The Two Major Classes of Wheat Proteins
Wheat grain proteins are broadly classified into gluten and non-gluten proteins. This classification, based on solubility, was established by Thomas Burr Osborne. Gluten proteins are most abundant and give dough its viscoelastic properties, while non-gluten proteins have metabolic and structural roles.
Gluten Proteins: Gliadin and Glutenin
Gluten, formed by gliadin and glutenin, provides wheat's characteristic elasticity. Mixing flour with water and kneading combines these proteins to create stretchy dough.
Gliadins: Viscosity and Extensibility
Gliadins are monomeric proteins soluble in aqueous alcohol. They contribute to dough's viscosity and extensibility, allowing it to stretch. Sub-types include $\alpha$-, $\gamma$-, and $\omega$-gliadins.
Glutenins: Elasticity and Strength
Glutenins are polymeric proteins linked by disulfide bonds, forming high-molecular-weight structures. This network provides dough strength and elasticity, enabling gas trapping and rising in baked goods. Glutenins have high-molecular-weight (HMW) and low-molecular-weight (LMW) subunits, crucial for bread quality.
The Gluten Network
The gluten network is a dynamic matrix formed by interacting gliadins and glutenins during kneading, allowing dough to retain CO2 gas. This network is essential for the texture of leavened baked goods. Dough strength is influenced by flour type, water, and kneading.
The Less Abundant Non-Gluten Proteins
Non-gluten proteins make up 15-20% of the total protein and have vital roles.
Albumins and Globulins
Albumins are water-soluble, while globulins are salt-soluble. They include metabolic and enzymatic proteins important for grain germination. Some, like amylases and enzyme inhibitors, can be allergenic. These proteins don't form part of the gluten network.
Other Non-Gluten Proteins
Minor non-gluten proteins like amylase/trypsin inhibitors and serpins can impact dough and act as allergens, potentially activating the innate immune system and causing sensitivities.
Comparison of Major Wheat Proteins
| Feature | Gliadins | Glutenins | Albumins & Globulins |
|---|---|---|---|
| Function | Viscosity, extensibility | Elasticity, strength | Metabolic, enzymatic, and structural |
| Structure | Monomeric polypeptide chains | Polymeric (subunits linked by disulfide bonds) | Monomeric, globular |
| Solubility | Soluble in aqueous alcohol | Insoluble in aqueous alcohol | Soluble in water (albumins) or salt solutions (globulins) |
| Molecular Weight | Lower (28-55 kDa) | Higher (30 kDa to >10 million Da) | Lower (<70 kDa) |
| Abundance | High, alongside glutenins | High, alongside gliadins | Relatively low (15-20% of total protein) |
The Function of Wheat Proteins in Food and Health
- Baking Performance: The ratio of gliadin to glutenin affects dough strength and extensibility, crucial for various baked goods.
- Nutritional Value: Wheat proteins are a global source of vegetable protein, though deficient in lysine.
- Allergenic Potential: Specific gliadins trigger celiac disease. Non-gluten proteins can also cause allergic reactions.
- Industrial Applications: Gluten's viscoelasticity makes it a binder, extender, and texture improver in various products.
- Dietary Implications: Whole-grain wheat offers nutrients for those without gluten disorders. Avoidance is needed for conditions like celiac disease.
Conclusion: The Intricate Balance of Wheat Proteins
Wheat proteins, primarily gliadins and glutenins forming gluten, are vital for this staple crop's functional properties in food production. Their composition influences dough elasticity and bread texture. Non-gluten albumins and globulins also play important biological and nutritional roles. The balance of these proteins defines wheat's versatility and characteristics. For more information, see this review.
