Modifying the Parboiling of Rice to Remove Inorganic Arsenic, WhileFortifying with Calcium

Rahman, H., Carey, M., Hossain, M., Savage, L., Islam, M. R., & Meharg, A. A. (2019). Modifying the Parboilingof Rice to Remove Inorganic Arsenic, While Fortifying with Calcium. Environmental science & technology, 53(9),5249. https://doi.org/10.1021/acs.est.8b06548

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1

Modifying the parboiling of rice to remove inorganic arsenic, while 1

fortifying with calcium. 2

3

4

Habibur Rahman1,2§, Manus Carey2§, Mahmud Hossain1, Laurie Savage1, M. Rafiqul Islam1, 5

Andrew A. Meharg2* 6

7

1. Department of Soil Science, Bangladesh Agricultural University, Mymensingh-2202, 8

Bangladesh. 9

2. Institute for Global Food Security, Queen’s University Belfast, David Keir Building, 10

Malone Road, Belfast, BT9 5BN, Northern Ireland. 11

12

13

* Corresponding author, aa.meharg@qub.ac.uk 14

§ Authors with equal contribution 15

16

17

2

ABSTRACT 18

Using village-based rice processing plants in rural Bangladesh, this study considered how 19

parboiling rice could be altered to reduce the content of the carcinogen inorganic arsenic. 20

Parboiling is normally conducted with rough rice (i.e. where the husk is intact) that is soaked 21

overnight at ambient temperatures and then either steamed or boiled for ~10 mins, 22

followed by drying. Across 13 geographically dispersed facilities it was found that a simple 23

alteration parboiling wholegrain, instead of rough rice, decreased the inorganic arsenic 24

content by 25% (P=0.002) in the final polished grain. Also, parboiling wholegrain had little 25

impact on milling quality of the final polished rice. The wholegrain parboiling approach 26

caused statistically significant median enrichment of calcium, by 213%; and a reduction in 27

potassium, by 40%; with all other nutrient elements tested being unaffected. Milled 28

parboiled rough rice had an enriched inorganic arsenic compared to non-parboiled milled 29

rice, but parboiling of wholegrain rice did not enrich inorganic arsenic in the final milled 30

product. Polished rice produced from the parboiling of both rough and wholegrain rice 31

significantly reduced cadmium compared to non-parboiled polished rice, by 25%. This study 32

also identified that trimethylarsine oxide and tretramethylarsonium are widely elevated in 33

the husk and bran of rice and, therefore, gives new insights into the biogeochemical cycling 34

of arsenic in paddy ecosystems. 35

36

37

3

INTRODUCTION 38

Concern regarding the elevated arsenic content of rice was first raised in Bangladesh in 39

20031,2. When a wider global survey of arsenic speciation in rice was conducted it was 40

realised that inorganic arsenic was specifically elevated in Bangladesh, and elsewhere3. In 41

comparison to wheat and barley, rice grain was typically ~10-fold elevated in inorganic 42

arsenic4. This elevation of inorganic arsenic is, primarily, due to rice being cultivated under 43

flooded conditions that results in anaerobic soil conditions that liberate inorganic arsenic as 44

arsenite; under aerobic conditions inorganic arsenic, as arsenate, is sequestered within iron 45

oxyhydroxide minerals5. As inorganic arsenic is a human carcinogen6, its elevation in rice is 46

problematic7. This is particularly true for Bangladesh where the populace is amongst the 47

highest per capita consumers of rice, ~450g per day4. Given that there is considerable 48

exposure to inorganic arsenic by the Bangladeshi populace from drinking water 49

consumption, due to naturally high arsenic in groundwaters8, rice ingestion adds a further 50

burden9. Rice is the dominant source of inorganic arsenic exposure in the lower arsenic 51

groundwater regions of Bangladesh9. Cadmium is also problematic for Bangladesh, and 52

elsewhere, as rice is the dominant exposure route to this carcinogen and renal toxicant at 53

concentrations that cause concern10. Rice grain nutritional content, in general, is also an 54

issue as subsistence rice diets are deficient in a range of essential elements and 55

vitamins11,12. 56

57

Given this background, there is considerable impetus and interest in reducing rice inorganic 58

arsenic13,14 and cadmium15,16, while fortifying mineral nutrients17,18. Altering agronomic 59

practices and breeding/selecting of new cultivars have been investigated but, as yet, there 60

