A quantitative competitive PCR system to detect contamination of wheat, barley or rye in gluten-free food for coeliac patients

The only treatment in coeliac disease is a lifelong avoidance of wheat, barley and rye (WBR) in the daily nutrition. According to the Coder Alimentarius Commission of the Joint Food and Agricultural Organisation and the World Health Organisation of the United Nations, 100 ppm gliadin (10 mg gliadin/100 g dry weight) is the maximum allowed in food labelled as 'gluten-free'. The present study describes the evaluation of a quantitative competitive polymerase chain reaction (QC-PCR) system as an indication of contamination of gluten-free food with the coeliac-toxic cereals. The QC-PCR system simultaneously detects WBR-DNA on the basis of a non-coding region of chloroplast trnL gene. An internal DNA standard was constructed by adding 20-bp to the original PCR product. This standard was calibrated to 0.02% and 0.2% wheat DNA corresponding to 10 ppm and 100 ppm gliadin, respectively. The QC-PCR system was applied to 15 commercially available products labelled as 'gluten-free'. QC-PCR and ELISA yielded identical results for most cases. By QC-PCR as well as by ELISA one sample was shown to contain more than the allowed maximum limit to be labelled as 'gluten-free'. Through the application of both methods, the origin of a contamination can be identified. A positive QC-PCR signal and a negative ELISA result indicates a possible gliadin-free wheat starch addition and vice versa a possible addition of wheat-free gliadin as a food additive. Both methods support each other in testing gluten-free products.