There is a stretch of river in Sidama with coffee in it.
The cherries are sealed into airtight bags, the bags are submerged, and the fermentation runs underwater, held at the river's temperature rather than the air's. Eliyas Dukamo and Atiklit Dejene produce it. RF stands for River Flow.
Twenty years ago that would have been the story of a washing station going badly wrong. Today it is a separated lot with a score on it, a bag design and a waiting list.
Processing has become the part of coffee production that producers control most deliberately. Variety takes years to plant and decades to judge. Altitude is fixed. Fermentation can be changed between one pick and the next — sealed or open, four hours or five days, with the cherry's own juice or with fruit thrown in alongside it — and the difference lands in the cup within weeks. That is why so much of the interesting work in the last decade has gone there.
What follows is ten of the most unusual lots in our range, and what was actually done to each of them. If the terminology is new, we have a separate guide to the words on the bag.
The one fermented underwater
TANAT – Elto [Elora RF8 Natural] 100g | Ethiopia
Eliyas Dukamo and Atiklit Dejene are a couple based in Sidama who produce small, carefully separated lots alongside the smallholder farmers around them. Washed, natural, honey and anaerobic all run separately, kept distinct, with the traceability afterwards to tell them apart. RF8 is the outlier even by their standards.
The cherries go into airtight bags and the bags go into a river. Fermenting underwater does one thing above all: it takes the temperature out of the producer's hands and gives it to the water, which is far more stable through a day than open air on a drying patio. A slow dry follows, and that is where the aromatics built during fermentation are either kept or lost.
The variety is 74158, one of the improved selections released by Ethiopia's Jimma Agricultural Research Center, growing at around 2,200 metres at Arbegona. Cherry leads the cup, ripe and direct. Dried rose follows, nearer petals than blossom. Underneath both sits bubblegum — sweet, frankly confectionery, and the note that tells you this was not a conventional natural.
The ones with fruit in the tank
People Possession – BPM Blueberry Pie Magic {Blend} 200g · People Possession – NBA New Berry Addiction {Blend} 200g
Co-fermentation is the technique that divides rooms. The principle is simple enough: the fermentation is not left to the coffee cherry alone. Fruit goes into the tank alongside the coffee, and the sugars and acids it carries feed into the same microbial activity that would otherwise be working only on the cherry's own mucilage. What comes out is a flavour the coffee would not have produced by itself. That is the whole point, and it is also the objection.
People Possession are unembarrassed about it. Their Chromatic Index blends are built to land on a flavour rather than to represent a farm, and each carries a three-letter name and a component list rather than an origin story.
BPM pairs an Ethiopian 74110 with a Colombian Caturra. One component is washed; the other is co-fermented with red fruits. Blueberry pie leads, with raspberry ripple ice cream behind it — as much a texture as a flavour, a rounded body with a sharper line of berry running through — and vanilla underneath holding the sweetness together.
NBA is built the same way, Ethiopia and Colombia, washed alongside a red-fruit co-ferment, and lands somewhere sharper. Wild strawberries first, less rounded than a cultivated strawberry, then candied lemon with the concentrated sweetness and slight chew that candying gives citrus. Red fruits fill the middle, broad rather than specific.
The ones fermented in their own juice
TANAT – Alo [Mosto Anaerobic Natural] 200g | Ethiopia · TANAT – Alo [DIGS Project Mosto Natural 72h] 200g | Ethiopia
Mosto is coffee juice. Where a tank fermentation would normally be topped up with water, these are sealed with the juice of the cherry itself, which concentrates the sugars available to the ferment instead of diluting them.
Both lots come from Tamiru Tadesse, who grows in the village of Alo in the Bensa highlands of southern Ethiopia. The variety in both is 74158. What separates them is time and elevation. The Mosto Anaerobic Natural runs 96 hours sealed, from parcels at 2,240 metres and above, and comes out intensely fruity — pineapple, lacto-fermented strawberry, coconut, firmly at the funky end of natural processing. The DIGS lot runs 72 hours, from higher still at 2,400 to 2,550 metres, and lands cleaner and more citric: bold lemon candy, then peach tea, finishing on a juicy redcurrant.
