Investigation of the Effects of Gluten-Green Tea Phenols Interaction on Immunogenic Gluten Peptides

Abstract

Celiac disease is an autoimmune disorder with the prevalence of 1% of the population and triggered by the consumption of gluten. Due to the high proline content of gluten, which makes it resistant to digestive enzymes, partial digestion results in the production of 33, 26, 19 amino acids-long gluten peptides. Subsequent to a series of reaction of gluten peptides, inflammation occurs through the epithelial cell wall. Therefore, gluten peptides produced as a result of partial digestion referred as 'immunogenic'.

Interaction between protein and polyphenols might take place through the covalent or non-covalent bonds. While the van der Waals interactions, hydrogen bonds, hydrophobic interactions are weaker than non-covalent interactions, covalent interaction takes place by electron pair sharing. Covalent interaction of protein with polyphenols occurs through the oxidation of polyphenols to electrophilic quinones and their subsequent binding to the protein via its amino, thiol side groups. Therefore, covalent interaction between protein and phenol is stimulated under the alkaline condition which is suitable for the oxidation of polyphenols.

This study aimed understanding the effect of interaction of gluten with green tea extract (GTE) on immunogenic gluten peptides. For this purpose, gluten was treated for 2 and 3 hours with GTE under following conditions; 1% and 2% GTE concentration; pH 7 and pH 9; at 50 °C free to air exposure. Treatment of gluten with GTE was confirmed by monitoring the changes in total antioxidant capacity, free amino and thiol compounds, thermal stability. Gluten treated with GTE at pH 7 exhibited more radical scavenging activity than pH 9, indicating that the binding of GTE phenol to the gluten occurred through the thiol groups at pH 7, whereas through the amino side groups at pH 9.

Following the in vitro digestion of native gluten and gluten samples treated with GTE, bioaccessible fractions of modified gluten exhibited less antioxidant capacity than their initial content which might indicate the delivery of GTE phenolic compounds to the colon. Digestibility of gluten decreased with the treatment with 2% GTE at pH 9. Inhibition of gluten peptide release were provided by the treatment of gluten with GTE at pH 9, whereas treatment of gluten with GTE at pH 7 stimulate the release of immunogenic peptides. Considering the inhibition of 33-mer (57%), which is widely known immunogenic gluten peptide, the most effective gluten treatment parameters were found as 2% GTE concentration at pH 9 and 50 °C for 2 hours.

In this study, it has been also investigated how treatment of gluten with GTE affected the bread quality characteristics. For the preparation of breads, gluten treated under the most effective conditions with the lowest immunogenic peptide release was used. Texture profile analysis showed that the interaction of gluten with GTE decreased textural properties of bread. Therefore, the recipe was modified by using soy protein isolate and guar gum to improve the textural properties of bread prepared with modified gluten. The textural properties, porosity and browning indexes of bread prepared with gluten treated with GTE were improved becoming closer to the values of control bread, however, still was significantly different.