4

has not been any practical solutions for reduction arsenic14, or for elevation in mineral 61

nutrients17, but there has been progress made for the reduction of cadmium15,16. 62

63

Post-harvest approaches have been more useful in removing inorganic arsenic. Cooking rice 64

in high volumes of water removes up to 70% of inorganic arsenic, does does not reduce 65

cadmium18. However, widespread implementation of new approaches to cooking is subject 66

to education and individual practice, as it is a domestic process. Arsenic contaminated 67

cooking water, as observed for Bangladesh, is also problematic when it comes using cooking 68

to remove inorganic arsenic19. Post-harvest rice processing technologies may be a suitable 69

route to explore to further reduce, in addition to cooking, rice grain inorganic arsenic and 70

cadmium content, as it is a way to alter rice before it reaches the consumer. It is known that 71

arsenic is elevated in the bran and germ of rice, and that these are removed through 72

polishing of wholegrain rice to white rice20. A pictorial representation of rice components 73

discussed in the text is given in SI Figure 1. The form of inorganic arsenic in rice grain, both 74

wholegrain and polished rice is, primarily, the more soluble form arsenite19. It is this 75

solubility of arsenite that makes cooking in high volumes of water an effective mitigation 76

strategy19, and which may translate into novel parboiling technologies to remove inorganic 77

arsenic. 78

79

In Bangladesh the vast majority of rice is consumed parboiled. Parboiling involves soaking 80

the rough rice (i.e. with husk intact) overnight in ambient water and then boiling or 81

steaming, followed by sun-drying, dehusking, and then milling21-27. This gelatinizes the 82

starch that constitutes the endosperm, causing the grain to expand, leading to the 83

separation of the tow-halves of the hull. Parboiling can be used to fortify rice with B-84

5

vitamins and mineral nutrient by adding them to the parboiling water21. The consequences 85

of parboiling procedures on rice arsenic content has been characterised in Bangladesh, 86

showing that there is little loss of total arsenic due to typical processing28. However, there 87

has been no attempt to alter parboiling protocols to see if arsenic or cadmium can be 88

removed. A recent study showed that parboiling wholegrain, as opposed to rough rice, 89

caused fortification with iron and zinc21. The migration of iron and zinc in wholegrain rice 90

during parboiling was attributed to removal of the husk that acts as a diffusion layer 91

between the external medium and the wholegrain. 92

93

The altered dynamics of iron and zinc migration on the parboiling of wholegrain rice is a 94

very interesting development21, and may have consequences for inorganic arsenic and 95

cadmium mobility in rough versus wholegrain rice during parboiling. The husk is specifically 96

high in inorganic arsenic29,30, and is potentially a diffusions barrier to the wholegrain it 97

encases, as for iron and zinc21. The study presented here set out to see parboiling 98

wholegrain versus rough rice could be implemented to effect inorganic arsenic removal, 99

while ascertaining what was the consequences for general mineral nutrition. In rural 100

Bangladesh rice parboiling is conducted locally in primitive plants (Figure 1), and it is these 101

that were tested for the development of altered parboiling approaches. 102

103

104

105

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MATTERIALS AND METHODS 106

107

Parboiling 108

Traditional small-scale par boiling plants throughout Bangladesh were used. (Figure 1). 109

These consist of concrete tanks for pre-soaking rough rice, a ~400 litre boiling/steaming 110

receptacle for parboiling, a cemented yard for sun drying parboiled grain before further 111

processing, and an adjacent mill to do the conduct dehusking and polishing of wholegrain. 112

Parboiling trials used 200 kg of rice per batch run, with runs conducted in triplicate. 113

