What is the Effect of Heat on Protein Structure (Relevant to Dry Fractionation Processing)

Plant storage proteins inside legume cotyledons have a native three-dimensional structure maintained by weak non-covalent bonds (hydrogen bonds, hydrophobic interactions, ionic bonds, disulfide linkages). Heat disrupts these stabilizing forces and triggers structural changes classified into two stages: reversible unfolding and irreversible thermal denaturation. This article links thermal damage to dry protein processing outcomes referenced from protein-mill.com dry fractionation standards.

1. Native Protein Structure Baseline

Functional native protein bodies rely on layered ordered structure:

1. Primary structure: Fixed amino acid peptide chain (unchanged by moderate heat).
2. Secondary structure: Alpha-helices and beta-sheets held by hydrogen bonds.
3. Tertiary structure: Folded 3D globular shape, balanced hydrophilic outer surfaces and hydrophobic inner cores that enable solubility, emulsification and foaming.
4. Quaternary structure: Multiple protein subunits assembled together to form intact protein bodies (2–5 μm).

Weak inter- and intramolecular bonds stabilize folded geometry; thermal energy breaks these weak bonds without breaking the peptide backbone.

2. Low mild heat (40–50 °C, controlled tempering in dry fractionation) – Reversible structural relaxation

This temperature range is the safe operating window for pre-conditioning dehulled peas:

• Minor thermal vibration weakens weak hydrogen bonds on the protein surface only.
• Slight loosening of surface folding, no permanent unfolding of core tertiary structure.
• After cooling, protein molecules spontaneously refold back to native geometry.
• Functional impact: negligible loss of solubility (NSI drop <3%).
• Processing benefit: Mild heating softens intercellular pectin, weakens adhesion between protein bodies and starch granules to improve separation efficiency, without permanent protein damage.

3. Critical threshold heat (>55 °C, sustained exposure) – Onset of irreversible thermal denaturation

Legume globular proteins reach a thermal transition point at approximately 55 °C under low-moisture dry processing conditions. Excess heat provides enough kinetic energy to break the full set of stabilizing bonds:

1. Secondary helical/sheet structures unwind.
2. Compact tertiary globular shape collapses; hydrophobic amino acid residues originally buried inside the protein core are exposed to the particle surface.
3. Disassembled protein subunits lose their ordered quaternary assembly; intact protein bodies split into fragmented polypeptide aggregates.

This unfolding is irreversible. Even after cooling, proteins cannot restore their original folded native shape.

Functional consequences of denaturation:

• Sharp drop in nitrogen solubility index (NSI): Exposed hydrophobic surfaces resist hydration in water, protein powder becomes poorly soluble in beverages.
• Lost emulsifying capacity: Cannot form stable oil-water interfacial films for plant-based meat and creamers.
• Degraded foaming ability: Unfolded protein aggregates cannot form thin, flexible foam films.
• Increased particle agglomeration: Exposed hydrophobic groups cause proteins to stick tightly to starch and fiber during air classification, worsening separation efficiency and lowering protein purity in fines.

4. High heat (>70 °C, prolonged heating) – Aggregation, cross-linking and permanent quality loss

If material temperature spikes above 70 °C (common in uncooled single-axis ACM or blocked recirculation loops), advanced structural damage occurs:

1. Unfolded denatured protein chains collide and form permanent inter-molecular disulfide cross-links.
2. Large insoluble protein polymer lumps form.
3. Maillard browning reaction accelerates between exposed amino groups and starch reducing sugars, generating bitter off-flavors and dark discoloration.
4. Severe loss of market value: Protein powder becomes gritty, poorly dispersible, unsuitable for high-value clean-label food applications.

5. Differences between dry-state thermal damage (dry fractionation) and wet-state heating

Moisture drastically amplifies heat-induced structural damage:

• Dry state (10–12% moisture): Limited free water slows molecular mobility; protein denaturation progresses slowly even at 55–65 °C, damage is mostly surface-only. Split-axis mills with cold air cooling easily hold temperatures under 50 °C to avoid this.
• Wet state (wet extraction): Abundant free water acts as a heat transfer medium and lubricates polypeptide movement; denaturation happens rapidly at 50–55 °C, which is why wet protein isolation inevitably produces lower-functionality protein isolates.

This is a core advantage of properly temperature-controlled dry fractionation over traditional wet processing.

6. Practical processing manifestations of heat damage in air classification lines

When milling outlet temperature exceeds 55 °C, operators observe these direct structural side effects:

1. Protein fines D90 shifts larger due to hydrophobic agglomeration of micro protein bodies.
2. More protein trapped in the coarse starch by-product stream, lower total protein recovery.
3. Secondary polishing classification cannot raise protein purity above 65%, as denatured protein-starch clumps cannot be aerodynamically separated.
4. Lab NSI testing shows a drop of 10+ points compared to low-temperature processed batches.

7. How split-axis ACM design mitigates heat-driven protein structural damage

1. Decoupled grinding and classifier rotors eliminate synchronized dual high-speed rotation, cutting total frictional heat generation.
2. Independent cold circulating airflow continuously removes thermal energy from the grinding zone to maintain material below 50 °C.
3. Separate speed tuning avoids forcing high grinding rotor speeds alongside fast classifier wheels, a major heat source in single-axis equipment.
4. Intermediate cooling sections in closed recirculation loops prevent cumulative heat buildup during multiple regrinding cycles.

Conclusion

1. Mild heat (40–50 °C, short-duration tempering): Only temporary, reversible surface relaxation of protein structure; no permanent functional loss, even beneficial for separation.
2. Sustained heat above 55 °C triggers irreversible thermal denaturation: loss of native folded globular structure, exposed hydrophobic residues, impaired solubility and core functional properties.
3. High heat over 70 °C causes irreversible protein cross-linking, aggregation and off-flavor formation, rendering protein powder low-value.
4. Strict temperature control below 50 °C throughout split-axis dry fractionation preserves complete native protein structure, which is the key benefit of dry physical separation compared to heat-prone wet protein extraction.