1. Executive Summary
Achieving a particle size of D90 < 20μm in pea protein dry grinding is a high-precision ultra-fine processing requirement for high-purity pea protein dry fractionation. Unlike conventional grinding (D90 30–60μm), ultra-fine grinding requires strict control of raw material pretreatment, moisture content, milling equipment parameters, temperature, and closed-loop classification. The core goal is to fully liberate pea protein bodies and starch granules, produce uniform ultra-fine powder without starch over-crushing and protein denaturation, and stably obtain powder with D90 less than 20μm for high-efficiency air classification separation.
2. Strict Pre-Milling Raw Material Preparation (Foundation of D90 < 20μm)
Unqualified raw materials are the main cause of failure to reach ultra-fine particle size and uneven powder distribution. Standardized pretreatment is mandatory.
2.1 Complete Cleaning and Dehulling
Pea husk contains tough crude fiber, which cannot be crushed to ultra-fine particles synchronously with protein and starch. Residual husk will cause uneven particle size and block milling equipment.
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Remove impurities: Screen, aspirate and destone raw peas to eliminate stones, dust, moldy grains and foreign debris.
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Full dry dehulling: Use an abrasive dehuller to completely remove seed coats, with residual husk content controlled at <1% to avoid fiber coarse particles mixing in finished powder.
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Purify cotyledons: Screen dehulled pea cotyledons to remove oversized and undersized particles for uniform feed.
2.2 Precise Moisture Conditioning (Most Critical Factor)
Moisture content directly determines the brittleness of pea cotyledons and grinding fineness stability. The ideal moisture range for D90 < 20μm ultra-fine grinding is 8.5%–9.5% (wet basis).
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<8.0% moisture: Materials are overly brittle, causing a large number of ultra-fine starch fragments (<5μm), resulting in particle size disorder and easy protein denaturation due to excessive grinding heat.
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8.5%–9.5% moisture: The material has optimal brittleness; cell walls are fully broken by mechanical shear, protein and starch are completely liberated, and particle size grinding is uniform.
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>10.0% moisture: Materials are tough and elastic, unable to be crushed to ultra-fine size, easy to form agglomerates, leading to D90 far exceeding 20μm.
Conditioning method: Adopt low-temperature constant-humidity tempering for 12–24 hours to ensure batch moisture uniformity with a deviation of ±0.3%.
2.3 Pre-Crushing and Particle Size Homogenization
Direct ultra-fine grinding of whole peas causes inconsistent mechanical force reception. Pre-crush dehulled pea cotyledons into uniform fine particles of 2–3mm to ensure each material unit receives equal impact and shear force during ultra-fine milling, laying a foundation for uniform D90 particle size.
3. Optimized Ultra-Fine Milling Equipment & Core Operating Parameters
Conventional hammer mills cannot meet D90 < 20μm requirements. Specialized ultra-fine grinding equipment and precise parameter matching are required.
3.1 Recommended Core Equipment
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High-speed pin mill (preferred): Suitable for ultra-fine liberation of pea protein-starch matrix, with uniform particle size and low starch damage.
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Air Classifying Mill (ACM): Integrated grinding and classification, real-time screening of coarse particles, stable output of D90 < 20μm powder.
3.2 Key Parameter Settings for D90 < 20μm
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Processing Parameter
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Optimal Range
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Function & Effect
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Main shaft rotating speed
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13,000–15,000 rpm
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Provide sufficient mechanical shear to break cell walls and crush materials to ultra-fine size
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Feeding rate
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30–80 kg/h (industrial small-medium scale)
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Slow and uniform feeding avoids material accumulation and incomplete grinding
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Grinding chamber temperature
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≤38°C (constant cooling)
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Prevent protein denaturation and starch secondary adhesion caused by frictional high temperature
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Classifier wheel speed (ACM)
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8,500–10,000 rpm
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Cut off coarse particles >20μm, ensure only ultra-fine powder passes through
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Circulating airflow
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20–25 m³/h
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Stably carry ultra-fine particles, avoid coarse particle residue
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4. Closed-Loop Regrinding & Classification Process (Guarantee Stable D90 < 20μm)
Primary ultra-fine grinding inevitably produces a small amount of incompletely ground coarse particles (>20μm). A closed-loop circulation process is required to ensure full batch compliance:
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After primary grinding, the powder enters the air classifier for size screening;
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Qualifying ultra-fine powder (D90 < 20μm) is collected as finished material;
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Unqualified coarse particles and middlings are automatically returned to the pin mill/ACM for secondary ultra-fine regrinding;
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Repeat the circulation 2–3 times to ensure the D90 of the final batch powder is stably controlled below 20μm with uniform particle size distribution.
5. Critical Quality Control Standards
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Finished powder particle size:D90 < 20μm, D50 ≈ 8–12μm, no coarse particles larger than 30μm;
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Powder uniformity: Span value <1.5 (narrow particle size distribution);
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Protein activity: No denaturation, maintain native solubility and functional properties;
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Starch integrity: Avoid excessive pulverization of starch granules to ensure subsequent air classification separation effect.
6. Common Problems & Troubleshooting
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Common Issue
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Root Cause
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Solution
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D90 cannot reach <20μm
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Excessive material moisture, low grinding speed, or fast feeding rate
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Adjust moisture to 8.5–9.5%, increase main shaft speed, and reduce feeding volume
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Uneven particle size
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Ununiform pre-milling particle size, unstable airflow
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Re-homogenize raw materials, calibrate classifier airflow and rotating speed
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Powder agglomeration
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Grinding temperature too high or material moisture excessive
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Turn on circulating cooling, re-condition material moisture
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Excessive ultra-fine starch powder
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Over-grinding, material moisture too low
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Properly increase material moisture and reduce grinding speed appropriately
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7. Conclusion
Stably achieving D90 < 20μm in pea protein ultra-fine grinding relies on three core conditions:standardized pretreatment (full dehulling + 8.5–9.5% constant moisture), high-speed ultra-fine milling with precise parameter control, and closed-loop regrinding classification. This process not only meets the ultra-fine particle size standard but also ensures the structural integrity of protein and starch, providing high-quality raw powder for subsequent high-precision dry fractionation and air classification separation.
