Target optimal range: 8.0 % ~ 11.5 % (wet basis); preferred set point = 9 %–10 % for most legumes (pea, faba bean, lentil, soybean)
1. Reason for this optimum moisture
- Optimal cell wall brittleness for liberation
At 9–10% moisture, legume cotyledon tissue turns suitably brittle: mechanical impact from pin mill/ACM easily fractures cell walls to free discrete protein bodies (1–10 μm) and intact starch granules (15–60 μm), no excessive shattering of starch into ultrafine dust.
- Too dry (<7%): material becomes overly brittle, starch pulverizes into micro-fines; ultra-fine starch mixes into protein fraction during air classification and reduces protein purity.
- Too wet (>12%): grains turn pliable/tough, grinding produces flaky agglomerates instead of discrete fine powder; protein remains trapped inside starch clusters and cannot be separated by air classifier.
- Control grinding temperature & avoid starch gelatinization / protein denaturation
Moisture above 12% + frictional heat from milling easily triggers partial starch gelatinization; gelatinized starch sticks to protein to form composite agglomerates and ruins dry separation. Over-dry feed creates extreme localized friction heat >45 ℃, denaturing native protein and changing particle density. - Stable powder flow for air classification
9–10% MC powder has good free-flow properties without caking inside classifier feed inlet or cyclone piping; high-moist feed sticks on classifier wheel and chamber walls, blocking airflow and disrupting separation balance.
2. Raw material differentiation of moisture setpoint
- Pea / Faba bean: 9.0–10.0 % (best standard)
- Lentil / Small grain legume: 8.0–9.5 %
- Soybean (higher oil): 10.0–11.5 % (slightly higher moisture to offset oily sticky tendency)
3. Quick adjustment rule
- Raw grain <7% MC: controlled tempering with low-volume humidified air to add moisture gradually (12–24 h conditioning).
- Raw grain >12% MC: low-temperature hot-air drying (≤45℃) to downsize moisture, avoid high heat damage to protein/starch structure.
The industry standard moisture for dry grinding before air classification is maintained at 9% ±1%, balancing particle liberation, grinding thermal control and downstream air separation performance.
