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Rethinking Rice Preparation for Highly Efficient Removal of Inorganic Arsenic Using Percolating Cooking Water

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Overview

This PLOS ONE study tested three percolating-cooking approaches against the conventional absorption method for removing inorganic arsenic (iAs) from rice: (1) a laboratory cooking experiment at water-to-rice ratios of 3:1, 6:1 and 12:1 under reflux to capture evaporative losses; (2) a Soxhlet apparatus where cooking water was continuously recycled through distilled steam; and (3) an off-the-shelf domestic coffee percolator running near-boiling water through the rice. The 12:1 ratio removed an average 57±5% of iAs across all rice types; the Soxhlet method removed 59±8% (polished) and 69±10% (wholegrain); the coffee percolator removed 49±7% on average and up to 85% in the best individual sample. Dimethylarsinic acid (DMA) was not significantly reduced by any cooking method tested. Potassium was the only major mineral lost in substantial quantity (53% on coffee-percolator cooking).

Key numbers

Values are in mg/kg dry-weight unless noted; arsenic species are inorganic arsenic (iAs = arsenite + arsenate) and dimethylarsinic acid (DMA), measured by IC-ICP-MS.

Cooking volume experiment (n=6 wholegrain + 6 polished; Fig 1) — rice cooked at 3:1, 6:1 and 12:1 water:rice ratios in a sealed-flask reflux setup that returned evaporated steam to the cooking vessel (no water discarded; rice freeze-dried whole).

  • 12:1 ratio: 57±5% iAs removed across all rice; 53±5% for polished, 61±3% for wholegrain.
  • 3:1 ratio: 29±5% iAs removed across all rice.
  • 6:1 ratio: 29±4% iAs removed across all rice.
  • Uncooked wholegrain iAs roughly two-fold higher than uncooked polished on average; wholegrain showed a steeper removal slope at higher cooking volumes.
  • DMA was not significantly reduced by cooking; only the rice-type term was significant (P<0.001), with wholegrain having higher DMA than polished.

Soxhlet recycled-condensation experiment (all 41 samples; Fig 2) — rice (2 g) held in a Soxhlet thimble while cooking water cycled through three reflux passes; receiving flask started with 200 mL deionized water.

  • iAs removal: 59±8% for polished (n=27) and 69±10% for wholegrain (n=13).
  • Individual samples reached >80% iAs removal in some cases.
  • For polished rice, higher initial iAs correlated with higher percentage iAs removed; for wholegrain the relationship was slightly inverse (saturation at high initial iAs).
  • DMA removal was poor; the rice-type and initial-DMA terms were not significant in the percentage-removal GLM.

Coffee percolator experiment (home-cooking trial, n=6 wholegrain + 6 polished; Fig 3, Fig 4) — 500 g rice in a Bravilor Bonamat catering coffee percolator with paper filter; 2 L of tap water passed through twice (polished; 20 min total) or three times (wholegrain; 30 min total); inlet water temperature 98 °C. Each rice type cooked in triplicate.

  • iAs removal: 49±7% across all 12 samples; up to 85% in the best individual sample.
  • Rice type (wholegrain vs polished) not a significant factor for percentage iAs removal.
  • iAs in the tap water was 0.050±0.02 µg/L (LOD 0.036 µg/L).
  • Mineral co-losses (paired t-test cooked vs uncooked): potassium 53% loss (P<0.001); phosphorus 7% loss (P=0.016); calcium, copper, iron, manganese, sulphur and zinc not significantly changed.

Uncooked rice iAs ranges (read across Figs 1, 2 and 3):

  • Polished: approximately 0.01 to 0.23 mg/kg (highest values in the coffee-percolator subset, Fig 3).
  • Wholegrain: approximately 0.085 to 0.46 mg/kg (highest values in the coffee-percolator subset, Fig 3).

