Heavy Metal Contamination in Food: The Perspective of the Sub-Saharan Informal Food Trade

Abstract

Food safety is often threatened by the intended or unintended introduction of contaminants. Street foods are susceptible to microbial, chemical and physical contaminants. Due to their ubiquitous existence in the environment, heavy metals are among the majority of food material contaminants and they are believed to have carcinogenic properties. Heavy metals have been a source of contamination in the informal food value chain mainly due to their bioaccumulation and existence in voluminous amounts. Several factors account for the rate of contamination and the continued failure to prevent it. Carcinogens such as lead and arsenic are often present in high levels in some vended foodstuffs, and their prolonged ingestion could have injurious effects on consumer health. Heavy metal contamination in street-vended foods sometimes occurs as a result of leaching from poorly designed or old and inadequately cleaned utensils. Moreover, aluminium pots used in cooking may leach aluminium ions, especially when acidic foods such as grains and seafood are cooked.

1. Introduction

Street-vended foods are classified as those foods cooked, prepared and served by vendors in streets and/or other public places and are directly consumed without further preparation [1]. Street foods provide essential nutrients and are a source of energy for millions of consumers worldwide, not only this, but they are also a means of income for many low-income earners around the world [2]. These foods are inexpensive and readily accessible and as a result, street-vended foods have experienced an economic boom in the past few years [1]. Although street foods offer a great deal of advantages to food security, concerns about their safety and sustainability are far too great to be ignored. Food safety entails the handling, preparation and processing of food in a manner that ensures that any food consumed does not cause illness or any health risks to consumers [2]. More often, food safety is threatened by the intended or unintended introduction of contaminants. Street foods are susceptible to microbial, chemical, and physical contaminants. Due to their ubiquitous existence in the environment, heavy metals are among the majority of food material contaminants and they are believed to have carcinogenic properties [3]. This thereby presents a far greater challenge that goes beyond the control of extrinsic and intrinsic conditions to avoid undesirable changes in food.

1.1 Food adulteration

Lakshan [4] states that “there is a major concern in nonfood-grade chemical additives such as coloring agents, flavor enhancers, preservatives and contaminants like pesticide residues and the use of adulterants.” A range of non-nutritional substances can be found in food, be it those that have been tested for safety-in-use or those which little may be known (contaminants), and it is within these, that carcinogens may unexpectedly be found [5].

Cancer has been identified as one of the globally recognised causes of death and it is sometimes attributed to consumption of a particular food type such as red meat or barbequed meat [6]. Food is often manipulated through the addition of substances called food additives that have technological importance or needs. The technological needs include maintaining or improving safety, freshness, taste, texture and appearance of food [7]. However, over the years there has been speculation and research done relating to the health risks of food additives and much is still open for debate and further investigations [8]. Anthropogenic stressors have contributed to the overuse or misuse of food additives in some parts of the world and this has further raised alarm on the issues pertaining to food additives and food adulteration [9].

Food adulteration has gained momentum in recent years, posing not only an economic problem but a health risk for consumers as well [10]. Food adulteration entails the use of prohibited substances that can depreciate or injuriously affect the health of consumers. This is achieved through partial or whole substitution of the food’s original nature by an abstraction of a portion of vital substance from it and by also presenting a food substance as an imitation of another [11, 12]. Food adulteration is committed for various reasons which include food insecurity, increasing profitability, illiteracy of the general public as well as increased food demand and its changing trends [13].

The prevalence of food adulteration practices in modern days is contributing to abating public health to a larger extent, especially through the widespread use of contaminants containing toxic trace elements. The use of adulterants such as metanil yellow, chalk powder and brick powder to enhance food products such as turmeric, coffee and wheat flower has proven to have caused illnesses such as anaemia, brain damage, paralysis, stomach disorders and cancer [14, 15].

