Screen Size and Bean Density

A screen is a perforated plate. Green coffee is passed over a stack of them, largest holes first, and what stays on each plate is that screen size.

The unit is 64ths of an inch. Screen 18 is 18/64″, about 7.1 mm.

What screen actually tells you

Uniformity. This is the real value. A batch of beans of similar size absorbs heat at a similar rate and reaches a given stage together. A wide size distribution roasts unevenly: the smalls are ahead, the larges behind, and the batch as a whole has a shorter window where most of it is right.

A weak proxy for maturity. Larger beans generally come from well-nourished trees with adequate water, which loosely correlates with better development. Loosely.

What screen does not tell you

Quality. A large bean from an under-ripe cherry is a large under-ripe bean.

Density. Two lots on screen 17 can differ substantially in mass per bean, and density is the property that governs how heat moves into the bean.

Roast behaviour on its own. Screen plus density predicts roast behaviour. Screen alone does not.

Range versus floor

The single most useful question to ask about a stated screen: is that a range or a minimum?

• "17/18" means the lot sits predominantly on those two screens. That is a uniformity claim.
• "15+" or "screen 15 and above" tells you only about the bottom. The lot may run from 15 to 19, which is a four-screen spread and will not roast evenly.

Origins and exporters use both conventions, sometimes in the same document. Get it clarified in writing before contracting.

Density

Density is usually expressed in grams per litre, and for arabica commonly falls somewhere in the 650–750 g/L region depending on origin and altitude, with high-grown coffees at the upper end.

Denser beans conduct heat more slowly into the centre. In practice:

• Dense coffee tolerates and often needs a higher charge temperature, and takes longer to develop through the middle.
• Less dense coffee takes heat readily, can run away through first crack, and generally wants a gentler approach.

This is where Indian coffee catches people out. Shade cover slows maturation, which raises density at a given altitude — so an Indian lot from 1,100 m can behave more like a higher-grown coffee than its stated altitude suggests. And anything monsooned goes the other way: the process lowers density considerably, so a monsooned lot on screen 18 is much lighter than an unmonsooned one on the same screen. See monsooned Malabar.

Practical use

When contracting: get the screen as a range, and get the defect count alongside it. Screen without defects is half a specification.

When roasting a new lot: weigh a known volume before the first batch. A litre measure and a scale gives you density in a minute, and it tells you more about how to approach the roast than the screen number will.

When comparing offers: a tighter screen range at the same price is a better lot, and it is one of the few uniformity claims that is objectively checkable on arrival.

Peaberry sits outside the ladder

Peaberries are round rather than flat, so they behave differently in a screen and are separated out entirely rather than graded with the flats. See what is peaberry coffee.

Hoysala states a screen for every SKU on the offer list, alongside grade, moisture, altitude and harvest. Blocks run from 900 m to 1,450 m under shade, which is worth weighing against the stated altitude when you judge density.