Robusta G1, Screen 16 and 18, Wet Polished: Reading Vietnamese Coffee Specs

“Robusta G1 S18 wet polished” is probably the most common product line in any Vietnamese coffee offer sheet. It looks precise, but it compresses several separate parameters into a few characters, and two exporters can use the same words for coffee that grades and cups quite differently. This article unpacks the naming so you can read an offer line by line, compare quotes on equal terms and write a spec that leaves no room for interpretation.

If you are new to buying from Vietnam, the roaster guide to Vietnamese green coffee covers regions, processing and logistics first.

The standard behind the names

Vietnamese green coffee grades are based on TCVN 4193:2014, the national technical standard for green coffee. It sets out how a sample is drawn, how beans are classified by size and how defects are counted and limited for each grade. Alongside the standard, the export trade has developed its own shorthand (G1, G2, S13, S16, S18) that refers to those limits in a compact way.

In practice, three variables make up a Robusta grade:

1. Bean size, expressed as a screen number and a minimum retention percentage.
2. Moisture, as a maximum percentage.
3. Defects, expressed either as a count of black and broken beans per sample or as a percentage by weight, plus a limit on foreign matter.

A fourth parameter, polishing, describes appearance after hulling and is added to the name when it applies.

Screen size: what the numbers mean

Screen numbers refer to sieve hole diameters in 64ths of an inch. A bean “on screen 18” does not pass through a sieve with round holes of 18/64 inch.

Screen	Hole size (inch)	Hole size (mm)	Typical use
13	13/64	5.16	Commercial grades, instant and extraction
16	16/64	6.35	Standard export grade, blends
18	18/64	7.14	Top Robusta export grade, espresso blends
20	20/64	7.94	Rare in Robusta, some exporters sell S20 separately

The grade usually requires 90% retention. S18 means at least 90% of the beans by weight stay on screen 18, and the remaining 10% or less can be smaller. Most contracts also cap the amount passing through the next screen down (for example max 2% through screen 16 for an S18), which is worth asking for explicitly.

Defects: black, broken and foreign matter

Vietnamese Robusta specs usually track three defect categories:

• Black beans: fully or partly black beans, typically caused by over-fermentation, disease or cherries that dried on the tree or ground. They give harsh, dirty and sometimes phenolic flavours.
• Broken beans: fragments from hulling or over-dried coffee. They roast unevenly and burn.
• Foreign matter: stones, sticks, husk, dust. Measured as a percentage by weight.

Exporters express the black and broken limit in two ways. Some count beans in a 300 g sample (for example max 30 black and broken per 300 g). Others give a weight percentage (for example black and broken 2% max). The two are not directly interchangeable, so ask the supplier which method their quality certificate uses.

Commonly used limits look like this:

Grade	Screen retention	Moisture max	Black and broken	Foreign matter
S18 natural or washed	90% on 18	12.5%	30 per 300 g, or about 2%	0.1%
S16	90% on 16	12.5%	60 per 300 g, or about 2% to 3%	0.1% to 0.2%
S13 commercial	90% on 13	13%	150 per 300 g, or about 5%	0.5% to 1%

Our green coffee grades page has the full reference table, including Arabica grades.

G1 and G2: the defect class

“G1” (grade 1) and “G2” (grade 2) describe the defect class rather than the size. In most export usage:

• G1 means the tighter defect limits: low black and broken content, low foreign matter and moisture at or below 12.5%.
• G2 allows higher black and broken content (often 5% or more), more foreign matter and sometimes moisture up to 13%.

So “G1 S18” means top defect class and large beans, while “G2 S13” means relaxed defect limits and small beans. You can also see combinations such as “G1 S16” that are clean but smaller. Because different exporters draw the G1 and G2 boundaries slightly differently, always ask for the numeric limits behind the label.

Wet polished vs unpolished

After hulling, green Robusta still carries a thin layer of silver skin. Wet polishing runs the beans through a polisher with a small amount of water, removing the silver skin and giving a cleaner, more uniform blue-green or grey-green look. The beans are then dried back to target moisture.

What wet polishing does:

• Improves appearance and uniformity, which matters for buyers who resell green coffee or display it.
• Reduces chaff during roasting.
• Usually adds a small premium per tonne.

