Some Metals Found in Stored Canned Fish Products Sold in Nigeria

Abstract

Trace amount of some metals like zinc, iron, manganese is normally constituent of natural water bodies, canned products and fishes. Excessive levels of metals can become detrimental to consumers of fish and fish products especially those stored for prolonged period at varied temperatures. Toxicological and environmental concerns have therefore prompted interest into the investigation of some of these metals especially in canned fish stored in different media, sold in Nigeria. In order to curb preventable damage to consumers’ vital organs as a result of unsuspecting elevated levels in consumed fish and fish products, this study will determine the presence of some metals in different storage media and compare their values with safe set-standard levels. Monthly mean concentration of metals was determined using Inductive Coupled Plasma-optical emission spectrometer (ICP-OES), and values compared with permissible set-values by different moderating organizations like the Codex, World Health Organization (WHO) among others when considering metal contamination. Based on all the different storage media, an average concentration of 2.88–29.45 mg/kg Fe, 7.04–72.09 mg/kg Zn, 0.09–0.67 mg/kg Mn, 0.55–5.61 mg/kg Ni, 0.02–0.27 mg/kg V were detected during the period of study. This study is therefore intended to encourage comprehensive periodic monitoring of canned products in Nigeria market.

1. Introduction

Many food-insecure nations are located in Africa, which happen to be the net importer of fish in terms of volume [1]. The Nigeria population which is estimated to be growing at 2.41% annually, depends massively on fish to meet the over 40% animal protein intake needs of its population. Demand for fish from industrial fishing countries around the world in the form of frozen and canned fish is valued at about $500 Million US Dollars or N250 Billion Naira per annum [2]. The state of World Fisheries and Aquaculture in 2022 reported that that global merchandise trade which increased at a rate of 6.8% per year in nominal terms between 1976 to 2020 and by 3.75% in real terms.

Fish consumption can supply over 50% high quality protein, low fat, essential poly unsaturated fatty acids, micro and macro nutrients [3]. World population growth has outweighed fish production due to increased fish and sea food consumption which have increased per capita across the world from as reported for as at 2017 [4].

Federal Department of Fisheries (FDF) and Food and Agriculture Organization (FAO) has previously reported Nigeria’s self-sufficiency in fish production ratio of 98.8% in 1983, which dwindled to between 40% and 19.2% in 2005–2014. And with the present economic realities in Nigeria foreign exchange, weak institutional reforms and economic measures without the well withal to implement them; the challenge to increase protein consumption in Nigeria appears to be more urgent [5].

According to FAO statistics as reported [6], 6% of the 11% global population’s animal protein intake is derived from fish. Presently, more than 30% of the world population is suffering from malnutrition, this is more evident in developing countries like Nigeria, where people experience just over one quarter of all possible deprivation as highlighted in the 2022 multidimensional poverty index survey which revealed that 63% (133 million people) of persons living within Nigeria are multidimensionally poor [7].

Carbohydrate which is the stable food in Nigeria is complemented richly with fish because of its provision of fatty amino acids, minerals and trace metals and it is also believed to be a healthier alternative to many other sources of animal protein. Until recently in Nigeria, fish was regarded as the cheapest and easily accessible animal protein source. And canned fish which has become very popular, makes marine fish available in different part of the world irrespective of season.

Humans are usually more exposed to toxic elements via food as they tend to absorb many of these pollutants from the natural environment, storage medium, storage conditions before sales among others [8]. Therefore, the determination of safety levels has become paramount due to the health challenges which hereto were not frequently diagnosed, but has become a recent trend in the poorly revitalized health sector of the country coupled with the dwindling value of the naira due to the high demand for dollars as a result of Nigeria’s mono-economy.

Studies on the quality of frozen and canned fishes has attracted attention from relevant agencies and experts, as the beneficial and toxic level range of any product are very small [9]. Therefore, effects of exposure above certain threshold can be potentially life threatening especially if not properly diagnosed for immediate medical treatment.

Canned food are popular food sources all around the world [10], and it is a modern technological advancement in food processing, helps to increase shelf life and allow storage for several years. Operations in canning industries are manually and mechanically carried out, the final heat treatment gives protection from spoilage organisms. But this process of heat treatment is done in such a way as not to alter the flavor and texture of fish considerably [11].

