Survey of tea for the presence of gluten

In 2013, the U.S. Food and Drug Administration conducted a survey of green and white teas marketed in the northeastern United States for the presence of undeclared wheat. Based on the requirement for concurrence between the RIDASCREEN gliadin (R5) enzyme-linked immunosorbent assay (ELISA) and the Morinaga Institutes of Biological Science (MIoBS) wheat protein ELISA, none of the 20 products included in the survey tested positive for wheat, rye, barley, or gluten. However, eight of the teas generated responses indicative of the presence of gluten with the RIDASCREEN gliadin (R5), AgraQuant gluten G12, and Aller-Tek (Skerritt) sandwich ELISAs. Five of the eight teas generated responses indicative of >20 ppm of gluten using the RIDASCREEN and AgraQuant ELISA test kits, and all eight had ≥ 20 ppm based on the Aller-Tek ELISA. Extracts prepared using the RIDASCREEN validated protocol and the MIoBS validated sodium dodecyl sulfate plus β-mercaptoethanol (overnight) protocol were analyzed using both test kits. The extracts prepared using the RIDASCREEN protocol tested positive for gluten with both test kits. Western blot analyses of the two sets of extracts using the R5 and MIoBS antibodies to visualize the bands revealed the presence of antigenic proteins in both sets of extracts, although the profiles and band intensities were different and inconsistent with the ELISA results. These results raise questions regarding the screening procedures used to detect gluten and how the observation of a homologous antigenic element is defined.