
In the last ten years, the demand for plant-based proteins has greatly increased, motivated by the triple factors of sustainability, health, and nutritional adequacy for the impending necessity to feed the global population at a lower environmental cost. In order to be able to match the functional performance of animal proteins, however, one must account for molecular, structural, and processing factors at the levels of extraction, functionality, digestibility, and bioactivity. Modern chemical biology offers tools and insights to rationally design, optimize, and apply plant proteins for food, nutraceutical, and biomedical uses.
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What are Plant-Based Proteins—at the Molecular Level?
Storage Proteins and Seed Biology
In plants, the major fraction of what we broadly consider as "plant protein" is stored as seed storage proteins of the legumin-type 11S globulins, the vicilin-type 7S globulins, the prolamin, and the albumin fractions.
For example, in the Brassica species (crucifers), the main seed storage protein is cruciferin, an 11S globulin that represents a large fraction of the total seed protein content and is present as α/β subunits with disulfide linkages of ~20–40 kDa molecular weight. Storage proteins in general are naturally selected to act as nitrogen reserves for plant germination and are therefore not necessarily designed to be readily digested by humans nor soluble in aqueous food environments.
Figure 1. Structural characteristics of seed storage proteins[1].
Beyond Storage: Functional and Stress-Responsive Proteins
Plants also encode proteins with roles in stress tolerance, signaling, or defense—for instance, dehydrins (members of the late embryogenesis abundant, or LEA, family) are intrinsically disordered proteins that protect against cold, drought, and osmotic stress.
Such proteins are not primary dietary proteins but showcase how structural flexibility and amino acid composition (rich in polar residues) may be leveraged in engineering protein performance (e.g. solubility, stability, emulsification).
Bioactive Peptides Embedded in Plant Proteins
Bioactive peptides may be latent in the seed storage proteins until released during enzymatic hydrolysis (digestion). A well-known example of this is the soybean peptide lunasin, a non-allergenic peptide with antioxidant and anti-inflammatory activity that can be obtained from soybean. These peptides may have a defined motif of amino acid sequence as well as length (5–10 kDa) for maintaining activity after gastric digestion.
Figure 2. Amino acid sequence of lunasin and its peptide activity[2].
How Are Plant Proteins Extracted and Processed?
A. Conventional Fractionation: Alkaline Extraction & Acid Precipitation
The standard method begins with milling raw material into a slurry, followed by alkaline solubilization (pH ~8–10) to dissolve protein, and then acid precipitation (pH ~4–5) to recover the protein fraction. Impurities and non-protein matter are removed via centrifugation or filtration. This approach is ubiquitous but risks denaturation, loss of solubility, and destruction of labile bioactives.
B. Advanced / Green Extraction Methods
To preserve functional integrity, newer strategies include:
- Enzyme-assisted extraction: proteases, carbohydrases, or cellulases break down cell walls or protein–matrix interactions, allowing milder solubilization.
- Ultrasonic/microwave-assisted extraction: applying acoustic or electromagnetic energy to accelerate mass transfer and improve yield while reducing harsh conditions.
- Natural Deep Eutectic Solvents (NADES): these are combinations of choline, organic acids, sugars, or amino acids that act as benign solvents for plant metabolites; there is emerging interest that NADES phases in plant cells also facilitate metabolite dissolution and extraction.
Figure 3. Production of plant protein-derived peptides by enzymatic hydrolysis[3].
C. Purification, Ultrafiltration, and Membrane Technology
After extraction, ultrafiltration (UF), nanofiltration (NF) or diafiltration is commonly applied to concentrate the proteins and remove smaller unwanted components (salts, phenolics). Membrane technology can be used to tune molecular weight cutoff to achieve better separation and increase purity without excessive heating or chemical treatment.
D. Drying, Denaturation, and Functional Preservation
Drying (freeze-drying, spray-drying or vacuum drying) to produce a powder is a final step. A major concern in drying steps is to limit protein denaturation (which can adversely affect solubility, emulsification, and gelling properties). This can be achieved by limiting heat input and exposure and maintaining moderate pH conditions during drying. Attention should also be paid to moisture content and prevention of oxidation of sensitive amino acids (tryptophan, methionine).
What Are the Key Nutritional & Functional Challenges?
Amino Acid Balance and "Completeness"
Many plant proteins are low or imbalanced for one or more EAAs, most commonly lysine (in cereals) or methionine (in legumes). To achieve "complete protein" status, formulations often combine complementary sources (e.g., cereal + legume).
