You've probably clocked it on one of our product pages, a coffee labelled as "Mosto Washed" or "Mosto Natural", and wondered what on earth that means. It's one of those terms that's starting to appear more and more in specialty coffee, and it deserves a proper explanation. Here's the thing: once you understand what mosto is, a whole new layer of what's happening in your cup starts to make sense.
Start with the word itself
Mosto is a Spanish word meaning fresh fruit juice, specifically the juice of crushed or pressed fruit before it has fully fermented. Wine drinkers might recognise the same root in the English term "must", which describes freshly pressed grape juice at the start of the winemaking process. It's the same idea: a raw, living, sugar-rich liquid that's teeming with potential.
In coffee, the meaning is equally specific. Mosto is the liquid produced when ripe coffee cherries ferment. It's not water. It's not a processed by-product. It's a concentrated, microbe-rich juice that's drawn out of the cherry during fermentation, and it turns out it has some remarkably useful properties when you're processing the next batch.
A quick word on fermentation
If you're familiar with how washed or natural coffees are made, you'll already know that fermentation plays a central role in coffee processing. After picking, the coffee cherry goes through various stages of pulping and drying, and at some point in that chain, fermentation happens. Sometimes it's deliberate and controlled; sometimes it's more of a natural consequence of the process.
In a standard washed coffee, the cherry is depulped (the outer skin is removed) and the bean is left in a tank to ferment. The fermentation breaks down the mucilage (that sticky, sugary layer clinging to the bean) so it can be rinsed away cleanly. What you're left with is a clean bean that, when dried and roasted, expresses the terroir and varietal character very directly. It's the processing method that gets out of the way.
What mosto processing does is turn that fermentation step into something far more deliberate.
So what exactly is mosto processing?
Here's where it gets interesting. Mosto processing uses the fermentation liquid from one batch of coffee as a starter culture for the next. Think of it a bit like sourdough: you're not starting from scratch each time. You're introducing a living, active microbial environment that gives the fermentation a head start and shapes the flavour outcome.
The technique even has a name borrowed from food science: backslopping. It's the same principle behind a sourdough starter, a kombucha SCOBY, or a yoghurt culture, where a portion of a finished, microbially active batch is carried over to seed the next one. Coffee is simply a recent arrival to a method that fermented-food cultures have relied on for centuries.
In practice, a mosto washed coffee is typically made like this:
- A first batch of coffee is depulped and placed in a closed fermentation tank. As the cherries ferment, they release liquid, the mosto. This is collected and set aside, with the pH and microbial activity carefully monitored.
- A second batch of depulped coffee is placed in a new tank. The mosto from batch one is added to this tank.
- The mosto jumpstarts the fermentation, introducing a ready-made population of yeasts and bacteria. Fermentation breaks down any remaining mucilage on the bean.
- Once fermentation is complete, the coffee is rinsed thoroughly and taken to the drying beds. From here, it follows the standard washed process.
A useful rule of thumb: the mosto drawn off a batch typically amounts to around 10 percent of the starting cherry mass, so a producer is never working with much of it. It's usually collected once fermentation is well underway, often around the 72 hour mark, when microbial activity is at its most vigorous. And because most mosto is generated and reused in sealed, low-oxygen tanks, what you're really looking at is a self-induced anaerobic fermentation, or SIAF, with a starter culture added. The low oxygen and the established microbial population work together: less room for spoilage organisms, more direction in the flavours that develop.
The result is a coffee that has all the clarity of a washed process (clean, bright, defined) but with an added layer of complexity that comes from that enriched fermentation environment.
What's actually in the mosto?
This is where the science gets properly fascinating. The mosto isn't just water and cherry juice. It's a substrate packed with naturally occurring yeasts, bacteria, organic acids, and sugars, all produced during that first fermentation. The mucilage of a coffee cherry is more than 80% water by composition, but the remaining fraction is full of fermentable compounds that microorganisms thrive on.
When you introduce this liquid to a new batch of coffee, you're effectively seeding it with a living culture. The microorganisms in the mosto go to work immediately, breaking down the sugars and producing a range of flavour-active compounds along the way: esters, organic acids, and other metabolites that directly influence what ends up in your cup.
If you want to get specific about it, the community at work is well documented. On the yeast side, research into coffee fermentation repeatedly points to Saccharomyces cerevisiae alongside non-Saccharomyces species like Pichia, Hanseniaspora, Torulaspora and Candida. These are the organisms that drive mucilage breakdown and throw off the fruity and floral aromatics, the esters and higher alcohols, that define a lively cup. Working alongside them are lactic acid bacteria such as Lactobacillus plantarum and Leuconostoc mesenteroides, which produce lactic and acetic acid and shape the brightness and structure of the acidity. The interesting part is that these groups aren't just coexisting. Studies on co-inoculated ferments show a kind of metabolic teamwork between yeast and lactic acid bacteria, with the combination producing more body, cleaner aromatics and more defined acidity than either does alone. A healthy mosto is essentially a ready-made version of that partnership, handed straight to the next batch.
Temperature is a critical variable here. Different temperatures favour different microbial populations, and different microbial populations produce different flavours. Producers working with mosto need to keep temperatures consistent and controlled, because too much variation makes the results unpredictable. The pH of the mosto is also monitored closely; the right pH not only encourages beneficial fermentation but helps suppress anything unwanted.
Why does it taste the way it does?