Most of the marine fish are canned, thus, making it more available for consumption by humans living far away from sea sites [12]. Fish are constantly exposed to chemicals in polluted and contaminated waters; therefore, fish products need to be well established to be relatively free of excessive metals as some metals according to [13] can act as catalysts in the oxidative reactions of biological macromolecules resulting in induced alterations in almost all body functions and organ integrity. Metals can be classified as potentially toxic, probably essential and essential [14].

Many priority pollutants as classified by the United States Environmental Protection Agency (US EPA) have made their way into aquatic systems through anthropogenic sources and natural weathering. The major sources of pollution of surface waters include effluent discharges by industries, agricultural run offs, atmospheric depositions of pollutants and accidental spillage. Trace elements may be immobilized within the stream sediments and result in absorption, co-precipitate and form complexes by co-absorbing with other elements or form particulates as oxides hydroxides of Fe, Mn.

Toxicity and the resultant adverse effect based on its’ concentration on humans has been of concern especially in developed countries which has resulted in the imposition of new and more restrictive regulations [15, 16]. Regulations in Nigeria are still not strictly followed due mostly to poor will-power and inadequate manpower. This has resulted in increased level of health concerns in the country, as more and more persons now embark on medical pilgrimages and increase pressure on the available medical facilities and personnel.

Canned fish and fish products considered by Nigerians as conventional, hygienic and tasty has resulted in their preference and patronage, but some of these products may be laden with deleterious contaminants whose effects may take a long time to manifest especially when consistently taken in large quantities.

During these last decades, environmental research on metal pollution on marine and freshwater environments has been conducted, focusing especially on the distribution of trace metals in freshwater as well as in seawater fish [17]. Since most of the marine fish are canned, it therefore becomes more available for consumption by humans living far away from sea sites [12].

Since metals present in seafood, at low concentration may be essential; however, they can become toxic having harmful effects when their intake exceeds the recommended quantities significantly [18, 19, 20].

Human exposure to heavy metals has risen dramatically in the last 50 years as a result of an exponential increase in the use of heavy metals in industrial processes and products [21]. Therefore, the lethal thresh of substances like zinc while working on the toxicity of zinc to Clarias gariepinus can be defined after a long period of exposure [22], which is characteristic of substances that may be slowly excreted or metabolized and at the same time a decreasing concentration may not appreciably increase the survival period of the fish. It is essential to identify the interaction between the foodstuff and its package, particularly when it is being purchased and consumed nationwide on a regular basis [23]. The most important heavy metals causing severe toxicity to fish are arsenic, lead, nickel, zinc, tin among others, as they are considered the most important form of pollutants of the aquatic environment because of their toxicity and accumulation by marine organisms [24]. Heavy metal pollution of the marine environment has long been recognized as a serious environmental concern as toxic elements can become very harmful even at low concentration when ingested over a long period of time [25, 26]. Heavy metals can be accumulated by marine organisms through a variety of pathways, including respiration, adsorption and ingestion [27, 28].

The aquatic environment is a major place for contamination by metals, as a result of bio-accumulation by fish during feeding, it may become bio-magnified in consumers of such contaminated fishes. The growing concern over the increasing level of heavy metals contamination, in agricultural, sea foods and industrially processed food products has therefore birthed this study. This study therefore aims to determine the levels of some metals (Fe, Zn, Mn and Ni) in six different brands of canned fish products stored in curry sauce, tomato sauce and soybean oil with a view to evaluating how safe these canned fish products in these storage media, sold in the Nigerian market are against set safe standards.

2. Materials and methods

2.1 Sample collection

Six different brands of canned fish products manufactured in three different Asian countries were purchased from open markets, wholesales shops and super markets (chain-stores) in Nigeria. Their storage media were tomato sauce, soyabean oil and curry sauce and the species canned were mackerel, sardine and skipjack tuna with an average weight of between 106 and 155 g/Can respectively. The expiration date of samples was between two to four years as at the time of study and bought samples were taken to the laboratory, kept in clean dry/cool place prior to digestion and analysis.

