moisture contents, followed by frozen samples, while smoked samples had the lowest values. For instance, moisture content in C. gariepinus<\/em> decreased from 75.42 \u00b1 0.31% in the fresh sample to 48.63 \u00b1 0.42% in the smoked sample, while the frozen sample recorded 72.81 \u00b1 0.28%.<\/p>\n\n\n\n

Crude protein content ranged from 17.92 \u00b1 0.21% in fresh O. niloticus<\/em> to 37.84 \u00b1 0.38% in smoked S. scombrus<\/em>. The smoked samples recorded considerably higher percentage protein contents than their corresponding fresh and frozen samples. Crude fat content similarly varied from 3.24 \u00b1 0.09% in fresh O. niloticus<\/em> to 14.62 \u00b1 0.21% in smoked S. scombrus<\/em>. Irrespective of preservation method, S. scombrus<\/em> generally exhibited higher crude fat contents than the other species.<\/p>\n\n\n\n

Ash content ranged from 1.38 \u00b1 0.04% in fresh S. scombrus<\/em> to 3.81 \u00b1 0.07% in smoked O. niloticus<\/em>. The smoked samples generally exhibited higher ash contents than their fresh and frozen counterparts. Carbohydrate content remained comparatively low across all samples, ranging from 0.66 \u00b1 0.03% in fresh C. gariepinus<\/em> to 1.87 \u00b1 0.04% in frozen O. niloticus<\/em>. <\/p>\n\n\n\n

Note: CF = Clarias gariepinus<\/em>; TF = Oreochromis niloticus<\/em>; MF = Scomber scombrus<\/em>; 01 = fresh; 02 = smoked; 03 = frozen. Values are expressed as mean \u00b1 standard deviation of triplicate determinations (n = 3).<\/p>\n\n\n\n

3.2 Oxidative Stability of Fresh, Smoked and Frozen Fish Samples<\/strong><\/p>\n\n\n\n

The oxidative stability indices of the selected fish samples are presented in Table 2. Peroxide values ranged from 2.46 \u00b1 0.09 meq O\u2082\/kg in fresh O. niloticus<\/em> to 8.46 \u00b1 0.22 meq O\u2082\/kg in smoked S. scombrus<\/em>. Fresh samples consistently recorded the lowest peroxide values, frozen samples exhibited intermediate values, and smoked samples recorded the highest values.<\/p>\n\n\n\n

Acid values ranged from 1.31 \u00b1 0.05 mg KOH\/g in fresh O. niloticus<\/em> to 3.42 \u00b1 0.11 mg KOH\/g in smoked S. scombrus<\/em>. A similar pattern was observed for free fatty acid concentrations, which ranged from 0.66 \u00b1 0.02% to 1.71 \u00b1 0.06%. Smoked S. scombrus<\/em> recorded the highest acid value and free fatty acid concentration, whereas fresh O. niloticus<\/em> recorded the lowest values.<\/p>\n\n\n\n

TBARS concentrations ranged from 0.74 \u00b1 0.03 mg MDA\/kg in fresh O. niloticus<\/em> to 2.68 \u00b1 0.08 mg MDA\/kg in smoked S. scombrus<\/em>. The smoked samples exhibited higher TBARS concentrations than the corresponding fresh and frozen samples. <\/p>\n\n\n\n

Note: TBARS = Thiobarbituric Acid Reactive Substances; MDA = malondialdehyde. Values are expressed as mean \u00b1 standard deviation of triplicate determinations (n = 3).<\/p>\n\n\n\n

3.3 Comparative Effects of Preservation Methods on Nutritional Quality<\/strong><\/p>\n\n\n\n

The comparative effects of preservation methods on the nutritional composition of the selected fish species are presented in Table 3. Significant differences (p < 0.001) were observed among preservation methods for moisture, crude protein, crude fat and ash contents, while the difference in carbohydrate content was not statistically significant (p = 0.087).<\/p>\n\n\n\n

