Chlorate Residues in Plant-Based Food: Origin Unknown
Dr. Ingrid Kaufmann-Horlacher, Ellen Scherbaum, Dr. Diana Stroher-Kolberg, Cristin Wildgrube
Chlorate is a universally effective herbicide. Until 1992 several herbicides containing sodium chlorate were authorized for use in Germany, the most well-known of which was "Unkraut Ex" (weeds ex). Since 2010, however, there has been an EU-wide ban on the application of plant protection products containing chlorate. According to EU regulations, the standard maximum residue limit (MRL) of 0.01 mg/kg for unauthorized substances is valid for chlorate residues in food.
After CVUA Stuttgart discovered perchlorate residues in plant-based foods in 2012 and 2013, and quantities greater than 0.1 mg/kg were detected in 2.3 % of the samples, a method was developed with which both chlorate and perchlorate residues could be quickly and easily detected in food. Using this method, CVUA Stuttgart analyzed fruit, vegetable and cereal*) samples for the presence of chlorate in a special statewide monitoring program. Among the 1,087 samples analyzed thus far 266 (24.5%) have been found to contain chlorate residues in a range of 0.01 to 2.7 mg/kg. The median value for all of the samples with chlorate residues was 0.022 mg/kg. The highest levels of chlorate measured were in beans (0.84 mg/kg), broccoli (0.97 mg/kg), basil (1.4 and 1.9 mg/kg) and coriander (2.7 mg/kg), all from Cambodia, and chili peppers (0.84 and 0.92 mg/kg) from Uganda. Among the 197 samples from organic food production 51 (25.9%) contained chlorate levels of 0.01 to 0.15 mg/kg, with a median value of 0.023 mg/kg. In 88 samples (8.1 % of all the analyzed samples) not only chlorate but also perchlorate residues were detected, whereby high levels of chlorate were not found in conjunction with high levels of perchlorate. Samples whose chlorate content are verified as exceeding the MRL of 0.01 mg/kg may not be marketed. According to the assessment made thus far, unwanted health problems (mainly negative effects on thyroid function and damage to the erythrocytes (red blood cells) such as methaemoglobin formation and hemolysis) are not of concern for the great majority of samples. However, the chlorate content in three samples (broccoli from Cambodia, prepared carrots and grapefruits from the USA) was so high that health effects cannot be ruled out.
The cause of the residues has not yet been determined. In the EU it cannot be precluded that unauthorized applications leading to the accumulation of chlorate residues are also taking place.
*) Subsequent analyses of cereals and cereal products did not confirm the positive findings from the monitoring program for these product groups.
Substance, Occurrence and Application
Illustration 1: Chemical
Structure of Chlorate.
Chlorates are the salts from chloric acid. They are strong oxidizers and are used, e.g. in the bleaching of paper, tanning of leather, surface treatment of metals and in pyrotechnics. Due to their disinfecting properties, they can also be used in cosmetics and, for oral hygiene purposes, in mouthwash and toothpaste.
Chlorates are also effective as herbicides and disinfectants (biocides). The herbicidal effect is based on the oxidative destruction of the plant tissue via the release of oxygen. Chlorates don't work selectively, but rather as total herbicides. Until 31 December, 1992 several plant protection products (herbicides) containing sodium chlorate were authorized for use in Germany. Since 2010, however, there has been an EU-wide ban on the use of herbicides containing chlorate.
Chlorate is a by-product of using chlorine or chlorine dioxide for the disinfection of drinking- and non-potable water, as well as of bleaching agents (sodium hypochlorite solution).
The source of the chlorate residues detected in the analyzed samples is not yet known. In general, various points of entry are conceivable:
- residues from the use of chlorinated washing water, the use of chlorine dioxide to disinfect washing water, or an illegal, direct chlorination of food by means of a procedure from which chlorine is formed "in situ", e.g. chlorine electrolysis;
- environment, e.g. atmospheric deposits, contaminated sprinklers or irrigation water, etc.;
- unauthorized application of chlorate as an herbicide;
- absorption of chlorate via plants from soil contaminated either from the environment or due to an earlier application of chlorate as an herbicide or from contaminated fertilizer.
Chlorates are substances that are effective as herbicides. In 2008 authorization for the use of chlorate was denied and since 10th May, 2010 the application of plant protectors containing chlorate has been banned in all EU countries. Sodium chlorate is also no longer permitted for use in biocide products. A direct chlorination of food is forbidden in the EU.