2.2 Digestion of samples

The canned fish preservative medium (tomato sauce, curry sauce and soybean oil) was carefully decanted in the laboratory, rinsed with deionized water and samples were oven dried to constant weight at a temperature of 80°C for 72 hours. Each sample in three replicates per brand/month was milled separately to powder form using a porcelain mortar and pestle. They were stored in labeled plastic packs, sealed and stored at −10°C prior to digestion and analysis during the period of study. Fish samples were digested using the organic extraction technique described by Sreedevi et al. [29].

2.3 Statistical analyses

Data obtained were analyzed using computer software (SPSS version 21). One way analysis of Variance (ANOVA) test was used in all cases to test for significant differences between means at 5% probability level. Significant treatment means were separated using the New Duncan’s Multiple Range Test.

3. Results

The results of the mean concentration of study metals (Fe, Zn, Mn, Ni and V) detected in six brands of canned fish products and at different months of study stored in curry sauce, tomatoes sauce and soyabean oil are as shown in Tables 1 and 2.

Month	Fe	Zn	Mn	Ni	V
February	7.30 ± 1.70^d	19.86 ± 4.24^c	0.17 ± 0.04^d	1.39 ± 0.32^d	0.07 ± 0.02^b
April	8.22 ± 1.92^bcd	22.37 ± 5.21^bc	0.19 ± 0.04^bcd	1.57 ± 0.37^bcd	0.08 ± 0.02^b
June	9.87 ± 2.30^bc	26.84 ± 6.26^ab	0.23 ± 0.05^bc	1.88 ± 0.44^bc	0.10 ± 0.02^b
August	10.30 ± 2.40^b	22.37 ± 5.21^bc	0.24 ± 0.05^b	1.96 ± 0.46^b	0.10 ± 0.03^b
October	14.72 ± 3.43^a	31.98 ± 7.46^a	0.35 ± 0.08^a	2.80 ± 0.65^a	0.15 ± 0.04^a

Table 1.

Mean and standard deviation of concentrations of study metals (mg/kg) in different months.

Note: Different superscripts in the same column are significantly different.

Spp/storage media	Fe	Zn	Mn	Ni	V
Sardine/CurrySauce1	29.45 ± 2.09^a	72.09 ± 3.64^a	0.67 ± 0.05^a	5.61 ± 0.51^a	0.22 ± 0.02^b
Mackerel/TomatoSauce2	6.15 ± 0.42^c	15.05 ± 0.72^c	0.14 ± 0.01^d	1.17 ± 0.08^c	0.05 ± 0.00^c
Mackerel/TomatoSauce3	3.20 ± 0.23^de	7.83 ± 0.41^de	0.09 ± 0.01^d	0.61 ± 0.04^d	0.02 ± 0.00^e
Mackerel/TomatoSauce4	2.88 ± 0.20^e	7.04 ± 0.34^e	0.66 ± 0.01^ab	0.55 ± 0.04^d	0.02 ± 0.00^e
Sardine/SoybeanOil5	15.18 ± 1.05^b	37.17 ± 1.80^b	0.35 ± 0.24^c	2.89 ± 0.20^b	0.27 ± 0.02^a
Skipjack/SoybeanOil6	3.64 ± 0.25^d	8.92 ± 0.43^d	0.11 ± 0.01^d	0.69 ± 0.05^d	0.04 ± 0.00^cd

Table 2.

Mean and standard deviation of concentration of metals (mg/kg) in the different species storage media.

Note: Different superscripts in the same column are significantly different.

Different subscript in spp./storage media column refers to different brands.

In the months of study (February–October) the highest concentration of all the metals were recorded in October, while the least concentrations were observed at the beginning of the study as shown in Table 1.

Concentration of the study metals in the six brands of canned fish analyzed varied within the months and between the storage medium, although some were not significantly different during the period of study. The highest concentration value of Fe (29.45 mg/kg), Zn (72.09 mg/kg), Mn (0.67 mg/kg), Ni (5.6 mg/kg) and V (0.27 mg/kg) in sardine all stored in curry sauce except V which was in sardine stored in soybean oil. The least value of Fe (2.88 mg/kg), Zn (7.04 mg/kg), Mn (0.09 mg/kg), Ni (0.55 mg/kg) and V (0.02 mg/kg) were all observed in mackerel stored in tomato sauces_(2–4) as shown in Table 2. The results obtained were statistically tested.

