INTRODUCTION

Smoked fish offers an alternative high quality animal protein source required by man. Anonymous (2005) and Eyo (2001) reported that smoked fish is one of the most widely distributed and the cheapest sources of protein and micronutrients in Nigeria. According to Eyo (2001), 45% of total fish catch in Nigeria are utilized as smoked fish. However, studies have shown that a high proportion of dried fish in Nigeria is usually infested by insect pest, such as Dermestes maculatus and Necrobia rufipes (Odeyemi et al., 2000; Osuji, 1974). Osuji (1974) further reported that about 71.5% of dried fish infestation in most of the producing areas was caused by D. maculatus.

Some synthesized insecticides, for example, pyrethroid, deltamethrin and permethrin have proved to be effective against stored product pests, if used at the right time, quantity and with the correct application (Golob et al., 1987). However, the general use of such chemicals to protect stored fish has been hampered by reports of health hazards and high cost of purchase. Odeyemi et al. (2000) reported that fish treated with chemical insecticides adversely affected consumers, causing blurred vision, dizziness and vomiting. In addition, Amusan and Okorie (2002) noted that dermestid larvae and adults, unlike many other beetles, are less susceptible to synthesized insecticides that normally attack pests of stored products. Therefore, attention is currently being focused on the use of natural preservation materials that are cheap, easily accessible and that have long term protecting ability. In this regard, a host of plant materials are presently considered as a promising alternative to synthetic insecticides in the control of pests of crops of plant origin (Onifade and Alabi, 1998; Saxena, 1989; Ivbijaro, 1983). However, information on the use of plant materials to control insect pests of stored fish is recent and is just growing.

In view of reports of the unattractive nature of stored product treated with some plant materials (Boeke et al., 2001; Amusan and Okorie, 2002; Onu and Baba, 2003) and because dried fish is often eaten without further processing by most people in the tropics, this study sought to evaluate the effect of powders and extracts of four plant materials, Dennettia tripetala, Eugenia aromatica, Monodora myristica and Piper guineeense on the palatability and acceptability of smoked fish, Clarias gariepinus by fish consumers.

MATERIALS AND METHODS

Preparation of Plant Materials, Insect Culture and Fish Samples
Dry fruits of four commonly-eaten species and locally-used medicinal plants: pepper fruit (D. tripetala: Annonaceae), clove (E. aromatica: Myrtaceae), black pepper (P. guineense: Piperaceae) and African nut-meg (M. myristica: Annonaceae) which were obtained from Erekesan Market in Akure, south western Nigeria were processed into powders and extracts according to the methods described by Adedire and Lajide (2000). To obtain the plant powders, each of the plant materials was washed with clean tap water, oven-dried at 40°C for 8 h, ground thoroughly in an electric 5.0 HP grinder and sieved through a 40 holes mm^-2 mesh screen. The plant powder was then kept in a plastic container with a tightly fitted lid at ambient laboratory conditions before use. To obtain the plant extracts, 100 mL of absolute ethanol was added to 10 g of each plant powder in a round-bottomed flask and soaked for 24 h. The mixture was boiled at 60°C for 30 min in UNISCOPE SM801A Laboratory Water Bath. The solution was filtered with Whatman No. 1 filter paper and the resulting filtrate was kept in a tightly covered dark brown bottle prior to use.

Samples of 100 g smoked fish used for the experiments were obtained from Erekesan Market in Akure, Nigeria and disinfested by treatments in the Gallenkamp oven at 60°C for 1 h and allowed to cool at room temperature before use. The initial source of culture of D. maculatus and N. rufipes was obtained from naturally infested smoked C. gariepinus. New insect generations were obtained after 2-3 weeks oviposition period and were kept in Kilner jars covered with muslin cloth and maintained at 30±2°C and relative humidity 65±5 %.

Investigation of Plant Materials on the Organoleptic Properties of Smoked Fish
Each of the plant powders at 2.5, 5.0 and 10.0 g and 5 mL of extracts at 2.5, 5.0 and 10% was thoroughly rubbed onto the body of a 10 g disinfested smoked fish. The treated fish was placed in a plastic jar, ten 45 h starved D. maculatus and N. rufipes larvae were introduced and the perforated lid covered with muslin cloth and was left on the shelf for 30 days. Disinfested fish without plant treatments served as the control. The protected fish at each concentration level was examined for palatability and physical attractiveness by a panel of three fish consumers within 24 h of exposure to plant products and also at 30 days post-treatment using the modified criteria of Okonkwo and Okoye (2001) and Clucas (1982) (Table 1). Tests were in triplicates for each treatment.