Chlorates used in plant protection products are covered under Regulation Nr. 396/2005 (see also Info Box "Definition of Residues"). Since the substance is no longer authorized and, thus far, no specific MRL for chlorate residues in food has been established, the MRL of 0.01 mg/kg provided in Regulation Nr. 396/2005, valid for all such cases in the EU, is applicable. According to the German Food and Feed Code (LFGB), food whose chlorate content has been verified as exceeding this generally applicable MRL may not be marketed.
defined in Article 3 2. (c) of Regulation (EC) No 396/2005:
’pesticide residues’ means residues, including active substances, metabolites and/or breakdown or reaction
products of active substances currently or formerly used in plant protection products which are present in or on the products, including in particular those which may arise as a result of use in plant protection, in veterinary medicine and as a biocide;
Based on this definition, residues from plant protector substances that find their way into food by means of contamination or application other than via pesticide products are also included.
Chlorate reversibly inhibits the absorption of iodide in the thyroid and, at high doses, especially in sensitive groups such as children, pregnant women or people with thyroid dysfunction or iodine deficiency, can cause unwanted health effects. In addition to affecting the thyroid function, chlorate can also cause damage to erythrocytes (red blood cells) such as the formation of methaemoglobin and hemolysis. Nevertheless, there is currently no acute reference dose (ARfD) for chlorate available. As an initial estimation, the Federal Institute for Risk Assessment (BfR), recommends applying the acceptable daily intake (ADI) of 0.01 miligram Chlorat per kilogram (kg) bodyweight, as derived by the World Health Organisation (WHO), as the tentative basis for calculating both chronic and acute risk. For risk assessment the general procedures for pesticide residues and the PRIMo Model from EFSA (the European Food Safety Authority's Pesticide Residue Intake Model) should be applied (see: http://www.bfr.bund.de/cm/343/vorschlaege-des-bfr-zur-gesundheitlichen-bewertung-von-chloratrueckstaenden-in-lebensmitteln.pdf).
When applying the EFSA PRIMo Model relevant to young children, there are no health risks associated with 99.7% of the samples analyzed. In three samples (0.3%), however, the calculated intake of chlorate was greater than 100% of the BfR-recommended acute reference dose of 0.01 mg/kg bodyweight. In these cases, unwanted health effects cannot be ruled out. In two further samples the amount of chlorate consumed would barely fall short of the recommended acute reference dose.
Acute Reference Dose (ARfD)
For the evaluation of plant protector substances that have a high, acute toxicity and that can cause health damage after just a single or short-term intake, the Acceptable Daily Intake (ADI) value is appropriate to only a limited extent. Since the ADI is derived from long-term studies, it is possibly inadequate as a measure of acute risk from residues in food. Therefore, in addition to the ADI value, a further exposure limit has been established – the so-called Acute Reference Dose (ARfD). The World Health Organization defined the ARfD as the amount of a substance one can consume over the period of one day or in one meal without resulting in any discernible health risk. Other than for the ADI, the ARfD value is not determined for every pesticide, but only for such substances that, when taken in sufficient quantities, could cause damage to one's health even after just one exposure.
A description of the analytical method developed by CVUA Stuttgart in 2013 can be downloaded from the following link to the website of the European Reference Laboratory for Single Residue Methods (LC-Method 1.3, see: http://www.eurl-pesticides.eu/library/docs/srm/meth_QuPPe.pdf)
Our Investigative Results
Since April, 2013 a total of 1,087 plant-based food samples have been analyzed for the presence of chlorate. In 821 samples (75.5%) either no chlorate was detected or the amount was under the limit of quantification of 0.01 mg/kg. Chlorate residue levels between 0.01 and 2.7 mg/kg were found in 266 samples. The median value for samples with chlorate residues was 0.022 mg/kg.
Illustration 2: Overview of Results for All Samples Analyzed for Chlorate since April, 2013.
Chlorate residues > 0.02 mg/kg were detected in 147 samples (13.5%); quantities > 0.05 mg/kg were found in 47 samples (4.3%); and residue quantities > 0.10 mg/kg were found in 26 samples, with a maximum value of 2.7. mg/kg.
An overview of the samples containing chlorate residues is presented by food group, as seen in Illustration 4. The median value for each food category, as well as the average, minimum and maximum values for all residues in the categories, are shown in Table 1.
Illustration 3: Overview of Samples by Range of Concentration (mg/kg).
Illustration 4: Overview of Samples, Itemized by Major Food Groups.
