5.2 What are MRLs?

Pesticide residues on crops are monitored with reference to Maximum Residue Limits (MRL) and are based on analysis of quantity of a given AI remaining on food product samples.

The MRL for a given crop/AI combination is usually determined by measurement, during a number (in the order of 10) of field trials, where the crop has been treated according to GAP and an appropriate pre-harvest interval (see section 5.8) has elapsed.

For many pesticides, however, this is set at the Limit of Determination (LOD) – since only major crops have been evaluated and understanding of ADI is incomplete (i.e. producers or public bodies have not submitted MRL data – often because these were not required in the past).

LOD can be considered a measure of presence/absence, but true residues may not be quantifiable at very low levels.

For this reason, the Limit of Quantification (LOQ) is often quoted in preference (and as a ‘rule of thumb’ is usually approximately 2X the LOD). Useful further information on detection limits is provided by the European Commission. [1]

It follows that the adoption of GAP at the farm level must be a priority: including and especially the withdrawal of obsolete pesticides. With increasingly sensitive detection equipment, a certain amount of pesticide residue will often be measurable following field use.

In the current regulatory environment, it would be wise for cocoa producers to focus on pest control agents that are permitted for use in major importing countries.

Testing for residues should be carried out in accredited laboratories (e.g. to ISO 17025 standards), following internationally agreed and validated methods; specific good laboratory practice (GLP) methods also apply (e.g. DIN, ISO, FDA) in many countries. Procedures include extraction and “clean-up” from samples, followed by analysis using various instruments, depending on the residue being analysed. Appropriate equipment for individual compounds is included in the Pesticide Manual entries. Analysis techniques include: gas chromatography (GC), gas-liquid chromatography (GLC), gel permeation chromatography (GPC), high-pressure liquid chromatography (HPLC) and various mass spectrometry techniques, so such laboratories are expensive to set up and maintain.

(photos: Jean Ponce Assi, SACO-CHOCODI)

It should be stressed that MRLs are set on the basis of observations and not on ADIs, and it is also generally understood that MRLs would considerably over-estimate actual residue intakes. MRL studies take place after years of initial development and it is most unlikely that an agrochemical company would even carry them out (with a view to registering the product), were toxicological studies to raise serious question marks about a new compound.