Digestibility, Anti-Nutritional Factors, and Bioaccessibility
Plant proteins are often associated with anti-nutritional factors (ANFs) like tannins, phytic acid, protease inhibitors, or lectins that can interfere with proteolysis or mineral absorption. Thermal/enzymatic treatments can reduce ANFs, but over-processing may reduce functional properties.
The protein digestibility–corrected amino acid score (PDCAAS) or digestible indispensable amino acid score (DIAAS) can be lower for plant proteins than for animal references, depending on how accessible the peptide bonds are.
Allergenicity and Immunogenic Epitopes
Certain plant proteins (e.g., soy, peanut, wheat) contain allergenic epitopes. Also, lipid transfer proteins (LTPs) are a known class of pan-allergens, resistant to proteolysis and heat, posing immunologic risk in susceptible individuals.
Functional Properties: Solubility, Emulsification, Gelling, Texture
For use in food analogs or formulations, proteins must provide functionality:
- Solubility across a pH range
- Emulsifying capacity and stability (for oil/water systems)
- Foaming capacity and stability
- Gelation or texture-formation (for meat analogs, dairy analogs)
- Viscoelastic properties
Plant proteins often fall short compared to animal proteins in these areas. Protein engineering (site-directed mutagenesis, domain fusions) or physical modification (glycosylation, acylation, crosslinking) can help.
Figure 4. Schematic overview of plant protein sources, processing and production, and the properties and activities of plant protein-derived peptides[3].
What Are the Applications of Plant-based Proteins?
- Meat, Milk, and Egg Analogs
Replicating the texture, mouthfeel, appearance, and nutritional profile of animal-derived foods is a primary goal of industrial production. Given the varying molecular properties, formulators must combine proteins, lipids, polysaccharides, and processing operations (e.g., extrusion, shearing, heating) to mimic the fibrous nature of meat or the curd-like nature of dairy products.
- Edible Coatings, Films, and Encapsulations
Plant-based proteins (e.g., soy, pea, and rice) can be used as edible films or coatings for fruits, seeds, or controlled-release capsules. They offer advantages in film-forming, barrier, and biodegradability. In fact, a recently designed plant-based protein microcapsule was able to encapsulate both hydrophilic and hydrophobic payloads, degrade under digestive conditions, and release the payload in a controlled manner—all using pea protein as a matrix.
- Nutrients, Bioactives, and Clinical Applications
Plant-based peptides are being explored as functional ingredients for immunomodulation, antioxidant activity, cholesterol management, anti-inflammatory responses, and metabolic control. For example, soy peptides have been studied for lowering cholesterol levels.
Frequently Asked Questions (FAQs)
1. Are plant-based proteins "incomplete" compared to animal proteins?
While many single plant proteins may lack one or more essential amino acids, blending complementary sources (e.g. legumes + cereals) can produce a complete profile equivalent to animal proteins.
2. How do anti-nutritional factors affect plant protein use?
Compounds such as phytic acid, tannins, and protease inhibitors reduce digestibility and mineral absorption. These can be mitigated by thermal, enzymatic, or fermentation treatments.
3. Do plant protein-derived peptides retain activity after digestion?
Yes, carefully selected peptides (5–10 kDa) may survive gastric and intestinal conditions and remain bioactive (e.g. lunasin).
4. What are the biggest hurdles for plant protein in food analogs?
Achieving animal-like texture, mouthfeel, and flavor while preserving nutrition and minimizing off-flavors is challenging. Also, processing costs and scale remain limiting factors.
5. Can plant proteins trigger allergies?
Certain proteins, particularly lipid transfer proteins (LTPs) and other seed allergens, are heat-stable and protease-resistant, posing allergenic risk in predisposed individuals.
6. How does Alfa Chemistry ensure product purity and reproducibility?
Our stringent specifications (e.g. moisture, ash, microbial counts, heavy metals) and quality control practices guarantee consistency suited for research and formulation work.
References
- Jain A. Seed Storage Protein, Functional Diversity and Association with Allergy. Allergies. 2023, 3(1), 25-38.
- Kyle S., et al. Recombinant production of the therapeutic peptide lunasin. Microbial Cell Factories. 2012, 11(1), 28.
- Nirmal N., et al. Plant protein-derived peptides: frontiers in sustainable food system and applications. Front. Sustain. Food Syst.. 2024, 8.
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