Fresh samples recorded the highest mean moisture content of 73.98 \u00b1 3.18%, followed by frozen samples at 71.69 \u00b1 2.61%, whereas smoked samples recorded the significantly lowest value of 47.50 \u00b1 3.45%. In contrast, smoked samples exhibited the highest mean crude protein content of 36.14 \u00b1 1.53%, compared with 19.78 \u00b1 1.38% in frozen samples and 19.01 \u00b1 1.30% in fresh samples.<\/p>\n\n\n\n

The mean crude fat content was significantly highest in smoked samples at 11.59 \u00b1 2.75%, followed by frozen samples at 5.45 \u00b1 1.79% and fresh samples at 4.59 \u00b1 1.84%. A similar pattern was observed for ash content, with smoked samples recording the highest mean value of 3.50 \u00b1 0.33%, followed by frozen samples at 1.61 \u00b1 0.07% and fresh samples at 1.45 \u00b1 0.07%.<\/p>\n\n\n\n

Carbohydrate content ranged from 0.96 \u00b1 0.25% in fresh samples to 1.46 \u00b1 0.51% in frozen samples. However, the observed differences among preservation methods were not statistically significant (p = 0.087). <\/p>\n\n\n\n

Note: Values are expressed as mean \u00b1 standard deviation. Values within the same column bearing different superscript letters are significantly different at p < 0.05.<\/p>\n\n\n\n

3.4 Comparative Effects of Preservation Methods on Oxidative Stability<\/strong><\/p>\n\n\n\n

The comparative effects of preservation methods on oxidative stability are presented in Table 4. Significant differences (p < 0.001) were observed among the preservation methods for peroxide value, acid value, free fatty acid concentration and TBARS.<\/p>\n\n\n\n

Smoked samples recorded the significantly highest mean peroxide value of 7.01 \u00b1 1.32 meq O\u2082\/kg, followed by frozen samples at 4.82 \u00b1 0.82 meq O\u2082\/kg, while fresh samples recorded the lowest value of 2.97 \u00b1 0.59 meq O\u2082\/kg. Mean acid values followed a similar pattern, increasing from 1.50 \u00b1 0.23 mg KOH\/g in fresh samples to 2.21 \u00b1 0.25 mg KOH\/g in frozen samples and 2.97 \u00b1 0.40 mg KOH\/g in smoked samples.<\/p>\n\n\n\n

The mean free fatty acid concentration was significantly higher in smoked samples (1.49 \u00b1 0.20%) than in frozen (1.11 \u00b1 0.13%) and fresh (0.75 \u00b1 0.12%) samples. Similarly, the mean TBARS concentration increased from 0.88 \u00b1 0.18 mg MDA\/kg in fresh samples to 1.39 \u00b1 0.23 mg MDA\/kg in frozen samples and 2.11 \u00b1 0.50 mg MDA\/kg in smoked samples.<\/p>\n\n\n\n

Note: Values are expressed as mean \u00b1 standard deviation. Values within the same column bearing different superscript letters are significantly different at p < 0.05.<\/p>\n\n\n\n

4.0 DISCUSSION<\/strong><\/h3>\n\n\n\n

The present study demonstrated that preservation method significantly influenced the nutritional composition and oxidative stability of the selected commercial fish species sold in Port Harcourt. Significant differences (p < 0.001) were observed in moisture, crude protein, crude fat, ash content and all oxidative stability indices among fresh, smoked and frozen samples, confirming that post-harvest processing substantially alters the chemical composition and storage quality of fish. These findings are consistent with previous reports which established that preservation methods modify fish quality primarily through moisture removal, biochemical transformation and lipid oxidation, ultimately influencing nutritional value, shelf life and consumer acceptability [3,4,10]. Fish is highly perishable because of its high moisture content, active endogenous enzymes and susceptibility to microbial spoilage and oxidative degradation immediately after harvest. Consequently, appropriate preservation techniques remain essential for reducing post-harvest losses while maintaining nutritional quality [4,7,35].<\/p>\n\n\n\n