1.2 Heavy metal contamination of street-vended foods

Studies have highlighted the prevailing contamination of various food substances and some street-vended foods with heavy metals and various conditions leading to this contamination [2, 16, 17]. Some of the metals found in street-vended foods in their reports include Pb, Cd, Hg, Sb, Mn and Al. Although consumers are advised to visually inspect food before purchasing, contamination of food with heavy metals which are present in parts per million (ppm) cannot be seen with the naked eye [15]. Food may intentionally (through adulteration) or unintentionally be contaminated with trace metals before, during or after processing.

Throughout the street food production and vending processes, the unintentional contamination with toxic metals can occur and influenced by numerous factors including environmental conditions during growth to post-harvest handling, storage and preparation of meals, selection of raw materials and even the vending sites [18]. Heavy metal contamination may also arise from poor packaging, vehicular exhaust emissions, indiscriminate waste disposal, industrial pollutants and other environmental pollutants in the vicinity of the vending stalls [19]. Ankar-Brewoo et al. [20] further add that metals used for food processing equipment material may contribute to leaching of toxic metals into food.

Street-vended foods have also been found to have the potential to greatly contribute to the daily intake of essential trace elements otherwise known as essential minerals [21]. However, data on bioavailability of these essential minerals are not widely known. Sufficient intake of essential minerals throughout the human life is necessary—about 1–2500 mg per day depending on the mineral—and is essential for immune functions and biochemical processes that facilitate growth and chronic disease prevention [22]. The essential minerals are divided into two classifications, namely, macrominerals and microminerals. Macrominerals are those that are required in greater quantities and these include calcium, magnesium, potassium and sulphur, while micronutrients are those that are only necessary in smaller amounts, these include zinc, iron, selenium, manganese, copper, chromium, and boron [23]. This implies that the insufficient supply of macronutrients or overconsumption of micronutrients could be problematic to human health and development.

This narrative review compiles recently published scientific material on essential trace elements content and bioavailability in street-vended foods in African countries. Additionally, factors and variables on the presence of toxic metals and how they affect essential mineral bioavailability are also reported. The compilation of this data will contribute to the information on essential mineral and toxic metals content in street-vended foods, there is also a further intent to highlight some of the gaps, trends and common problems identified by researchers in relation to this topic.

2. Materials and methods

In order to achieve the objective of this review, the authors conducted a search to identify the relevant literature. As a measure to ensure quality and academic standard, databases were carefully searched for full-text articles, abstracts as well as conference proceedings focusing on heavy metals in food generally and street-vended foods. Additionally, competent authorities designated to provide food regulations such as permissible limits of substances in food were also consulted in order to obtain the maximum levels of heavy metals allowed in food as well as the daily intake required to maintain a healthy diet. For this purpose, the CODEX Alimentarius and the Foodstuff, Cosmetics and Disinfectant Act No. 54 of 1972 were consulted. Databases searched include Pubmed, Semantic Scholar, ScienceDirect and Google Scholar. A search strategy was developed using keywords related to the assessment of heavy metals in street-vended foods. These keywords included: “heavy metals,” “street vended foods” and “essential trace elements.” A number of search phrases such as “street vended foods in Africa,” “heavy metal toxicity and food” and “heavy metal contamination in the food chain” were also employed. Publications considered in this review were limited by (1) language (English), (2) year of publication (2014–2022), (3) literature presented should have been focusing on heavy metals in street-vended foods and non-vended foods and lastly (4) since the subsequent study that will follow after this review will be done in South Africa, the authors gave more consideration to literature published in studies conducted in African countries as there was little information from studies conducted locally.

3. Results

Chemical toxicants form a great portion of contaminants according to recent studies [24, 25]. These toxicants can lead to several undesired health effects for habitual consumers. Furthermore, the consumption of potential toxic metals negatively affects the bioavailability of some essential minerals causing a decrease in immunological defenses, intrauterine growth retardation, psychosocial behaviour disorders, disabilities associated with malnutrition, and high prevalence of upper gastrointestinal cancers [2, 24]. This makes heavy metal-induced toxicity a public health concern since the major pathway of human exposure to toxic substances is through the oral route. Most of the prevalent chemical toxicants obtained in street-vended foods are discussed below.