Two lots, one producer, one variety, one technique. Twenty-four hours of fermentation and a few hundred metres of altitude between them, and they taste nothing alike. That is a fair summary of why producers keep experimenting.
The one with wine yeast and a green apple in it
People Possession – Industrial Supply Series [GREEN] {Filter} 200g | Colombia
Jairo Arcila's experimental lots turn up across most of the roasters worth paying attention to. This is his Castillo, grown in Quindío between 1,400 and 1,500 metres, put through a honey co-fermentation run with wine yeast and green apple.
The wine yeast is the less visible half of that and arguably the more significant. Inoculating a ferment with a selected strain means the producer decides which organisms do the work, rather than leaving it to whatever happens to be living on the cherry and in the air — the same logic a winemaker applies, for the same reason. The apple is the obvious half. It goes into the ferment, and it shows.
Apple soda first, then candy, with berries underneath. It is bright and sweet in a deliberately artificial-leaning way, and it is not pretending otherwise.
The one that spent five days in a cold room, then went back for a week
Manhattan – Las Marías Wush Wush 125g | Colombia
Wush Wush is the most aromatic variety growing at Finca Las Marías, in Pijao, Quindío. José Julián Giraldo has chosen to push it rather than handle it gently.
Ripe cherries are hand-picked and sealed whole in a cold room at 14°C for 120 hours. Five days is an exceptionally long first fermentation, and holding it cold is what makes it survivable — low temperature slows the microbial activity enough that the extra time builds complexity instead of tipping into spoilage. It is still a bet on a delicate variety's cell structure taking the punishment.
They then go out onto raised beds to dry slowly in the shade. At day twenty of drying they are brought back into the cold room for a further seven days of enzymatic fermentation, then returned to the beds to finish. Interrupting a dry to ferment a second time is not something a conventional natural would ever do.
Pineapple, orange and bergamot, juicy and refreshing. Manhattan call the process Advanced Natural.
The one with sea salt and a thermometer
Manhattan – El Paseo 250g | Colombia
Until around 2015 Huver Castillo was teaching maths in a school in Nariño. He is a third-generation grower, but working coffee properly is recent. Finca El Paseo sits near Buesaco on the Suma Paz trail, on slopes looking over the valley, with native shade trees standing through it.
The Caturra is picked only at full ripeness, then held in cherry for a 36-hour pre-fermentation. It is floated and disinfected in a 2% sea salt solution before going into stainless steel tanks for six days of controlled anaerobic fermentation, with collected leachates and selected enzymes added during that stage to draw out the floral side. Drying is mechanical and deliberately cool, below 38°C, over two or three days, after which the coffee stabilises for a further fifteen.
Almost every step there is a decision that a traditional process does not offer. The salt float sets the microbial starting conditions; the stainless tanks hold them; the cool mechanical dry stops the aromatics being cooked off at the end. Violet, lychee and dragon fruit, with a smooth mouthfeel.
The hour counts: 48, 72, 96
Manhattan – Shoondhisa Anaerobic 250g | Ethiopia · TANAT – Elto Elora [Anaerobic 72h] 200g | Ethiopia · TANAT – Elto Hamasho [Anaerobic 96h] 200g | Ethiopia
Three Ethiopian anaerobic naturals, three different lengths of sealed fermentation, printed on the bag because the number matters.
Shoondhisa is the short one. Sookoo Coffee was founded in 2018 by Ture Waji and his brother Assefa at Odo Shakiso in Oromia, buying from the kebeles immediately around the site and keeping each lot separate. This one is from Shoondhisa kebele at 2,173 metres, Gibirinna 74110 and Serto 74112. The cherries spend 48 hours sealed in plastic barrels before going out to dry whole on raised beds over 15 to 20 days. Blueberry, orange and pink Fruittella, juicy and fruit-forward.
Elora runs 72 hours, from Dukamo and Dejene's parcels at 2,200 metres, planted with 74112 and 74110. Sealing out oxygen for that long changes how the sugars in the mucilage break down, and here it pushes the fruit towards precision rather than weight: mandarin and star fruit up front, strawberry flower in the middle, a zesty pomelo finish.
