What is the Effect of Heat on Protein Structure

Plant proteins possess a hierarchical folded structure maintained by weak non-covalent bonds (hydrogen bonds, hydrophobic interactions, ionic bonds, van der Waals forces). Heat supplies thermal energy that disrupts these stabilizing forces. This structural change directly explains losses in functionality, digestibility and nutritional integrity observed when dry fractionation runs at elevated powder temperatures. This article connects thermal exposure to molecular changes and practical processing outcomes for pulse protein production.

1. Four Levels of Protein Structure

1. Primary structure: Linear chain of amino acids linked by covalent peptide bonds.
2. Secondary structure: Local folding (α-helices, β-sheets) held by hydrogen bonds.
3. Tertiary structure: Overall 3D fold of a single polypeptide chain, defining native shape.
4. Quaternary structure: Assembly of multiple protein subunits (typical for globular storage proteins in peas, fava beans).

Critical distinction:
Heat rarely breaks primary peptide bonds under typical dry processing temperatures, but easily disrupts secondary, tertiary and quaternary folding.

2. Stepwise thermal unfolding mechanism (denaturation)

Stage 1: Reversible unfolding (mild heating, short duration, <60–65 °C)

Thermal vibration weakens weak intermolecular forces. The compact native globule opens partially; hydrophobic amino acid residues become exposed to the surface.

• If cooled rapidly: protein may refold back toward native conformation.
• In dry fractionation: brief temperature spikes below 60 °C often cause temporary, reversible structural disturbance.

Stage 2: Irreversible denaturation (sustained >60–65 °C; critical threshold for pulse proteins)

Once unfolding proceeds beyond a critical point, spontaneous refolding is no longer possible.
Exposed hydrophobic surfaces from separate protein molecules attract one another → protein aggregation.
Aggregates can be:

• Soft colloidal aggregates (partially soluble)
• Dense covalently cross-linked clusters (insoluble)

Stage 3: Advanced thermal damage (>75–80 °C, prolonged exposure)

Two secondary reactions occur alongside denaturation:

1. Disulfide bond rearrangement: New inter-protein disulfide bridges form, locking aggregates permanently.
2. Maillard reaction: Free amino groups on lysine react with reducing sugars present in the powder.
   • Destroys essential amino acids (lysine);
   • Lowers protein digestibility;
   • Produces brown pigments and off-flavours.

3. How structural changes translate into measurable functional losses

Native folded protein

Compact globular structure; hydrophobic regions buried internally.

• High solubility in water;
• Good emulsification (can orient at oil-water interfaces);
• Strong foaming capacity;
• Easily cleaved by digestive enzymes → high digestibility.

Heat-denatured, aggregated protein

Hydrophobic sites exposed and stuck together into large clusters.

• Sharp drop in aqueous solubility;
• Poor dispersibility (“fish eyes” during beverage mixing);
• Weak emulsifying and foaming properties;
• Enzymes cannot access cleavage sites → reduced protein digestibility;
• Creates gritty suspensions and accelerates sedimentation in plant-based drinks.

4. Differences between dry-state heating vs wet-state heating

This is vital for understanding dry fractionation:

• Wet heating (liquid slurries): Denaturation occurs at lower temperatures (~55–60 °C). Water acts as a plasticizer, accelerating unfolding.
• Dry heating (powder, low moisture 8–10%): Protein is more thermally stable. Denaturation threshold shifts higher, but damage becomes more permanent once initiated.
  In dry milling, trapped hot powder deposits experience dry thermal stress; even short residence times can trigger irreversible aggregation without visible moisture.

5. Relevance to dry fractionation process control

Heat sources in split/single-axis air classifier mills:

• Frictional heat from high-speed grinding impact;
• Stagnant powder adhering to hot rotor and housing surfaces;
• Multiple passes of middling recirculation accumulating thermal load.

Practical observations:

• Outlet powder consistently <60 °C: protein largely retains native structure, full functionality preserved.
• Sustained 65–72 °C: gradual unfolding and incipient aggregation; solubility slowly declines.
• Continuous >75 °C: severe denaturation + Maillard reaction; protein loses premium beverage-grade functionality.

6. Key Misconception

Many operators assume protein damage only happens above boiling temperature.
For plant storage proteins, sustained exposure above 60–65 °C is sufficient to drive irreversible denaturation over time — even at atmospheric ambient pressure, without liquid water present.

Summary

1. Heat disrupts hydrogen bonds, hydrophobic interactions and other weak forces maintaining secondary, tertiary and quaternary protein folding. Primary amino acid sequence remains intact initially.
2. Mild, short-duration heating may cause reversible unfolding; sustained temperature above 60–65 °C triggers irreversible denaturation and protein aggregation.
3. Aggregation exposes hydrophobic regions and drastically reduces solubility, emulsification and foaming performance.
4. Higher temperatures further initiate Maillard reactions, destroying essential amino acids and reducing nutritional quality.
5. In dry fractionation, controlling powder temperature below 60 °C is the primary strategy to preserve native protein structure and maintain high-value functional properties for plant-based beverage applications.