3.1 Lead

Lead (Pb) is a chemical element with high density that affects various body systems such as haematological, renal and neurological systems and is mostly harmful in young children [1, 26]. Additionally, Pb is regarded as a potentially toxic and carcinogenic metal and has no known physiological functions [1]. The widespread usage of this metal has presented adverse effects such as environmental contamination, human exposure and other public health issues [26]. According to the South African Foodstuffs, Cosmetics and Disinfectant Act (Act No. 54 of 1972) [27] the maximum permissible limit of Pb is 0.3 mg/kg for leafy vegetables with the exception of spinach, and 0.1 mg/kg for meats, processed meats, fruits as well as other non-leafy vegetables. A study conducted in Durban, South Africa fruits and vegetables found that the levels of lead ranged between 1.52–3.45 and 1.57–4.52, respectively [28]. These values exceeded the South African reference levels based on Act No. 54 of 1972. Bamuwamye et al. [3] observed that the levels of Pb in different meats procured for preparation in the street food market (fresh chicken, pork, beef and goat meat) were relatively low (0.006–0.054 mg/kg), however, these levels increase drastically after preparation (0.146–2.290 mg/kg). This presence of heavy metals prior to preparation can be attributed to earlier contamination which could have occurred from environmental and metal-based animal feeds. However, the increase in lead levels post-preparation can be due to a number of unhygienic handling practices, material used for preparation/storage as well as the type of fuel used for roasting (wood or charcoal). Grilled chicken intestines in a number of studies have been found to have high levels of lead content [1] and this is a cause for concern and highlights a need for further investigations into direct health effects as this type of food is widely consumed across African countries. Contrary to the results of the aforementioned studies, Ogunkunle et al. [29] found Pb to be absent in some of the fruit samples analysed in their study. Even at low levels, lead is considered to have high toxicity, thereby forming part of the group of heavy metals that are considered to be of great public health importance.

3.2 Cadmium

Cadmium (Cd) is of the ten chemicals considered to be of great public health importance by the World Health Organization and is present in most food stuffs [30]. Although Cd may be present in low concentrations, it is considered to be toxic when absorbed in the human body as it can result in long-term health problems such as brain damage, renal dysfunction and hypertension [31, 32]. Additionally, a correlation between Cd exposure and hepatocellular carcinoma (HCC) has been established in some ecological studies [31]. The maximum levels of Cd allowed in fruits and vegetables as well as meat and meat products are 0.05 mg/kg, and 1.0 mg/kg in fish and fish products [27]. Adeyeye [33] reported that the levels of Cd in warankasi cheese sold in Ibadan Nigeria were below the WHO maximum permissible limits of Cd (0.2 ppm) and ranged between 1.8 and 2.9 ng/g. A study conducted on street-roasted and -vended meats showed that cadmium levels before preparation were very low (undetected to 0.001 mg/kg), however, this changed substantially when tests were done on the meats after preparation (Cd levels 0.063–0.248 mg/kg). This increase in levels of Cd can be ascribed to the methods of preparation, handling practices or utensils used for preparation [3]. Maintaining Cd levels below tolerable limits is needed to prevent the looming health risks. Certain food types such as rice, meat (including edible offals) and vegetables which are often sold in street markets have been identified as some of the greatest contributors to Cd exposure in humans [32].

3.3 Mercury

Mercury (Hg) is a toxic element that comes in various forms in the environment and has the capability to induce a number of clinical presentations if excess amounts are consumed [34]. Often mercury is introduced into the environment through industrial sectors such as smelter or coal-fired power plants and may also come from the atmosphere due to distant pollutants [35]. All Hg taken up by fish is said to be methylmercury which is highly toxic and can be harmful to the nervous system and the developing brain [36]. Inorganic Hg, however, which is present in other foodstuffs besides fish and fish products is considered to be less toxic [37], this, however, does not discourage caution when handling or consuming such products. The maximum level of Hg in natural water is 0.001 mg kg and that of methylmercury is 0.5 mg/kg in fish and fish products [27, 38]. In a study by Obi-Iyeke [35], no mercury was detected in commonly consumed fruits and leafy vegetables. Obi-Iyeke [35] further suggests that since Hg is one of the highly toxic heavy metals, its presence in food should be very minimal. The same results were obtained in one other study carried out on street-vended fruits and vegetables [29].