Treatments always included rough rice versus wholegrain rice comparisons. Both parboiling 114

and steaming were initially trialled (data not shown) at Mymensingh, Bangladesh, focusing 115

on the length and temperature of soaking, length and temperature of heating stage, 116

steaming as compared to boiling water, and static as opposed to percolating boiling water, 117

using four cultivars. Optimal conditions for inorganic arsenic removal were derived and 118

these parameters and were: dehusking rough rice and then soaking the resulting wholegrain 119

for 12 hours in in ambient water, then parboiling using boiling water for 15 minutes, sun-120

drying the resultant grain, and finally milling to produce polished white rice. This differs 121

from traditional parboiling only in that wholegrain, as opposed to rough rice, was parboiled, 122

with all other parameters and treatments being the same, except that rough rice is 123

dehusked after sun drying, before polishing. The experiments were trialled at 13 locations 124

throughout Bangladesh (Figure 1). Cultivars used in these studies were recorded and 125

reported in Supporting Information (SI) Table 1. At one site, Faridpur, 6 cultivars (BRRI 126

dahn29, BRRI dhan33, BRRI dhan39, BRRI dhan56, BR3 and BR11) were used to determine if 127

there was any variance in processing due to rice breed. From all parboiling experiments the 128

water used to treat the rice was sampled. Husk, bran, wholegrain and polished rice was 129

7

obtained for the untreated rice. Polished rice was produced in the mill attached to the 130

parboiling plant for analysis. 131

132

Chemical analysis 133

Arsenic speciation and total elemental content was conducted using our published ICP-MS 134

(Thermo, ICAP Q) protocols for waters31 and rice18. Briefly, solid samples were freeze dried 135

and then ground to a fine powder using a Retsch ball mill with zirconium oxide lined milling 136

chamber and balls. For total elemental analysis, around 100 mg of sample was accurately 137

weighed into 50 ml polypropylene tubes to which 2 ml of concentrated Aristar nitric acid 138

was added and left to stand overnight. Prior to digestion 2 ml of reagent grade full-strength 139

hydrogen peroxide was added, and then the tube placed in a Mars CEM microwave digestor 140

using a rice digestion protocol derived in our laboratories18, left to cool on completion of the 141

protocol, and then diluted to 30 ml with double distilled deionized water. For arsenic 142

speciation, the same protocols was used except that 10ml of 1% nitric acid was used for 143

digestion/extraction. Prior to analysis 7µl of hydrogen peroxide added to 700µl of the 144

extract, to convert arsenite to arsenate for subsequent chromatography as arsenite elutes 145

at the solvent front. For speciation, authentic samples of arsenite, arsenate, 146

monomethylarsonic acid (MMA), dimethylarsinic acid (DMA), trimethylarsine oxide (TMAO) 147

and tretramethylarsonium were used as outlined previously18,31. An authentic arsenic 148

species chromatogram is presented in SI Figure 2. XRF (Rigaku NEX CG) was performed on 149

the ball milled samples to determine calcium, potassium, magnesium, sulphur and silicon 150

content, which are problematic by ICP-MS. CRM (rice flour, NIST 1568b) recoveries for all 151

8

reported parameters are detailed in SI Table 2 for ICP-MS, along with LoDs, and SI Table 3 152

for XRF. 153

154

Statistical analysis 155

Where any concentration was below LoD, the value of half-LoD was used in statistical 156

analysis. Statistical analysis was conducted using non-parametric comparisons of medians 157

using the software package GraphpadPrism, version 6. 158

9

RESULTS 159

Initial trials showed that steaming did not remove inorganic arsenic or cadmium, so trials 160

reported here focused on boiling to parboil rice. Concentrations of arsenic species, cadmium 161

and macro- and micro- element in polished grain were related to husk, bran, wholegrain and 162

polished rice from untreated grain, and are given in for the countrywide experiments (Figure 163

2), with percentage removal through comparing treatments of polished rice is also reported 164

(Figure 3), and the statistical analysis of these percentage removal findings given (Table 1). 165

