Bioaccessibility of trace elements and chromium speciation in commonly consumed cereals and pulses

ABSTRACT

Copper, manganese, and chromium content and their bioaccessibility were determined in commonly consumed cereals and pulses. Copper, manganese, and chromium concentration of these grains ranged from 0.19 to 1.22, 0.46 to 8.12, and 0.02 to 0.11 mg/100 g, respectively. Bioaccessibility of these minerals from the grains ranged from 6.6 to 32.5% (copper), 15.5 to 43.5% (manganese), and 2.66 to 36.4% (chromium). In terms of bioaccessibility of these minerals, pulses provided more than cereals. Speciation analysis of chromium by selective alkaline method demonstrated the presence of the toxic hexavalent chromium in lower concentration than trivalent form.

KEYWORDS:

• Copper
• Manganese
• Chromium
• Bioaccessibility
• Speciation

Introduction

Trace elements are inorganic substances, present in all body tissues and fluids, and their presence is necessary for the maintenance of several physicochemical processes that are essential for life.^[1] Copper (Cu), manganese (Mn), and chromium (Cr) possesses vital biological functions, and thus, plays the fundamental role in human metabolism. Cu helps to form hemoglobin in the blood, facilitates the absorption and use of iron so that the red blood cells can transport oxygen to the tissues, and assists in regulating the blood pressure and heart rate. Mn is essential for the proper formation and maintenance of bone, cartilage, and connective tissue. It contributes to the synthesis of proteins and the genetic material and helps to produce energy for the body from foods.^[2] However, Cr is involved in lipid and carbohydrate metabolism, and the most frequent manifestation of Cr deficiency is altered glucose tolerance.^[3] This nutrient is also associated with cardiovascular disease and gene expression.^[4]

Mineral deficiency is usually caused by a low mineral content in the diet and/or poor availability of minerals from the diet. In India, and most of the developing countries, cereals and pulses are the major dietary sources of these trace elements. However, the total levels of such elements in food grains can be misleading unless bioavailability is also taken into account, because natural mineral levels are sometimes amplified by environmental contamination that does not wash out, especially in plants.^[5] Thus, the present study was undertaken with the objective of determining the Cu, Mn, and Cr content and their bioaccessibility from various cereals and pulses.

Oxidation state is the most important factor determining the toxicity of Cr and its essentiality toward living organisms.^[6] It is considered to be a bioelement in its trivalent form (Cr³⁺), and hexavalent form (Cr⁶⁺) is known to have genotoxic and carcinogenic properties.^[7] Cr is usually present in food in its trivalent form (Cr³⁺), and its bioavailability depends on the chemical and physical properties of Cr compounds and complexes.^[8] Determination of total Cr concentration provides data which is important for estimating its contribution to daily intake, but to evaluate the biological effects of Cr on humans or its potential toxicity, speciation analysis is mandatory.^[6] Thus, in the present investigation, speciation of Cr from food grains was also carried out, since there is a notable scarcity of information on this.

Materials and methods

Materials and reagents

Cereals—rice (Oryzasativa), finger millet (Eleusinecoracana), sorghum (Sorghum vulgare), wheat (Triticumaestivum), and maize (Zea mays), and pulses—chickpea (Cicer arietinum)—whole and decorticated, green gram (Phaseolus aureus)—whole and decorticated, decorticated black gram (Phaseolus mungo), decorticated red gram (Cajanuscajan), cowpea (Vignacatjang), and French bean (Phaseolus vulgaris) were procured from the National Seeds Corporation, Mysore, Karnataka, cleaned and used for the study. Cr (III) nitrate, pepsin, pancreatin, and bile extract of porcine origin were procured from Sigma-Aldrich Chemicals Co. (St. Louis, MO, USA). All solutions were prepared with MilliQ water, and the chemicals used, HCl, HNO₃, NaOH, and NH₄NO₃ in the study were of Suprapure grade (Merck).

For Cr speciation, total Cr standards were made from a 1000 mg/L solution of Cr (III) nitrate (Sigma-Aldrich sol) in HNO₃ (1% v/v). An aqueous stock solution of hexavalent Cr at 1000 µg/mL was prepared by dissolving 0.2829 g of potassium dichromate (Riedel-de-Haen, Germany) in 100 mL of MilliQ water. The diluted standard solution was prepared from these stock solutions. MilliQ water was employed during the entire study.

