Milling

Coffee milling has evolved from manual hulling with mortars and pestles to sophisticated mechanized mills. The first mechanical hullers were developed in the 19th century, dramatically increasing processing efficiency. Over time, milling equipment became more specialized, with separate machines for hulling, polishing, grading, and sorting. The development of optical sorters in the late 20th century revolutionized defect removal, using cameras and air jets to remove defective beans at high speed. Today, modern coffee mills use a combination of mechanical and optical technologies to produce clean, uniform, export-ready green coffee.

Milling involves several mechanical processes. Hulling removes the parchment layer (the papery casing around the green bean) using friction. The huller uses either a rubbing action (disc huller) or a rotating cylinder (cylinder huller) to crack and remove the parchment. Polishing, an optional step, removes the silver skin (the thin, silvery membrane adhering to the bean). Grading separates beans by size using screens with different hole diameters (measured in 1/64 inch increments, e.g., screen 18, screen 16). Density grading uses air or water to separate beans by density, with denser beans being higher quality. Optical sorting uses cameras to identify and remove defective beans based on color, with air jets ejecting the defectives. Each step must be calibrated to avoid damaging the beans, as cracked or broken beans are considered defects.

1. Receiving and Storage: Dried parchment coffee is received at the mill and stored in breathable bags. Moisture is checked to ensure it is 10 to 12%. 2. Hulling: The parchment coffee is fed into a huller, which removes the parchment layer through friction. The output is green coffee with the silver skin still attached. 3. Polishing (Optional): The green coffee passes through a polisher, which removes the silver skin through gentle friction. This step is optional and is skipped by some mills. 4. Size Grading: The green coffee passes through screens with different hole sizes, separating beans by size. Common screen sizes are 20, 19, 18, 17, 16, and 15 (measured in 1/64 inch). 5. Density Grading: The graded coffee passes through a density separator (catador), which uses air to separate beans by density. Heavier (denser) beans are higher quality. 6. Color Sorting: The coffee passes through an optical sorter, which uses cameras to identify defective beans (black, sour, insect-damaged, etc.) and air jets to eject them. 7. Hand Sorting (Optional): Some specialty coffees receive additional hand sorting to remove any remaining defects. 8. Bagging: The sorted green coffee is bagged in jute or vacuum-sealed bags for export. 9. Cupping and Quality Control: Samples are cupped to verify quality before export.

Milling is not related to fermentation. By the time coffee reaches the mill, fermentation has been completed during processing and drying. However, milling quality affects how the coffee roasts and brews, which in turn affects how fermentation-derived flavors are expressed in the cup.

Milling occurs after drying is complete. The coffee must be at 10 to 12% moisture before hulling. If the coffee is too dry (below 9%), hulling can cause excessive breakage. If too wet (above 13%), the parchment may not remove cleanly and there is risk of damage. After hulling, the green coffee should be stored at 10 to 12% moisture in appropriate packaging.

After milling, green coffee is stored in jute bags (for short-term) or vacuum-sealed bags and GrainPro hermetic bags (for long-term or export). Store at 15 to 25 degrees Celsius with 50 to 60% relative humidity. Green coffee should be rested for 30 to 60 days after milling before export, allowing flavors to settle and stabilize.

• •Transforms dried parchment into export-ready green coffee
• •Removes defects through grading and sorting, improving cup quality
• •Creates uniform bean size for consistent roasting
• •Optical sorting removes defective beans at high speed and accuracy
• •Adds value to the coffee through quality improvement
• •Enables quality differentiation through screen size and density grading

• •Requires significant investment in milling equipment
• •Risk of bean damage (breakage, cracking) during hulling
• •Optical sorters require regular calibration and maintenance
• •Milling generates dust and chaff that require management
• •Small producers may lack access to quality milling facilities
• •Improper milling can introduce defects and reduce quality

Milling does not directly affect flavor, but milling quality indirectly affects the cup. Uniform bean size (achieved through grading) ensures even roasting, which is essential for optimal flavor development. Removing defective beans (black, sour, insect-damaged) prevents off-flavors from entering the cup. Polishing (removing the silver skin) can slightly reduce astringency in the cup, as the silver skin contains compounds that contribute to harshness. Overall, high-quality milling is essential for producing clean, consistent, and defect-free coffee.

• •Broken or chipped beans from aggressive hulling
• •Unhulled beans (parchment not fully removed) from under-hulling
• •Inconsistent sizing from worn or miscalibrated screens
• •Defects passing through if optical sorter is miscalibrated
• •Silver skin remaining if polishing is skipped or inadequate
• •Moisture damage if coffee is milled at incorrect moisture levels

Milling generates waste in the form of parchment hulls, silver skin, and rejected beans. These byproducts can be composted, used as biomass fuel, or repurposed as fertilizer. The main energy use is electricity for hullers, sorters, and conveyors. Modern mills increasingly use energy-efficient equipment and recycle byproducts. Dust management is important for worker health and safety.

• •AI-powered optical sorters with machine learning for defect identification
• •Multi-spectral sorting using UV and infrared to detect internal defects
• •Automated moisture and density tracking throughout the milling process
• •Energy-efficient hullers with reduced bean damage
• •Blockchain tracking for traceability from mill to consumer
• •Mobile milling units for small producers in remote areas

• •Chapter 3: Post-Harvest Processing
• •Chapter 2: Processing Methods

• •Specialty Coffee Association
• •Coffee Science Foundation
• •World Coffee Research
• •International Coffee Organization