Trials on the 6-cultivars at one site (Faridpur) showed there was little varietal variation in 166

parboiling results, and the data was conformational of the regional trials, with just less 167

variance (SI, Figures 3 and 4). 168

169

Inorganic arsenic was highly elevated in bran, 2.5-times higher than husk, 5-times higher 170

than wholegrain, and 7-times greater than polished rice (Figure 2). Copper and potassium 171

had similar elemental profiles. Other species and elements that had specifically high bran 172

concentrations, but low husk, were tretramethylarsonium, magnesium, molybdenum, 173

phosphorus (in particular), sulphur and selenium. Monomethylarsonic acid, calcium, iron 174

and manganese had high concentrations in both husk and bran. Silicon was very high is 175

husk, and low in other components. Zinc’s profile was more like that of 176

tretramethylarsonium. The elements and arsenic species that stand out are dimethylarsinic 177

acid, molybdenum and cadmium, which were lower in the husk, and have similar levels 178

across other compartments. 179

180

With respect to how parboiling procedures impact chemical composition of polished rice, 181

inorganic arsenic was removed by parboiling wholegrain rice by a median of 25% (P=0.0014) 182

10

across all sites (Figure 3). Neither parboiling approach significantly altered the inorganic 183

arsenic content compared to non-parboiled polished rice. The water used in parboiling had 184

inorganic arsenic concentrations ranging from 0.17 to 9.5 g/kg across the 13 study sites (SI 185

Table 1). No other arsenic species were detected. There was no correlation (P>0.05) 186

between treatment water an arsenic species in any rice compartment. Trimethylarsenic 187

oxide was reduced in wholegrain versus non-parboiled polished rice (by 20%, P=0.043), 188

while dimethylarsinic acid was enriched (by 20%, P=0.048), on comparing parboiled rough to 189

non-parboiled rice. Cadmium concentrations were not significantly affected on altering 190

parboiling procedures, but the parboiling of wholegrain rice reduced cadmium by 25% 191

(P=0.0014) compared to non-parboiled counterpart. 192

193

For beneficial nutrient elements, calcium was significantly enriched when parboiling 194

wholegrain was compared to rough rice, by 213% (P=0.0034); and to non-parboiled polished 195

rice, by 196% (P=0.0126); with parboiled rough rice being non-significantly different to non-196

parboiled rice (P>0.05). Parboiling, of both wholegrain and rough rice, significantly 197

enhanced copper (120-136%), iron (207-307%) and phosphorus (139-148%), while zinc 198

decreased (79-84%) compared to non-parboiled polished rice, with no difference in these 199

parameters between wholegrain versus rough rice parboiling. Potassium is significantly 200

decreased (by 40%, P=0.001) by parboiling wholegrain as opposed to rough rice, but not 201

when compared to non-parboiled polished rice. Molybdenum is significantly decreased by 202

~20% compared to both rough and non-parboiled rice when wholegrain is parboiled. 203

Parboiling of rough rice (P=0.0034) significantly increases magnesium, but parboiling 204

wholegrain did not have this result. Sulphur, selenium and silicon content of rice was 205

unchanged across the treatments. 206

11

207

Milling quality, i.e. the percentage of non-broken grains on polished rice production, 208

between regions, when assessed non-parametrically by a Mann-Whitney test found that the 209

new approach to parboiling was significantly lower than traditional (P=0.0037), with 210

medians of 74.4 and 88.1 %, respectively. For the Mymensingh cultivar variance trial, there 211

was no significant difference (P=0.589) in percentage broken grains, 74.5 and 73.4% for 212

wholegrain and traditional parboiling, respectively. 213

12

DISCUSSION 214

Rice, in its parboiled form, is the dietary staple of Bangladesh24,28. It is also a major 215

contributor to inorganic arsenic exposure in Bangladesh, calculated to be 56% in an arsenic 216

elevated region of Bangladesh9. Other studies confirm the importance of rice as an inorganic 217

arsenic exposure source for Bangladesh3,28. Here we find that parboiling wholegrain rice, as 218

opposed to rough rice, is an effective method of centrally reducing inorganic arsenic from 219

the diet of the Bangladesh populace, with a median reduction of 25%. As Bangladeshi adults 220

