The unique properties of wheat flour, which allow it to be transformed into everything from light bread to chewy pasta, are primarily due to two distinct storage proteins: glutenin and gliadin. These proteins reside within the endosperm of the wheat kernel, alongside starch, and remain mostly inert until they are combined with water. Once hydrated, glutenin and gliadin interact with each other to form a complex, interconnected substance known as gluten. This resulting network is what gives dough its characteristic elasticity and ability to stretch, a property that is fundamental to baking.
The Distinct Roles of Glutenin and Gliadin
Glutenin and gliadin possess different structures and functions, which ultimately combine to form the viscoelastic properties of gluten. Gliadin proteins are smaller, existing as compact, single-chain molecules (monomeric proteins). Gliadin contributes viscosity and extensibility, allowing the dough to flow and stretch easily.
In contrast, glutenin is a much larger protein, formed from multiple subunits linked into massive polymers. Glutenin is responsible for the strength and elasticity of the dough, providing the ability to spring back after being stretched. This difference in function is rooted in their chemical bonding; glutenin utilizes both intra- and interchain disulfide bonds to create its robust, interconnected network, while gliadin forms few or no disulfide bonds.
How Gluten Forms and Affects Food Structure
The formation of the gluten network is a multi-step process initiated by the addition of water to flour. Hydration causes the previously coiled glutenin and gliadin proteins to uncoil and begin to interact. Mechanical energy, typically provided by mixing or kneading, is also necessary to physically align the protein chains and encourage them to link up.
During this mechanical action, the large glutenin polymers connect through disulfide bonds, forming a continuous, three-dimensional mesh. Gliadin molecules, which are smaller and more fluid, become incorporated into this glutenin scaffold, acting as a plasticizer. This combined network gives dough its unique functionality in baking, as it has the ability to trap gases produced by leavening agents like yeast. The elastic gluten matrix stretches to accommodate the expanding carbon dioxide bubbles, which allows the dough to rise and gives baked goods their characteristic light and airy internal structure. Without this strong, flexible gluten network, a dough would collapse, yielding a dense product with a crumbly texture.
Gliadin as the Immune System Trigger
Beyond its role in food structure, gliadin contains specific peptides that can trigger adverse immune reactions in susceptible individuals. Celiac disease, an autoimmune condition, is caused by the body’s reaction to certain peptides within gliadin, particularly the alpha-gliadin fraction. These specific gliadin fragments are unusually resistant to digestion by the enzymes in the human gut, allowing them to pass into the small intestine largely intact.
Once in the intestine, an enzyme called tissue transglutaminase (tTG) modifies these peptides by a process called deamidation. This modification increases the peptide’s ability to bind strongly to specific immune system molecules (HLA-DQ2 and HLA-DQ8), which are present in most people with celiac disease. This binding event triggers an adaptive immune response, where T-cells recognize the gliadin peptide as a foreign invader. The resulting inflammatory cascade damages the small intestine’s villi, the finger-like projections responsible for nutrient absorption, leading to malabsorption and various symptoms. A different gliadin fragment, known as P31–43, is also involved in the process, triggering a more immediate innate immune response mediated by the signaling molecule Interleukin-15 (IL-15). In contrast, non-celiac gluten sensitivity is not characterized by this autoimmune damage, though gliadin peptides may still affect intestinal barrier function.