Rice as a source of microelements and toxic contaminants
On December 16, 2002, the United Nations General Assembly (UNGA) declared the year 2004 the International Year of Rice (IYR) under the concept “Rice is Life”. In declaring IYR, UNGA recognized that rice is the primary food source for more than half of the world’s population.
Rice (Oryza sativa L.) is considered the main staple food for several countries (Myanmar, Lao People’s Democratic Republic, Vietnam, Cambodia, Bangladesh, Indonesia, Thailand, Philippines, Nepal, P.R. China) and is a major source of nutrients. In developing countries, rice accounts for 715 Kcal/capita/day; 27% of dietary energy supply, 20% of dietary protein, and 3% of dietary fat. However, while rice provides a substantial amount of dietary energy, it has an incomplete amino acid profile and contains limited amounts of essential micronutrients.
Genetic factors, soil and weather conditions, and the use of fertilizers affect the final level of mineral and contaminant in rice. The human food chain is linked through vegetable, fruit, and tuber consumption to the nature of the soil which supplies, for example, mineral ions (copper (Cu), iron (Fe), zinc (Zn), as well as others.
On the other hand, many industrialized processes give rise to environmental problems with increased levels of contamination from such elements as arsenic (As), cadmium (Cd), chromium (Cr), mercury (Hg), and lead (Pb), which can have profoundly deleterious effects on health. The joint FAO/WHO (Food and Agriculture Organization/World Health Organization) requires detailed information on the concentration levels of elements in agricultural crops to assess the toxicological and nutritional significance of human and animal intake of these elements.
The normal “background” concentrations of such toxic elements as As, Cd, Cr, Hg and Pb must be known (that is, measured) to develop limitations on the intake of these elements from foods. Information on background levels provides guidance in evaluating the effect of such soil additives as phosphatic fertilizers and sewage sludge (that can contain As, Cd, Cr and Pb) as well as the effect of commercial food handling and processing steps which can result in food contamination. Intake of relatively low doses of these elements over a long period of time can lead to malfunction of organs and chronic toxicity. Determination of base-line levels of As, Cd, Cr, Hg and Pb in agricultural and horticultural crops is necessary to evaluate their toxicological significance and to establish action levels.
These potentially toxic metals can be present in the environment from both natural and anthropogenic sources and enter the human body via the two main routes of ingestion and inhalation. The latter pathways is of minor importance except for industrial workers and people living close to emission sources. For the general population, ingestion is the major route of intake with food being main contributor.
It is well known that in some countries, e.g., Bangladesh, irrigation of the paddy fields with arsenic-contaminated groundwater has led to arsenic buildup in paddy soil, with subsequent elevations in rice grain arsenic. But in 2005 some researchers reported high levels of As in rice from the United States. These high levels are attributed to the use of As herbicides in cotton fields in the early to mid-20th century and currently some of these areas are used for rice production. Some factors can have a great impact in the absorption of toxic metals by the human body. It has been demonstrated that under fasting conditions the gastrointestinal absorption of toxic metals can be appreciably increased. For instance, Pb absorption in the human adult is in the order of about 10-15% (up to 50% for children) and under fasting circumstances it raises up to 45%. Also, studies have shown that certain dietary factors such as milk fasting result in low calcium and vitamin D intake, causing iron deficiency and may enhance Pb absorption from the gut. Since rice is considered a main staple food and a major source of nutrients for the poor who lack access to diverse foods, strict control should be enforced on the maximum level of toxic metals allowed in this important crop.
In terms of mineral composition, potassium (K ) is the most abundant mineral found in rice (brown, parboiled brown, milled and parboiled milled rice) followed by magnesium (Mg) and calcium (Ca). Among microelements, the presence of Cu, Fe, molybdenum (Mo), manganese (Mn), sodium (Na) and Zn in rice is outstanding. It is generally accepted that as greater amounts of rice bran are removed from the grain during milling and polishing, more vitamins and minerals are lost. Milled rice shows a significantly lower content of K, Mg, Mn, Na and Zn than brown rice, but the milling process seems to have little influence on the Ca, Co, Cu and Fe levels.
The distribution pattern of metal ions in the rice grain is not yet clearly known. Some authors point out that microelements (Cu, Fe, Mn and Zn) are likely to be uniformly distributed in the grain while macro elements (phosphorus (P), K, Ca and Mg) seem to be present mainly in the external layers of the grain (aleurone and pericarp). But research has shown that some element concentrations are highly affected by the milling process—for example, Fe content decreases with milling, which suggests that much of the Fe is in the outer layers of the grain. The effects of rice grain processing with regard to the mineral levels in the edible product (i.e., milled and parboiled rice) are still being evaluated.
On the other hand, the parboiling process seems to have none to little effect on the mineral content in rice. It has been reported that parboiled rice is of superior nutritional value in comparison to milled rice, mostly due to the retention of minerals and water-soluble vitamins. The higher retention of micronutrients in parboiled rice has bees assigned to their solubilization and migration to the center of the grain and their setting during the starch gelatinization process. However, it has been discussed that the concentration of some microelements dropped after parboiling process, then it can be assumed that the minerals spread to the external layers of the grain while soaked and steamed and are subsequently removed by milling.
Nevertheless, the significance of the nutritional benefits of parboiled rice is still arguable, mainly due to the lack of uniform commercial processes applied in different countries. It is believed that the retention pattern of some minerals is the result of the interaction of different factors such as mineral location in the grain and their solubility during soaking, different ratios of migration, as well as variations in the hydrothermal process and milling resistance of the parboiled grain. Further studies need to be carried out to achieve a more complete understanding of mineral retention.
The amount of people that the great supply of dietary energy comes from rice is colossal, therefore severe control and careful attention should be paid to its contamination level and mineral content. Rice is an important staple food, so any increase in mineral concentration might well have a significant impact on human nutrition and health.
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