consume ~450g rice per day4, and parboiling plants are present in every community in the 221

rice growing zones of Bangladesh, this simple, low-cost and low-technology intervention of 222

parboiling wholegrain offers an instantly implementable way of reducing inorganic arsenic 223

exposure. 224

225

Previous investigations on how parboiling affects grain arsenic have been limited28; based 226

on one cultivar at one site, with no alteration of procedures, and with only total arsenic 227

measurements reported. This older study did, however, suggest that parboiling enhanced 228

the arsenic content of the final polished rice as compared to non-parboiled polished rice 229

produced from the same grain28. This observation that parboiling elevates grain arsenic was 230

also found in the study reported here, for both the sum of arsenic species and inorganic 231

arsenic, in the traditionally processed rough rice. Thus, if non-parboiled rice was consumed 232

in Bangladesh, this would also decrease inorganic arsenic exposure in comparison to 233

traditional rough rice parboiling. The new wholegrain parboiled rice has the same inorganic 234

arsenic concentrations as non-parboiled polished rice. 235

236

13

Inorganic arsenic in grain is dominated by relatively soluble arsenite29,30. Arsenite is 237

concentrated in surface bran29,30. Thus, wholegrain parboiling allows the inorganic arsenic 238

diffuse directly out to the external media, where parboiling rough presumably disrupts 239

diffusion from the grain and, perhaps, enables husk derived arsenite to diffuse into the 240

encapsulated wholegrain. Given that the husk is also high in inorganic arsenic29,30, it 241

potentially contributes to wholegrain inorganic arsenic loading during rough rice parboiling. 242

243

Cadmium, which is typically elevated in rice10, was unaffected by the new parboiling 244

protocol, but parboiling per se reduced cadmium compared to non-parboiled polished rice. 245

Cadmium is a major issue in Bangladeshi rice10, and the fact that parboiling reduces its 246

content in rice grain is fortuitous. 247

248

Parboiling wholegrain, compared to rough, rice has the added benefit of fortifying the 249

polished rice grain with calcium by ~200%. Again, given that rice is the dietary staple, this is 250

an important finding as dietary calcium intakes, particularly amongst young children, are of 251

concern in Bangladesh11,12. Technological approaches to enhancing food biofortification 252

through breeding17 cannot approach the enhancement in calcium observed in the altered 253

parboiling protocols reported here. Given that the calcium has migrated into grain, this 254

suggests mobility, which may translate into gut bioavailability, although this has to be 255

tested. The reason why calcium migrates into the endosperm on parboiling wholegrain also 256

needs investigating. It is likely that calcium has enhanced perfusion from the bran to the 257

endosperm on parboiling wholegrain. 258

259

14

The presence of anti-nutrients is one of the major issues in elemental nutrition of grain, as 260

phytic acid immobilizes divalent cations17. Chemical fortification of rice through spraying 261

nutrients, such as calcium, onto the surface of rice is not widely practised outside the USA32. 262

Chemical fortification is particularly problematic as the common practice of rinsing (for 263

hygiene & culinary preference) removes these artificially applied nutrients32. Also, cooking 264

rice in large volumes of water, another effective way to reduce inorganic arsenic in rice18, 265

removes chemically fortified nutrients32. 266

267

The parboiling of wholegrain rice does remove a substantial portion of potassium, 268

confirming our earlier observation that this element is readily leached from rice grain18, with 269

others subsequently reporting a similar effect33. Evidence is also presented here that 270

molybdenum, in cases, may also be leached, but all other mineral nutrients remaining 271

unchanged. Parboiling, per se, reduces zinc concentration in polished rice. Enhancing zinc 272

nutrition being a key problem with respect to human nutrition in developing countries17. 273

