Mineral profile and arsenic accumulation in brown rice (Oryza sativa L.) from organic and conventional agriculture

Portugal is the largest consumer of rice in Europe, with a consumption of 17 kg per capita per year. The process that produces brown rice removes only the outermost layer, the hull, of the rice kernel. This process is the least damaging to the nutritional value of the rice and avoids the loss of nutrients that occurs with further processing. This cereal has a key role in the world's food. However rice could be one of the main source of arsenic. Therefore risks benefits associated with the consumption of rice in Portugal needs to be clarified. The aim of this study is correlate the profile of minerals with arsenic accumulation levels in brown rice (Oryza sativa L.) from organic and conventional agriculture. Mineral profile and the total and different species of arsenic levels were determined in 17 samples of different types of brown rice, from organic and conventional agriculture, grown in different areas of Portugal. The determination of minerals, manganese, copper, zinc, iron, magnesium, calcium, phosphorus, sodium and potassium was performed by Inductively Couple Plasma Atomic Emission Spectrometry – ICP-OES, while the arsenic was determined with a coupled mass spectrometry (ICP-MS / MS). Furthermore samples, with the highest content of arsenic, were analyzed to identify the different arsenic chemical species eventually present. The speciation analysis was carried by coupling an HPLC to the ICP-MS. Brown rice from organic agriculture, the highest mineral contents ranged from: 71.5 - 840 mg/100g (phosphorus), 69.6 - 497 mg/100g (potassium) and 37.8 - 301 mg/100g (magnesium). The content of copper and iron were the lowest and varied from 0.14 - 0.28 mg/100g and 0.76 - 0.97 mg/100g, respectively. Brown rice from conventional agriculture the highest mineral contents ranged from: 308 to 870 mg/100g (phosphorus), 221 to 278 mg/100g (potassium) and 111 to 290 mg/100g (magnesium). The content of copper and iron were the lowest and varied from 0.89 to 1.75 mg/100g and 0.17 to 0.77 mg/100g, respectively. Total arsenic ranged from 71.7 to 252 µg/Kg (organic agriculture) and 98.0 to 237 µg /Kg (conventional agriculture). The results, coherent for all samples, showed that the species present were mainly arsenite (AsIII), and dimethylarsinic acid (DMA). Using ANOVA and Kruskal-Wallis to find differences between organic and conventional brown rice, regarding arsenic levels, no differences between the characteristics under study were observed. Using Spearman correlation to compare total arsenic and mineral profile in organic and conventional brown rice, for a significant level of 95%, it was observed: positive correlation between arsenic and calcium, magnesium and phosphorus and a negative correlation between arsenic, copper and zinc, for organic brown rice. Conventional brown rice, showed a positive correlation between arsenic and phosphorus and a negative correlation between manganese and iron.