What is the relationship between fineness and mouthfeel

Fineness (quantified by D50, D90, particle size distribution width and coarse tail content) is the dominant physical factor determining the sensory mouthfeel of plant protein suspensions and plant-based beverages produced from dry-fractionated pulse protein. Fineness does not follow a simple “finer = better” rule; there is an optimal window. Too coarse generates grit, while excessive fineness introduces slippery, viscous or chalky off-notes. This article links particle size metrics directly to sensory perception, tailored to dry protein fractionation and beverage formulation.

1. Core Sensory Mechanism

Human oral tactile detection threshold: particles above approximately 10–15 μm can be perceived as discrete gritty particles inside the mouth.

• Large particles collide with tongue and palate, creating a noticeable sandy, grainy sensation.
• As particle size reduces below the detection threshold, discrete particle sensation disappears, and mouthfeel shifts toward viscosity-related texture.
• Particle size distribution width matters equally: even a small volume fraction of oversized particles creates grit, even if the average D50 is fine.

2. Mouthfeel Performance by Particle Size Zones

Zone 1: Coarse powder (D90 > 30 μm, significant coarse tail >30 μm)

• Sensory characteristic: Distinct gritty, sandy, grainy mouthfeel; sediment forms rapidly in liquid.
• Root cause: Undissociated protein–starch composite particles. These rigid particles exceed oral detection limits.
• Application limitation: Unsuitable for premium ready-to-drink plant beverages; only acceptable for bakery, extruded feed or low-cost functional powders where smoothness is not prioritised.

Zone 2: Optimal range for beverage-grade protein (D90 = 10–25 μm, narrow PSD, minimal coarse tail)

• Sensory characteristic: Smooth, creamy, clean mouthfeel after homogenisation; no obvious grit.
• Behaviour after hydration: Particles fall close to or below oral detection threshold once homogenised. Balanced viscosity; natural creamy impression without excessive thickness or chalkiness.
  This is the target fineness for UHT plant milk, ready-to-drink protein beverages and premium instant drink bases.

Zone 3: Ultra-fine powder (D90 < 8 μm)

Two main negative sensory shifts emerge:

1. Chalky, powdery aftertaste: High specific surface area binds large volumes of water, increasing apparent viscosity and creating a dry, lingering powdery sensation in the throat.
2. Higher solution viscosity: Beverages become thick, heavy, “sludgy”, losing refreshing fluid properties.
   Additional processing risk: Ultra-fine powder comes from over-grinding, which fractures starch granules. Elevated fine starch further boosts viscosity and restricts maximum achievable protein purity.

3. Critical influence of particle distribution width (beyond average fineness)

Two powders can share identical D50/D90 but deliver vastly different mouthfeel:

• Wide PSD: Contains both oversized grit particles and ultra-fine fractions. Outcome: simultaneous sandy grit + chalky heavy texture — the worst combination for beverage applications.
• Narrow PSD: Particles concentrated within the optimal size band. Consistent smoothness, predictable viscosity, no conflicting sensory signals.

Even a small volume of oversized tailings (>30 μm) dominates sensory perception. Consumers easily detect grit and ignore the smooth majority of fine particles. This explains why removing coarse tailings is higher priority than further reducing average particle size.

4. Interaction between fineness and homogenisation

Dry powder fineness sets the upper limit of beverage texture quality:

• Coarse dry composites cannot be fully eliminated by standard high-pressure homogenisation. Rigid protein-starch composites resist shear and remain as grit precursors.
• Well-controlled beverage-grade dry protein (D90 10–25 μm) allows homogenisers to break soft agglomerates efficiently, achieving a smooth stable beverage with moderate homogenisation pressure.

Poor upstream fineness control forces formulators to increase homogeniser pressure, raise stabiliser dosage, and accept higher operating costs.

5. Secondary linked effects that alter perceived mouthfeel

Fineness indirectly changes texture via these pathways:

1. Hydration rate: Coarse particles hydrate slowly; partially hydrated particles feel grainy. Ultra-fine powder hydrates instantly but builds high viscosity.
2. Suspension stability: Coarse particles settle; sediment creates inconsistent mouthfeel during consumption.
3. Air entrapment: Ultra-fine powder tends to trap air during mixing, leading to foamy beverages.

6. Practical production guidance for dry fractionation

1. Primary target: Eliminate oversized composite particles first (reduce coarse tail), rather than endlessly pursuing finer average size.
2. Maintain protein fines D90 within 10–25 μm with narrow PSD for beverage end-use.
3. Avoid pushing grinding intensity to achieve D90 below 8 μm; sensory degradation and purity loss outweigh minor smoothness improvements.
4. Use multi-stage polishing classification to strip coarse tailings, delivering consistent smooth mouthfeel batch-to-batch.

Summary

1. Coarse particles (D90 >30 μm) produce gritty, sandy mouthfeel due to discrete particle detection by the tongue.
2. The optimal fineness window for beverage-grade plant protein is D90 10–25 μm with narrow PSD, delivering smooth, creamy sensory properties.
3. Excessively fine powder (D90 <8 μm) triggers chalkiness, heavy viscosity and powdery after-feel.
4. Particle distribution width is often more important than average fineness; a small fraction of oversized particles dominates sensory defects.

Fineness tuning must balance separation performance, protein purity and target sensory requirements for plant-based beverage customers.