The month of October had the highest mean concentration values across the individual heavy metals and the least concentrations at the beginning of study as shown in Table 1 above. Using Analysis of variance (ANOVA) to statistically test the results obtained for significant difference and a further separation of means with the Duncan Multiple Range Test (DMRT) between brands storage media. It was observed that there was significant difference (p > 0.05) in the metal contents among them. The brand of canned sardine in curry sauce had higher concentration of all the study metals except for V, followed by sardine stored in Soybean oil within brands of canned fishes (Table 2); there was also in the different months significant difference (p > 0.05) in the metal concentration in the different months (Table 1). Mn, Ni and V were not significantly different (p < 0.05) during the months of study and in the brands irrespective of their storage media.

4. Discussion

Fish has been considered a good indicator for heavy metal contamination in aquatic systems because they occupy different trophic levels therefore the levels of toxic elements in fish are related to age, sex, season and habitat [30, 31]. Fish being widely consumed in many parts of the world as this has been made possible by advances in packaging technology especially the use of cans, such that any level of pollution in stored fish will endanger human health all over the world. Results from this study shows that Fe, Zn, Mn, Ni and V were present in the six brands of canned fish products in the different storage medium marketed and frequently consumed in the Nigeria market.

Iron is an essential metal and it is an important constituent of hemoglobin, myoglobin and enzymes such as catalases and peroxidases. WHO set iron permissible limit at 0.30 mg/day and in the UK, women during their reproductive age and post-menopausal estimated daily iron requirement are set at 11.4 mg and 6.7 mg respectively, while 6.7–8.8 mg was for young male-adults. Estimated average daily requirements for children of between 0 and 10 years varied between 1.3 mg and 6.7 mg [32]. Although, there are no set permissible limits to be used as a standard in Nigeria with regards to ferrous concentration, the results obtained during the study period and in the different storage media exceeded the WHO set limit in all the brands. Except for sardine stored in curry sauce and soybean oil, all other values were within the UK and COMA set permissible limit. However previous studies in Nigeria [33], revealed that 0.0379 mg/kg and 0.0302 mg/kg were obtained while working on two different brands of canned fish. Results from this study are much higher than their findings but similar to results obtained earlier [34] while working on canned fish stored in brine and sunflower.

Zinc has been implicated in most metabolic pathways in humans; its deficiency can lead to loss of appetite, growth retardation, skin changes and immunological abnormalities. Maximum zinc level permitted for fish is 50 mg/kg according to Food Codex, while the US recommended daily intakes of zinc are 15 mg and 12 mg for adult male and female respectively. Results obtained shows two brands of mackerel in tomato sauce_(2,3) and skipjack in soybean oil₆ did not exceed these daily requirement Although, United Nations Environmental Protection Agency and the European Commission (US-EPA and EC) have not set any standard limits for zinc concentrations, Codex [35] maximum permissible limit of 40 mg/kg was exceeded in the mean zinc concentration of 72.09 mg/kg observed in sardine in curry sauce₁ but all other values were within the permissible limit during the different months. and the lowest average zinc concentration was observed in costa mackerel in tomato sauce₃ 7.04 mg/kg. This value (7.04 mg/kg) obtained in this study, compare favorably with the result of [36] who also observe a low Zn content in mackerel (6.9 mg/kg), binito (5.8 mg/kg) and trout (6.4 mg/kg) and a higher content in anchovy and sardine (27.3 mg/kg and 17.6 mg/kg respectively). In canned tuna flakes stored in brine a zinc concentration level of 28.95 mg/kg had earlier been reported [34]. While working on four imported fish species stored in commercial cold store in Nigeria [37] it was observed that no value exceeded the WHO/FAO maximum permissible limit as the highest mean value of 21.52 mg/kg was obtained.

