The Classification of Wheat Proteins
Wheat grain proteins are typically categorized into four main groups based on their solubility, a method pioneered by chemist Thomas Burr Osborne. These groups are broadly divided into the primary storage proteins that form gluten and the non-gluten functional proteins. This diverse protein composition is responsible for wheat's unique baking properties and overall nutritional value.
Gluten Proteins: Gliadin and Glutenin
Gluten, the well-known protein complex in wheat, is not a single protein but a mixture of two main types: gliadin and glutenin. When water is added to wheat flour, these two proteins combine to form the cohesive, viscoelastic network known as gluten. The specific ratio and composition of these proteins vary between wheat varieties and directly influence baking quality.
- Gliadin: Monomeric proteins that are soluble in 70% ethanol. Gliadins are responsible for the viscosity and extensibility of dough, allowing it to stretch. There are three main types of gliadin—alpha, gamma, and omega—and specific variants within these categories are known to trigger celiac disease in sensitive individuals. Gliadins contain only intramolecular disulfide bonds.
- Glutenin: Polymeric proteins that are insoluble in alcohol and provide the elasticity and strength to dough. Glutenins are held together by both inter- and intramolecular disulfide bonds, forming a complex polymeric network. They are classified into high-molecular-weight (HMW) and low-molecular-weight (LMW) subunits.
Non-Gluten Proteins: Albumins and Globulins
Making up about 15–20% of the total grain protein, albumins and globulins are distinct from gluten proteins in both function and location.
- Albumins: These proteins are soluble in water and typically perform metabolic or enzymatic roles within the wheat kernel. They are a mixture of structural, metabolic, and protective proteins, such as enzymes and enzyme inhibitors. Albumins are located mainly in the embryo and outer layers of the grain.
- Globulins: Soluble in dilute salt solutions, globulins also serve metabolic and protective functions. Some high molecular weight globulins can have storage functions. Like albumins, they have a better-balanced amino acid profile, including higher levels of lysine, which is often deficient in gluten proteins.
Functions of Wheat Proteins in Baking and Nutrition
The functional properties of gliadins and glutenins are what make wheat unique for applications like bread, pasta, and pastries. The balance between the viscous gliadins and the elastic glutenins determines the dough's final texture. For instance, flours used for bread-making (hard wheat) have higher protein content, particularly high-molecular-weight glutenins, for strength and chewiness, while flours for cakes and pastries (soft wheat) are lower in protein for tenderness.
From a nutritional standpoint, wheat proteins serve as a nitrogen and amino acid source for the germinating seed. However, wheat proteins have a relatively low nutritional quality for humans compared to animal proteins due to their deficiency in certain essential amino acids, most notably lysine.
Health Implications: Celiac Disease and Wheat Allergy
It is crucial to differentiate between celiac disease and a wheat allergy, as they involve different immune responses to wheat proteins.
- Celiac Disease: An autoimmune disorder triggered by the ingestion of gluten. It causes an immune reaction that attacks the small intestine, leading to nutrient malabsorption and other health issues. This reaction is primarily caused by specific gliadin peptides that are resistant to digestion. The treatment is a lifelong, strict gluten-free diet.
- Wheat Allergy: An allergic reaction where the immune system overreacts to one or more proteins in wheat. This can cause symptoms like hives, stomach cramps, or, in severe cases, anaphylaxis. The allergen is not always gluten; other wheat proteins, including albumins and globulins, can also be responsible. An allergist can diagnose it, and the treatment involves avoiding wheat.
Comparing Gluten vs. Non-Gluten Wheat Proteins
| Feature | Gluten Proteins (Gliadin & Glutenin) | Non-Gluten Proteins (Albumins & Globulins) |
|---|---|---|
| Primary Role | Storage proteins for seed germination; form viscoelastic network in dough for baking. | Functional/metabolic proteins (e.g., enzymes); protective functions. |
| Location | Primarily in the starchy endosperm of the kernel. | In the embryo, bran, and aleurone layer. |
| Solubility | Gliadins are soluble in aqueous alcohol; glutenins are insoluble. | Albumins are water-soluble; globulins are salt-soluble. |
| Amino Acid Profile | Rich in proline and glutamine, but deficient in essential amino acids like lysine. | Higher nutritional value due to more balanced amino acid composition. |
| Key Characteristic | Forms a rubbery, elastic network when hydrated, enabling gas retention in dough. | Lack viscoelastic properties; many are enzymes that regulate grain growth and metabolism. |
Conclusion
Wheat contains a complex array of proteins, from the abundant gluten-forming gliadins and glutenins to the less-common but functionally significant albumins and globulins. The unique interaction between gliadin and glutenin is the key to wheat's versatility in baking, providing the elasticity and structure necessary for various food products. While nutritionally valuable in many respects, understanding the specific proteins involved is crucial for managing health conditions like celiac disease and wheat allergies. The balance and composition of these proteins ultimately dictate the end-use quality of the flour and its impact on human health.
A list of major wheat protein types
- Gliadins: Monomeric, alcohol-soluble proteins that add viscosity to dough.
- Glutenins: Polymeric, alcohol-insoluble proteins that provide elasticity and strength.
- Albumins: Water-soluble proteins with metabolic and enzymatic functions.
- Globulins: Salt-soluble proteins involved in metabolic and protective roles.
- HMW-GS: High-molecular-weight glutenin subunits, which form polymers.
- LMW-GS: Low-molecular-weight glutenin subunits, which also contribute to glutenin polymers.
How the different proteins interact to form gluten
When flour is mixed with water, the gliadin and glutenin proteins interact to form a network called gluten. Gliadins act as plasticizers, contributing viscosity and extensibility, while glutenins, linked by disulfide bonds, provide elasticity and strength. This viscoelastic structure is what allows dough to be kneaded and rise during baking. The more the dough is mixed and kneaded, the more developed the gluten network becomes. Non-gluten proteins, lipids, and starches are also integrated into this complex matrix.
What factors influence the protein content and quality of wheat?
The protein content and quality of wheat are influenced by a combination of genetic and environmental factors. Agronomic practices, such as nitrogen fertilization, can significantly impact the total protein percentage in the grain. Additionally, factors like temperature and water availability during grain filling can affect protein synthesis and composition. Genetic makeup, including the specific alleles for glutenin subunits, is a key determinant of overall protein quality. The milling process also affects the final protein concentration in different types of flour.