Instrumentation

The total and bioaccessible Cu and Mncontentin foods were determined by using flame atomic absorption spectrometry (FAAS; Shimadzu AAF-6701). However, inductively coupled plasma-atomic emission spectrometry (Horiba JobinYvon JY-2000) was used for the analysis of total and bioaccessible Cr.

Contamination avoidance

Glassware used throughout the study was dipped in 20% (v/v) nitric acid in water for 24 h and then rinsed thoroughly with MilliQ water. The food grains were cleaned and washed with MilliQ water, to avoid any soil contamination, prior use.

Total Cu, Mn, and Cr content

Grain samples were finely ground into particles that pass through 280-µm size wire mesh. Approximately 1.0 g of dried powder of powdered grain samples was subjected to acid digestion with a mixture of 65 % of HNO₃ (Sp.Gr.1.42; 5 mL) and 30% H₂O₂ (2 mL) at room temperature overnight. This process was followed by refluxing with gradual heating on a hot plate. Digested solutions were diluted to 50.0 mL using MilliQ water. Cu and Mn contents were determined by FAAS. However, Cr content was determined by inductive coupled plasma-atomic emission spectroscopy (ICP-AES). Calibration of measurements was performed using commercial standards.

Bioaccessibility of Cu, Mn, and Cr

Bioaccessibility of Cu, Mn, and Cr from the selected food grains was determined by an in vitro method described by Luten et al.,^[9] involving simulated gastrointestinal digestion with suitable modifications.^[10] The ground samples were subjected to simulated gastric digestion. For this, 10 g of samples were added to 60 mL of MilliQ water and pH was adjusted to 2.0 with 6 mol/L of HCl and pepsin was added and the weight of the samples was brought up to 100 mL. The mixture was then shaken in the water bath at temp 37ºC for 2 h at 110 rpm. Gastric digests were stored at 0 ºC until further analysis. Titratable acidity was measured in an aliquot (20 ml) of gastric digest by adjusting the pH 7.5 with 0.2 M sodium hydroxide (NaOH) in the presence of pancreatin – bile extract mixture. Titratable acidity was defined as the amount of 0.2 M NaOH required attaining a pH of 7.5.

To simulate intestinal digestion, segments of dialysis tubing (molecular mass cut off: 10 kDa) containing 25 mL sodium bicarbonate solution, being equimolar in moles of sodium hydroxide needed to neutralize the gastric digest (titratable acidity) determined as above were placed in Erlenmeyer flasks containing the gastric digest and incubated at 37ºC with shaking for 30 min or longer until the pH of the digest reached 5.0. Pancreatin–bile extract mixture (5 mL) was added, and incubation was continued for 2 h or longer until the pH of the digest reached 7.0. At the end of the incubation period, the dialysis tubing sections were removed and rinsed with water carefully, and the contents of the tubing (dialysate) were weighed. The minerals present in the dialysates (representing the bioaccessible fraction) were analyzed by FAAS and ICP-AES.

Bioaccessibility (%) was calculated as follows:

where Y is the element content of the bioaccessible fraction (mg mineral element/100 g of grain), and Z is the total Cu, Mn, or Cr content (mg mineral element/100 g of grain.). Bioaccessibility results are expressed as mg/100 g of native grain.

Extraction of hexavalent Cr

For the selective extraction of hexavalent Cr, a procedure described by Soares et al.^[7] was adapted to analyze hexavalent Cr in cereals and pulses. Approximately 1.0 g of dried grain powder was accurately weighed and placed into a 10 mL polypropylene tube; then, 9 mL of 0.01 M NaOH solution was added, the cap was fitted, and the tubes were shaken horizontally in an oscillating agitator for 17 h at 300 oscillations per minute, at room temperature, to selectively extract the hexavalent Cr. After this period, 1 mL of 1M NH₄NO₃ solution was added, and the sample was shaken briefly and then centrifuged for 30 min at 12,500 rpm. Hexavalent Cr present in the supernatant was measured by ICP-AES. Alkaline hexavalent Cr standard solutions and blank reagents were subjected to the same pretreatment procedure as for the flour samples.

pH determination of food grains

The pH of the food grains was evaluated to discard any possible effect in the extraction procedure of the hexavalent Cr.^[7] Aliquots of 10 g of dried grain powder of cereals and pulses were taken, suspended in 100 mL of MilliQ water, and stirred; the suspension was allowed to stand for 30 min. After sedimentation and centrifugation for 30 min at 12,500 rpm, the pH was measured in the supernatant.^[11]

Statistical analysis

All determinations were carried out in five replicates, and the average values were reported. Data analysis was conducted using Graph Pad Prism (Version 5). Initial descriptive statistics includes mean, and standard error mean (SEM). Pearson correlation analysis was carried out between the total mineral content and bioaccessible mineral of cereals and pulses. p-values were two-tailed and one significant level was used, p ≤ 0.05.

