EFFECTS OF SOME PLANT ESSENTIAL OILS ON THE KEEPING QUALITIES OF SMOKE-DRIED AFRICAN CATFISH, CLARIAS GARIEPINUS (BURCHELL, 1822)

Abstract

The effects of the essential oils of Ginger rhizomes (Zingiber officinale, Roscoe), African nutmeg seeds (Monodora myristica, Gaertn Dunal), Black pepper seeds (Piper guineense, Schumach & Thorn) on the keeping qualities of smoke-dried African catfish, Clarias gariepinus (Burchell, 1822) were evaluated. Dried spices were purchased and pulverized and the nutritional composition determined. The Essential oil (EOs) were extracted and characterized. The physicochemical characteristics and the antimicrobial activity of the EOs were determined and the anti-oxidant activities were assayed. Two hundred and seventy C. gariepinus, (350 ± 50 g), were sacrificed and soaked for 10 minutes in a 15% brine solution and smoked. Fish samples were divided into five (5) groups: A - without coating (control), B -coated with a synthetic antioxidant (BHT 0.02%), while C- coated with 150 mg/mL, D- coated with 300 mg/mL and E- 600 mg/mL of the EOs respectively and smoked. Samples were packed in perforated sterile polyethylene bags and stored for six (6) weeks at room temperature (28±2oC). Sensory, chemical and microbiological analyses were carried out on weekly basis. Data obtained were expressed as mean ± standard error (SE) and subjected to one way ANOVA. Means were separated using Duncan multiple range test at 5% level of significance. There were significant differences (P≤0.05) in the proximate, mineral and vitamin components of the spices. The EOs varied from colorless for M. myristica and P. guineense to yellow for Z. officinale. There were significant differences (P≤0.05) in all physicochemical parameters except for the refractive index and specific gravity Z. officinale contained thirty-three compounds, most abundant compound was β-sesquiphellandrene (16.53%), M. myristica oil contained twenty-five components with the most abundant being limonene (17.58%), while twenty-four compounds were detected in the EO of P. guineense β-linalool (10.32%) was the most abundant. Z. officinale oil elicited maximal antimicrobial activity compared to the control (positive). Z. officinale exhibited maximal bactericidal and fungicidal attributes at 150 mg/mL. The EOs significantly differed (P≤0.05) in all antioxidant assays. The EOs showed a significantly lower (P≤0.05) antioxidant activity compared to Butylated hydrotoluene. There was steady decline in overall acceptability as storage time increased. There were significant differences (P≤0.05) in the overall acceptability among coated and uncoated samples. There was significant increase (P≤0.05) in the moisture and ash content but a significant decrease (P≤0.05) in protein, lipid content, and pH value. There was significant increase (P≤0.05) in the PV, TBAR, FFA, TVN-B, total bacteria, and fungi counts. This study showed that coating of smoke-dried catfish with BHT and EO resulted in significant differences in sensory, chemical and microbial load compared to uncoated (control) sample. BHT was the most effective in reduction of microbial load and lipid oxidation. M. myristica EO was the most acceptable in organoleptic assessment, P. guineense EO was the most effective in retarding microbial load, and Z. officcinale EO was the most effective in retarding microbial load at 600 mg/mL. The antibacterial and antioxidant effects of the EOs as a fish preservative, both in-vitro and in-vivo, gave comparable efficacy to the synthetic preservative.