Subsequent analyses of cereals and cereal products did not confirm the positive findings from the monitoring program for these product groups.
None of the 19 analyzed potato samples contained chlorate. Of the 220 samples of leafy vegetables 40 were detected with chlorate. Nine of these samples contained quantities yet over 0.1 mg/kg, including lamb's lettuce from Germany at 0.29 and 0.16 mg/kg and from Italy at 0.18 mg/kg, rocket from Germany, at 0.19 mg/kg, spinach from Italy, at 0.24 mg/kg and basil from Israel, at 0.11 mg/kg. The highest quantities were in samples from Cambodia: two samples of basil, at 1.4 and 1.9 mg/kg and one of coriander, at 2.7 mg/kg.
(Samples with Chlorate
|Minimum mg/kg||Maximum mg/kg|
|Tree Nuts/Sweet Chestnut||14||5||42||0.020||0.033||0.01||0.07|
Chlorate residues were detected in 23.7% of the fruiting vegetable samples, whereby several of the samples contained higher amounts; the highest residues were in beans from Cambodia (0.84 mg/kg) and chili peppers from Uganda (0.92 and 0.84 mg/kg). A German cucumber contained 0.17 mg/kg, a French tomato, 0.075 mg/kg and a Spanish zucchini, 0.083 mg/kg chlorate.
|Land of Origin||No.
|Median (Samples With
High Quantities mg/kg
0.16 and 0.29
|Non-EU Countries||11||9||82||Basil (Israel): 0.11
Basil (Cambodia): 1.4 and 1.9 Coriander
|Other Non-EU Countries||36||8||22||Long Bean
0.84 and 0.92
|USA||4||4||100||Peeled Carrots: 0.28|
|Other Non-EU Countries||37||13||35||Pineapple
|Other Non-EU Countries||14||5||36||Pear (South Africa): 0.038|
|Other Non-EU Countries||44||7||16||Grapefruit (USA): 0.14|
|Turkey||9||7||78||Organic Sultana: 0.11 and 0.15|
|Other Non-EU Countries||3||3||100|
Morello Cherries: 0.15
|Germany||7||2||29||Pear Juice: 0.05|
The values for sprout vegetables were between 0.011 and 0.045, with the exception of a Chinese broccoli (Chinese kale, also named as kai-lan, consist mainly of leaves) from Cambodia, at 0.97 mg/kg. In the food category of processed vegetables - mainly deep-frozen spinach and peeled carrots - one sample of deep-frozen spinach contained 0.12 mg/kg. All four of the analyzed peeled carrot samples from the USA contained chlorate residues; one sample was detected with as much as 0.28 mg/kg. Among the citrus fruits 20.7% contained chlorate residues, with one grapefruit sample containing a high value of 0.14 mg/kg. One of the fruit product samples, cherry preserves from an unknown origin, contained 0.15 mg/kg. Most of the fruit products, however, were dried fruits, such as dried apricots and sultanas. In this group, high levels of up to 0.15 mg/kg were detected, although one must consider that the drying process itself causes an increase in residues. Two rooibos tea samples, one from South Africa, the other from an unknown source, also contained high values of 0.12 and 0.43 mg/kg. More details regarding countries of origin and conspicuous residue findings in reference to various food categories can be found in Table 2.
Chlorate and Perchlorate Residues
In 88 samples (8.1 % of all samples) both chlorate and perchlorate residues were detected.
Illustration 5: Samples with Chlorate vs. Chlorate and Perchlorate Residues.
High levels of chlorate residues do not necessarily go hand in hand with high perchlorate levels. The situation is inconsistent, however; leafy and fruiting vegetables are particularly noteworthy in that they are often contaminated with both chlorate and perchlorate residues (see Table 3).
However, the remaining food categories are significantly less likely to contain both types of residues.
Perchlorate as % of all Chlorate Findings*
|Tree Nuts /
Food from Organic Production
Among the 1,087 analyzed samples were 197 from organic cultivation. There was no significant difference between the organic and conventionally produced food samples in terms of chlorate residues. Values above 0.01 mg/kg were also found in the organic products. An overview of the chlorate findings in organic products is presented in Table 4.
|Median mg/kg (Samples with Chlorate
|Minimum mg/kg||Maximum mg/kg|
The investigative results do not give any suggestion as to possible causes of the chlorate residues.
The investigations will continue.
See also our Internet article "Chlorate Residues in Carrots Traced to Chlorinated Water Used in Post-Harvest Treatment"