3.4 Arsenic

Arsenic (As) is a metalloid that is often taken up in the food chain from the use of contaminated water for drinking and irrigation purposes or through the burning of arsenic-containing coal for the purposes of cooking, which releases arsenic into the air and consequently coats and permeates food [39, 40]. This therefore discourages human consumption as chronic exposure to arsenic has adverse health effects such as skin pigmentation changes, various skin cancers and liver diseases [40]. Analysed As in raw materials used by street vendors in Oshodi Expressway Nigeria was reported to be within the range of 0.25–0.809 mg/kg [41]. Similar to these results were those of a study conducted on different types of roasted-vended meats which all had levels of As above the set limits by the WHO (0.1 mg/kg) and below those set by the South African Foodstuffs, Cosmetics and Disinfectants Act (1972) (1.0 mg/kg) for meat and processed meat [3].

3.5 Aluminium

Aluminium metal has a wide range of uses across the food industry as well as households. These uses include manufacturing of cooking utensils, food packages (cans, foil) and in some instances, food preparation surfaces. The use of such cooking utensils can result in the leaching of aluminium into the food prepared or stored [42]. Additionally, aluminium can be introduced into the food chain through the use of aluminium-containing food additives [43]. Although there have been controversial debates about the health implications of aluminium exposure/consumption, recent studies have demonstrated a weak but emerging link between aluminium exposure/consumption and events leading to Alzheimer’s disease [44, 45, 46] and kidney insufficiency [20]. The Joint Food and Agriculture Organization/World Health Organization Expert Committee on Food Additives (JECFA) [47] re-evaluated the safety of aluminium in food and concluded that the Provisional Tolerable Weekly Intake (PTWI) should be 1 mg/kg body weight as a result of its potential to affect both the reproductive and nervous system in experimental animals. In a study by Manan et al. [47] on heavy metals in street foods, the highest recorded concentrations of Al were 0.270 mg/kg which were below the permissible limits by JECFA [47]. Nuapia et al. [48] reported from their study on the presence of heavy metals in raw samples from open markets that Al levels obtained were within safe limits when compared to recommended limits by WHO/FAO [49]. Nevertheless, it is important to note that the prolonged consumption of street-vended food contaminated with heavy metals may lead to undesirable health effects.

3.6 Chromium

It is an essential trace element that enhances bodily functions and influences carbohydrate and fat metabolism. There are two types of chromium, namely trivalent (Cr (III)) and hexavalent chromium (Cr (IV)). Cr (IV) is widely used for industrial functions and is a reported carcinogen that can lead to lung cancer [50]. Cr (III) however is biologically active and found in food. It has functions such as enhancing the action of insulin which is essential to the metabolism and storage of carbohydrates, fat and protein in the human body [51]. The daily requirement for Cr is about 0.005 mg/day [51] and the permissible limit in food is 0.025–0.2 mg/kg [38]. Studies conducted on street-vended foods have shown the amount of Cr to be high in protein-rich foods such as meat. Bamuwamye et al. [3] reported levels of 1.36–2.35 mg/kg in street-roasted vended meat, and these levels were higher than the recommended permissible limits.