According to USEPA [38], there is no information on the carcinogenicity of manganese; COMA and WHO have been unable to set a specific recommendation for manganese intake. The EU scientific committee for food (EU-SCF) considered an adequate intake to be 1–10 mg/person/day. Results obtained from this analysis shows that the maximum and minimum concentration range of Mn was 0.09–0.67 mg/kg with a mean monthly value which was basically below the recommended values by the US National Research Council (NRC) [39] specified estimated safe and adequate daily dietary intakes of 0.3–1, 1–3 and 2–5 mg/day for infants, children and adults respectively. This result is comparable to [34] reported range of 0.07–0.51 mg/kgMn and 0.002 mg/kgMn and 0.0016 mg/kgMn [33] while working on brands of Geisha and Founty canned fish respectively. But it was however lower than the corresponding maximum level of 15.77 mg/kg reported for canned sardines in Brazil [40]. Although small daily intake of manganese is needed for a total wellbeing, inability to remove excess amount of manganese from the system can result in nervous system problems.

Nickel which can act as an activator of some enzyme systems at high levels can also cause respiratory problems because it can accumulate in the lungs and it is carcinogenic [41]. In addition to environmental contamination of nickel in foods, it can get into foods through processing activities such as canning and cooking. The upper tolerable intake level of nickel for children and adult are 7 mg/day and 40 mg/day respectively [42] but Codex [35] set the permissible limit in foods at 3.0 mg/day. The mean concentration range of 5.61 mg/kg-0.55 mg/kg Ni observed in this study was higher than that earlier obtained [43], which was in the range of 0.0 to 0.78 mg/kg while working on canned fishes sold in Brazil. Except for sardine stored in curry sauce, all other values obtained during this study were below set permissible limit in foods [35] especially in all the brands stored in tomato sauces_(2–4).

Vanadium is a transition metal, with an estimated ˃ 60 thousand tons of this element being emitted into the atmosphere yearly from human activities as oxides or sulphates. But it also occurs naturally in soil, waters and air [44, 45]. Obtained values from this study showed a V concentration range of 0.27 mg/kg to 0.02 mg/kg which were much less than the 1.8 mg/kg upper tolerance intake level of vanadium for adults of between 19 and 70 years as set by Institute of Medicine [42], and 2 mg/kg and 2.9 mg/kg set by Codex and EC respectively.

5. Conclusions

Metal poisoning though can be diagnosed and treated, the best option however is to prevent poisoning. As metal consumption beyond certain threshold can be potentially life threatening as a result of their bio-toxic effects,

Canned fish brands used for this study can be certified as relatively safe for consumption haven detected concentrations lower than most maximum tolerance concentration set by many foods and fish regulatory bodies. Storage media did not significantly affect the fishes stored in the different media but for their brands, as mackerel stored in tomato sauce in three different brands showed a wide variation especially in one of the brands as compared to the others which were not significantly different. Sardine and skipjack both stored in Soybean oil were significantly different as sardine stored in the soybean oil had much higher percentage metal concentration compared to skipjack. Same brand of sardine stored in soybean oil and curry sauce respectively, were not significantly different from each other but both were significantly different from all the other brands.

The trend observed during this study also shows that these study metals concentration showed some level of variation as the storage period increased. These variations with time may be due to sales and storage temperatures to which these products are exposed to. Temperatures in Nigeria can be extremely high, and no deliberate attempt is made to adhere to manufacturer instructions on storage and keeping details by marketers, quality managers and regulatory bodies. Good management of the harvesting waters, production process and adequate hygienic sales outlet, with controlled atmosphere where possible for handling canned fish products must be advocated in Nigeria. Food regulatory bodies in Nigeria should ensure that good quality canned fish products are not only imported into Nigeria, but that the quality integrity of these imported products are kept at optimum level by monitoring possibly the chain line where products are likely to loss quality. Because of their storage and keeping conditions during sales and before consumption, so as to ensure the safety of unsuspecting consumers. Therefore, continuous monitoring of the metal levels in imported products distributed within the country at point of arrival and periodic evaluation of products in circulation and where possible other variables, will help sanitize the market and safe guild the health of her people.