The percentage acceptable was calculated as follows:

The sensory evaluation of the treated fish was classified according to the criteria in (Table 2). Data obtained were subjected to analysis of variance (ANOVA) and where significant differences existed at 0.05 significance level, the treatment means were separated using Tukey’s Test.

Table 1:	Description of the sensory quality used in grading treated smoked fish

Table 2:	Quality classification of treated fish based on sensory evaluation

Table 3:	Sensory evaluation of smoked fish treated with plant powers at 1 day post-treatment
Values are means of triplicate samples followed by standard error of means; Means in the same row with different superscripts are significantly different (p<0.05) by Tukey’s Test; * GA: General Acceptance; CF: Colour of fish; OF: Odour and flavour; TA: Taste; TE: Texture; FG: Fragmentation

RESULTS

The sensory evaluation and consumers’ acceptance of smoked fish treated with plant powders at 1 day post-treatment and at 30 days’ storage period are shown in Table 3, 4, respectively. The results (Table 3) showed that irrespective of powder concentration, smoked fish treated with each of the plant powders was generally ranked excellent by fish consumers. Generally, the percentage acceptance declined as the dosage of powers increased. The organoleptic rating of the treated fish at 30 days post-treatment was significantly high (p<0.05) in comparison with untreated fish (control) (Table 4). The values obtained at the concentration levels in each admixture showed significant differences.

The acceptability of smoked fish protected with plant extracts when freshly applied (Table 5) and at 30 days post-treatment (Table 6) was rated excellent. There was a significant difference between the ratings of the extract-protected fish and the control at 30 days post-treatment. In general, high values were recorded for General Acceptance (GA), Colour of Flesh (CF), Odour and Flavour (OF), Texture (TF) and Fragmentation (FG) of fish protected with all the plant products.

Table 4:	Sensory evaluation of smoked fish treated with plant powders at 30 post-treatment
Values are means of triplicate samples followed by standard error of means; Means in the same row with different superscripts are significantly different (p<0.05) by Tukey’s Test; * GA: General Acceptance; CF: Colour of fish; OF: Odour and flavour; TA: Taste; TE: Texture; FG: Fragmentation

Table 5:	Sensory evaluation of smoked fish treated with plant extracts at 1 day post-treatment
Values are means of triplicate samples followed by standard error of means; Means in the same row with different superscripts are significantly different (p<0.05) by Tukey’s Test; * GA: General Acceptance; CF: Colour of fish flesh; OF: Odour and flavour; TA: Taste; TE: Texture, FG: Fragmentation

Table 6:	Sensory evaluation of smoked fish treated with plant extracts at 30 days post-treatment
Values are means of triplicate samples followed by standard error of means; Means in the same row with different superscripts are significantly different (p<0.05) by Tukey’s Test; * GA: General Acceptance; CF: Colour of fish flesh; OF: Odour and flavour; TA: Taste; TE: Texture; FG: Fragmentation

DISCUSSION

The treated fish were accepted by the fish consumers, as there was no adverse evidence of taint, smell or change in taste, texture or flavour of fish. This is in contrast to Onu and Baba (2003) who observed that the use of some plant insecticides rendered the fish unattractive to the consumers. In this study, the admixtures were all highly accepted but preference for fish treated with low dosages of plant materials was demonstrated. The hedonic scores of all fish samples treated with the lowest dosage were high, in contrast to lower scores obtained in fish protected with higher dosages. The score for taste, which was generally ranked average at higher dosages, was considered good in fish treated with the lowest dosage. The average rating of the taste of all treated fish also confirms the peppeerish and pungency characteristics of the plant materials. The pungency is caused by the presence of ligname, cubebin and several other related compounds, such as hinokinin, clusin, dihydroclusin, dihydrocubebin, sylvatine and trichostachine (Oliver-Bever, 1986).

It was evident that the application of the studied plant materials prevented the unwholesomeness of the smoked fish during storage and thus its acceptance by fish consumers. On the other hand, the unprotected fish (control) became unhygienic and therefore unfit for human consumption, thus its low hedonic rating. According to Elhag (2000), the unfit nature of such a spoilt fish is attributable to the presence of high amounts of uric acid. Insect infestation could also cause an increase in the anti-nutritional factors, such as phytic acid, trypsin inhibitor activity and crude fibre as well as a decrease in starch and proteins contents of stored produce, thereby affecting the nutritional quality and consumers’ acceptability of stored products (Modgil and Mehta, 1996; Clucas, 1982).

In this study, each of the plant products is organoleptically assessed as a palatable and acceptable protectant of smoked C. gariepinus against dermestid and clerid infestation by fish consumers, thus enhancing the consumer friendliness of the studied material.