The protein content of wheat is more complex than a single value suggests, consisting of distinct protein fractions with different solubilities and functions. These fractions are categorized into four main types, based on a system developed by biochemist Thomas Burr Osborne over a century ago. Understanding these types is crucial for nutrition, especially regarding the properties of dough and dietary health issues like celiac disease.
The Four Osborne Fractions of Wheat Protein
1. Gliadins: The Viscous Components
Gliadins are monomeric proteins, meaning they consist of single polypeptide chains. Soluble in aqueous alcohols, gliadins are responsible for the extensibility and viscosity of wheat dough. When wheat flour is mixed with water and kneaded, the individual gliadin molecules contribute to the flow and flexibility of the resulting dough, allowing it to be stretched. There are several subtypes of gliadins (alpha-, gamma-, and omega-gliadins), some of which are known to be a primary trigger for celiac disease.
2. Glutenins: The Elastic Backbone
In contrast to gliadins, glutenins are large, polymeric proteins consisting of multiple polypeptide chains linked together by disulfide bonds. This large, complex structure provides the elasticity and strength that gives dough its firmness and ability to hold its shape. Glutenins are further categorized into high-molecular-weight (HMW) and low-molecular-weight (LMW) subunits, with the HMW subunits having a more significant impact on dough elasticity. The balance between gliadins and glutenins is what creates the unique viscoelastic properties of gluten.
3. Albumins: The Water-Soluble Fraction
Albumins are a group of water-soluble proteins that primarily contain metabolic and regulatory proteins, such as enzymes. These non-gluten proteins make up a smaller percentage of the total wheat protein compared to gliadins and glutenins. While not responsible for dough structure, albumins play critical roles in the grain's biological functions, particularly during germination. They are also a significant source of wheat hypersensitivities, contributing to certain allergic reactions.
4. Globulins: The Salt-Soluble Components
Globulins are another group of non-gluten proteins, characterized by their solubility in dilute salt solutions but not in water. Like albumins, globulins are a mix of metabolic, protective, and some storage proteins. They also constitute a minor but important fraction of the total wheat protein, with some globulins being linked to allergic responses.
Nutritional and Functional Implications of Wheat Proteins
While wheat is a notable source of plant-based protein, it's considered to have relatively low protein quality for human nutrition due to a deficiency in the essential amino acid lysine. Whole grains offer more nutrients, but white flour, which is primarily endosperm and gluten, is more deficient in lysine. To compensate for this, a diet rich in other protein sources, particularly legumes, is recommended to achieve a more complete amino acid profile. The digestibility of wheat protein is generally high, though processing methods can sometimes reduce it. For example, texturized wheat proteins are developed to improve nutritional quality and can be comparable to meat when processed and supplemented correctly.
Gluten vs. Non-Gluten Proteins in Wheat
| Feature | Gliadins (Gluten) | Glutenins (Gluten) | Albumins (Non-Gluten) | Globulins (Non-Gluten) | ||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Primary Function | Viscosity and extensibility of dough | Elasticity and strength of dough | Metabolic and enzymatic activity | Metabolic and protective functions | ||||||
| Solubility | Soluble in aqueous alcohol | Insoluble in water and alcohol; soluble in dilute acid/alkali | Soluble in water | Soluble in dilute salt solutions | n | Structure | Monomeric (single polypeptide chains) | Polymeric (multiple chains linked by disulfide bonds) | Monomeric; various structures | Monomeric; globular structure |
| Molecular Weight | Lower (30-55 kD) | High to very high (>700 kD polymers) | Very low to low | Low to medium | ||||||
| Role in Celiac Disease | Primarily responsible for triggering immune response | Also contains peptides that trigger immune response | Can be involved in some wheat hypersensitivities | Can be involved in some wheat hypersensitivities | ||||||
| Relative Abundance | Significant portion (~30% of total protein) | Significant portion (~50% of total protein) | Minor fraction (<20% of total protein) | Minor fraction (<20% of total protein) |
The Impact on Baking and Dough Quality
The unique properties of wheat dough that make it suitable for bread-making—its ability to rise and its chewy texture—are almost entirely due to the gliadin and glutenin proteins. When flour is hydrated, these two fractions interact to form the gluten network. Gliadins contribute the extensibility, allowing the dough to stretch, while glutenins provide the strength and elasticity, enabling it to trap the gases released during fermentation. The ratio of gliadin to glutenin, along with the composition of HMW glutenin subunits, largely determines the dough's characteristics and the final product's quality.
Wheat Proteins and Dietary Health
For the vast majority of the population, wheat protein is a nutritious and affordable part of their diet. However, for individuals with celiac disease or a non-celiac gluten sensitivity, the consumption of gluten can cause significant health problems, including damage to the small intestine. This is because the immune system mistakenly identifies gluten proteins, primarily gliadins and glutenins, as a threat. For these individuals, a lifelong gluten-free diet is the only effective treatment.
Conclusion
Wheat protein is not a single entity but a complex mixture of four distinct types: gliadins, glutenins, albumins, and globulins. While gliadin and glutenin are the components that form the elastic and extensible gluten network central to baking, the smaller, non-gluten proteins like albumin and globulin serve important metabolic and enzymatic roles. Acknowledging the diversity of these proteins enhances our understanding of wheat’s nutritional profile, its functional properties in food production, and its implications for dietary health. This knowledge is essential for making informed dietary choices and for the continuous innovation in food science and nutritional health.
Supporting the Muscle System
Wheat protein can also play a role in supporting the muscle system, especially for vegetarians and vegans looking for plant-based protein sources. While studies show that wheat protein may not stimulate muscle protein synthesis as robustly as animal-sourced proteins like whey, its effectiveness can be enhanced. Combining it with other plant proteins to create a more balanced amino acid profile is key, as wheat protein is typically deficient in lysine. Innovative food processing has also led to texturized wheat protein products with higher digestibility and improved nutritional quality, making them more comparable to animal proteins when supplemented with deficient amino acids. This offers a more economical and environmentally sustainable option for the nutrition industry and consumers.
