One of the most relished animal-protein sources the world over is fish [1]. Fish and fish products bring in a dimension of flavour to the diet of man. However, fish is highly perishable, particularly while fresh. This essentially is the basis for preservation of fish products with the aim of maintaining quality, safety, and shelf life. Oreochromis niloticus, commonly known as Nile tilapia, is a widely consumed freshwater fish that is prone to spoilage due to microbial growth and enzymatic activity. Fish spoilage is mainly caused by microbial proliferation, oxidative rancidity, and enzymatic degradation, which can result isn undesirable changes in texture, color, and odor [2]. Traditional preservation methods, such as smoking, salting, and drying, have been employed to extend the shelf life of fish products [3]. However, the use of natural preservatives can further enhance preservation by inhibiting microbial growth and delaying oxidative changes.

Garlic (Allium sativum) and onion (Allium cepa) are two commonly used culinary ingredients known for their antimicrobial properties. Previous studies have indicated that both garlic and onion possess bioactive compounds, such as allicin in garlic and quercetin in onion, which can inhibit the growth of spoilage microorganisms [4]. Allicin is known for its antibacterial and antifungal properties, which disrupt microbial cell functions [5,6]. Similarly, quercetin exhibits strong antioxidant activity and has been shown to suppress microbial growth [7]. This study aims to compare the preservative effects of garlic and onion pastes on treated smoke-dried Oreochromis niloticus, assessing their impact on microbial load, sensory attributes, and overall shelf life.

Source of Fish Sample

Fresh Oreochromis niloticus (Nile tilapia) was obtained from fishermen at Zaria Dam. Fish selection criteria included freshness indicators such as clear eyes, firm flesh, and absence of off-odours [8].

Source of Preservatives (Garlic and Onion)

Fresh garlic and onion were both sourced from Tudun Wada Market, Zaria Local Government Area, and Kaduna State, Nigeria. The garlic and onion were selected based on uniform size and freshness, as these factors influence the concentration of bioactive compounds [9].

Preparation of Fish

Fresh Oreochromis niloticus were cleaned, gutted, and filleted according to standard fish handling protocols to reduce initial microbial contamination [8]. The fish fillets were treated with a 5% brine solution (saltwater) for 30 minutes to enhance flavour and preservation. Brining has been reported to improve texture and reduce microbial load in smoked fish [10].

Preparation of Garlic and Onion Pastes

Fresh garlic and onion were peeled, chopped, and blended into a smooth paste using a sterile blender to prevent contamination [4]. The pastes were stored in sterile containers at refrigeration temperature (4°C) until use to maintain their bioactive properties [9].

Smoking Process

The fish were smoked using a traditional smoking method at a controlled temperature of 60–70°C for 4 hours to ensure proper drying and flavour infusion [3]. Smoking not only imparts flavour but also reduces water activity, making the fish less susceptible to microbial spoilage [10].

Treatment Groups

The smoked fish were divided into three groups: the control group with no paste treatment, the garlic paste group with 100 g of fish coated with 5g, 10g, 15 g, and 20 g of garlic paste, and the onion paste group with 100 g of fish coated with 5g, 10g, 15 g, and 20 g of onion paste. Each treatment in the garlic and onion paste groups, including the control, was replicated three times. All treated fish samples were stored in sterile containers at room temperature and monitored for microbial growth and sensory attributes over a period of 14 days [4].

Storage

All treated fish samples were stored in sterile containers at room temperature (25±2°C) and monitored for microbial growth and sensory attributes over a period of 14 days [10].

Microbial Analysis

Microbial load was assessed by plating fish samples on nutrient agar using the pour plate method. Samples were incubated at 37°C for 24 hours, and colony-forming units (CFUs) were counted to determine bacterial load. The microbial analysis was conducted in triplicate to ensure statistical reliability [8].

Sensory Evaluation

A panel of trained sensory evaluators (n=10) assessed the fish samples for appearance, aroma, taste, and overall acceptability using a 9-point hedonic scale. Sensory attributes were evaluated at baseline and at intervals of 3, 7, 10, and 14 days to determine consumer acceptability over time.

Microbial Load Analysis

Table 1: Microbial Load (CFU/g) On Treated and Control Smoke-Dried Fish from Days 0-14.

The microbial load results demonstrated a significant difference between the control and treatment groups, with garlic and onion pastes effectively reducing microbial growth in smoke-dried Oreochromis niloticus. The control group exhibited the highest microbial proliferation over the 14-day period, exceeding 2.0×10⁵ CFU/g by day 14. Conversely, fish samples treated with garlic and onion pastes showed reduced bacterial counts, with the effect being dose-dependent [11].

