Submitted to: Asian-Pacific International Society on Toxinology Congress
Publication Type: Proceedings
Publication Acceptance Date: 10/29/2008
Publication Date: 12/2/2008
Citation: Shier, W.T., Abbas, H.K. Mycotoxin Contamination of Rice Grown in the Southern USA. Proceedings of the 8th International Society on Toxinology Asian-Pacific Meeting on Animal Plant and Microbial Toxins, Hanoi, Vietnam, Dec. 2-6, Page 90. 2008 Interpretive Summary: Rice is the most significant staple food cereal crop around the world. Rice is considered least susceptible crop to mycotoxin contamination, however under unusual weather conditions, rice becomes susceptible to fungal (mold) toxins called aflatoxin and fumonisin. This study showed that unpolished rice and rice fractions showed different degree of contamination with these toxins. Rice is usually resistant to contamination from toxins such as aflatoxin and fumonisin, two poisons that can be present in food. The study showed that this resistance is due to resistance of rice to fungal infection. Also, rice that had gotten wet during storage contained aflatoxin, showing that near ideal storage conditions are required. This work allows us to make recommendations to growers and industry regarding storage. Also, this study indicates that white polished rice, the most consumable form, is relatively free from mycotoxins.
Technical Abstract: 1. Background Rice is widely regarded as unusually resistant to mycotoxin contamination, despite having kernels that provide an exceptionally favorable medium for fungal culture and mycotoxin production. We are interested in learning why rice is mycotoxin resistant, and if any of the resistance mechanisms can be transferred to mycotoxin-susceptible crops such as maize (corn). Among the plausible mechanisms for mycotoxin resistance in rice are the following: (1) rice kernels develop and mature in a structure anatomically resistant to fungal infection; and (2) all upland rice and most paddy rice is harvested in the dry season, which ensures effective drying and most of its storage time under near ideal conditions. 2. Material and methods Plausible explanations for resistance to mycotoxin contamination in rice have been investigated under field conditions by examining rice harvested in situations in which one of the proposed resistance mechanisms was not operative. 3. Results To examine the importance of mechanism (1) (resistance of kernels to fungal infection) in enabling mycotoxin resistance in rice, we examined rough rice samples (67 in Arkansas in 1998; 33 in Mississippi in 1999 and 2000) harvested from fields exhibiting Fusarium sheath rot disease caused by F. proliferatum that infected rice plants despite their resistance. Of the samples 50% contained Aspergillus flavus (47% toxigenic); 26% contained aflatoxins (20-270 ppm); 27% contained F. proliferatum (84% toxigenic); 11% contained fumonisins (0.5-6.2 ppm). A sample of rough rice with 5.13 ppm fumonisin yielded hulls with 16.8 ppm; brown rice, 0.87 ppm; bran, 3.53 ppm; but no fumonisin was detected in polished rice. To examine the importance of mechanism (2) (storage under near ideal conditions) in enabling mycotoxin resistance in rice, we measured mycotoxin levels in lots of rice that had gotten wet during storage. No fumonisin was detected, but aflatoxin was present in all samples of rough rice studied (average, 143 ppb; range: 50-272 ppb). A sample of rough rice with 61.8 ppb aflatoxin yielded hulls, 16.3 ppb; brown rice, 88.3 ppb; bran, 587 ppb; polished rice, 63.5, cooked rice, 28.4 ppb. 4. Conclusions The results obtained so far are consistent with mycotoxin resistance in rice being due at least in part to resistance to fungal infection and near ideal processing and storage conditions.