Coffees processed with mosto tend to have a particular character: ripe, juicy fruit notes, bright and complex acidity, and a roundness that sits somewhere between a classic washed coffee and a more overtly fermented style. You'll often find flavours that echo fresh fruit cordials, stone fruits, and winey grape-like sweetness, flavours that feel vibrant and alive rather than heavy or overworked.
That's because the mosto is essentially an accelerator. It doesn't introduce entirely foreign flavours. It amplifies and focuses the natural fermentation compounds already present in the cherry. The existing microbial ecosystem of the fruit gets a significant boost, which means the fermentation is both faster and more directional. Producers can use this to their advantage, guiding the flavour development in a way that's simply not possible with a standard fermentation process.
It's also worth noting that mosto isn't a wildcard. It's a precision tool. Producers who use it well are monitoring every variable carefully and running microbiological analyses to make sure the culture is healthy and beneficial. When it's done right, the process is remarkably consistent.
Variations on the method
Mosto doesn't only apply to washed coffees. You'll sometimes see it used in natural processing too, where the mosto is applied to whole or depulped cherries that are then dried on raised beds with the fruit intact. This produces a more intense, deeply fruity result, as the mosto compounds interact with the cherry flesh during the extended drying phase, adding another dimension of complexity.
There's also variation in where the mosto comes from. In some cases, it's generated from the same batch of cherries and reintroduced, a kind of closed-loop fermentation. In others, the mosto is drawn from a different variety or even a different lot entirely, with producers experimenting to see how the flavour compounds of one coffee's fermentation can influence another. At its most experimental, producers have used mosto from coffees known for particularly vivid or prized flavours, essentially trying to transfer some of that character across batches.
Worth knowing too that a given batch of mosto has a working life. Producers will generally cycle it through only a couple of fermentations before its vigour drops off and it needs refreshing with a new pressing. Backslopping is powerful, but it isn't perpetual motion.
You may occasionally see mosto naturals sold under other names. Some roasters label the fruit-forward, bag-fermented version a "Candy Natural", which is the same backslopping idea applied to whole cherries rather than depulped coffee.
Where does it come from?
Mosto processing has its roots in Latin America, and the washed mosto method in particular is widely credited to Finca Los Rodriguez and the Rodriguez family's Agricafe operation in Caranavi, Bolivia, where we've sourced several mosto processed coffees. It was here, processing thousands of tiny single-day microlots at a central wet mill, that the approach was refined: depulped coffee submerged in the cherries' own mosto and fermented in sealed tanks, often for somewhere between 16 and 48 hours, with the liquor carried forward to seed subsequent batches. The same family sit behind Sol de la Mañana, Bolivia's first producer mentoring programme, which they founded in 2014 to share that technical knowledge with neighbouring smallholders. It's a reminder that a lot of the most influential processing innovation comes not from a lab, but from a producer obsessively iterating on their own harvest.
Costa Rica, with its long history of processing innovation (it pioneered honey processing, after all), has also been at the forefront of mosto experimentation. It's partly that independent, micro-mill spirit, so many producers processing their own coffee and pushing each other to try new things, that helped the technique spread and evolve.
And it is spreading. We're starting to see it adopted by producers far beyond Latin America. In Thailand, our partners at Sirinya Coffee in Doi Chaang (the same region that was growing opium until the 1980s before being redirected to coffee) have been experimenting with their own mosto natural process, inspired directly by what's been coming out of Costa Rica. Producer Supon Chaosuwanvilai ferments whole cherries in closed tanks using the liquor from previous batches, cycling it through successive fermentations so the culture grows richer with each pass. The results have been vivid and characterful. The fact that a producer in northern Thailand is taking cues from Costa Rican micro-mill innovation tells you quite a lot about how far this technique has travelled.
The broader context is the rise of controlled fermentation across specialty coffee, a movement that has drawn heavily on parallels with winemaking, whether that's the sealed-tank principles behind anaerobic fermentation, the CO2-flushed environments of carbonic maceration, or the starter-culture approach of mosto. Just as winemakers have spent centuries learning to manipulate fermentation to express terroir and varietal character, coffee producers are increasingly doing the same. Mosto is one of the more elegant expressions of that approach: not adding anything external to the coffee, but redeploying what the coffee itself produces.
It isn't without its challenges
For all its elegance, mosto asks a lot of a producer. Backslopping a live culture means that if something undesirable takes hold, it can be carried straight into the next batch, which is exactly why the better operations run microbiological checks rather than working on instinct alone. There's an environmental dimension too. Fermentation leachate is a high-strength organic liquid, and if it's allowed to run off into waterways untreated it strips out oxygen and harms aquatic life. The flip side is that mosto processing reuses a liquid that would otherwise be waste, so handled responsibly it can actually tighten up a mill's water story rather than worsen it.
What it means when you see it on a bag
If you spot "Mosto Washed" or "Mosto Natural" on one of our coffees, you can expect something a little more layered than a standard washed or natural. The clean, defined structure of a washed coffee will still be there (mosto processing doesn't turn everything into a funky natural) but there'll be more going on. More acidity, more fruit complexity, more of that sense that the cup is reaching for something rather than settling.
It's a technique that rewards curious drinkers. And if you've had one of our Bolivian mosto washed coffees, the cherry cordial, white grape, and tangerine profile of the Pedro Flores, for instance, you'll have a pretty good sense of what the method is capable of.
Expect to see more of it.