Moisture content was the parameter most affected by preservation method. Fresh fish recorded the highest mean moisture content (73.98 \u00b1 3.18%), whereas smoked fish had the lowest value (47.50 \u00b1 3.45%), demonstrating the substantial dehydrating effect of smoking. Moisture reduction inhibits microbial proliferation by lowering water activity and simultaneously prolongs shelf life. Similar reductions have been reported in Clarias <\/strong>gariepinus, Pseudotolithus species, Chrysichthys nigrodigitatus and Polypterus bichir bichir, where smoking significantly reduced moisture through evaporation of free and bound water from muscle tissues [2,3,10,11,37]. Comparable observations have also been reported in rainbow trout, lais fish, Mediterranean mussels, crayfish and hilsa shad, indicating that moisture loss remains a universal consequence of thermal processing irrespective of fish species [8,17,18,32,16]. In contrast, frozen samples retained moisture contents closer to fresh fish, supporting previous findings that freezing effectively preserves muscle integrity by slowing microbial growth and enzymatic degradation [1,7,20,23].<\/p>\n\n\n\n

The significant increase in crude protein content observed in smoked fish (36.14 \u00b1 1.53%) compared with fresh (19.01 \u00b1 1.30%) and frozen samples (19.78 \u00b1 1.38%) is principally attributable to moisture removal rather than true protein synthesis. As water is removed during smoking, the remaining nutrients become proportionally concentrated, resulting in higher apparent protein concentrations. Similar findings have been reported for African mud catfish, Nile tilapia and several marine fish species, where dehydration significantly increased measured protein content following smoking [3,10,14,37]. Investigations by Jules Christophe et al. and Karimian-Khosroshahi et al. equally demonstrated that thermal processing increased protein concentration without increasing absolute protein quantity [16,17]. Therefore, the higher protein values obtained in smoked samples reflect concentration effects associated with dehydration rather than biochemical enrichment of fish tissue.<\/p>\n\n\n\n

Crude fat content exhibited a similar pattern, with smoked fish recording significantly higher values (11.59 \u00b1 2.75%) than fresh and frozen samples. This observation agrees with previous reports that smoking concentrates lipids following moisture reduction rather than increasing lipid synthesis [<\/strong>3,8,17,30,41]. Among the three species examined, Scomber scombrus consistently exhibited the highest fat content, reaching 14.62 \u00b1 0.21% in smoked samples. This finding was expected because Atlantic mackerel is naturally classified as an oily fish with relatively high concentrations of polyunsaturated fatty acids compared with freshwater species [4,41]. Similar species-specific differences have been reported in red carp, freshwater fishes and commercially important marine fish species, where lipid composition varied considerably according to species and habitat [16,23,33]. Although higher lipid concentrations improve nutritional value by increasing essential fatty acid intake, they simultaneously increase susceptibility to oxidative deterioration during processing and storage.<\/p>\n\n\n\n

Ash content also increased significantly after smoking, with smoked samples recording a mean value of 3.50 \u00b1 0.33% compared with 1.45 \u00b1 0.07% in fresh fish. Since ash represents the total mineral fraction, this increase reflects concentration of mineral constituents following dehydration rather than additional mineral accumulation. Similar increases have been reported in smoked Oreochromis niloticus, Pseudotolithus species and Chrysichthys nigrodigitatus, while Kiczorowska et al. similarly observed greater mineral concentrations in smoked fish than in fresh samples [10,11,14,18]. Conversely, carbohydrate content remained low and was not significantly influenced by preservation method (p = 0.087), reflecting the naturally low glycogen reserves in fish muscle that are rapidly depleted after harvest [1,4,20]<\/strong>.<\/p>\n\n\n\n

The oxidative stability indices demonstrated that preservation method significantly affected lipid deterioration. Smoked samples recorded the highest peroxide value (7.01 \u00b1 1.32 meq O\u2082\/kg), acid value (2.97 \u00b1 0.40 mg KOH\/g), free fatty acid concentration (1.49 \u00b1 0.20%) and TBARS value (2.11 \u00b1 0.50 mg MDA\/kg), whereas fresh fish consistently exhibited the lowest values. These findings indicate that thermal processing accelerates both primary and secondary lipid oxidation because elevated temperatures promote decomposition of unsaturated fatty acids and increase oxygen interaction with lipids [12,35,36]. Similar increases in peroxide value and oxidative deterioration following smoking have been reported in freshwater and marine fish species processed under traditional smoking conditions [10,11,22,38].<\/p>\n\n\n\n

