Speciation Of Selenium In Rice And Its Bioaccessibility

Objective: Selenium(Se)is a necessary trace element in human body,and it participates in the formation of many important enzymes in human body.Rice is an important source of daily Se intake for human body,and the Se content and its speciation in rice determines the utilization efficiency and nutrient level of human selenium.In this paper,we compared the difference of speciation and bioaccessibility of Se in rice between Yichun typical longevity area and the adjacent natural villages.At same time,we compared the difference of Se speciation and bioaccessibility of Se in rice by root-box experiment with adding exogenous selenium or not.In addition,we discussed the effects of cooking process on Se loss and bioaccessibility of Se in rice.It may provide certain scientific evidences for improving human Se nutrition level and strengthening effectivity of supplying Se for human.Method: In this study,the rice samples were collected and processed from field investigation and greenhouse root-box experiment,and the speciation and bioavailability of Se in the samples were analyzed.Field investigation and sampling were carried out mainly in Wentang Town and adjacent natural villages(regared as control)in yichun typical longevity area.The root-box experiment carried out in the Guangdong academy of agricultural science.And the Se-enriched rice varieties were used as material.The root box experiment set four soils sodium selenite treatment levels(0.0,0.5,1.0,and 0.0 mg/kg).Related indicators of Se in rice samples were analyzed after hulled(not polishing)and smashed.The total concentration of Se in rice grains was determined by hydride generation-atomic fluorescence(HG-AFS),and the organic Se in rice was determined by the continuous water extraction with dialysis bags(with the retention molecular weight of 8000 ~ 14000 Da).The concentration of protein in rice was determined by biuret method,and the glutelin was extracted by alkaline extraction and acid precipitation.The speciation of Se in rice and its glutelin were analyzed by liquid chromatography with atomic fluorescence spectrometry(LC-HG-AFS)after enzymatic hydrolysis.At the same time,the bioaccessibility of Se in rice was determined by in vitro simulation.Inaddition,the concentration and speciation of Se in cooked and uncooked rice grains were determined.Results:(1)The concentration of Se was no significant difference between the longevity area and the control group rice grains(P > 0.05).In general,the speciation of Se in rice grains is mainly organic selenium(about 50%).The percentage of organic rice Se in Yichun longevity area rice(78.67 ± 13.52%)was significantly higher than that in the control(65.19 ± 8.73%).Although the percentages of glutelin Se in longevity area rice(53.73 ± 8.27%)was lower than the control(66.42 ± 12.48%)(P < 0.05),the percentage of Selenomethionine(SeMet)in longevity area rice(65.29± 14.54%)was significantly higher than the control group(38.50 ± 6.83%)(P < 0.05).The results of in vitro simulation showed the bioaccessibility of Se in longevity area and control group rice respectively were 55.58 ± 10.53 % and 37.42 ± 9.13%,the former was significantly higher than the latter(P < 0.05).(2)The organic Se,glutelin Se and SeMet concentration increased with increasing of soil Se levels(0.0、0.5、1.0 and 5.0 mg/kg),and the percentage of SeMet in gluten increased while the percentage of organic Se decreased.At the same time,the percentage of glutelin Se in rice was closely related to the percentage of rice protein,both of which presented the change rule of inverted "U" with the increaseing of exogenous Se treatment levels.In general,the bioaccessibility of Se varied in the range of 42.08% ~ 76.16%.The Se species,SeMet,selenite,and some unknown Se forms in rice grains could be utilized by human body,and the percentage of SeMet and the bioaccessibility of Se in rice increased with increasing of Se treatment levels.(3)The percentage of organic selenium in Se-enriched rice from the root-box experiment was lower than non Se-enriched rice from longevity area(P < 0.05),but the percentage of protein and glutelin Se was not significant difference(P > 0.05).The percentage of SeMet in glutelin in non Se-enrich rice(65.35 ± 14.94%)was significantly higher than Se-enriched rice(38.82 ± 10.80%)(P < 0.05).But the bioaccessibility of Se in non Se-enrich rice(55.34 ± 10.77%)slightly lower than Se-enrich rice(57.04 ± 9.00%)(P > 0.05).The percentages of SeMet in Se-enrich rice(25.20 ± 16.83%,n = 4)was approximately twice as high as that in glutelin(12.63 ± 4.98%,n = 4),which indicated that only half of the SeMet derived from therice glutelin.(4)The concentration of Se in cooked rice decreased about 5% compared with uncooked rice.At same time,the bioaccessibility of Se in uncooked rice,respectively named rice 1(77.99 ± 5.45%)and rice 2(77.44 ± 3.69%),were higher than cooked rice(respectively 70.49 ± 2.59% and 68.06 ± 4.45%)(P < 0.05).Accordingly,the percentage of SeMet in the simulated digestive solutions of uncooked rice(respectively 60.62 ± 2.94% and 63.47 ± 1.32%)also higher than cooked rice(respectively 52.87 ± 0.12% and 50.45 ± 2.84%)(P < 0.05).Conclusion: Although the concentration of Se in rice in longevity area was not high,the percentage of organic Se and SeMet was relatively higher.Also,organic Se and SeMet were easily absorbed by human.So it is likely to be closely related to the local people longevity and healthy.The exogenous sodium selenite can,to a certain extent,increased the concentration of total Se,organic Se,glutelin Se and SeMet.At same time,the percentage of SeMet both in rice and glutelin increased gradually with increasing of Se treatment levels,as well as the bioaccessibility of Se in rice.The percentages of both organic Se and SeMet in glutelin in non Se-enrich rice higher than Se-enrich rice.That maybe related to the decrease in the efficiency of inorganic Se converse to organic Se.Therefore,the exogenous sodium selenite can,to a certain extent,increased the concentration and percentage of SeMet in rice,as well as the bioaccessibility of Se in rice.In addition,the cooking process may cause some loss of Se in rice and reduce the absorption and utilization of Se for human body.