Screen Size Grading¶
Screen Size Grading Classification system based on bean physical size. Uses perforated metal screens with specific hole sizes. Beans sorted by which screens they pass through. Measured in 1/64-inch increments (screens 10-20). Common in international coffee trade. Screen Measurement Screen Number Represents hole size in 1/64-inch increments. Screen 16 = 16/64 inch = 6.35mm holes. Screen 18 = 18/64 inch = 7.14mm holes. Higher number = larger beans. Range typically 10-20. Sorting Process Stacked screens, largest holes on top. Green coffee poured into top screen. Shaken/vibrated to sort. Beans remain on screen they can't pass through. Multiple sizes separated simultaneously. Measurement System North American system (64ths of inch). European system (millimeters). Conversion: Screen number × 0.397 = mm. Example: Screen 16 = 6.35mm, Screen 18 = 7.14mm. Common Screen Sizes Screen 20 (7.94mm) Extra-large beans. Rare, specialty designation. Maragogipe (elephant beans). Premium pricing sometimes. Limited commercial significance. Screen 19 (7.54mm) Very large beans. "Supremo" in Colombian grading. Quality indicator (usually). Premium grade. Limited supply typical. Screen 18 (7.14mm) Large beans. "AA" in Kenya. "Supremo" threshold Colombia. High quality association. Consistent roasting. Desirable commercial size. Screen 17 (6.75mm) Large-medium beans. "AB" range in Kenya. Colombian "Excelso." Good quality. Most common premium size. Roasts evenly. Screen 16 (6.35mm) Medium beans. Lower end AB Kenya. Acceptable quality. Common size. Standard commercial. Good for blends. Screen 15 (5.95mm) Medium-small beans. Lower grading tier. "C" grade in some systems. Still usable. Lower price. Blend component. Screen 14 (5.56mm) Small beans. Lower quality association. Challenges in roasting. Faster roasting. Often separated out. Budget commercial. Screen 13 and below Very small beans. "Peaberries" possible. Often defective or immature. Usually separated/discarded. Low commercial value. Exception: Intentional peaberry separation. Regional Grading Systems Kenya (Screen-Based) - AA: Screen 17-18 (6.75-7.14mm) - AB: Screen 15-16 (5.95-6.35mm) - C: Below screen 15 - E (Elephant): Extra large, rare - PB (Peaberry): Round beans, separated - TT/T: Lower grades, broken, defects Colombia (Screen-Based) - Supremo: Screen 17+ (6.75mm+) - Excelso: Screen 14-16 (5.56-6.35mm) - UGQ (Usual Good Quality): Mix of sizes Supremo commands premium over Excelso. Tanzania (Similar to Kenya) - AA: Largest beans - A: Medium-large - B: Medium - C: Small - PB: Peaberry Screen-based like Kenya system. Size vs Quality Relationship General Correlation Larger beans often (not always) indicate better quality. High-altitude, slow maturation creates density and size. Better development generally. Exceptions exist. Density More Important Size alone insufficient. Dense small bean > less dense large bean. Altitude and growing conditions matter more. Screen size is convenience, not quality guarantee. Even Roasting Main practical benefit of screening. Uniform sizes roast evenly. Mixed sizes problematic (small beans burn while large under-roast). Consistency crucial for roasting. Market Perception Larger beans command premiums. Not always justified by cup quality. Marketing and tradition drive pricing. Informed buyers look beyond size. Screen Grading Limitations Doesn't Measure Quality Size ≠ quality necessarily. Defects can be any size. Doesn't assess flavor. No processing information. No defect detection. Complementary grading needed. Doesn't Measure Density Dense small bean vs. large hollow bean. Density more important for quality. Separate density grading exists. Screen sizing misses this critical factor. Peaberry Separation Round peaberries don't screen like flat beans. Mixed in with regular beans. Requires separate sorting. Shape matters as much as size. Variable by Origin Different origins have different typical sizes. Ethiopian heirloom: Smaller beans typical. Maragogipe variety: Naturally huge. Context matters for interpretation. Density Grading Complement Gravity Separation Uses air or water to separate by density. Dense beans sink/resist air. Light beans float/blow away. More accurate quality indicator. Often combined with screen sizing. Electronic Sorting Modern optical sorters. Detect color, size, density, defects. More sophisticated than screens. Expensive equipment. High-end processing. Combined Approach Screen + density + visual + cupping. Multiple quality checks. No single system sufficient. Comprehensive grading essential. Commercial coffee standard. Practical Applications Roasting Consistency Primary reason for screen grading. Uniform size = even roasting. Mixed sizes create problems. Small beans scorch, large under-develop. Quality control essential. Commercial Trading Standard commodity trading specification. Contracts specify screen sizes. Price differentials by size. International trade requirement. Communication across supply chain. Blending Different screen sizes for different purposes. Larger for single origin showcase. Smaller for blend components. Economic efficiency. Quality/cost optimization.
Related Notes: - Coffee Terminology MoC