Advice to rinse fruits and vegetables before eating them simply will not wash for apples coated with pesticide residues, research has shown.
Only dunking in a baking powder solution for up to 15 minutes will do the job, scientists found, and even then, some of the potentially toxic chemicals will be left behind.
Researchers in the US carried out laboratory tests of different methods used to remove two common pesticides from apples. One was the fungicide thiabendazole and the other the insecticide phosmet.
Washing with ordinary tap water removed none of the chemicals that had penetrated the skins of the apples, while a commercial bleaching agent, Clorex, only had limited effectiveness.
A mixture of 1% bicarbonate of soda, or baking powder, and water was better at getting rid of the residues than Clorex, which is approved in the US to treat newly picked fruits and vegetables before they are sold to consumers.
After 12 and 15 minutes immersion in the baking powder solution respectively, 80% of the thiabendazole and 96% of the phosmet, was removed.
Imaging showed that thiabendezole had penetrated up to 80 micrometres deep into the apples compared with 20 micrometres for phosmet, making it more difficult to extract.
The scientists, led by Dr Lili He, from the University of Massachusetts, pointed out that pesticides that reached the cells below an apple's waxy and cuticle outer layers were "irreversibly bound".
The researchers wrote: "For apples, the peel can easily be removed along with most of the internalised pesticide residues; however, important nutrients (eg polyphenolic compounds, fibres, pigments, vitamins, and minerals) will also be lost."