The Dominant Proteins: Globulins and Albumins
Pulses contain two major protein fractions: globulins and albumins. The protein content and composition can vary by variety, growing environment, and processing. However, in most pulses, globulins are the predominant protein type, making up 50–80% of the total protein content. Albumins constitute a smaller portion, typically around 10–20%.
Globulins: The Primary Storage Proteins
- High concentration: Globulins are the main storage proteins, providing the seed with energy during germination.
- Amino acid profile: They contain high levels of specific amino acids, including glutamine, aspartic acid, and lysine.
- Further classification: Globulins are further divided into two types: legumins (11S) and vicilins (7S). Legumins have more sulfur-containing amino acids (methionine and cysteine) than vicilins, though still limited compared to animal sources.
Albumins: The Metabolic Proteins
- Metabolic role: Albumins play a more functional, metabolic role within the plant, acting as enzymes and inhibitors.
- Amino acid quality: Nutritionally, albumins are considered a better source of amino acids and tend to have higher concentrations of cysteine and methionine compared to globulins.
- Nutrient inhibitors: Albumins can also include some antinutritional compounds, such as trypsin inhibitors, though their effects are significantly reduced or eliminated through proper cooking.
The “Incomplete” Protein Profile and Complementary Pairing
Pulse proteins are often referred to as 'incomplete' because they are typically low in the sulfur-containing essential amino acids methionine and cysteine, and in some cases, tryptophan. This is in contrast to animal proteins, which generally provide a complete profile of all nine essential amino acids. However, this limitation is not a major concern for those on a balanced diet, including vegetarians and vegans, for several reasons.
Firstly, the body maintains a pool of essential amino acids throughout the day, so combining different plant foods that offer complementary amino acid profiles, like pulses with cereals, can create a complete protein meal. Pulses are rich in lysine, which is often the limiting amino acid in cereals like rice and wheat. A classic example is the combination of beans and rice, a pairing found in many global cuisines. Secondly, research has shown that as long as enough calories from a variety of foods are consumed daily, the body can meet its essential amino acid needs from different plant-based sources.
Digestibility and the Impact of Processing
Another important aspect of pulse protein quality is its digestibility. The digestibility of plant proteins is generally lower compared to animal proteins due to several factors, including the presence of antinutritional factors (ANFs) and the complex structure of the seed. ANFs such as phytic acid, tannins, and trypsin inhibitors can interfere with nutrient absorption and protein digestion.
However, these issues can be largely mitigated through processing methods commonly used in food preparation:
- Cooking and soaking: Boiling, soaking, and other forms of heat treatment denature protein structures, making them more accessible to digestive enzymes and inactivating heat-sensitive ANFs.
- Germination and fermentation: These biological processes activate enzymes within the pulse, further breaking down ANFs and complex proteins to improve nutrient bioavailability and digestibility.
- Milling and fractionation: Grinding pulses into flour can increase protein digestibility by disrupting the plant's cell walls and separating protein from starches and fibers.
Comparing Protein Types: Pulses vs. Animal and Cereal Proteins
| Feature | Pulse Protein (e.g., lentils, beans) | Animal Protein (e.g., meat, eggs) | Cereal Protein (e.g., rice, wheat) |
|---|---|---|---|
| Primary Proteins | Globulins and Albumins | All essential amino acids | Prolamins and Glutelins |
| Amino Acid Profile | High in Lysine; limiting in Methionine, Cysteine, and Tryptophan | Complete profile of all nine essential amino acids | High in Methionine, Cysteine, and Tryptophan; limiting in Lysine |
| Completeness | Incomplete, but can be combined with cereals to form complete proteins | Complete protein source | Incomplete, can be combined with pulses for completeness |
| Digestibility | Good, especially after cooking; can be affected by ANFs | Excellent digestibility | Good, but varies by type |
| Saturated Fat & Cholesterol | No cholesterol and naturally low in saturated fat | Contains saturated fat and cholesterol | Generally low in fat |
| Dietary Fiber | Excellent source of dietary fiber | Contains no dietary fiber | Source of dietary fiber |
Conclusion
The protein in pulses is predominantly composed of globulins and albumins, providing a rich source of plant-based protein, particularly high in lysine. While they are considered 'incomplete' due to lower levels of certain sulfur-containing amino acids, this can be easily addressed by consuming a varied diet that includes complementary protein sources like cereals. Proper cooking and preparation methods effectively improve the digestibility and bioavailability of pulse protein by breaking down anti-nutritional factors. For those seeking sustainable, affordable, and versatile plant-based protein options, pulses like lentils, chickpeas, and beans are an excellent choice, offering numerous health benefits beyond just protein content.