Results and discussion

Various factors lead to changes in food composition, including introduction of new food varieties and the use of soil fertilizers and agrochemicals in crop production. For this reason, and also due to developments in analytical techniques involving more accurate and precise methods of analysis, it is necessary to periodically re-evaluate food composition.^[12] Cereals and pulses are the main sources of trace elements, especially in developing countries. As the nutritional adequacy of trace elements depends on their amount and bioavailability in the foods, it is important to determine how much of these minerals are bioavailable. Therefore, the objectives of the present study were to analyze the total Cu, Mn, and Cr content as well as bioaccessibility of these minerals in cereals and pulses.

Cu content and bioaccessibility from cereals and pulses

Cu content and its bioaccessibility from the cereals and pulses studied are summarized in Tables 1 and 2. The inherent Cu content in analyzed cereals ranged from 0.19 to 0.45 mg/100 g, while that in pulses ranged from 0.36 to 1.22 mg/100 g. Thus, the inherent Cu content in cereals is nearly half of that in pulses, except in the case of cowpea (0.36 mg/100 g). All the pulses examined here, with a higher amount of Cu compared to cereals, are a better source of this micronutrient. Foods with higher levels of Cu are those high in proteins, such as beans, justifying our findings.^[12] Also, Sandberg,^[13] reported that the total content of iron and other minerals are generally high in legumes. The mean Cu concentration in wheat, rice, and maize were found to be 0.41, 0.187, and 0.25 mg/100 g, respectively. A previous study by Roychowdhury et al.^[14] reported similar mean Cu concentration in wheat of 0.378 mg/100 g cultivated in the western part of India. In agreement with the findings of the present study, the other study,^[15] reported similar total Cu content, in rice (0.17 mg/100 g) and maize (0.23 mg/100 g). The data for Cu content in wheat (0.37 mg/100 g) are in agreement with the data published by Srikumar.^[16] However, the author had reported lower Cu content in black gram (0.27 mg/100 g) and green gram (0.67mg/100 g). Contrary to the present study, 0.35 mg/100 g of inherent Cu in chickpea was reported,^[17] lower than value reported by us. Tripathi et al.^[18] reported the mean concentration of Cu (mg/100 g) in cereals: 0.222, pulses: 0.675, respectively, which is lower than values reported by us. However, Patra et al.^[19] reported 0.35 and 1.0 mg/100 g total Cu content in cereals and pulses, which is comparable to our study. The observed variation in metal concentrations for analyzed foodstuffs might be due to variable capabilities of absorption and accumulation of metals by the crops.^[20] Cu content of food grains observed in this study is comparable to the values reported by Gopalan et al.^[21] where the Cu content of cereals ranged from 0.24 to 0.68 mg/100 g and that of pulses, ranged from 0.39 to 1.45 mg/100 g, respectively. Similar values for the total Cu content, 0.18 mg/100 g in milled rice (variety—sonamasuri) used in our study, was reported by a previous study.^[22]

Table 1. Total content and bioaccessibility of Cu, Mn, and Cr from cereals.

Cereals	Cu content (mg/100 g)	Bioaccessible Cu (%)	Mn content (mg/100 g)	Bioaccessible Mn (%)	Cr content (mg/100 g)	Bioaccessible Cr (%)
Wheat	0.41 ± 0.004	20.5 ± 0.74	2.23 ± 0.07	36.0 ± 3.0	0.056 ± 0.004	5.49 ± 0.35
Rice	0.19 ± 0.01	32.5 ± 2.82	0.62 ± 0.02	28.7 ± 2.3	0.06 ± 0.005	4.67 ± 0.3
Maize	0.25 ± 0.03	8.92 ± 0.89	0.46 ± 0.02	20.2 ± 1.6	0.08 ± 0.007	2.66 ± 0.2
Sorghum	0.38 ± 0.04	10.8 ± 1.06	1.10 ± 0.03	15.5 ± 1. 4	0.02 ± 0.002	15.2 ± 1.02
Finger Millet	0.45 ± 0.023	6.6 ± 0.66	8.12 ± 0.29	31.2 ± 1.7	0.07 ± 0.004	4.23 ± 0.22

Values are mean ± SEM of five replicates.

