Grinding fineness is positively correlated with water absorption capacity up to an optimal particle size; beyond this threshold, water absorption may plateau or even decline for dry-fractionated legume protein powders.
Increased specific surface area drives higher water binding
Finer grinding breaks cotyledon cell walls, liberates protein bodies and exposes more hydrophilic functional groups on protein and polysaccharide surfaces. Higher specific surface area provides more sites to trap and hold water molecules, which increases water absorption and water-holding capacity. Coarse particles have sealed internal cell structures; much of the hydrophilic matrix remains inaccessible to water.
Capillary effect in fine particle packing
Micronized powders form a dense network of small inter-particle voids. These capillaries physically retain water, further boosting water absorption. This physical water entrapment adds to the hydration from protein and fibre molecules.
The turning point: over-grinding may reduce water absorption
When particles are excessively fine:
Severe mechanical stress and frictional heat can partially denature proteins. Protein unfolding may expose hydrophobic amino acid residues, lowering overall water-binding ability.
Extremely fine particles tend to agglomerate tightly. Dense agglomerates reduce accessible surface area and limit water penetration into particle clusters.
Damage to carbohydrate/fibre structures can also degrade their water-holding capacity.
Dual contributions from protein and residual fibre
For dry-fractionated protein fines, residual fine fibre is also micronized alongside protein. Fibre has high water affinity. Finer fibre fragments contribute significantly to total water absorption, so measured water absorption is a combined effect of protein and residual fibre.
Moderate ultra-fine grinding improves water absorption, which is desirable for many food applications. However, excessive fineness risks protein denaturation, poor powder flow, and agglomeration that negates hydration benefits. The target particle size must balance water absorption, separation efficiency and handling properties.
