Evaluation of Iodine Content of Some Commonly Consumed Foods in Zaria Metropolis, Nigeria
Aim and Objectives
The aim of this research is to study the iodine contents of some commonly consumed foods in Zaria metropolis.
The aim of the study was achieved through the following set objectives:
- Determining the iodine contents in foods consumed by the people of different economic status (income) in Zaria metropolis.
- Determining the concentrations of iodine in agricultural soil, crops, vegetables and foods.
- Comparing the iodine contents in local diets with the Required Dietary Intake (RUI). Identifying the foods that are rich in iodine.
- Compare the results obtained with Recommended Daily Allowance (RDA) and Upper Tolerable Intake Level (UTI).
- LITERATURE REVIEW
Iodine is an essential element of human nutrition (Moehlmann, 2005). Even though it is very important for mammalian well-being, it is not an essential element of plant nutrition (Welch and Graham, 1999).
The concentration of iodine in most natural foods is quite low and coupled with its physiochemical property, it is quite difficult to analyse accurately (Dasgupta and Shelor, 2011). Numerous analytical methods have been applied to determine iodine mainly in the form of iodide, such as Spectrophotometry (Van Stadenet al., 2004; Gamallo-lorenzoet al., 2005), Electro-potentiometry (Masadomeet al., 2000; El-Hallag, 2010), Atomic Absorption Spectroscopy (Adams et al., 1974; Bermejo-Barreraet al., 1999; Yebra and Cespon, 2000) and Chromatography (Rendlet al., 1994; Below and Kahlert 2001; Hu et al., 2009).
However, only a few methods are currently used routinely for analysis. These methods are often at the end of sophistication, requiring expensive instrumentation with qualified personnel (inductively coupled plasma-mass spectrometry, instrumental neutron activation analysis) or by kinetic colorimetric method requiring oxidative sample digestion (Dasgupta and Shelor, 2011).The kinetic method has many merits, for example, high sensitivity, good selectivity and simple experimental procedures as well as good reproducibility and low budget apparatus. The kinetic method has been widely applied for determination of iodide, (Van Stadenet al., 2004; Gamallo-lorenzoet al., 2005). Most of the kinetic methods are based on the catalytic effect of iodide on oxidation (Gamallo-lorenzoet al., 2005).
The Sandell-Kolthoff reaction is based on the Ce4+ -As 3+ iodine catalyses redox reaction, with rate constant that is stable over the first 3 minutes. The iodide-catalysed reaction is mainly free from significant interference. Ru(IV) and Os(VIII) can catalyse the reduction of Ce(IV), while Ag(I) and Hg(II) ions significantly inhibit the reaction at low concentrations (Sandell and Kolthoff, 1937; Rodriguez and Pardue, 1969). Other more common ions, such as PO 3−, SO 2−, ClO−, Cl−, Br− and compounds such as Fe O , Al O , MgO, Cu O, Zn O have no effect on the assay although any species that absorbs in the region of 350 nm will interfere (Rodriguez and Pardue, 1969).
Travniceket al., (2006) used Sandell-Kolthoff method after alkaline ashing to analyse milk samples collected at dairy farms, from 14 areas of South-western Bohemia. Their result shows that the average iodine content in milk samples was 442.5 ± 185.6 μg/ L, with minimum and maximum value of 68.6 and 1000.6 μg/L respectively. About 82% of the samples contained more than 250 μg/ L. The average iodine content in milk from the collecting areas ranged from 230.2 ± 133.0 μg/ L to 702.7 ± 166.2 μg/ L, with about 36% of collecting areas being higher than 500 μg/L. Higher mean values were recorded in the period of winter feed rations (April 495.9 ± 50.8 and October 494.3 ± 176.4 μg/ L), while lower values were measured during the period of summer feed rations (September 350.9 ± 178.4 μg/ L). If converted per dry matter, 1 kg of dry milk matter contained, on average, 3.428 ±1.497 mg iodine, maximum and minimum values were 0.543 and 7.995 mg respectively (Travniceket al., 2006).
Gamallo-Lorenzoetal., (2005), employed microwave assisted alkaline digestion with tetramethylammonium hydroxide (TMAH) to digest seaweed samples. The determination of iodide was performed directly in the alkaline digest by Sandell–Kolthoff reaction. The linear calibration range was 0.30–20.0 μg/ kg and the limit of detection was 9.2 μg/ g while the relative standard deviation for 11 determinations was 2.6% for 196.3 mg/kg of iodide.