Table 2. Total content and bioaccessibility of Cu, Mn, and Cr from pulses.

Pulses	Cu content (mg/100 g)	Bioaccessible Cu (%)	Mn content (mg/100 g)	Bioaccessible Mn (%)	Cr content (mg/100 g)	Bioaccessible Cr (%)
Red gram decorticated	0.94 ± 0.02	14.6 ±1.2	0.97 ± 0.035	42.8 ± 1.3	0.045 ± 0.003	11.2 ± 1.2
Chickpea whole	0.76 ± 0.04	19.5 ± 1.9	1.78 ± 0.04	33.5 ± 1.60	0.055 ± 0.004	8 ± 0.7
Chickpea decorticated	1.22 ± 0.05	18.4 ± 0.8	1.19 ± 0.03	39.3 ± 2.6	0.071 ± 0.002	3.7 ± 0.2
Green gram whole	0.91 ± 0.005	13.8 ± 1.3	0.73 ± 0.02	31 ± 1.6	0.035 ± 0.003	9.6 ± 0.6
Green gram decorticated	1.13 ± 0.01	14.7 ± 0.6	0.58 ± 0.02	27.7 ± 1.40	0.065 ± 0.002	15.6 ± 1.4
Black gram	0.62 ± 0.004	14.3 ± 0.50	1.04 ± 0.00	36.2 ± 2.9	0.108 ± 0.002	2.8 ± 0.1
French bean	0.82 ± 0.015	27.4 ± 1.9	0.91 ± 0.03	35.1 ± 1.5	0.046 ± 0.003	36.4 ± 2.9
Cowpea	0.36 ± 0.01	21.2 ± 0.45	1.01 ± 0.035	43.5 ± 3.5	0.023 ± 0.002	16.6 ± 1.2

Values are mean ± SEM of five replicates.

Bioaccessibility of Cu varied significantly among the cereals examined, ranging from 6.6 to 32.5% and that of pulses ranged from 13.8 to 27.4% of the element present (Tables 1 and 2). Thus, the bioaccessible Cu in pulses is comparatively higher than that from cereals, except in the case of rice. Rice, with lowest concentration of total Cu, showed highest bioaccessibility of Cu, while finger millet with the highest content of Cu showed lowest bioaccessibility of Cu, which indicates that Cu bioaccessibility is not necessarily dependent on its concentration in the food grains. Hunt and Vanderpool,^[20] determined apparent Cu absorption from controlled lactoovovegetarian and non-vegetarian diets, and found that women absorbed 33.0% of Cu from a vegetarian diet, which is close to the percentage of dialyzable Cu found in food grains in our study. The most important factors that inhibit Cu absorption are sugars, animal proteins, S-amino acids, and histidine, rather than dietary fibers c.f.^[23] A previously published study reported 46.7 to 69.1% of bioaccessible Cu in 10 different types of biscuits, containing wheat flour and observed that lowest Cu solubility was found in the sample with the highest polyphenolic content, while fibers have negligible effect on Cu bioaccessibility.^[23] The lower bioaccessibility of Cu from finger millet observed in our study could be attributed to the higher polyphenols content of this millet. The reduced apparent Cu absorption from vegetarian diets may be due to dietary inhibitors of Cu absorption that reduced the Cu bioavailability of the diet.^[20]

Mn content and bioaccessibility from cereals and pulses

Mn levels in the analyzed cereals and pulses are summarized in Tables 1 and 2. The Mn concentration of cereals ranged from 0.46 to 8.12 mg/100 g while in pulses it ranged from 0.58 to 1.78 mg/100 g. The total Mn content of both cereals and pulses is somewhat similar except in the case of wheat and finger millet.

