Coffee Industrial Process_Paola Muggia_V2 - Coffee Grinding – Comprehensive Study Notes

Concept of Grinding

Definition
- Grinding = mechanical operation that converts roasted coffee beans into powder.
- Follows roasting; relies on brittleness created during roasting (roasted beans fracture easily).
Purpose
- Increases total surface area of coffee → enhances mass transfer during extraction.
- Breaks internal cellular structure → opens pores → water penetrates more easily.
- Enables controlled, method-specific particle‐size distributions (PSD).

Gap-Grinding Principle
- Beans drop by gravity through a gap formed by two moving, corrugated cutting tools.
- Gradually decreasing gap applies compressive + shear forces.
Distinct Mechanical Stages
- Crushing (pre-breaking)
- Brittle beans shattered into fragments ≈ 1\ \text{mm}.
- Grinding / Finishing
- Fragments subjected mainly to shear → reduced to target fine sizes.
- Multi-stage industrial grinders repeat these two actions in successive tool pairs.

PSD Basics
- Ground coffee contains a spectrum of particle sizes, not a single value.
- Characterised by:
- Mean/median particle diameter (often expressed as “x_50”).
- Proportion of fines (sub-100 µm fraction).
Definition of Fines
- Particles with equivalent spherical diameter <100\ \mu\text{m}.
- In specialty contexts, ultra-fines = <30\ \mu\text{m} (used to boost flavour in instant products).
Why PSD Matters
- Surface area ↔ extraction rate.
- Excess fines can clog filters (drip, cold brew) but are essential for body/crema in espresso.
- Every brewing method specifies a target PSD window (mean size + fines %).
- Espresso: bi-modal—mixture of coarse support particles & fines dispersed within bed.
- Cold brew / long-steep: coarse grind, minimal fines (time compensates for low area).
- Drip / pour-over: medium grind; fines must be controlled to prevent channeling & filter blocking.

Thermal Effects
- Compression + shear raise local temperature of cutting tools & grounds.
- Heat can re-trigger roasting reactions (e.g., Maillard), altering flavour.
- Industrial solutions: water-cooled, insulated tool housings to maintain thermal stability.
Degassing & Volatiles Loss
- Fracturing opens pore network → rapid release of \text{CO}_2 and aroma compounds.
- Resulting powder becomes more “sticky” as released oils migrate to surfaces.
Consequence
- Timing between grinding and brewing/packaging is critical to flavour retention.

Common Design Principle: two mating cutting tools create an adjustable gap; coffee fed by gravity.

Structure & Motion
- Multiple pairs (2–4) of parallel, counter-rotating cylinders; each pair = stage (break, grind, finish).
- Variables: individual gaps, roller speed, diameter, length, corrugation pattern.
Characteristics
- Continuous, high-capacity industrial system.
- Requires pre-run “warm-up” to reach thermal/mechanical stability.
- Recipe-driven via computer control (gap, speed, direction).
Pros/Cons
- + Precise PSD control; high throughput.
- – Large footprint; unsuitable for stop-and-go operation.

Geometry
- Two coaxial, flat discs with truncated-cone cavities & variable corrugations.
- Coffee drawn in by screw feed; expelled via centrifugal force.
Operation
- One main adjustable parameter: disc spacing (gap).
- Corrugation pattern can be swapped to create built-in two-stage action (pre-break + finish).
Usage
- Medium capacity; found in both industrial lines and professional shop grinders.
- Frequently paired with packaging machines (e.g., pods, bags) thanks to easier start/stop.

Design
- Male & female cone rotate coaxially, forming tapered gap.
Application
- Low-capacity, on-demand grinding in cafés/bars; integrated with dose/weight sensors.
- Rapid gap adjustment lets baristas tune grind for different extraction methods in real time.

Gap Stability
- Thermal expansion of metal alters gap; cooling jackets or water circulation maintain constancy.
Start-Up Phase
- Equipment must reach steady state (thermal + mechanical) before production coffee is collected.
Buffering & Packaging
- Continuous grinders demand downstream buffers to accommodate high flow before final packaging.

Espresso
- Needs bimodal PSD: coarse particles build the puck; fines (10–30 % of mass) enhance extraction, body, crema.
- Grinder choice often flat or conical burrs with narrow tolerance.
Drip / Pour-Over
- Medium grind; fines limited to avoid filter clogging & over-extraction at bottom of brew bed.
Cold Brew
- Very coarse (>600\ \mu\text{m} typical); long contact time compensates for low surface area, so fines avoided.
Instant Coffee Flavor Boosters
- Manufacturers may deliberately blend in ultra-fines <30\ \mu\text{m} to intensify aroma release when rehydrated.

Freshness vs Waste
- Grinding on demand (bar setting) preserves aroma but increases energy use & requires precise dosing to reduce waste.
Worker Safety
- High-speed grinders produce dust; ventilation & explosion-prevention measures are critical.
Sustainability
- Energy-intensive grinding underscores need for efficient motors & heat recovery in large plants.

Links Back to Roasting Lecture
- Brittleness, volatile formation, Maillard precursors—all dictate grindability & flavour outcomes.
Preview of Lab Session
- Hands-on measurement of PSD via sieving, laser diffraction, or image analysis.
- Comparison of grinder types & resulting extraction yields.

Evaluate:
- Sieve analysis to determine x_50 and fines %.
- Temperature mapping of grinders during operation.
Experiment:
- Adjust gap settings; brew coffee; record TDS (Total Dissolved Solids) & sensory notes.
Objective:
- Correlate grinder parameters → PSD → extraction → sensory profile.

Pre-crush fragment size ≈ 1\ \text{mm} (1000 µm).
Fines: <100\ \mu\text{m}; Ultra-fines for instant: <30\ \mu\text{m}.
PSD described via statistical measures (e.g., d{50}) & fines fraction \text{w}{<100\mu m}.
Surface area ∝ 1/\text{particle diameter} (simplified sphere model).