Garlic paste at 20g per 100g of fish demonstrated the highest efficacy in microbial inhibition, with microbial counts of 9.0×10³ CFU/g on day 0, increasing to 8.0×10⁴ CFU/g by day 14. This reduction is likely attributed to the presence of allicin, a potent antimicrobial compound in garlic, which disrupts microbial cell function [5,6]. Similar trends were observed in fish treated with onion paste, though microbial inhibition was slightly less effective than garlic, particularly at lower concentrations. Onion paste at 20g per 100g fish resulted in 1.1×10⁴ CFU/g on day 0 and 9.0×10⁴ CFU/g by day 14, reinforcing previous findings that quercetin and other sulfur-containing compounds in onions contribute to antimicrobial activity [7,11].

The findings indicate that garlic paste had a stronger preservative effect compared to onion paste, particularly at higher concentrations. This aligns with previous research suggesting that allicin in garlic has superior antibacterial properties compared to quercetin in onions [5,7]. However, both pastes exhibited dose-dependent efficacy, with higher concentrations (15g, 20g) significantly extending shelf life [11].

The statistical analysis confirmed these findings. The one-way ANOVA test (F=80.08, p < 0.001) indicated significant differences in microbial load across treatment groups. Tukey’s HSD post-hoc test further established that microbial counts in the garlic (15g, 20g) and onion (15g, 20g) treatments were significantly lower compared to the control and lower-dose treatments (5g, 10g). These results support the hypothesis that higher concentrations of garlic and onion pastes enhance microbial inhibition, extending the shelf life of smoked fish products.

Sensory Evaluation Analysis

The evaluation is based on a 9-point hedonic scale (1 = extremely disliked, 9 = extremely liked) for appearance, aroma, taste, and overall acceptability. Each value represents the average rating given by the panel of trained evaluators (n=10) for each replicate.

Table 2: Sensory Evaluation Mean Scores of Treated and Control Smoke-Dried Fish from Days 0-14.

Where AP=Appearance; AR=Aroma; TE=Texture; TA=Taste (mean scores for smoked-dried Oreochromis niloticus)

Sensory evaluation scores provided additional insights into the effectiveness of garlic and onion pastes in preserving the quality of smoke-dried Oreochromis niloticus. Panelists assessed appearance, aroma, taste, and overall acceptability using a 9-point hedonic scale at intervals of 0, 3, 7, and 14 days. The control group exhibited a significant decline in sensory attributes over time, with overall acceptability dropping from 8.7 (day 0) to 3.0 (day 14), indicating noticeable spoilage [12].

In contrast, fish treated with garlic and onion pastes retained higher acceptability scores. Garlic 20g-treated fish maintained a relatively high overall acceptability score (9.0 on day 0, 6.0 on day 14), showing better retention of aroma, taste, and texture compared to the control [12]. Similarly, onion-20g-treated fish had an overall acceptability score of 9.0 on day 0 and 5.0 on day 14, demonstrating notable preservation benefits. Lower paste concentrations (5g, 10g) led to moderate improvements in sensory attributes but were less effective over time [13].

The two-way ANOVA test revealed significant differences among treatments (p<0.001) and across different storage days (p<0.001). The interaction effect between treatment and storage time was also significant (p < 0.001), indicating that the sensory attributes of the samples were influenced by both the type of treatment and the duration of storage. Garlic (20 g) and onion (20 g) treatments were significantly better than lower concentrations and the control in maintaining sensory quality [12]. These results align with previous findings that garlic and onion compounds not only inhibit spoilage microorganisms but also mitigate oxidative rancidity, preserving sensory attributes [3,4].

While smoking alone is effective in reducing microbial growth, combining smoking with natural antimicrobial pastes enhances fish preservation [14]. This study supports the integration of garlic and onion pastes as natural preservatives to improve the shelf stability of smoke-dried fish products. The organoleptic benefits also suggest that these treatments enhance consumer acceptability, reducing spoilage-related wastage.

The study demonstrated that garlic and onion pastes effectively reduced microbial load and preserved the sensory attributes of smoke-dried Oreochromis niloticus. Higher concentrations (15g, 20g per 100g of fish) exhibited the most significant preservative effects, with garlic outperforming onion in microbial inhibition and sensory preservation. These findings suggest that natural plant-based preservatives can be viable alternatives to synthetic additives, enhancing both food safety and quality. Future research should focus on optimizing concentrations for industrial applications and evaluating consumer acceptability on a broader scale.

Recommendations

The use of garlic and onion pastes can be adopted in the fish processing industry to improve shelf life and reduce dependence on artificial preservatives.

Further studies should explore the specific mechanisms by which bioactive compounds in garlic and onion inhibit spoilage and oxidative processes.

Research into optimizing concentration levels for larger-scale fish preservation is needed to balance efficacy and cost-effectiveness.

Future studies could assess whether the flavour impact of garlic and onion pastes influences consumer preferences beyond microbial inhibition.

Acknowledgement

I want to acknowledge the staff of Veterinary Microbiology Laboratory of the Faculty of Veterinary Medicine, Ahmadu Bello University, and Zaria for their contribution to the output of this research.

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