274

Polished rice is a poor source of vitamins, with the exception of the B-complex11,12. Evidence 275

suggests that parboiling fortifies the final polished rice with B-vitamins21,34. The effect of 276

parboiling wholegrain rice on B-vitamins needs further investigation. Given that potassium 277

and B-vitamins are an issue with the altered parboiling protocol, if it is to be widely 278

implemented it must be done so with advice to enhance the consumption of foods that will 279

compensate for this loss. 280

281

15

Rice milling and eating quality also needs to be considered when changing parboiling 282

protocols23,34. The number of broken grains did increase on parboiling wholegrain versus 283

rough rice in the study presented here across all sites, but where very similar at the site 284

used in the multiple cultivar study. Also, Pantindol et al.21 found that wholegrain and rough 285

rice give the same head rice (i.e. polished rice) yield. Broken rice is sold as is for 286

conventional cooking, and used to produce flour, also a valuable product. The culinary 287

properties, taste, texture, colour etc., should not be disrupted too much on changing rice 288

processing protocols21. Pantindolet et al.21 did find some differences in their studies, but 289

ultimately this needs consumer testing, or education of consumers to accept a, hopefully 290

slightly, different product because of health benefits. 291

292

Despite the large body of work conducted on arsenic speciation in rice3,5,7,14,18,20,29,30, 293

questions remain regarding the full complexity of arsenic’s chemical form in grain35. An 294

interesting outcome of this study is that, because analysis used a mixed anion/cation 295

chromatography column, a better separation of arsenic species was obtained compared to 296

the usual HPLC-anion exchange chromatography approach normally used to characterise 297

rice7,18. In particular, cationic species trimethylarsenic oxide and tretramethylarsonium 298

would be lost at the solvent front unless a mixed anion/cation column is used36, or that both 299

anionic and cationic columns are chromatography would need performed on each sample to 300

given better detailed speciation35. While the mixed ion exchange approach has been used 301

on wholegrain and polished rice previously7,18, it had not been used to characterize arsenic 302

speciation specifically in husk and bran that relates to polished rice. Tretramethylarsonium 303

has been reported in a limited number of wholegrain samples35, however, this current study 304

reports for the first time that the presence of trimethylarsenic oxide is widespread, in both 305

16

husk and bran. It also showed that tretramethylarsonium is present in bran. 306

Monomethylarsonic acid is not systematically reported in rice grain studies, but here was 307

found to be above the LoD in husk and bran. 308

309

The observation that rough rice components, primarily husk and bran, contain a wider range 310

of methylated arsenic species than previously realised raises questions regarding arsenic 311

biogeochemical processing arsenic in paddy fields. Methylated arsenic species can only 312

derive from microbial metabolism37. Studies on microbial metabolism of arsenic in soils, for 313

analytical reasons outlined above for grain, overlook any trimethylarsenic oxide and 314

tretramethylarsonium that may be present. Our unpublished results, using mixed ion 315

exchange chromatography, routinely detects trimethylarsenic oxide and 316

tretramethylarsonium in soil porewaters. These missing species are important components 317

of arsenic’s environmental fate; trimethylarsenic oxide is the precursor of trimethylarsine, 318

the major arsine emitted from paddy soils36. Also, another route of trimethylarsenic oxide 319

entry into paddy soils is wet deposition, with ~20% of arsenic deposited into Bangladeshis 320

(and Sri Lankan) paddy soils being comprised of this species36. 321

322

The uptake and transport of trimethylarsenic oxide is unknown in rice and other plants, but 323

transport of other methylated species has been characterised; for monomethylarsonic and 324

dimethylarsinic acids38. Dimethylarsinic acid has a distinctly different mode of arsenic 325

transport to other arsenic species, attributed to not being readily complexed by 326

phytochelatins that are known to retard transport of arsenite and monomethylarsonic acid. 327

The pattern of trimethylarsine oxide in rough rice components is more akin to that of 328

17

monomethylarsonic acid, than dimethylarsinic acid. The pattern of dimethylarsinic acid is 329

highly distinctive compared to other arsenic species and most other elements, with 330

molybdenum being the exception; both being low husk, and having bran concentrations 331

similar to that in polished rice. Previous physiological studies had shown that 332

dimethylarsinic acid is readily unloaded into grain30. 333

334

Acknowledgements–The authors wish to thank the Nestlé Foundation who funded this 335

work. 336

337

SUPPORTING INFORMATION. 338

Supporting Information contains site locations and their characterization, CRM recoveries 339

for arsenic species, and for elements, analysed, and the findings of the parboiling trials with 340

respect to cultivar variation. . 341

342

343

344

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Table 1. Median and significance (P) as ascertained using a Wilcoxon Signed Rank Test, using

a theoretical mean of 100% for polished rice that resulted from wholegrain (WG) and rough

rice (RR) parboiling, and from non-parboiled polished (NPBP). Significantly different results

are given in bold.

