Comprehensive analysis of gluten in processed foods using a new extraction method and a competitive ELISA based on the R5 antibody

The only treatment for coeliac disease is to follow a strict, life-long gluten-free diet. It is therefore essential to use a highly sensitive, specific technique for gluten analysis in foods. Nowadays, the usual method for determining gluten content in gluten-free foods, internationally accepted by the Codex Alimentarius Commission, is the R5 antibody-based sandwich ELISA, combined with the cocktail-extraction solution. This technique requires at least two epitopes in the protein, but in hydrolysed foods, proteins are fragmented during food processing and converted into peptides in which only one toxic epitope may appear. Consequently, it was necessary to develop a new competitive immunoassay that, together with a reliable, compatible extraction solution, would provide a complete gluten analysis in any kind of food. We analysed commercial foods and home-made maize breads spiked with a known amount of gliadins using the sandwich R5 ELISA and the new competitive R5 ELISA that has been developed. These foods had previously been extracted with 60% ethanol/water, the cocktail solution or the new extracting solution called UPEX (universal prolamin and glutelin extractant solution). The complementary SDS-PAGE and western blot techniques were also used to confirm the gluten content. The limits of detection and quantification of the competitive R5 ELISA were 0.36 and 1.22 ng/ml of gliadins, respectively. The intra- and inter-assay precisions based on two samples were, respectively, 7.3% and 5.4% for the first sample and 9.9% and 6.3% for the second. This new assay was a better technique than the sandwich R5 ELISA for detecting gliadins quantitatively in hydrolysed foods. Regarding the extraction procedure, we did not find any significant interference from components of the UPEX solution at the concentration used. In addition, the UPEX solution extraction was compatible with the R5 western blot and mass spectrometry techniques. The competitive R5 ELISA we developed, combined with the UPEX solution described here, is a very useful tool for detecting and quantifying gluten in any kind of food samples, including heat-treated and/or hydrolysed ones.