The mean values for Mn concentration in wheat and rice were 2.23 and 0.62 mg/100 g. These results are in line with those of Roychowdhury et al.^[14] who reported, Mn concentration in rice (2.54 mg/100 g) and wheat (0.599 mg/100 g), respectively. Mn content of cereals observed in this study is comparable to the values reported by Gopalan et al.^[21] ranging between 0.48 and 5.49 mg/100 g while that in pulses is between 0.69 and 2.47 mg/100 g, respectively. A different study,^[24] have reported, total Mn content in wheat, maize, and sorghum to be 2.97, 0.34, and 2.31 mg/100 g, which is comparable to our finding. Mn content of rice (0.62 mg/100 g) reported by Dash et al.^[22] was also similar to our study. However, other studies^[25] have reported 1.70 mg/100 g of Mn content in green gram, higher than the value reported by us. The data for Mn content in black gram (1.3 mg/100 g) are in agreement with the data published by Srikumar.^[16] However, author had reported higher Mn content in wheat (3.7 mg/100 g) and green gram (1.2 mg/100 g).

Mn bioaccessibility from the cereals and pulses is presented in Tables 1 and 2. Bioaccessibility of Mn from the five cereals examined ranged from 15.5 to 36%, and in the case of pulses ranged from 27.7 to 43.5%. In agreement with the findings of the present study, Kulkarni et al.^[26] have found the bioaccessibility of Mn in wheat to be 37.7%. Bioaccessible Mn was reported to be 22% from meals consumed in Spain.^[27] A different study have reported, the bioaccessible Mn in 10 different types of biscuits, containing wheat flour enriched with various dietary fibers, ranged from 19.1 to 60%, and also found that phytic acid, and especially polyphenols, limits the solubility of Mn, resulting in significant decrease of Mn bioaccessibility, while the effect of proteins was negligible.^[23] These values are somewhat similar to the mean percentages of bioaccessible Mn found in food grains observed in this investigation. Like in the case of Cu, the bioaccessibility of Mn from pulses was also higher than cereals. In general, the bioaccessibility of Mn from all the food grains studied was higher than that of Cu, this difference being more prominent in the case of pulses.

Cr content and bioaccessibility from cereals and pulses

Cr content and its bioaccessibility from the food grains studied are presented in Tables 1 and 2. The inherent Cr content in cereals was from 0.02 to 0.08 mg/100 g, and in pulses it ranged from 0.023 to 0.108 mg/100 g, respectively. The Cr concentrations found in cereals and pulses was thus similar. In our study, the mean Cr concentration in rice, wheat and maize was 0.06, 0.056, and 0.08 mg/100 g. Mean Cr concentration in Bangladeshi rice, wheat and maize was reported to be 0.18, 0.13, and 0.19 mg/100 g,^[15] which is higher than our values. However, Fu et al.^[28] reported mean Cr concentration in Chinese rice to be 0.02 mg/100 g. In the present study, the highest Cr concentration among all cereals and pulses was found in black gram (0.11 mg/100 g). Cr content of cereals observed in this study is comparable to the values reported by other study^[21] that ranged from 0.004 to 0.028 mg/100 g; while in case of pulses it is from 0.001 to 0.029 mg/100 g. Dash et al.^[22] reported 0.048 mg/100 g of inherent Cr in rice, which is lower than value reported by us. These differences among the total Cr content in cereals and pulses in different studies could be due to variations in the soil Cr content, as well as the method of determination of Cr. A different study^[29] reported a statistically significant correlation between Cr and the protein/carbohydrate content of the diet and energy intake, a finding that was not observed by us. The Cr content in chickpea reported by Cabrera et al.^[17] was 0.012 mg/100 g, lower than the value reported by the current study.

