Determination Of Natural Level In Vegetables And Toxicity Of VSCs And Ethyl Formate And Degradation Of VSCs' Precursor

Research on methyl bromide alternatives is one of the hot issues in the area of current pest control. To study and develop new alternative fumigant, re-evaluate current fumigants are the focus of methyl bromide alternatives research. The widely used methyl bromide, ethyl formate (EtF) and volatile sulfide compounds (VSCs) such as carbonyl sulfide (COS), carbon disulfide (CS2), dimethyl disulfide (DMDS) are all important fumigants for pest control. Carbonyl sulfide has been registered in Australia in the 1990s to control pests in food products. As an environment-friendly fumigant, VapormateTM, a formulation of ethyl formate with CO2 has been registered in Australia. Meanwhile volatile sulfide compounds (VSCs), having a low boiling point, low-threshold olfactory (PPb level), being chemically active and with poor stability characteristics, makes sampling and analytical techniques publicly known international problems. In order to solve the technical problems of detection of volatile sulfide compounds (VSCs) and ethyl formate, to clarify the impact of fumigant residual detection on the natural level of Volatile Sulfide Compounds (VSCs) and ethyl formate in vegetables, to keep the toxicity of fumigants from being enhanced by carbon dioxide (CO2) and to find out methyl bromide alternative fumigants, to solve the difficult technical problems of wide usage and application of ethyl formate, to clarify the degradation process of VSCs'precursors and reveal the metabolism relationship between VSCs and its precursors, the author done research on determination of natural level in vegetables and toxicity of VSCs and ethyl formate and degradation of VSCs'precursor. This study could provide technical support for the residue detection of VSCs and ethyl formate in imported foodstuffs. This study also could provide technical support for methyl bromide alternatives research and toxic and harmful substances control. It will be of significance for new fumigants of methyl bromide alternatives to be developed and applied. The main results are as follows:1. Ethyl formate and DMDS have good control over storage pests, and should be strongly promoted for treatment of storage pests as methyl bromide alternative. At the same time, DMDS is of great potential value as a fumigant for storage pest control. It is necessary to carry out a thorough study on pest control technology with DMDS, with an expectation of obtaining a new fumigant of methyl bromide alternative.2. Investigation of the toxicity of fumigation on adult and mixed stages of the three most tolerant insect species -- Sitophilus oryzae (L.), Rhyzopertha dominica (F.) and Tribolium castaneum (Herbst) -- under a range of EtF,DMS and DMDS doses in a Flow-through Fumigation System (FTFS) shows that the FTFS procedure offers a constant concentration of fumigant with low CO2 concentrations and ensures that the fumigant toxicity is effectively not compromised by CO2. In comparison with chamber fumigation method, the toxicity data of EtF,DMS and DMDS obtained from this experiment is more accurate by using FTFS.Ethyl formate could provide good disinfestations of all three adults'species at 25°C and 70% r.h. for the 6 h exposure. The Ct products of ethyl formate for adult Sitophilus oryzae, Tribolium castaneum and Rhyzopertha dominica at 25°C and 70% RH for the 6 h exposure were respectively: Ct50 107.8, 108.8 and 72.8 mg h L-1.It was found that under same experimental conditions high control over mixed stage cultures of T. castaneum and R. dominica can be obtained by ethyl formate at 25°C and 70% RH for the 6 h exposure to 43.34,80.72,99.08,117.74 and 146.49mg/L. The dose rates of EtF of 80.72,99.08,117.74 and 146.49mg/L gave good control of mixed stage cultures of S. oryzae in 6 h exposures at 25°C and 70% RH, but poor control of S. oryzae pupae.The LC50 of DMDS for adult S. oryzae, T. castaneum and R. dominica can be obtained by DMS at 25°C and 50% RH for the 6 h exposure to15.69,17.02,17.80 and 18.02 mg/L were respectively: 16.86 mg/L,16.95 mg/L,15.31mg/L. It was found that high control of adult T. castaneum 25°C and 50% RH for the 6 h exposure to 51.81,53.02,53.51 and 54.50 mg/L. While adult S. oryzae had no response and adult R. dominica remained more tolerant to DMS. In this condition, LC50 of adult R. dominica was 51.11 mg/L.3. To establish technicality of HS-GC-PFPD with good separation of five species of VSCs'chromatographic peak and HS-GC-FID in good chromatographic peak separation of EtF, formic acid and ethanol by methods adopted for temperature programming, optimizing chromatography conditions and determining response's index and so on.4. To determine natural level of VSCs and EtF in broccoli, cabbage, cauliflower, radish, lettuce and garlic and emission of VSCs and EtF in acidity by using the above technicality of HS-GC-PFPD and HS-GC-FID.Natural level of EtF and VSCs varies in different vegetables and parts. The different vegetables also contain various VSCs'species. The major releases of VSCs'species in vegetables were COS, DMS and DMDS. High natural level of COS and DMS in radish and high natural level of H2S, CS2, DMDS and EtF in different vegetables are respectively: radish leaves, broccoli, garlic and lettuce. Natural level of H2S, COS, DMS, DMDS and EtF in vegetables ranged between 0.12～1.11mg/kg,0.05～4.98mg/kg,0.07～8.11 mg/kg,0.08～3.53mg/kg,0.001～3.25mg/kg and 0.04～0.71mg/kg respectively. The major releases of vegetables in acidity were EtF, DMS, COS, DMDS and part of CS2. The emission of VSCs in acidity was smaller within five hours and different vegetables released different species and various emission of VSCs. High COS and DMDS emission were found in radish, high CS2 and EtF in lettuce and high DMS in broccoli in acidity. Compared to natural level of VSCs and EtF in vegetables, DMS and EtF emission of vegetables in acidity were much higher than in water, being respectively, 0.08 ~ 12.18 mg / kg and 0.04 ~ 7.32mg/kg.5. Initial detection for the VSCs'precursor degradation trends in vegetables has been found. To simplify the complex process of metabolism and degradation of SCAA and SMM with"Black Box method", choosing Met, Cystine, Cys of three SCAA and SMM as possible precursor of VSCs, to research VSCs'precursor degradation in vegetables. The results indicate that SCAA and SMM are important precursor substances of VSCs in vegetables. There were obvious differences in the variety and emission of VSCs under different conditions, and in the release of VSCs at different times. SMM is the main precursor substance of DMS, and Cystine is one of the precursor substances of DMS in vegetables. Cys and Cystine can notably reduce the release of COS in cell-free or enzyme-free extraction. In cell-free extraction, the emissions of COS, DMS, DMDS ranged between 1.06mg/L～26.79mg/L,0.58mg/L～9.16mg/L, 0.68mg/L～11.23mg/L respectively. In enzyme-free extraction, the emissions of COS, DMS, DMDS ranged between 0.61mg/L～5.71mg/L,0.27mg/L～3.55mg/L, 0.08mg/L～2.49mg/L respectively.