Hamasho, from the same producers at Arbegona, runs 96 hours. TANAT describe that length as a revealer of texture rather than a chase for aromatic fireworks, and the cup holds together in a way long ferments often do not. Blueberry, blackberry and black cherry lead, jammy without tipping over, with Earl Grey and florals emerging behind.
Longer is not better. Longer is different, and harder to control.
The one that's anaerobic and then washed anyway
DAK – Silky Way 250g | Colombia
Most of the coffees here are naturals, dried whole in the fruit. Silky Way is the counter-example: pulped first, then fermented in a sealed, oxygen-free environment, then washed. The anaerobic stage builds brighter and more layered acidity than a standard washed process, without any of the jammy weight that comes from drying in cherry.
It is grown by Jhoan at Finca Las Flores in Acevedo, the Huila municipality where Pink Bourbon was first identified, in the San Adolfo neighbourhood. The variety remains a genuine puzzle. Despite the name, genetic testing in 2017 and again in 2023 found no relation to the Bourbon family at all, pointing instead to Ethiopian landrace ancestry — the same broad group that includes Geisha. How it ended up in southern Huila is still not understood.
Lemongrass, crisp green apple and white flowers. For anyone who finds co-ferments too loud, this is the experimental end of the range with the volume down.
The one with no water in it at all
DAK – Rumba 2026 Edition 200g | Ethiopia
Rumba comes round every year. Ethiopia matters to DAK more than most origins — they travel there annually and buy in enough volume to hold multiple lots through to the following harvest, which is why Rumba can exist as an annual release rather than a one-off.
This year's lot is from Tulise Washing Station in the Gedeo zone, at 2,100 metres, planted with heirloom varieties and local landraces. The processing is a dry anaerobic natural: fermentation with no water added to the tank. DAK point out the practical reason that is the more sensible route in Ethiopia, where clean water at a washing station cannot be taken for granted. Experimental processing is not always a flavour decision; sometimes it is the method that fits what the site actually has. The cherries then dry for around 23 days in semi-shade.
Stewed strawberries, apricot and brown sugar, with a syrupy texture. Sweet and fruit-driven rather than bright.
Three more, without the theatre
DAK – Blueberry Beats 250g | Ethiopia · TANAT – Nestor Lasso [Typica Mejorado Natural] 200g | Colombia · DAK – The Alchemist 250g | Rwanda
Not every anaerobic lot is an exhibit. These three use sealed fermentation as a tool rather than a headline.
Blueberry Beats comes from the Baturo washing station in the Bura area of Sidama, grown at an unusually high 2,360 to 2,440 metres, heirloom varieties, whole cherries sealed in tanks before drying. Blueberry and strawberry jam alongside vanilla and a broader ripe, almost stewed-fruit character.
Nestor Lasso's Typica Mejorado is the variety story rather than the process one. Genetic testing has identified Typica Mejorado as a cross between Bourbon and an Ethiopian landrace, and the grains are elongated, unusually dense and high in sugar — which is precisely what makes a controlled natural anaerobic worth doing on it. Lasso seals whole cherries away from oxygen and lets that sugar ferment slowly before drying. Blackberry, raspberry, black cherry, violet and watermelon, layered rather than one-note.
The Alchemist is the espresso of the group: Red Bourbon from Gasharu Farms in Nyamasheke, western Rwanda, at 1,750 metres, fermented sealed and dried whole in the fruit. Tropical fruits, ripe fruits and cacao nibs.
Why any of this matters
The argument against experimental processing is that it flattens origin — that a coffee co-fermented with red fruits tastes of red fruits rather than of Huila, and that something is lost when the farm stops being audible in the cup.
It is a fair argument, and it does not cover the whole field. A 96-hour ferment at Arbegona is not an additive; it is a producer finding out what their own cherry does under different conditions. Rumba's dry anaerobic is a method chosen around a water supply. The mosto lots are fermented in nothing but coffee. What links them is not artifice but control — producers deciding the variables instead of accepting them, in a part of the process that was treated as a chore for most of coffee's history.
Some of it will not last. Co-ferments in particular are moving fast enough that the techniques being sold this year may look crude in five. That is what an experimental phase looks like from inside it.
The full experimental range is here. If you want the vocabulary first, start with the processing terms guide, and if a funky lot has ever come out muddy in your brewer, there's a post on that too.