Before a single coffee cherry is pulped at a washed-process mill, most well-run stations do something deceptively simple: they tip the day's harvest into water and watch what sinks. The dense, ripe, sound fruit drops to the bottom, while a scum of lighter material — unripe and overripe cherries, insect-bored and hollow "floaters," dried and raisined fruit, plus leaves, twigs and stones — rises to the surface, where it is skimmed away and sent down a separate channel. This is flotation, and it is the very first quality gate in wet processing.
Flotation is cheap, fast and remarkably effective. It removes a large share of potential defects before they ever reach the pulper, and it doubles as a wash and a conveyor, carrying clean cherry through channels toward the next machine. It is not a magic filter — it sorts by density, not by ripeness directly — but for the small cost of some water and a tank, it does more to protect cup quality than almost any other single step at intake. This guide explains the physics, the equipment, why it matters, and how it differs from the color sorting and gravity-table grading that come much later.
What flotation is and the simple physics of density
Flotation is a water-based density separation. A coffee cherry that is fully ripe and structurally sound is dense — its seeds are well developed and its flesh is heavy with sugar and water — so it is heavier than the water it displaces and sinks. Material that is less dense than water floats. That single rule does the sorting.
Several things make a cherry less dense than a good one. An insect-damaged or diseased seed often has a void or an underdeveloped bean inside, trapping air. Overripe and dried, raisined cherries have lost water and shrunk away from their skins, leaving air gaps. Truly hollow cherries — where the seed never formed — are essentially little floats. Sticks, leaves and empty husks are lighter still. All of this rises, and everything genuinely heavy and intact sinks.
It is worth being honest about the limits. Flotation separates by density, which correlates with quality but is not identical to it. Unripe green cherries are a notorious blind spot: they are still full of water and therefore sink alongside ripe fruit, so a flotation tank alone will not fully de-green a lot. For that reason flotation is best understood as a complement to careful selective picking of ripe cherry, not a substitute for it. What it excels at is pulling out the low-density defects that hand-sorting on a moving harvest would miss.
What floats and what sinks
Knowing what ends up in each stream is the whole point, so it is worth spelling out.
The sinkers — the fraction you want — are dense, ripe, sound cherries with fully developed seeds. These go on to be pulped and processed as the mill's main lot.
The floaters are a mixed bag of things you do not want fermenting and pulping alongside good fruit:
- Overripe, dried and raisined cherries that have lost moisture and gone light.
- Insect-damaged fruit — for example cherries attacked by the coffee berry borer — which are often hollowed out.
- Cherries with hollow or malformed seeds that never developed properly.
- Some proportion of underripe fruit, though not reliably, since young cherries can still be water-heavy enough to sink.
- Plain foreign matter: leaves, twigs, sticks, husks and other sources of defects that arrive mixed in with the harvest.
Because floaters are not all worthless — some are simply overripe rather than rotten — many origins do not throw them away. They are frequently dried on their own as a lower-grade natural or "floater" lot, composted, or diverted to local consumption. Keeping them out of the premium stream is the goal; how the floaters are then used is a separate commercial decision at the washing station.
How flotation is done at the mill intake
The equipment ranges from a bucket to a purpose-built machine, but the logic is identical everywhere. Flotation happens at the mill intake, before pulping, as the first thing that touches the day's delivery.
Tanks and receiving hoppers
At its simplest, cherries are dumped into a water-filled receiving hopper or tank. The mass is stirred or left to settle for a moment; floaters rise and are skimmed or raked off the top, while the sinkers are drawn from the bottom. Larger, heavier debris and stones settle lowest of all, which is why intakes on many stations are built so the good cherry is pulled from just above the very bottom, leaving stones behind.
Siphon tanks and float channels
More developed mills use a siphon tank or a system of water channels. A siphon draws ripe and unripe cherry out from near the bottom of the tank — above the layer where stones collect — while lighter floaters stay at the surface and are carried off through a separate float channel. In channel-based setups, flowing water does two jobs at once: it separates by density and it transports the good cherry along the channel toward the pulper. The current itself becomes the conveyor, which is why flotation is so often described as both a cleaning and a moving step.
This is also where flotation earns its reputation as a wash. Running the harvest through water rinses off field dirt, dust and some surface microbes before the fruit is opened, giving the subsequent washed-process ferment a cleaner starting point. Stations that recirculate water do so to save it, though very dirty recycled water can itself become a hygiene risk, so the cleanest operations manage that carefully.
