Chlorate Residues in Carrots Traced to Chlorinated Water Used in Post-Harvest Treatment

Dr. Ingrid Kaufmann-Horlacher, Dr. Diana Ströher-Kolberg, Cristin Wildgrube, Giovanna Cerchia


Photo: Prepared carrots.

The search for the cause of chlorate residues detected in plant-based foods has led to a possible path of contamination. In the course of analyzing fruit and vegetables for the presence of chlorate residues under our food monitoring program, prepared carrots from the USA caught our attention with their chlorate residues of up to 0.54 mg/kg. The manufacturer declared in an initial statement not to have used any chlorine-containing pesticides. However, he pointed to their Hydro-Cooling procedure as a possible cause of the chlorate findings, for which chlorinated water is used. The chlorination of water can cause the formation of chlorate as a byproduct. Food that has been treated with chlorinated water can thusly become contaminated.


"Hydro-cooling" with Chlorinated Water

"Hydro-cooling" is a post-harvest procedure in which freshly harvested and, in some cases, prepared vegetables/fruit are quickly cooled in ice water to a temperature of 10 °C, to extend the shelf life and minimize the loss of moisture. To reduce the contamination of food with microorganisms during and after treatment, chlorine, sodium - or calcium hypochlorite is sometimes added to the cold water, depending on the procedure, handler, or the chilled goods. This procedure serves to kill off any pathogenic germs and contaminants.  
Post-harvest treatment with chlorinated water is permitted in the USA1). Based on generally available information on hydro-cooling procedures, the addition of chlorine or chlorine compounds provides a constant concentration of chlorine in the cooled water. According to the manufacturer in the USA, the chlorine concentration and redox potential is measured several times per day. However, the concentration of chlorate in hydro-cooler water is apparently not measured.   


Chlorate can be formed as a byproduct from the treatment of water with chlorine and the application of chlorine bleach (sodium - or calcium hypochlorite solution). The formation of chlorate as a byproduct is dependent on the water temperature, the length of application, and also on the level of organic contamination in the water. The more contaminated the water is, the more chlorine that is needed to purify it, thereby increasing the amount of chlorate formed as a byproduct. If only the chlorine concentration is monitored during the treatment of food in a hydro-cooler, the resulting chlorate concentration can rise substantially, unnoticed by the user, especially under constant, repetitive chlorination. The end result is the existence of chlorate residues in our food.


Illustration 2: Quantity of Chlorate in Prepared Carrots from Various Manufacturers and Countries.

Illustration 2: Quantity of Chlorate in Prepared Carrots from Various Manufacturers and Countries.


Investigatory Results

A total of 13 prepared, packaged and refrigerated carrots were analyzed for the presence of chlorate residues. Eight of the samples came from the USA and four samples from the Netherlands; the origin of the final sample is unknown. All of the prepared carrots originating in the USA had chlorate residues, one of which contained the highest amount of chlorate, at 0.54 mg/kg (see also Illustration 2). Four other samples from the USA, marketed under the importer's name, contained chlorate residues of between 0.036 and 0.071 mg/kg. In the same timeframe as with the prepared carrots, a total of 16 fresh carrots from various commercial enterprises were also analyzed in order to compare fresh vs. prepared carrots in terms of chlorate residues. The levels of chlorate residues detected in all of these samples were under the limit of detection of 0.01 mg/kg. 

Info Box

Maximum Residue Level

Chlorates are herbicide effective substances. Sodium chlorate, for example, was authorized for use as a pesticide in Germany until 1992. However, since 10 May, 2010, the application of pesticides containing chlorate is no longer permitted in EU countries.

In Regulation (EG) Nr. 396/2005 pesticide residues are defined thusly: residues, also from substances and their metabolic and/or degradation or reaction products, either used presently or earlier in pesticides, which fall among the products listed in Appendix I of this regulation. Especially included are residues in veterinary medicine and biocide products that can stem from the use of plant protectors.

Based on this definition, all residues from plant protector substances that find their way into food, regardless of the cause, also by means of contamination other than via pesticide products, are included.


There is no specific maximum residue level established for chlorates. Therefore, in accordance with Article 18, Paragraph 1b of the above regulation, the universal standard limit value of 0.01 mg/kg is valid. Foods that contain chlorate residues verified as being above this universal maximum may not be marketed, as stipulated by the German Food, Consumer Goods and Feed Code (LFGB).


Need for Action:

Chlorate residues in carrots, at least in high amounts, are not harmless to one's health. Three samples containing amounts of 0.24, 0.28 and 0.54 mg/kg respectively were considered "unfit for human consumption" and thereby judged as "unsafe food" in accordance with Article 14 of Regulation (EG) 178/2002, in which the general principles and requirements of food law in Europe were established. A detailed presentation of the problem of chlorate residues can be found in our Internet article Chlorate Residues in Plant-Based Food "Chlorate Residues in Plant-Based Food: Origin Unknown " 2)





1)Guidance for Industry: Guide to Minimize Microbial Food Safety Hazards for Fresh Fruits and Vegetables

2) Chlorate Residues in Plant-Based Food: Origin Unknown


Photo Credits:

Prepared Carrots: Andrea Karst, CVUA Stuttgart.


Artikel erstmals erschienen am 11.03.2014