Signes-Pastor et al. 2016 — Inorganic arsenic geographic variation in Iberian Peninsula paddy fields and commercial rice
This Food Chemistry study characterized total arsenic (tAs) and inorganic arsenic (iAs) speciation across eight main rice-producing regions of the Iberian Peninsula (Andalucía, Aragón, Catalunya, Extremadura, Murcia, Navarra, Portugal, Valencia) using soil (n=40), shoot (n=40), field-collected grain (n=20), and commercial rice (n=144: 20 brown, 11 parboiled, 113 polished) samples. IC-ICP-MS speciation revealed significant regional differences in iAs in commercial polished rice: Extremadura/Portugal had the highest iAs (median 0.087–0.088 mg/kg), while Andalucía had the lowest (0.054 mg/kg). About 26% of commercial rice samples exceeded the EC permissible concentration for infant food production (0.100 mg/kg iAs). Soil arsenic had a geogenic/mining origin. Cadmium concentrations in rice grain were consistently low and near the LOD.
Key numbers
- Commercial polished rice (n=113 total):
- Entire dataset median iAs: 0.071 mg/kg; range 0.027–0.175 mg/kg
- By region (median iAs, mg/kg):
- Andalucía (n=20): 0.054 (range 0.027–0.130)
- Aragón/Navarra (n=10): 0.067 (range 0.044–0.154)
- Catalunya (n=14): 0.080 (range 0.044–0.147)
- Extremadura/Portugal (n=23): 0.087 (range 0.066–0.138)
- Murcia (n=11): 0.057 (range 0.039–0.075)
- Valencia (n=29): 0.063 (range 0.028–0.175)
- Significant regional differences (Kruskal-Wallis P < 0.001)
- ~26% of polished rice samples exceeded EU 100 µg/kg infant production ML
- Brown rice: higher median iAs than polished; median iAs (all Spain/Portugal brown) 0.157 mg/kg (range 0.083–0.247)
- Parboiled rice: median iAs 0.083 mg/kg (range 0.022–0.170)
- Commercial rice iAs vs. tAs regression: slope 0.186, R² = 0.38 across entire polished dataset
- DMA (dimethylarsinic acid) in polished rice: median 0.055 mg/kg (range 0.003–0.291)
- Highest median DMA: Extremadura/Portugal 0.139 mg/kg; lowest: Murcia 0.009 mg/kg (P < 0.001)
- iAs percentage of ΣAs in polished rice: median 57% (range 14–95%); Murcia 87% iAs proportion (highest), Extremadura/Portugal lowest (41%)
- Soil tAs (8 paddy regions): range 2.3–17 mg/kg; Portugal highest (median 15 mg/kg)
- Shoots iAs: range 0.257–17.1 mg/kg (dry weight); Extremadura highest (11.0 mg/kg median), Murcia lowest (1.4 mg/kg median)
- Cadmium in all rice grain samples: consistently low, near LOD (LOD for Cd analysis 0.009 mg/kg)
- Field-collected grain iAs: median iAs percentage 85% (41–97%), range 0.052–0.161 mg/kg; Extremadura highest (0.100, 0.061–0.130), Murcia lowest values
- LOD: Cd 0.009 mg/kg (soil CRM), As speciation 0.002 mg/kg (rice CRM from DMA calibration)
Methods (brief)
Soil (n=40), shoot (n=40), field grain (n=20) collected September 2014 from 8 paddy regions (5 replicates per region). Commercial rice (n=144) purchased from supermarkets and local shops. IC-ICP-MS for As speciation (Thermo IC5000 + Thermo AG7 guard column + Thermo ICAP Q ICP-MS); ICP-MS direct acquisition for tAs and Cd. Freeze-dried, ball-milled samples; 1% HNO₃ microwave digestion (55°C/75°C/95°C). CRM: NIST 1568b rice flour (As species and Cd certified). Statistical: Kruskal-Wallis rank non-parametric; R statistical software.
Limitations
Commercial rice samples (n=113 polished) do not uniformly represent all Spanish rice-growing regions — Valencia n=29 is well-represented, Extremadura n=3 in field-collected grain is small. Regional commercial rice samples were purchased from supermarkets and include rice of uncertain origin within the labeled region. The very high DMA proportion in Extremadura/Portugal rice (which reduces the iAs% of total arsenic while maintaining absolute iAs near the top) reflects complex soil-plant arsenic speciation interactions not fully explained. Cadmium data for commercial rice grain are largely below LOD, providing little quantitative information for Cd risk assessment.
Implications
- Certification: The regional breakdown of Spanish polished rice iAs (Andalucía lowest 54 µg/kg; Extremadura/Portugal highest 87 µg/kg) is directly usable as a geographic_breakdown entry for the rice ingredient page. The finding that ~26% of commercial rice exceeded the EU infant food ML of 100 µg/kg is a supply-chain risk signal. Sourcing from Andalucía or Murcia within Iberia provides a lower-iAs baseline.
- Courses: Strong case study for geographic variance within a single agricultural region. Soil tAs gradient (Portugal/Catalunya highest, Murcia/Extremadura lowest as Murcia) driving plant uptake demonstrates the soil → shoot → grain pathway.
- App: For Spanish/Portuguese market rice ingredients, regional origin matters. Andalucía (54 µg/kg median) and Murcia (57 µg/kg median) polished rice are substantially lower than Extremadura/Portugal (87 µg/kg) or Valencia (63 µg/kg). Brown rice median of ~157 µg/kg is the relevant figure for brown rice ingredient scoring.
- Microbiome: Not primary topic.