Bioaccessibility of Cr varied significantly among the cereals examined, ranging from 2.6 to 15.2%, and that of pulses ranged from 2.8 to 36.4% of the total element present in them (Tables 1 and 2). Thus, the bioaccessible Cr in pulses is comparatively higher than that from cereals. Sorghum, have the lowest concentration of total Cr, with the highest bioaccessibility of this mineral, while maize with the highest content of Cr have the lowest bioaccessibility, which indicates that the Cr bioaccessibility is not necessarily dependent on its concentration in the food grain. These findings are in agreement with those of Mateos et al.^[30] who concluded that the Cr dialysability was independent of the total amount of this mineral in the breakfast cereals. A study,^[31] have reported that Cr absorption was higher for low levels of daily dietary intake (>40 µg) than for levels of 40–80 µg; for high levels (>80 µg) there was an increase in the dialyzable fraction. Anderson^[4] has reported that iron deficiency stimulates the net absorption and bioavailability of Cr, which could be due to competition between both minerals at the same intestine absorption sites. Cr present in meals interacts with other nutrients that influence its net absorption, specifically; amino acids, starch, and ascorbic acid c.f.^[32] Significant correlations was reported by Velasco et al.^[32] between the dialyzable Cr fraction and levels of other nutrients, such as proteins, iron, sodium, iodine, fluorine, ascorbic acid, fiber, and vitamin A. Mateos et al.^[30] reported the percentage absorbable Cr fraction in the breakfast cereals ranged from 0.48 to 3.26%. A different study^[29] reported the bioaccessible Cr from the diet in between 0.40 to 1.60%. These low bioaccessible values for Cr are in agreement with our data, with few exceptions justifying that only a small percentage of Cr contained in food and drink is absorbed, most of it being excreted in urine.^[33]

Our results thus suggested that the bioaccessibility of Cu, Mn, and Cr was independent of their total content. To prove this observation, Pearson correlation analysis was carried out between the total mineral content and their bioaccessibility from cereals and pulses. In the case of cereals, the total Cu content was strongly positive correlated with bioaccessible Cu only in case of sorghum (r = 0.659), in remaining other cereals there was no visible correlation, whereas in the case of pulses, strong positive correlation was found in chickpea decorticated (r = 0.615) and strong negative correlation was found in French bean (r = –0.657) and in others no visible correlation was found. The correlation between total Mn content and bioaccessible Mn was found to be negative in all the cereals, except sorghum. However, in case of pulses, a very strong positive correlation was found only in green gram decorticated (r = 0.853) while, in all other pulses, no visible correlation was found. In the case of Cr, a negligible correlation was found in the cereals, whereas in the pulses very strong negative correlation was found only for red gram decorticated (r = –0.964). These observations are supported by several researchers who have noted that the efficiency of Mn absorption declines with the enhancement of the total intake of Mn c.f.^[31] However, Cabrera-Vique and Bouzas,^[31] have ascribed no correlation between dialyzable Mn fraction and Mn levels but found a positive correlation between Cr dialysate fraction and Cr levels.

There is a wide variation in the concentration of these trace elements among all the cereals and pulses studied in this investigation. It was observed that the concentration of Mn was comparatively more in all the cereals and pulses followed by Cu and Cr. The observed variation in metal concentrations in different foods could be due to variation in absorption and accumulation capabilities.^[34] The results of this investigation suggest that the bioaccessibility of Cu, Mn, and Cr is subject to limitations and is probably influenced by dietary factors such as inhibitors and enhancers of mineral bioavailability. Because of the paucity of data on the bioaccessibility of these minerals in pulses, no comparisons with the results obtained by other researchers can be drawn.

Speciation of Cr present in cereals and pulses

The need to determine different species of trace elements in environmental and biological materials is important because the effects and toxicity of an element depend to a great extent on its chemical form and concentration.^[35] The alkaline extraction method adopted for the speciation of Cr that is, the selective separation of Cr³⁺ was applied to the cereals and pulses under study.

Hexavalent Cr content of the food grains studied is presented in Tables 3 and 4. Cr ⁶⁺ in cereals ranged from 0.02 to 0.033 mg/kg, whereas in pulses, it was between 0.017 to 0.036 mg/kg. The mean values found for Cr (VI) were 0.028 mg/kg for cereals and 0.03 mg/kg for pulses, which were 6.3 and 8.1% of the total Cr content. These values of percent hexavalent Cr are somewhat similar to those of a previously published study^[7] in which the authors determined the total and hexavalent Cr content in 150 bread samples and found that Cr⁶⁺ was slightly above 10% of the total Cr contents.

Table 3. Hexavalent and trivalent Cr content of cereals.

Cereals	Trivalent chromium (mg/kg)	Cr (III) % of total content	Haxavalent chromium (mg/kg)	Cr (VI) % of total content
Wheat	0.54 ± 0.04	96.4 ± 7.4	0.02 ± 0.001	3.3 ± 0.3
Rice	0.57 ± 0.05	95.7 ± 8.2	0.033 ± 0.002	5.6 ± 0.4
Maize	0.776 ± 0.07	97 ± 9.6	0.03 ± 0.003	4.04 ± 0.4
Sorghum	0.16 ± 0.001	82.2 ± 6.0	0.029 ± 0.002	14.3 ± 1.1
Finger Millet	0.67 ± 0.04	96.5 ± 6.60	0.023 ± 0.002	4.01 ± 0.4

Values are mean ± SEM of five replicates.

