Inlet air temperature and relative humidity are often overlooked but critical operating variables for air classification in pulse protein dry fractionation. They alter powder surface properties, particle interactions, airflow density, and even protein body integrity. Together, they directly impact particle dispersion, agglomeration tendency, cut-point stability, protein enrichment factor, and overall protein recovery yield.
1. Effect of inlet air humidity
Humidity controls the surface moisture of fine protein and starch particles. Pulse flour is highly hygroscopic.
High inlet air humidity
- Adsorbed moisture forms thin liquid bridges between fine protein particles or between protein fines and coarse starch granules. This triggers particle agglomeration. Tiny protein bodies clump together, behaving aerodynamically like larger particles. These agglomerates are rejected by the classifier wheel and report to the coarse starch stream.
- Result: protein loss in coarse fraction rises; protein enrichment and recovery drop.
- High moisture also increases powder adhesion. Material sticks to classifier wheel blades, chamber walls and internal baffles, causing fouling. Fouling distorts airflow patterns, shifts cut point unpredictably, and creates inconsistent separation batch to batch.
- Extreme high humidity may cause partial flour caking, disrupting steady feed flow.
Low inlet air humidity
- Surface moisture is minimal. Inter-particle liquid bridges disappear, keeping particles well dispersed. Individual protein bodies and starch granules maintain their native aerodynamic size, supporting sharp separation.
- However, excessively dry air can generate more static electricity. Fine protein particles build static charge, leading to electrostatic agglomeration. Static adhesion also causes powder to cling to metal surfaces, creating localized buildup.
- Very low humidity may also slightly alter protein surface properties; in rare cases it increases friability and creates extra ultra-fine debris.
Target operating window for pulse protein dry fractionation: controlled low relative humidity, typically 30–50% RH, adjusted according to feed flour moisture.
2. Effect of inlet air temperature
Temperature affects air density, powder moisture equilibrium, particle plasticity, and native protein functionality.
Higher inlet air temperature
- Higher air temperature reduces air density. At fixed volumetric airflow, lower gas density reduces aerodynamic drag force on particles. This shifts the classifier cut point toward larger particle sizes, unless wheel frequency is compensated.
- Elevated temperature accelerates surface moisture removal from powder. It helps break weak moisture-based agglomerates and improves particle dispersion.
- Critical limit: Too high inlet temperature introduces thermal risk. Excess heat denatures native pulse protein, damaging functional properties such as solubility, emulsification and foaming capacity, even if separation efficiency looks acceptable. High heat can also soften starch granules, making them more prone to sticking.
Lower inlet air temperature
- Cooler air has higher density and stronger drag force. With unchanged wheel speed, finer particles are more easily carried through the rotor. Cut point shifts smaller.
- Low temperature slows moisture evaporation. If incoming flour is slightly damp, cool inlet air cannot reduce surface moisture effectively. This leaves liquid bridges intact and agglomeration risk higher.
- The advantage is thermal safety: low inlet air avoids protein denaturation, preserving the native functionality of the protein concentrate.
3. Combined interaction of temperature and humidity
Temperature and humidity do not act independently. At the same absolute moisture content, rising air temperature lowers relative humidity.
- Example: Warming inlet air reduces RH, which mitigates moisture-caused agglomeration, but must stay below the protein denaturation threshold.
- Operators must track absolute moisture rather than only RH, especially when adjusting temperature. Changing temperature without humidity compensation can unexpectedly trigger agglomeration or static buildup.
4. Impacts on key classification metrics
- Cut point and Tromp curve: Humidity-induced agglomeration widens particle size distribution, flattens the Tromp curve and reduces separation sharpness. Temperature indirectly shifts cut point by changing air density.
- Enrichment factor: High humidity usually lowers enrichment because protein fines are trapped inside agglomerates and sent to coarse stream.
- Mass yield: Agglomeration reduces fine fraction yield. Severe fouling can cause unstable yield drift over long production runs.
- Product quality: Excessive temperature preserves separation performance at the cost of native protein functionality.
5. Practical operating considerations for pulse protein dry fractionation
- Maintain stable inlet air conditions rather than letting temperature/humidity fluctuate; fluctuations cause continuous cut-point drift.
- Pre-condition process air before feeding into classifier: dehumidification is often required in humid climates.
- Avoid rapid temperature swings, which cause condensation inside the classification chamber. Condensation leads to sudden caking and equipment fouling.
- Monitor both inlet air and powder outlet moisture as paired control parameters, together with PSD and periodic protein assays.
High inlet air humidity promotes particle agglomeration and fouling, reducing protein recovery and separation sharpness. Increasing inlet air temperature can reduce relative humidity and improve particle dispersion, yet overheating risks protein denaturation. Low temperature protects native protein functionality but may leave moisture-driven agglomeration unaddressed. Successful air classification requires stable, tightly controlled inlet air temperature and humidity, balanced to optimise aerodynamic sorting while preserving the functional quality of pulse protein concentrates.