The elevated peroxide values observed in smoked fish indicate increased formation of lipid hydroperoxides, which represent the primary products of lipid oxidation. Although hydroperoxides are initially odourless and tasteless, their subsequent decomposition produces secondary oxidation products responsible for rancidity, deterioration of sensory quality and nutritional losses. The peroxide values obtained in the present study remained within acceptable limits for processed fish products but clearly demonstrated that smoking accelerated oxidative reactions compared with freezing and fresh storage. Similar observations were reported by Tenyang et al., who found significantly higher peroxide values in smoked Chrysichthys nigrodigitatus, while Dama et al. also reported progressive increases in peroxide values following smoking and frozen storage of Pseudotolithus species [10,38]. Likewise, Koral et al. demonstrated that oxidative deterioration increased progressively during storage of salted anchovies, particularly when preservation conditions promoted lipid oxidation [19]. Shahidi and Wanasundara explained that the abundance of polyunsaturated fatty acids in fish makes them particularly susceptible to oxidative degradation during thermal processing and storage [35].<\/p>\n\n\n\n

Acid value and free fatty acid concentrations followed trends similar to peroxide value, with smoked samples recording significantly higher values than fresh and frozen fish. These parameters are reliable indicators of hydrolytic rancidity because they reflect the liberation of free fatty acids following triglyceride hydrolysis. The higher acid values obtained in smoked samples suggest greater hydrolytic degradation of lipids during thermal processing, which may subsequently accelerate oxidative deterioration. Similar findings have been reported in smoked freshwater fish and traditionally processed fish products, where prolonged heating promoted hydrolysis of lipids and increased free fatty acid concentrations [11,22,38]. Shantha and Decker further explained that increasing acid value reflects progressive deterioration of edible oils and fish lipids during storage and processing, thereby reducing nutritional quality and consumer acceptability [36]<\/strong>. The relatively low acid values observed in frozen samples indicate that freezing effectively slows hydrolytic reactions by suppressing enzyme activity, thereby preserving lipid quality during storage [7,20].<\/p>\n\n\n\n

Secondary lipid oxidation, measured using TBARS, also differed significantly among preservation methods. Smoked fish exhibited the highest TBARS concentration (2.11 \u00b1 0.50 mg MDA\/kg), whereas fresh samples recorded the lowest value (0.88 \u00b1 0.18 mg MDA\/kg). TBARS quantifies malondialdehyde, one of the principal secondary oxidation products formed following decomposition of lipid hydroperoxides, and is therefore widely accepted as a reliable indicator of oxidative rancidity [12]. The higher TBARS values recorded in smoked samples indicate that heat exposure accelerated lipid peroxidation, resulting in greater formation of aldehydes and ketones capable of adversely affecting flavour, aroma and nutritional quality. Similar increases in TBARS following smoking have been reported in freshwater fishes, Nile tilapia and several commercially important marine fish species [13,14,22,38]. Conversely, freezing significantly limited secondary oxidation, corroborating reports that low-temperature storage suppresses oxidative reactions by reducing oxygen diffusion and enzyme-mediated lipid degradation [7,20,23].<\/p>\n\n\n\n

Among the fish species investigated, Scomber scombrus consistently exhibited the highest crude fat content and the greatest oxidative deterioration under all preservation methods. This observation agrees with previous studies demonstrating that oily fish undergo more rapid lipid oxidation because their tissues contain larger proportions of highly unsaturated fatty acids that readily react with oxygen during processing and storage [4,16,23]. In contrast, Oreochromis niloticus exhibited comparatively lower oxidative indices, suggesting greater oxidative stability associated with its lower lipid content. Such species-specific differences have important implications for processing because fatty fish require stricter control of smoking temperature, storage duration and antioxidant protection than lean fish to minimise oxidative quality loss [17,18,21].<\/p>\n\n\n\n

