Screen size: What does Screen 14, 16, 18 really say?
When buying coffee beans, descriptions like Screen 14, Screen 16, or Screen 18 often appear in product specifications. Many people see Screen 18 as large beans, Screen 16 as smaller beans, and assume that the higher the screen size, the better the quality.
That understanding is only partially correct. Screen size reflects the ability to pass through a system of holes with a specific size. It is an important physical criterion in classifying coffee beans, but it cannot replace checking for defects, moisture, aroma, and sensory quality.
1. What are Screen 14, 16, and 18?
Screen is the term for the screen size used in classifying coffee beans. Each screen size corresponds to a specific hole size. In the method using round-hole screens, the beans are separated based on their ability to pass through the holes during the test.
According to Appendix B of TCVN 4193:2014 – Coffee Beans, the hole sizes are specified as follows:
| Screen size | Hole size | Approximate conversion |
|---|---|---|
| N°13 | 5.00 mm | 5,000 µm |
| N°14 | 5.60 mm | 5,600 µm |
| N°15 | 6.00 mm | 6,000 µm |
| N°16 | 6.30 mm | 6,300 µm |
| N°17 | 6.70 mm | 6,700 µm |
| No. 18 | 7.10 mm | 7,100 µm |
| No. 19 | 7.50 mm | 7,500 µm |
| No. 20 | 8.00 mm | 8,000 µm |
Thus, the No. 18 screen aperture is larger than the No. 16 screen aperture. However, this test should not be interpreted as a direct measurement of the geometric length or diameter of individual beans. Coffee beans are not perfectly round, and their orientation when contacting the screen aperture also affects their ability to pass through.
2. Principle of classification using a sieve system
In the test, sieves are arranged in order of decreasing aperture size. The coffee sample is placed on the top sieve and then sifted manually or mechanically. Beans retained on each sieve are collected separately and weighed.
ISO 4150:2011 specifies the method for analyzing green coffee bean particle size using manual and mechanical sieving. The principle of the test is to separate the sample into fractions and express the results as a percentage by mass.
3. Calculation of mass proportion for each sieve size
Mass proportion (%) = Mass of fraction / Sample mass × 100
Illustrative example with a hypothetical test sample weighing 300 g:
| Fraction | Mass | Mass proportion |
|---|---|---|
| Retained on No. 18 sieve | 90 g | 30% |
| Retained on No. 16 sieve | 150 g | 50% |
| Retain on No. 14 screen | 45 g | 15% |
| Pass through No. 14 screen | 15 g | 5% |
| Total | 300 g | 100% |
This is just an example calculation, not the result of actual coffee testing. In quality control, it is necessary to clearly record the type of screen, testing method, sample code, and results of each segment to enable comparison between inspections.
4. Why is screen size not synonymous with quality?
TCVN 4193:2014 does not classify coffee by a single Screen number. The standard also specifies requirements for color, aroma, moisture, percentage of other coffee types, percentage of defects, and impurities.
A notable point is that the standard distinguishes between product codes such as R1 18a, R1 18b, R1 16a, R1 16b, and R1 16c. These are all codes belonging to Grade 1 Arabica coffee, but with different requirements for screen size and defect limits.
| Robusta classification code | Maximum defect rate | Maximum impurity rate |
|---|---|---|
| R1 18a | 10% | 0,1% |
| R1 18b | 15% | 0,5% |
| R1 16a | 14% | 0,5% |
| R1 16b | 16% | 0,5% |
| R1 16c | 18% | 0,5% |
The above limits are parameters in Table 4 of TCVN 4193:2014, not sensory standards for specialty coffee. The standard also specifies separate minimum percentages on round-hole screens and specific defects in subsequent tables.
Same screen size but may differ in particle structure
Two coffee lots with a similar 16 Screen rating can still differ in density, moisture, bean development level, and defect status. These factors affect the behavior of the beans during storage and roasting.
For example, two beans of similar size but one with a porous structure, low density, and different moisture levels. When roasted under the same conditions, they do not necessarily absorb heat and change in the same way.
Therefore, uniform screen size can help control the uniformity of the input material, but it is not enough to ensure uniform roasting.
5. When buying green coffee, what should be noted in addition to Screen?
If the contract only states 'Robusta Screen 18', the buyer and seller may still have different understandings of the percentage of beans meeting the screen size, defect limits, and sampling method.
A clear commodity specification should define:
- Coffee variety or type: Arabica, Robusta, or other.
- Grading code and applicable standards, if any.
- Minimum mass percentage for each screen size.
- Limits for small beans, broken beans, and foreign matter.
- Moisture content and determination method.
- Sampling method, sample mass, and handling of discrepancies between trials.
- Sensory requirements and cupping protocol if cup quality is a condition of sale.
6. Distinguishing screen size from particle size distribution after grinding
Green coffee screen size and post-grind particle size distribution are two different measurements. Screen size is used to classify raw beans before roasting; Particle Size Distribution (PSD) describes the size of particles after grinding.
Do not use Screen 16 or Screen 18 results to infer the amount of fines in espresso. To understand differences in PSD after grinding, see the article Flat and conical burrs: How grinder burr geometry shapes particle distribution and Espresso flow?
References
- TCVN 4193:2014 – Green coffee, especially Table 1, Table 4, Table 6, and Annex B on screen sizes.
- ISO 4150:2011 – Green coffee or raw coffee: Size analysis – Manual and machine sieving.
Note on standard citations: TCVN 4193:2014 lists TCVN 4807:2013 (ISO 4150:2011) in its normative references, while some content in Section 5.6 and Annex B still refers to TCVN 4807:2001 (ISO 4150:1991). When applying this for contracts or official inspection, cross-check the original standard and the current version of the test method.