High-Selenium Pinto Beans as a Value-Added Product
April 18, 2005
1. Determine the Selenium (Se) content of field-grown pinto beans from the vicinity of Jamestown, North Dakota.
2. Quantify the contributors to variation of the Se content in pinto beans.
3. Develop a map of the Jamestown area that predicts the feasibility of growing high-selenium beans in that area.
Benefits to North Dakota/Minnesota bean growers
A recent conference hosted by the Grand Forks Human Nutrition Research Center highlighted the potential benefits to human health of enhancing the food supply with the essential trace element selenium (Se). Selenium is needed in moderate amounts (55 micrograms per day) for nutritionally essential functions such as producing enzymes that guard against oxygen stress in cells. However, a clinical study conducted in humans found that consumption of an additional 200 micrograms of selenium per day reduces the incidence of cancer, especially prostate and colo-rectal cancer. A large study (approximately 32,000 subjects) is currently being conducted in an attempt to confirm the previous finding of selenium-mediated reduction of cancer.
The implications of these studies for agriculture are substantial; a demand is developing for selenium-enriched foods, and this demand will certainly increase if the present prostate cancer trial yields positive results. Plant foods accumulate selenium partially in direct relationship to the selenium concentration of the soil, so soils enriched in selenium may produce selenium-enriched crops. North Dakota has areas with very high concentrations of selenium in the soil, and many of these areas are in regions where pinto beans are produced. Consequently, pinto beans from North Dakota are a potentially valuable source of supplemental selenium; this attribute could be used to produce a value-added product and/or help to market pinto beans from North Dakota.
Pinto beans were collected from 78 different fields and 37 different producers broadly organized into five different geographical regions across North Dakota (Figure 1). Dried beans were crushed, dissolved in acid and analyzed for total selenium concentration (by hydride generation atomic absorption spectroscopy).
The mean selenium concentration of all samples was 486 nanograms selenium/g (or 0.49 micrograms per gram), but there was wide variation (136 to 983 nanograms per gram; standard deviation of 253ng/g). Samples were analyzed as five distinct geographic clusters; selenium concentrations in 4 of the 5 clusters were similar while the Gilby area was lower than the rest (however, there were only 4 samples from this area). Wide variation was noted in all areas except Gilby.
These results are what we predicted based on our understanding of selenium accumulation in plants.
First, as expected beans grown on North Dakota soils had selenium concentrations well above average. The USDA nutrient database gives average nutritional values for many foods, including canned pinto beans; the selenium content is listed as 71 nanograms per gram. Because canned beans are 75% water, this corresponds to a dry bean selenium concentration of 280 nanograms per gram (0.28 micrograms per gram). By comparison, 4 of the 5 regions in North Dakota had selenium concentrations almost twice that.
The second expected result is the extreme variation in selenium concentrations. High selenium soils occur where a selenium-rich rock layer reaches the surface and becomes weathered, consequently high and low selenium soils may occur within a few miles of each other.
The geographical origination of the samples was determined by having producers record field coordinates using either the Public Land Survey System (PLSS) or global positioning system (GPS); this information was converted to latitude/longitude coordinates using a program developed by the North Dakota State Water Commission. This procedure allowed the results to be mapped to specific geographical points in North Dakota and development of a preliminary map of average selenium concentrations in pinto beans.
There are no doubt many other geographical factors that may contribute to the selenium concentration of pinto beans. An excellent tool for assessing other geological and geographical factors is the Geographic Information System (GIS), a database that allows analysis of extensive environmental data for a specific geographic coordinate. As a first step in this analysis, the present data have been entered into a mapping program, allowing development of a preliminary contour map of selenium concentrations.
Information regarding topography, precipitation, soil type, and planting history, among other factors is being incorporated into the GIS to develop a model that may then be used to more accurately predict spatial patterns of selenium concentration.
The data collected thus far demonstrate that pinto beans grown in North Dakota do accumulate sufficient selenium to make them, on average, an excellent source of dietary selenium. However, the initial data also show a large degree of variation in selenium concentrations. Additional analyses of a greater number of samples collected during a second growing season is needed to determine factors that result in this variation, and such information will allow bean growers to predict with more accuracy where and under what conditions high-selenium crops may be grown.