Single-pass air classification is the core dry fractionation step for pulse proteins (pea, faba bean, lentil, chickpea). It separates liberated fine protein bodies from coarse starch granules in one classification stage, without re-grinding or recycling the coarse stream back into the classifier. Two key terms are used to describe performance: protein shift (absolute gain in protein percentage, dry basis) and enrichment factor (ratio: fine fraction protein content / feed flour protein content).
Typical Ranges for Optimized Single-Pass Operation
For properly pre-treated, dehulled pulses that have been ultra-fine milled to liberate protein bodies:
- Feedstock baseline: Dehulled pulse flour typically runs 20–26% protein (db) for yellow pea, faba bean, lentil.
- Fine fraction protein after single pass: 40–60% protein (db); well-tuned lab/pilot conditions can touch ~61% for selected pulse varieties.
- Enrichment factor: 1.6× to 2.3× is the standard industrial single-pass window.
- Conservative industrial production: 1.6–1.9×
- Optimized pilot/lab trials: 2.0–2.3×
- Protein shift (absolute increase): +20 to +35 percentage points above feed protein content.
Example: 23% protein pea feed → fine fraction at 48% protein = 25-point protein shift, enrichment factor = 2.09×.
Important trade-off: Higher enrichment factor usually comes with lower fine fraction mass yield. If you push rotor speed to maximize protein concentration, the fine yield drops and more protein reports into the coarse starch stream, reducing total protein recovery.
Practical Boundaries & Raw Material Variability
These numbers assume the critical prerequisite: adequate liberation by fine milling. If the flour is insufficiently milled, protein remains bound inside starch-cell agglomerates. The enrichment factor may fall to only 1.2–1.5× even with identical classifier settings.
- Yellow pea: single pass enrichment commonly 1.8–2.2×
- Faba bean: ~1.7–1.9×
- Red lentil: up to ~2.1–2.3× under optimized milling and classification
- Chickpea: generally lower single-pass enrichment, ~1.6–1.8× due to seed microstructure and fiber content
How Single-Pass Compares Against Multi-Stage Classification
Single-pass air classification cannot reach the purity levels of cascaded multi-pass dry fractionation.
- Single pass: max practical fine protein ~60% db, enrichment ≤2.3×
- Multi-pass (re-grind coarse stream + re-classify): can push fine fraction protein toward 65–70% and higher enrichment factors, at the cost of extra energy and lower overall yield.
Definition Recap for Your Technical Documentation
- Enrichment Factor = Protein content of fine fraction ÷ Protein content of incoming feed flour (dry basis)
- Protein shift = Protein content of fine fraction − Protein content of incoming feed flour (dry basis, percentage-point difference)
Main Factors Limiting Single-Pass Protein Enrichment
- Incomplete cell liberation during milling: protein bodies remain attached to starch granules
- Particle agglomeration: fine protein sticks onto coarse starch particles and escapes to coarse discharge
- Cut-point selection: sharpness of the Tromp curve; cross-contamination between fractions
- Residual hull fiber, seed variety, moisture content and particle size distribution of feed flour
For optimized single-pass air classification of dehulled, finely milled pulses, the typical enrichment factor ranges from 1.6× to 2.3×, with a protein shift of +20 to +35 percentage points. Most commercial dry-fractionation lines operate at 1.7–2.0× enrichment in single pass to balance protein purity, fine mass yield and protein recovery.
