Detection Methods for Organic Sulfur Pesticide Residues in Organic Vegetables and the Hazards of Sulfur Residues in Food

Testing for organic sulfur pesticide residues in organic vegetables - do you know how to do the testing and what the methods are? If not, let's take a look together.

        Pesticide residue testing reports for organic vegetables have appeared in consumer reports for a second time, and the pesticides detected were all "dithiocarbamate" fungicides, commonly known as "organic sulfur." Organic sulfur is different from sulfur powder. Sulfur powder is the earliest pesticide used by humans for pest and disease control. The fungicidal action of sulfur powder works as follows: after microorganisms come into contact with sulfur powder, the sulfur element enters the cell's electron transport system and is reduced to hydrogen sulfide, which then binds with various proteins, causing damage and death. When the temperature is above 25 degrees C, sulfur powder also has a fumigation effect, which helps control downy mildew. Organic sulfur fungicides include Farbam, Dithane, Maneb, Mancozeb, Zineb, Zinc Manganese Zineb, Methyl Zinc Zineb, and others. The most famous is the 80% wettable powder "Dithane 45" produced by the former Rohm and Haas Company, commonly called "Dithane powder" by farmers. These pesticides belong to the ethylenebisdithiocarbamates (EBDCs) class, which is where the name "dithiocarbamate" comes from. These compounds decompose to produce isothiocyanate groups (-N=C=S), which react with -SH groups on amino acid molecules that make up proteins, causing the proteins to lose their function. In addition, the heavy metal element portion undergoes chelation with enzyme proteins within cells, which also enhances the fungicidal effectiveness. Because of their good efficacy, low likelihood of generating pesticide resistance, broad control spectrum, and low toxicity to organisms, they are very commonly used either as single agents or in mixtures. Since these pesticides were introduced in the 1950s, they remain the most heavily used fungicides to this day.

       There are three analytical methods for "dithiocarbamates." The earliest and most widely used is the colorimetric method published by Dr. G. E. Keppel in 1969. In this method, the pesticide is heated in an acidic matrix to generate carbon disulfide (CS2) gas, which is introduced into a solution containing amines and copper acetate to form a yellow complex, then detected with a spectrophotometer at a wavelength of 435 nm. This method was announced as a national standard testing method around 1980. It cannot distinguish between different types of "dithiocarbamate" pesticides, and during analysis, steps must be added to remove sulfur-containing interfering substances using adsorption tubes or adsorbents. Some vegetables such as dried shiitake mushrooms, onions, and radishes easily cause interference. The second method is headspace sampling chromatography. The pretreatment principle is the same: the generated carbon disulfide is directly introduced into the chromatograph and detected using a flame photometric detector with a sulfur filter at a wavelength of 393 nm. This method was announced as a national standard testing method in 2000. It is faster than the previous method and can avoid most interference. The third method is high-performance liquid chromatography (HPLC), in which metal ions are chelated with a chelating agent, then the dithiocarbamate ions are methylated and detected using a high-performance liquid chromatograph. This method can classify dithiocarbamates into four groups for analysis and is not subject to interference from sulfides and other substances. However, the analytical steps are complex and difficult, requiring an experienced analyst to perform. This method was announced as a national standard testing method in 2003.

        When the colorimetric method was widely used, the problems caused by interference were constantly mentioned. In addition to sulfur-containing compounds in crops, glucosinolates commonly present in cruciferous vegetables, especially Brassica crops such as cauliflower and rutabaga, are easily converted by enzymes within the plant to produce thiocyanates, which in colorimetric analysis produce the same color reaction as dithiocarbamate fungicides, resulting in false positive detection results.

        The differences in detection among these three methods can be compared using the analysis of 200 organic samples collected and submitted for testing in May of year 93 of the Republic of China calendar. Of the 200 samples, 29 showed positive reactions when tested by the colorimetric method. When these 29 samples were retested using headspace sampling chromatography, 18 were detected. After the 18 samples were tested using high-performance liquid chromatography, it was confirmed that none had any dithiocarbamate fungicide residues.

        In order to make the use of the three methods complement each other, we suggest that for dithiocarbamate residue analysis in organic vegetables, if the colorimetric method detects a positive result, one can first compare whether there is a difference before and after washing, because organic sulfur fungicides are non-systemic pesticides and are very easily removed by washing. If there is no difference, it may be due to interfering components in the plant itself, and high-performance liquid chromatography should then be used for confirmation.