Beyond preservation practices, environmental quality also influences the nutritional composition and safety of commercially available fish. Several studies conducted in the Niger Delta have shown that petroleum hydrocarbons, heavy metals and other environmental contaminants can accumulate in aquatic organisms, thereby affecting fish quality and posing potential public health risks [24\u201331]. Furthermore, inappropriate smoking techniques may increase the formation of polycyclic aromatic hydrocarbons, creating a nutritional\u2013toxicological trade-off despite improvements in shelf life [13]<\/strong>. Consequently, maintaining good environmental quality, adopting improved smoking technologies and implementing effective quality assurance measures are essential for protecting both nutritional value and consumer health. The use of controlled smoking kilns, rapid freezing systems and hygienic handling practices would substantially reduce post-harvest deterioration while improving product safety and marketability [2,5,6,39,40,41].<\/p>\n\n\n\n

CONCLUSION<\/strong><\/h2>\n\n\n\n

This study comparatively evaluated the effects of fresh, smoked and frozen preservation methods on the nutritional quality and oxidative stability of selected commercial fish species (Clarias gariepinus<\/em>, Oreochromis niloticus<\/em> and Scomber scombrus<\/em>) sold in Port Harcourt, Nigeria. The findings demonstrated that preservation method significantly influenced both the proximate composition and oxidative stability of the fish samples. Smoking substantially reduced moisture content while increasing the relative concentrations of crude protein, crude fat and ash due to moisture loss. However, smoked fish also exhibited significantly higher peroxide value, acid value, free fatty acid and TBARS concentrations, indicating greater lipid oxidation and reduced oxidative stability. Frozen samples retained nutritional characteristics closer to those of fresh fish but exhibited moderate oxidative changes during storage. Among the fish species studied, Scomber scombrus<\/em> consistently recorded the highest crude fat content and the greatest susceptibility to oxidative deterioration following smoking.<\/p>\n\n\n\n

Therefore, the study established that although smoking enhances the apparent nutritional composition of fish by concentrating nutrients through dehydration, it simultaneously accelerates lipid oxidation, which may compromise product quality during storage. Freezing proved to be a more effective preservation method for maintaining nutritional quality and limiting oxidative deterioration compared with smoking. The results provide valuable information for consumers, fish processors, public health authorities and fisheries stakeholders regarding the selection of appropriate preservation methods for maintaining fish quality and extending shelf life. Adoption of improved smoking technologies, appropriate frozen storage conditions and routine quality monitoring is therefore recommended to enhance the nutritional value, safety and marketability of commercially processed fish in Port Harcourt and other parts of Nigeria.<\/p>\n\n\n\n

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      <\/p>\n","protected":false},"excerpt":{"rendered":"

      INTRODUCTION Fish is one of the most important sources of animal protein worldwide and contributes substantially to global food and nutritional security. It provides high-quality proteins containing all essential amino acids, long-chain omega-3 polyunsaturated fatty acids (PUFAs), vitamins (A, D and B-complex) and essential minerals such as calcium, phosphorus, selenium and iodine, which are indispensable […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[2,1],"tags":[],"article-archive":[39],"class_list":["post-893","post","type-post","status-publish","format-standard","hentry","category-original-research-article","category-uncategorized","article-archive-volume-5-issue-1-2026","entry"],"acf":[],"_links":{"self":[{"href":"https:\/\/academicsociety.org\/zoologicalreports\/wp-json\/wp\/v2\/posts\/893","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/academicsociety.org\/zoologicalreports\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/academicsociety.org\/zoologicalreports\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/academicsociety.org\/zoologicalreports\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/academicsociety.org\/zoologicalreports\/wp-json\/wp\/v2\/comments?post=893"}],"version-history":[{"count":3,"href":"https:\/\/academicsociety.org\/zoologicalreports\/wp-json\/wp\/v2\/posts\/893\/revisions"}],"predecessor-version":[{"id":899,"href":"https:\/\/academicsociety.org\/zoologicalreports\/wp-json\/wp\/v2\/posts\/893\/revisions\/899"}],"wp:attachment":[{"href":"https:\/\/academicsociety.org\/zoologicalreports\/wp-json\/wp\/v2\/media?parent=893"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/academicsociety.org\/zoologicalreports\/wp-json\/wp\/v2\/categories?post=893"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/academicsociety.org\/zoologicalreports\/wp-json\/wp\/v2\/tags?post=893"},{"taxonomy":"article-archive","embeddable":true,"href":"https:\/\/academicsociety.org\/zoologicalreports\/wp-json\/wp\/v2\/article-archive?post=893"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}