Analysis On Irradiation-induced Degradation Mechanism And Radiolytic Products Of Aflatoxin B₁

Aflatoxins (AF) is the secondary metabolites formed mainly from the Aspergillusflavus and Aspergillus parasiticus, and the International Agency for Research on Cancer(IARC) has classified aflatoxin Bl(AFB₁) as a Group1A human carcinogen, so theresearch and security management about it has been paid more and more attention. Sinceaflatoxins are widely distributed in food and feed, the World Health Organization hasconsidered the control of AFB₁pollution in food security as an very important goal of theglobal public health management. Ionizing radiation technique (IRT) is a new techniquewhich could degrade AFB₁in food effectively. However, the degradation mechanism ofAFB₁in food and the research about radiolysis products were rarely reported. Thisresearch has been designed from simple to complex and we studied AFB₁in the methanol-water system, water system and peanut meal. The effects of the degradation in differentsystems were stated and radiolysis products of AFB₁in different mediums were studiedrespectively. Thus, we preliminarily discussed the degradation mechanism by irradiation ofAFB₁in water and the results are listed as follows:(1) We confirmed that gamma rays irradiation can depredate AFB₁in water-methanolsystem (V:V=60:40) effectively. Results showed that the greater the radiation dose was, thehigher the degradation rate of AFB₁could be. The degradation rate of AFB₁(0.1mg/L)were over80%with the irradiation dose of4kGy in methanol-water system, anddegradation rate of AFB₁(0.1mg/L) in methanol-water solution reached to96%at6kGy.We have also confirmed that the lower initial concentration of AFB₁was, the easierdegradation of AFB₁could be. The degradation rate of AFB₁in methanol-water solution(0.1mg/L) at3kGy was58.8%, which was approximately consistent with the degradationrate (58.8%) of AFB₁in methanol-water solution (10) mg/L) at6kGy.(2) LC-Q-TOF was used as a tool for detection of radiolysis products. We studied2₅radiolysis products of AFB₁which was produced in methanol-water system at differentirradiation dose. These radiolysis products which have many types and low concentrationswere analyzed by TOF to obtain the basic information of ion fragments (molecular weightand pieces) and then this result was combined with the supplementary information ofstructures by tandem mass spectrum to find structural information of₇radiolysis products(C₁₇H₁₄O₇, C₁₆H₁₀O₅, C₁₄H₁₀O₆, C₁₈H₁₆O8, C₁₆H₁₄O₇, C₁₇H₁₄O8(a)and C₁₇H₁₄O₈(b)) at differentirradiation doses. Through the structure analysis, it was preliminarily showed that theradiation-induced degradation of AFB₁was followed by free radical reaction.(3) We assessed the toxicity of radiolysis products of AFB₁. Comparison study showedthat six out of the seven main radiolysis products had been through addition reactionhappened on the Furan Ring, which led to the reduce of unsaturation. According to theanalysis of quantitative structure-activity relationship, the double bond of Furan Ring is thetoxic activity structure and also the main combination part which can cause primary cancer of humanbeings. Therefore, the toxic of the radiolysis products were lower than AFB₁withthe same concentration.(4) Results showed that the degradation of AFB₁in water solution is much easier, andafter irradiation the turbid liquid became clarified. Because the collision between AFB₁turbid liquid and water was not always homogeneous, irradiation degradation effects can tkeep constant. However, the general trend is that the bigger the radiation dose is, the higherthe degradation rates are. The degradation rate of AFB₁(20mg/L) turbid liquid afterirradiated twice at₁₀kGy is94.3%and₇4.9%, respectively. Compared to methanol waterdissolved AFB₁, the species of radiolytic products of AFB₁suspension was less, whichindicated that it might be water induced free radicals that caused the main reaction.Molecular structure information of four radiolytic products from water dissolved AFB₁wasobtained as well by the detection of LC-Q-TOF-MS. The molecular formulas of all theradiolysis products were C(17)H₁₄O₇, C₁₇H₁₂O₇, C₁₆H₁₂O₇and C₁₆H₁₀O₆, in which only C(17)H₁₄O₇has the same structure in methanol-water system(5) We studied the irradiation degradation mechanism of AFB₁in methanol-watersystem. The radiolysis procedure was regulated by setting different atmosphere andadditives including saturated N₂, saturated N₂O, saturated N2+isopropanol, saturatedN₂+H₂O₂and saturated N₂+tert-butyl alcohol. The radiation chemical yield G-values andthe irradiation degradation effects of each atmospheres were compared. The main freeradicals involved in the irradiation of AFB₁were determined. The contribution order offree radicals in radiolytic products of AFB₁was: HO₂,O₂->OH>eaq->H.(6) We studied the contamination of AFB₁in foodstuffs-peanut meal. Studies provedthat high levels of AFB₁in peanut meal were still able to degrade effectively. Thedegradation rate of sample (20mg/kg) after irradiation by20kGy was14.4%, and waterplayed a powerful role in radiation degradation process after the comparison with thedegradation rate of water, methanol-water and solid AFB₁. After the optimized extractionpurification steps in a very special way, the25radiolysis products was detected by usingLC-Q-TOF-MS in ion monitoring mode. Finally we foundtwo major products,319.0813m/z and331.0821m/z, which had same structure with those in methanol-water system.