A second pass after pulping
Density sorting in water is not confined to whole cherries. In some flows the same principle is applied again after pulping, once the beans are in parchment and still coated in mucilage or freshly washed. Lower-density parchment — floaters, poorly filled beans — can be skimmed in wash channels, giving a second density cut before drying. The common thread is that density is being read through water at more than one stage.
Why this cheap first pass matters
Flotation punches far above its cost. A tank and some water remove a meaningful fraction of the material most likely to spoil a lot — hollow, insect-eaten and over-fermented fruit — before it can bleed off-flavors into the batch during fermentation and drying. Removing defects at the front of the line is always cheaper than trying to pick them out at the end, when they are hidden inside parchment or, worse, roasted.
The benefits stack up:
- Defect removal. Many potential cup faults leave with the floaters before pulping, improving consistency.
- Cleaning. The harvest is rinsed of dirt, debris and stones that would otherwise reach — and damage — the pulper.
- Transport. Water channels move cherry through the mill with less handling.
- Machine protection. Removing stones and sticks up front spares the pulper blades and screens.
Producers who perform this step diligently are commonly reported to achieve more uniform, higher-scoring coffee, because their fermenting mass is cleaner and more even to begin with. It is one of the highest-return, lowest-tech moves available to a smallholder or a large mill alike, and it sits alongside hand-picking and later sorting stages as part of a layered defect-removal strategy.
Flotation versus color sorting and gravity-table density grading
Because flotation, color sorting and density grading all "sort" coffee, they are easy to confuse. They are entirely different steps at different points in the chain, working on different material in different media. Getting the distinction right is essential to understanding a coffee's journey.
Flotation is the wet, water-tank cherry step at intake. It reads density through water on whole fruit, at the very start of processing. It is the subject of this guide.
Optical color sorting comes much later and is dry. It runs on hulled green beans (or sometimes dried cherry), using cameras and light to spot beans of the wrong color — blacks, sours, faded whites — and blows them off the stream with jets of air. It reads appearance, not density, and it works on a single bean at a time.
Dry-mill density grading is later still and also dry. On a gravity or densimetric table, air is blown up through a vibrating, tilted deck so that dense beans "climb" to the high side while light beans drift to the low side. It reads density like flotation does — but through air on hulled beans, at export preparation, not through water on whole cherries at intake.
| Step | When | Medium | Works on | Sorts by |
|---|---|---|---|---|
| Flotation | Mill intake, before pulping | Water tank / channel | Whole cherries | Density |
| Color sorting | Dry mill, near the end | Dry, optical + air jet | Green beans | Color / appearance |
| Density grading | Dry mill, export prep | Dry, gravity table + air | Green beans | Density |
The through-line is density: flotation and the gravity table are cousins that both exploit it, just at opposite ends of the process and in different media. Color sorting is the odd one out, judging by sight rather than weight.
Frequently asked questions
What is flotation in coffee processing?
Flotation is a water-based sorting step at the start of wet processing. Coffee cherries are dumped into a tank or channel of water; dense, ripe, sound cherries sink and go on to be pulped, while lighter material — unripe, overripe, insect-damaged, hollow and dried cherries, plus sticks and debris — floats and is skimmed off as "floaters." It cleans and transports the cherry at the same time.
Why do some coffee cherries float and others sink?
It comes down to density. A fully ripe, sound cherry is heavier than the water it displaces, so it sinks. Cherries that are insect-damaged, hollow, dried out or overripe trap air or have lost water, making them lighter than water, so they float. One catch: unripe green cherries are still water-heavy and often sink too, so flotation alone will not remove all of them.
When does flotation happen in the coffee process?
Flotation happens right at the mill intake, before pulping — it is usually the first thing done with the day's harvest. In some flows the same water-density principle is applied a second time after pulping, in the wash channels, to skim off low-density parchment before drying.
Is flotation the same as density grading on a gravity table?
No, although both use density. Flotation is the wet, water-tank step on whole cherries at intake. Gravity-table (densimetric) density grading is a dry step much later at the dry mill, blowing air up through a tilted, vibrating deck to separate hulled green beans by weight before export. Same physical property, very different point in the chain and different medium.
Does flotation remove all defective coffee?
No. Flotation is a strong first pass that removes many low-density defects, but it misses defects that do not change a cherry's density — most notably unripe green fruit, which still sinks. It works best combined with selective hand-picking beforehand and with later color sorting and density grading, as part of a layered approach to removing defects.