Tomato quality control: Determination of pesticides present in tomatoes intended for consumption using the QuEChERS procedure

The tomato (Solanum Lycopersicum) is an important source of vitamins A and C, as well as carotenoids, lutein and lycopene, its consumption being linked, among other things, to a lower risk of developing cancer and cardiovascular diseases. Due to these positive properties, the tomato is consumed regularly in daily diets, which means that it must be produced on a large scale in the agricultural industry. To ensure this production, the plants must be treated with certain pesticides, since the tomato plant is very susceptible to the development of certain diseases caused by fungi and bacteria that, if occurred, would invalidate the harvest. 

In most cases, the pesticides used during cultivation disappear due to the effect of external agents, such as wind and rain. However, sometimes pesticides can be absorbed by the surface of the plant, remaining retained in the leaves and fruit, to later be ingested by the consumer. Because long-term exposure to pesticides has been linked to various acute and chronic human health problems, it is especially important to ensure that pesticides do not exceed the optimal limits in the final product of the production chain; process known as quality control.

First step extraction by the “QuEChERS” method

The analysis of pesticides for tomato quality control consists of two steps: extraction of pesticides and detection. For the extraction, the most used method is the “QuEChERS” method, while for the detection method, GC-MS (gas chromatography – mass spectrometry) or LC-MS (liquid chromatography – mass spectrometry) is used.

This process requires a centrifuge that allows working with 15 and 50 ml samples, reaching RCF of more than 3000 xg, such as the Bioprocen 22 R.

For this process, approximately 8-15g of homogenized sample is resuspended in 10-15mL of water in a 50mL tube. Then 10-15mL of ACN (acetonitrile) are added, which is the organic solvent used for the extraction. After shaking the tube, the QuEChERS salts, which are composed of MgSO4, NaCl, and citrate buffer, are added. The tube is shaken again and then centrifuged for 5 minutes at > 3000 G.

Second step GC-MS or LC-MS analysis

As a result of this centrifugation, the components of the sample are separated into phases based on their density and their affinity to the different solutions of the mixture. Pesticides are concentrated in the upper phase, made up of ACN. Thus, the next step consists of washing the sample, taking the ACN phase and transferring it to a 15mL tube containing dSPE (dispersive solid phase extraction). 

The tube is centrifuged for 5 minutes at> 3000 G, after which two phases form in the tube: a dSPE pellet containing the proteins, lipids, and chlorophyll; and an ACN supernatant containing the pesticides. Finally, the supernatant is transferred to an appropriate vial for analysis by GC-MS or LC-MS.

The tomato quality control process is carried out through the analysis of pesticides, due to the negative health effects that prolonged exposure to them has shown. The QuEChERS method, which requires two previous centrifugation steps, is the method of choice for this quick and easy analysis.