WG/

RR (%)

WG /

NPBP (%)

RR /

NPBP (%)

media

n P median P median P

iAs 74.4 0.002

4 116 0.088

8 112 0.057

4

MMA 95 0.677

2 97.9 0.812

2 135 0.176

3

DMA 71 0.190

9 102 0.869

3 120 0.047

9 TMAO 100

0.3125 79.5

0.0429 100

0.3125

Tetra 100

> 0.999

9 104 0.170

8 100

> 0.999

9

Ca 213 0.003

4 196 0.012

6 99.9 0.909

7

Cd 106 0.969

7 74.3 0.001

4 83.5 0.850

1

Cu 75.5 0.077

1 120 0.033

1 136 0.012

2

Fe 162 0.101

6 303 0.004

3 207 0.024

4

K 59.5 0.001 101 0.956

4 158 0.064

Mg 96.3 0.622

1 155 0.011

2 155 0.034

2

Mn 82.3 0.233

4 85.4 0.056

2 103 0.791

Mo 83.3 0.042

5 79.1 0.006

7 95.4 0.969

7

P 82.2 0.052

2 148 0.002

7 139 0.003

4

24

S 93.9 0.380

4 110 0.217

7 112 0.052

2

Se 100 0.296

9 92.6 0.518

4 100 0.312

5

Si 117 0.203

6 180 0.056

8 130 0.203

6

Zn 81.9 0.203

6 79.1 0.004

6 84.9 0.003

4

25

Figure 1. Image of one of the village-based parboiling plants used in this study (a-d) and the

location of the parboiling plants (e). a. shows the boiler, b. the top of the ambient water

soaking tanks, c. the drain of the ambient water soaking tanks, and d. the boiling/steaming

vessels.

a.

b.

c.

d.

e.

26

Figure 2. Arsenic species and elemental concentrations between the different components

that constitute rough rice, and with different parboiling (PB) approaches across 13 locations

across Bangladesh. iAs denotes inorganic arsenic; MMA, monomethylarsonic acid; DMA,

dimethylarsinic acid; TMAO, trimethylarsenic oxide; Tetra, tretramethylarsonium. Median

values are presented with bars showing the 75th percentiles.

27

Figure 3. Ratios for each element concentrations in wholegrain vs. traditional parboiling (A),

wholegrain vs. non-parboiled polished (B) and traditional parboiled vs. non-parboiled

polished (C). Median values are presented with bars showing the 25th and 75th percentiles.

The statistical significance of this data is given in Table 2. iAs denotes inorganic arsenic;

MMA, monomethylarsonic acid; DMA, dimethylarsinic acid; TMAO, trimethylarsenic oxide;

Tetra, tretramethylarsonium.

iAsM

MADM

A

TMAO

Tetra Ca

Cd Cu Fe K Mg

Mn

Mo P S Se Si

Zn0

100

200

300

400

500

wh

ole

gra

in/t

rad

itio

na

l p

roto

co

l (%

)

A

iAsM

MADM

A

TMAO

Tetra Ca

Cd Cu Fe K Mg

Mn

Mo P S Se Si

Zn0

100

200

300

400

500

600

700

wh

ole

gra

in/n

on

-pa

rbo

iled

p

olis

he

d p

roto

co

l (%

) B

iAsM

MADM

A

TMAO

Tetra Ca

Cd Cu Fe K Mg

Mn

Mo P S Se Si

Zn0

100

200

300

400

500

tra

diti

on

al/n

on

-pa

rbo

iled

p

olis

he

d p

roto

co

l (%

) C