Green coffee bean screen size: What is Screen 18, and does it indicate quality?

When reviewing the specifications for a lot of green coffee beans, buyers often encounter notations such as Screen 16, Screen 18, 16/18, or 18+. These numbers describe bean size according to the screening classification method. However, in coffee trading, screen size is sometimes mistakenly interpreted as a direct measure of quality.

A Robusta Screen 18 lot is not inherently superior to a Screen 16 lot. Larger beans do not necessarily imply higher density, fewer defects, or better sensory potential. To interpret this specification correctly, one must understand what the screen measures and which factors lie outside this measurement.

1. What does screen size actually measure?

Screen size is a method for classifying green coffee beans based on their retention or passage through screens with holes of defined sizes. Beans retained on a screen with larger holes than another screen belong to the size category corresponding to the screening system in use.

In traditional commercial notation, the Screen number usually corresponds to the hole diameter in units of 1/64 inch. Under this convention, Screen 18 nominally corresponds to 18/64 inch, approximately 7.14 mm.

Example:

The above values are conversions based on the 1/64-inch convention commonly used in trade. ISO 4150:2011 specifies the method for analyzing green coffee bean size using laboratory test sieves, including manual and mechanical sieving. When drafting contracts or inspection reports, clearly specify the sieve set, actual aperture sizes, and testing method; do not assume that all metric sieve sets have dimensions that exactly match commercial conversions.

The key point is that Screen size does not directly measure the absolute length of individual beans. Beans may change orientation during vibration or shaking; their ability to pass through an aperture also depends on their cross-sectional area and shape.

2. Why distinguish between flat beans and peaberries?

Standard coffee beans are typically flat on one side with a central crease. Peaberries form when only one seed develops inside the fruit instead of the usual two, resulting in a rounder shape.

Due to their different shapes, these two groups of beans may require different sieving methods. Round-hole sieves are commonly used to grade flat beans; slotted sieves may be more suitable for grading peaberries. If the two types are mixed and graded using only one sieve set, the results may not accurately reflect the size distribution of each group.

3. Interpreting a screen size distribution report

A single Screen 18 value does not indicate the uniformity of the entire lot. Buyers need to examine the mass percentage retained on each sieve, the percentage of beans smaller than the specified threshold, and the contract tolerances.

Assume a 300 g sample is taken to illustrate how to record results:

These are hypothetical data provided to guide the interpretation of the table, not measurement results from an actual coffee lot or a mandatory grading threshold.

The FAO has republished the former SCAA method for grading green coffee beans, in which the sample is sieved through sizes 14, 15, 16, 17, and 18, and the mass percentage on each sieve is recorded. The document also states that coffee must be roasted and cupped to evaluate its in-cup characteristics. Therefore, size metrics are part of physical classification and do not replace sensory evaluation.

4. Bean size, density, and physical defects are three distinct parameters.

A large bean may be light, hollow, or damaged. A small bean can still be dense, clean, and have good cup quality. Size grading screens do not directly measure density, moisture content, internal condition, or flavor.

In the processing line, size classification is often combined with density or aerodynamic sorting and color sorters. Each stage addresses a different group of variations:

• Size screens: separate beans based on their ability to pass through screen apertures.
• Density table: assists in separating bean groups based on specific gravity and movement behavior in an air stream.
• Color sorter: identifies beans with abnormal coloration for removal according to settings.
• Manual or laboratory defect inspection: checks for defects that equipment or automated sorting systems may miss.

5. Does Screen 18 help achieve more uniform roasting?

A narrow size distribution can help roasters control raw material more easily, as differences in size and shape can affect how beans absorb heat. However, one cannot rely solely on screen size to predict roasting rate or bean development.

Moisture content, density, cellular structure, variety, growing region, primary processing method, and storage conditions also affect how beans respond during roasting. Two lots both classified as Screen 18 but with different moisture levels and densities may still require different roast profiles.

6. How to record screen size specifications in purchase orders

A clear product description should include more information than just “Screen 18”. It can follow this structure:

• Coffee species or group: Arabica / Robusta.
• Measurement method: type of screen, aperture system, and manual or machine method.
• Size distribution: mass percentage retained on each screen.
• Tolerance: maximum allowable percentage outside the required size range.
• Other criteria: moisture content, physical defects, density, or cupping requirements if applicable.

For example, instead of just writing “Robusta Screen 18”, the specification could state “Robusta, size analysis according to designated screen set, mass percentage retained on Screen 18 and above, maximum undersize percentage as per contract”. Tolerance figures must be agreed upon by both parties and verified according to applicable standards, not inferred from the screen name alone.

Screen size is a useful language in green coffee trade, but it is only valuable when placed in the proper context: it describes classification size, not overall quality grade. Buyers, roasters, and processors need to consider size, moisture content, density, physical defects, and cupping results together to correctly understand the value of a lot.

References

• International Organization for Standardization. ISO 4150:2011 — Green coffee or raw coffee — Size analysis — Manual and machine sieving. ISO standard page.
• Food and Agriculture Organization of the United Nations. Annex 7: Green Coffee Classification and Grading. FAO document.
• Bicho, N. C. et al. Application of colour parameters for assessing the quality of Arabica and Robusta green coffee. DOI: 10.9755/ejfa.v26i1.17190.
• Luna González, A. et al. Cup quality attributes of Catimors as affected by size and shape of coffee bean (Coffea arabica L.). DOI: 10.1080/10942912.2019.1603997.