Xiashi and Yiaqin(2008), satisfactorily applied inhibition kinetic method, which was based on iodine inhibition effects on the reaction between rhodamine and KBrO3 in H2SO4 medium at 45±0.5 OC for 15 min to determine iodide concentration in kelp. The linear range was 2.0–6.0 ng/ L and the detection limit was 0.06 ng/ L.
Van Stadenet al., (2004), proposed a kinetic method for the determination of iodide based on its catalytic effect on the 4, 4- methylenebis (N,N-dimethylaniline)-chloramine-T reaction in acidic medium. This method involves sequential aspiration of tetra base and then chloramine- T solutions into a carrier stream to be stacked inside a holding coil and flow reversed through a reaction coil towards a detector. The resulting coloured compound is measured at 600 nm using an UV/Vis-spectrophotometer, with a linear calibration curve over a range of 0.1– 6.0 μg/ L of iodide concentration with detection limit of 0.05 μg/ L (Van Stadenet al., 2004).
A catalytic kinetic method for the determination of trace amount of iodine was developed by Liu and Zhang, (1998). The method is based on the inhibitory action of iodide ion on the reduction of toluidine blue with N2H4·2HCl in hydrochloric acid medium. It has high sensitivity and selectivity for determination of trace iodine. The linear range of determination in the method was 0-8 μg/ L with a detection limit of 0.41µg/ L.
Instrumental Neutron Activation Analysis, INAA, has been used extensibly for the determination of iodine. However, due to Compton interference from 28Al, 24Na, 56Mn, 38Cl, and 82Br, Epithermal NAA along with Radiochemical NAA are often employed for lower detection (Dasgupta and Shelor, 2011).
- MATERIALS AND METHODS
- Description of the study area
Zaria is the capital city of the ancient Zazzau Kingdom as well as the headquarters of Zaria Local Government Area. However, the name Zaria now applies to a large metropolitan area that includes the ancient city as well as neighboring towns/ villages such as Sabongari, Samaru, Shika and Soba. Zaria metropolis has a population of 1.5 million, situated in Kaduna State, Nigeria, and located on longitude N11 and latitude E7 with an approximate land mass of 300 sq. Km. Zaria is about 950 Km from the coast on an altitude of 653.6 Km. (Ahmadu Bello University website, 2013).
A significant percentage of the population comes from all over Nigeria due to the presence of many Federal Institutions. As such, Zaria can be divided into four broad regions, the ancient city and Tudunwada which hosts mostly the indigenous population, the TudunJukun- Sabongari Area which hosts quite a diverse set of people from all over Nigeria, the Government Residential Area (GRA), which accommodates both the natives and non-natives, mostly of higher income earners and the Samaru Area which have both the indigenous population from villages around as well as a significant population that are mostly students and civil servants from the surrounding Federal Institutions (Abbas and Arigbede, 2011).
The people of the ancient city of Zaria are mostly farmers, traders and skilled artisan, with some civil servants. The people of Sabongari are mostly traders, with a significant portion that were/ are staff of agro-allied industries and the railway, with some more civil servants. However, the population of Samaru is predominantly civil servants, with some of the indigenous population being farmers and artisans (Abbas and Arigbede, 2011).
- Recovery Experiments and Calibration
Recovery experiments for both Pre-concentration Neutron Activation Analysis (PCNAA) and Sandell- Kolthof Spectroscopy (S-K spectroscopy) were carried out using standard reference materials (SRM). Table 4.1 compares the obtained iodine concentrations of the SRMs analysed in this work by PCNAA with the certificate values. NIST 1548a (Typical diet) is the only SRM that have certified iodine concentration. Table 4.2 present the comparison of iodine concentration of SRMs obtained in this work by S-K spectroscopy with certificate values. Table 4.3 however, compares the obtained sodium, potassium and bromine concentrations of the SRMs analysed in this work by INAA with the certificate values. The SRM analysed in Table 4.3 are NIST 1573a (Tomato leaves), NIST 1570a (Trace element in spinach leaves), NIST 1515 (Apple leaves), IAEA 359 (Cabbage), and IAEA SL 3 (Lake sediment). Bromine was not reported in NIST 1570a and IAEA 359, and it was not certified in all the reported SRMs except IAEA SL 3.