Quality control:

  • Certified reference material: NIST SRM 1568b Rice flour, certified for both iAs (0.092 mg/kg) and DMA (0.182 mg/kg); recovery (n=11) was 88.2±3.3% for iAs and 91.6±4.4% for DMA, sum-of-species recovery 90.3%.
  • LOD for iAs and DMA in rice: 0.0028±0.001 mg/kg dry-weight (n=5 calibration).
  • All samples were above LOD for iAs; two were below LOD for DMA (½ LOD substitution used).

Methods

Rice was purchased from major UK retailers in Belfast and via UK online retail; 41 packets in total (13 wholegrain, 28 polished), with the country-of-origin labels listed under sample_population above.

Three cooking systems were compared:

  1. Standardised laboratory cooking at controlled water:rice ratios. 20 g rice in a 250 mL round-bottomed Pyrex flask heated in an electrothermal mantle; the flask neck connected to a 25 cm dimpled Vigreux condensing tube (42 dimples) that returned steam-as-water to the flask. Double-distilled, deionized water added to give 3:1, 6:1, or 12:1 water:rice ratios. After cooking, samples were freeze-dried and powdered in a Retsch PM100 ball mill using a zirconium-oxide-lined vessel and zirconium-oxide grinding balls.
  2. Soxhlet recycled-condensation cooking. 2 g rice was held in a VWR Soxhlet thimble inside a 25 cm × 3.5 cm Quickfit Soxhlet attached to a 250 mL receiving flask containing 200 mL of double-distilled, deionized water; cooking timed by Soxhlet reflux cycles, with three cycles sufficient for full cook.
  3. Home coffee-percolator cooking. Off-the-shelf Bravilor Bonamat catering coffee percolator (no hotplate; filter unit only); 500 g rice packet placed in the manufacturer-supplied paper filter; the 2 L reservoir took 10 min to discharge per cycle. Polished rice cooked with 2 × 2 L (4 L total, 20 min); wholegrain cooked with 3 × 2 L (6 L total, 30 min). Inlet water entered the filter at 98 °C. Cooking experiments were run in triplicate for each rice type.

Analytical chemistry: cooked and uncooked rice was freeze-dried and milled, then 0.1 g powder digested overnight in 2 mL of 1% Aristar HNO3, microwave-digested in a CEM MARS 6 (30 min staged ramp to 95 °C), diluted to 10 mL, and treated with 10 µL H2O2 to convert arsenite to arsenate prior to ion-chromatography. Arsenic speciation by IC-ICP-MS used a Thermo Scientific IC5000 ion-chromatography system with a Thermo AS7 (2 × 250 mm) column and AG7 (2 × 50 mm) guard, a 20–200 mM ammonium-carbonate linear gradient over 15 min, interfaced to a Thermo iCAP Q ICP-MS monitoring m/z 75 with He collision-cell mode; calibration species DMA, iAs, MMA, tetramethyl arsonium and arsenobetaine. Total elements were measured by the same iCAP Q in direct-solution mode with Rh as internal standard; additional elements were measured by bench-top XRF (Rigaku CG). Multi-element digestion used a more aggressive program to 180 °C with 2 mL concentrated HNO3 plus 2 mL H2O2. Blanks and NIST SRM 1568b rice-flour CRM were included in each batch of 40 samples.

Statistics: General Linear Modeling (GLM) using Minitab v.16; residuals tested for normality with Anderson–Darling and found normal so no transformation was applied except for percentage data, which were rank-transformed before analysis.

Implications

The source supports rice mitigation context for iAs reduction by cooking-water management. Findings should be reported with the cooking-water regime (water:rice ratio, percolating vs static, distilled vs tap) preserved, because both percentage-removal and final cooked iAs vary substantially with the regime. The paper measures rice as a raw grain commodity cooked in a laboratory or domestic setting; it does not measure or test commercially manufactured infant rice cereal or rice-based plant milk, so any application of these removal percentages to manufactured rice products is downstream extrapolation, not direct evidence.

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