What it does not do:

• It does not change screen size or defect class.
• It does not fix flavour defects from overripe picking, ferment or slow drying.
• It adds a re-drying step, so moisture control after polishing matters. Ask for moisture and, ideally, water activity measured after polishing.

Unpolished (sometimes called “clean” or “normal”) Robusta is common for industrial roasting and extraction, where appearance matters less.

Reading a real spec line

Take a typical line:

Robusta G1 S18 wet polished, moisture 12.5% max, black and broken 2% max, foreign matter 0.1% max

Read it piece by piece:

• Robusta: Coffea canephora, most likely Central Highlands origin unless stated.
• G1: top defect class.
• S18: at least 90% retained on screen 18 (7.14 mm). Ask for the tolerance through screen 16.
• Wet polished: silver skin removed, re-dried.
• Moisture 12.5% max: measured at loading. Specialty buyers often ask for 11 to 12%.
• Black and broken 2% max: by weight. Confirm whether the certificate reports by weight or by count per 300 g.
• Foreign matter 0.1% max: by weight.

What the line does not tell you: processing method (natural, semi-washed or washed), crop year, region, cup profile, packing and the inspection body. Those need to be added to the contract. A complete version might read:

Robusta G1 S18 wet polished, natural process, crop 2026/27, Dak Lak origin, moisture 12.5% max, black and broken 2% max, foreign matter 0.1% max, max 2% through screen 16, clean cup free of ferment and earthy notes, 60 kg jute bags, 1 x 20ft, quality and weight final at loading per independent inspection.

Other variants you will see on offer sheets

Beyond the core grade, exporters add several descriptors. The most common ones and what to ask about each:

• Washed Robusta (WR): pulped and washed before drying. Cleaner cup and usually a higher differential than natural. Ask whether it is fully washed or semi-washed, since both are sometimes sold under the same label.
• Semi-washed or honey Robusta: pulped with mucilage left on. Confirm drying method and time.
• Red cherry or ripe-picked Robusta: indicates selective picking, often a step toward fine Robusta. Ask what percentage of ripe cherry the lot was picked to.
• Double or triple sorted: extra passes through gravity tables and colour sorters to remove light and discoloured beans. It helps the defect count but adds cost.
• Machine cleaned vs hand picked: hand picking after colour sorting is used for the cleanest export lots.
• Crop year: “new crop” and “old crop” affect freshness and price. Old-crop Robusta stored through the wet season can pick up woody or baggy notes, so ask when it was hulled.

Each of these can move the price by a few tens of USD per tonne or more, so ask every supplier to quote on the same descriptors when comparing offers.

How this differs from SCA grading

The Specialty Coffee Association grading system (see sca.coffee) works differently:

Aspect	Vietnamese TCVN and trade grades	SCA green grading
Sample size	Often 300 g per grade test	350 g
Defect logic	Count or weight of black, broken, foreign matter	Primary and secondary defects converted to full defect equivalents
Size	Screen retention percentage defines grade	Screen size reported, not the main grade driver
Sensory	Not part of the grade	Cup score required (80+ for specialty)
Moisture	Max 12.5% to 13%	Typically 10% to 12% expected

This means a Vietnamese G1 S18 is a clean commercial Robusta, not automatically a specialty coffee. A fine Robusta lot, by contrast, is graded physically and then cupped under a fine Robusta or SCA-style protocol and needs a cup score of 80 or more.

Practical tips for buyers

• Ask for the quality certificate format before contracting, so you know whether defects are by count or by weight.
• Specify screen tolerance through the next screen down.
• Put moisture at loading and, if you roast for specialty, a water activity target of roughly 0.55 to 0.65 aw.
• Include a cup clause: “clean cup, free of ferment, mould and earthy taints”.
• Name the inspection body (Cafecontrol, SGS or Cotecna are common) and whether their results are final.
• Keep a retained portion of your approved sample for arrival comparison.

Get matched with suppliers for your grade

If you already know the grade you need, send your spec with screen, defect limits, polishing, volume and shipment window. coffeebeanvn.com is free for buyers: we reply within 24 business hours and match you with up to 3 Vietnamese exporters or processors whose standard grades fit your line. They quote and contract with you directly.