- Quality Assurance
The employment of multiple standard reference materials brought confidence to the analysis as well as providing multiple base for validation and comparison of the results obtained. Tables 4.1 and 4.2 present the obtained results from the PCNAA and spectroscopic analysis of the SRM in this work, and comparing it with certified values. The analyses of the SRMs were done in batches along with the samples. Percentage deviation (% Dev.) and student’s t- test (t test) of the SRM verify that the results obtained from this work were significantly the same as the values reported in the certified reference materials. However, comparing the two methods employed for the food analysis, it shows that PCNAA records have better accuracy than the spectroscopic technique. NIST 1548a (Typical diet) reports a student’s t-test of 1 across the two sample categories in PCNAA as against 0.72 and 0.92 for duplicate and total diet sample respectively from the spectroscopy. PCNAA also records lower percentage standard deviation (Table 4.1, % Dev.) as against S-K spectroscopy (Table 4.2, %Dev.). Duplicate diet SRMs from Table 4.2 were digested with alkaline ashing while the typical diet SRMs were processed with microwave assisted acid digestion. The obtained results suggest that microwave assisted acid digestion is more reliable in providing accurate result as compared to alkaline ashing because it show less deviation from the certified value. Microwave assisted acid digestion in this study records lower percentage deviation than alkaline ashing for the same SRMs, NIST 1548a (typical diet), NIST 1573a (tomato leaves) and NIST 1515 (apple leaves) in the order of 6.67%, 15.25% and 51.67 % respectively. The variation could be attributed to the infinitesimal concentration of iodine, and that the iodine could have been depleted by adsorption on the walls of the crucible when scorched owning to high temperature, while the PTFE material for the microwave vessel is non-scorching and non-absorbing. Generally, the SRM analysis results compare favorably to the certified values. Only NIST 1515 (Table 4.2) have percentage deviation greater than 10.
Several NIST reference materials of agricultural and food materials, with one of geological material were analysed by INAA along with samples for quality control purposes. The SRM NIST 1573a (Tomato leaves) is the only SRM that recorded deviation larger than 5%. Table 4.3 shows that the results obtained are in agreement with the uncertainty margins with NIST certified values whenever available, thus proving the accuracy of the results obtained in this work.
CONCLUSION AND RECOMMENDATIONS
The evaluation of iodine content of commonly consumed foods in Zaria metropolis was successfully carried out. Pre-concentration Neutron Activation Analysis and classic Sandell-Kolthoff Spectroscopy were both employed in the determination of iodine in micro concentration. While the spectroscopy is cheap with little labour, the accuracy of its result is not as excellent as that of the neutron activation analysis.
Iodine concentration of foods analyzed were found to range from 0.16 ± 0.02 to 2.04± 0.11 mg/kg for breakfast, 0.50 ± 0.03 to 1.56 ± 0.02mg/kg for lunch and
0.14 ± 0.04 to 1.99 ± 0.09mg/kg for dinner. The daily consumption of iodine was found to be from 187.36 to 1165.29 mg/day, with an average consumption of
531.0 mg/day. The foods analyzed supplied the recommended daily allowance for iodine and in some cases approaching the upper tolerable limit.
The iodine concentrations in soils and potable water were very low in the analyzed samples and could not be detected by the methods applied. However, the iodine concentration in cultivated crops were equally low but were successfully determined, with maize containing the least iodine at 0.22±0.08, rice 0.29± 0.05, wheat 0.31± 0.08, beans 0.32 ± 0.07 and spinach 1.27± 0.10 mg/kg respectively.
Table salts sold within the study area were found to be sufficiently iodized with concentration ranging from 14.87 ± 0.51to 74.78 ± 0.59 mg/kg with an average concentration of 45.12 mg/kg from home samples and 41.12 ± 0.51 to 93.31 ± 0.59 mg/kg with an average concentration of 61.01 mg/kg for retail samples respectively. Iodine concentration of foods was found to have positive correlation to iodine concentration of the table salts.
Iodine consumption by different income group is not significantly different since the major avenue for iodination of foods is iodized salt and is accessible to all at low cost.
The spectroscopic technique shows relatively poor correlation to PCNAA when dry ashing digestion method was employed (0.53). However, the correlation improved significantly to 0.89 when microwave assisted acid digestion was employed.
- There is a need for the National Agency for Food Drug Administration and Control (NAFDAC) and Standard Organization of Nigeria (SON) to reconsider the salt intake of the population and the iodine concentration in
- The university should carry out more community development projects and enlightenment campaigns so that the community can better understand and responds to needs of the university staff and students.
- The university library should endeavor to update its serial section for easy access to relevant information in any field of research.
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