Table 4. Hexavalent and trivalent Cr content of pulses.

Pulses	Trivalent chromium (mg/kg)	Cr (III) % of total content	Haxavalent chromium (mg/kg)	Cr (VI) % of total content
Red gram decorticated	0.415 ± 0.03	92.2 ± 7.0	0.035 ± 0.003	7.8 ± 0.7
Chickpea Whole	0.534 ± 0.04	97.2 ± 7.7	0.017 ± 0.001	3.2 ± 0.2
Chickpea decorticated	0.683 ± 0.02	96.2 ± 2.9	0.031 ± 0.002	4.43 ± 0.3
Green gram Whole	0.329 ± 0.03	93.0 ± 8.3	0.024 ± 0.001	6.8 ± 0.4
Green gram decorticated	0.617 ± 0.02	95.0 ± 4.5	0.031 ± 0.002	4.77 ± 0.4
Black gram	1.06 ± 0.02	98 ± 1.9	0.027 ± 0.001	2.54 ± 0.2
French bean	0.42 ± 0.03	91.5 ± 8.4	0.036 ± 0.003	7.73 ± 0.7
Cowpea	0.20 ± 0.02	84.5 ± 3.8	0.036 ± 0.002	17 ± 1.3

Values are mean ± SEM of five replicates.

The pH of all the cereals and pulses was evaluated to avoid any possible effect in the determination of Cr⁶⁺. For cereals, the mean value of pH was 6.03 and for the pulses, it was 6.31. Hence, the pH values of cereals and pulses were similar and no influence of the food grains pH in the extraction of hexavalent Cr can be seen. Since Cr is considered as bio-element in the trivalent form, it would be relevant to gather information regarding Cr³⁺ content of food grains. The difference between the total Cr content and Cr⁶⁺ is considered to be the Cr³⁺ content. The Cr³⁺ content found in the food grains is presented in Tables 3 and 4.

The Cr³⁺ content in cereals ranged from 0.16 to 0.776 mg/kg and in pulses it ranged from 0.2 to 1.06 mg/kg, respectively. Thus, trivalent Cr was predominant in both cereals and pulses, ranging from 82 to 98% of the total Cr in these foods. Since not many studies are available on the speciation of Cr in cereals and pulses (values for both hexavalent and trivalent Cr); no comparisons with the results obtained by other researchers can be drawn.

The alkaline extraction adopted for the speciation of hexavalent Cr enabled the selective separation of these species from the grain samples. Detailed understanding of the nutritional significance of essential elements in foodstuffs is not possible without the consideration of the chemical forms in which these elements occur in the foods. Not much attention has been paid to elemental speciation in plant-based foods. To the best of our knowledge, there is no information on the concentration of trivalent and hexavalent Cr in commonly consumed foods.

Thus, from the data obtained in this study, we can conclude that, in terms of mean values, 6.3 to 8.1% of the hexavalent Cr, which is toxic in nature, is present in the cereals and pulses. Speciation of Cr assumes importance, since this species is reported to be present in all parts of the plant, including the grains, hence proving that hexavalent Cr is not completely reduced either in the environment or the plant.^[36]

Conclusions

This study provides data on trace mineral contents and their bioaccessibility from cereal and pulses, which are consumed almost on daily basis in India. Information on bioaccessibility of minerals in cereals and pulses is essential to ascertain their optimal intake for improving human health. In our study, bioaccessibility of Cu, Mn, and Cr was found to be higher in pulses and hence, pulses are good sources of these essential minerals. Statistical analysis was done to confirm that there was no correlation between the total content and bioaccessibility of mineral, thus proving that bioaccessibility of minerals were independent of the total content. Speciation of Cr revealed that more than 90% of the total Cr in the food grains is in the trivalent form (bio-element) thus, ensuring the less exposure of consumer to the toxic hexavalent Cr. These findings would be of potential use in making recommendations for the optimal intake of these biologically important trace elements.

Acknowledgment

The first author acknowledges University Grants Commission, New Delhi, India, for the award of fellowship.