3.7 Copper

Copper (Cu) is an essential mineral in the human organism. About 2–5 mg of Cu is required daily for human consumption, and only 50% is absorbed in the gastrointestinal tract while the rest is excreted through bile and kidney [52]. The level of copper obtained in a study conducted on various roasted-vended meat was found to be within the range of 0.37–1.22 mg/kg [3]. The concentration of Cu generally in foodstuffs is ≤2 mg/kg [38]. Cu deficiency is said to exist when levels of Cu are <10 mg/kg in muscle, organs and tissues of animals [3]. Ogunkunle et al. [29] found that levels of Cu in all sampled fruits were below the maximum limits set by WHO/FAO. Ogunkunle et al. [29] further highlight that most studies show a trend that fruits generally have lower heavy metal contamination than vegetables. Mechanisms such as iron homeostasis, biosynthesis of neurotransmitters and peptide hormones are some of the biochemical processes facilitated by this redox-active metal [53]. However, insidious chronic diseases affecting the oral cavity and the pharynx such as oral submucous fibrosis can result from the increased dietary intake of copper [40, 54] This implies that although copper may have beneficial functions in the body, its intake must be regulated in order to avoid adverse health effects.

3.8 Zinc

Zinc (Zn) is one of the valuable multipurpose trace elements with varied functions in the human body. Some of its main functions include normal spermatogenesis and maturation, genomic integrity of the sperm, good epithelialisation in wound healing as well as the secretion of pancreas and gastric enzymes [52]. The average daily intake of Zn according to WHO/FAO [38] is 20 mg/day and the permissible value is 1.5 mg/kg. The levels of Zn observed in one study were substantially below the maximum values allowed [29]. Opeolu et al. [55] reported Zn levels in plantain in three different states and the concentrations ranged from 2.0 to 4.5 mg/kg. These high levels of contamination were attributed to low income, high population density and high traffic density [55].

3.9 Iron

Iron (Fe) is an essential element for nearly all living organisms due to its wide variety of metabolic processes, which include oxygen transport, deoxyribonucleic acid (DNA) synthesis as well as electron transport [56]. This mineral is obtained mainly through supplements and other food sources. An average person loses at least 1 mg through faeces alone, therefore, it is important that one consumes iron-rich foods in order to replace the one lost [57]. Iron deficiency is one of the predominant precursors of severe anaemia in children, women of childbearing age and pregnant women [3, 58]. The absorption of Fe in some organs such as the liver and kidney may be affected by toxic metals, thereby leading to deficiencies [1]. The average daily intake of Fe has been estimated to be 17 mg/day for males and 9–12 mg/day for females. Since street food provides daily out-of-home meals for most middle-working-class individuals, it is imperative that they are rich in iron in order to successfully cater for this need.

Street food vending in South Africa is among the fast-growing informal trades due to the high unemployment rate (27.6% since 2019). Although evidence on heavy metal contamination within this trade has not been explored, research from other African countries shows that these present a great public health concern [32, 48]. The present outcome of some laboratory investigations such as those of Ogunkunle et al. [29] show trends that fruits have the least heavy metals contamination when compared to vegetables. However, studies on heavy metals in street-vended meat, vegetables, dairy products and beverages mostly indicate contamination levels that are above permissible limits [1, 3].

4. Conclusions

With street food vending being the main source of income for many households as well as a means of affordable and readily available meals for a diverse group of consumers, it is imperative that food safety is ensured through various stakeholders. Based on the existing literature, contamination in the food chain by heavy metals is through both natural and anthropogenic sources. These heavy metals are introduced into the food chain mostly through processes of runoffs and leaching. Moreover, because of their bioavailabilty, bioaccessibility and excessive concentrations, several human impacts have been observed. Although existing research on heavy metal contamination in street-vended foods in African countries is fragmented, this review successfully showed that heavy metals do affect the food chain. Most of the reviewed studies show contamination levels of some carcinogens such as lead to be too high. Studies on heavy metal contamination were predominantly done in most parts of Nigeria than in other Sub-Saharan countries, with South Africa being among those with the least relevant literature. However, taking into consideration the similarities in the food types and methods of preparation mentioned in these studies, it is hypothesised that similar results may be obtained within the South African street food markets and therefore calls for investigation. The presence of heavy metals in food is not entirely avoidable; however, measures can be put in place to ensure that the health of habitual consumers is protected.

Acknowledgments

We would like to thank and acknowledge the Central University of Technology for providing access to resources needed for conduction this review and the Research Innovation Fund by the Central University of Technology for funding the study.

Conflict of interest

The authors declare no conflict of interest.