What Is Mossto? The Cherry Juice That Becomes a Fermentation Starter

In both coffee and wine, "mossto" (must) plays an important role. In wine, must is the pressed juice of the grape, which is fermented into wine. In coffee, mossto is the juice that forms during the fermentation of the cherry. Different products, but the way it forms and what it does have much in common. This article is about how coffee mossto forms, what it is like, and how it affects the flavour of the bean.

1. What mossto is

Mossto is the juice obtained by processing fruit. In wine, pressed grape juice is the must; it contains sugar, acids, enzymes and yeasts. Coffee mossto is the juice produced during the fermentation of the cherry. Coffee is not a fruit you can press for juice the way you press a grape. But the mucilage (the slippery fruit layer) is rich in sugar. So we ferment the cherries sealed (anaerobically); they soften and are crushed under their own weight, and a mossto packed with sugar and microbes collects. Think of it as "a fruit with little juice, so we squeeze it out anaerobically".

2. Wine must and coffee mossto

Wine must is the juice extracted after the harvested grapes are pressed and the pulp, seeds and skins separated. It contains sugar, acids, enzymes and yeasts; fermentation produces alcohol and carbon dioxide, and the wine goes on to age. Coffee mossto is a by-product formed during fermentation, containing sugar, acids, organic compounds, enzymes and microbes. The path is different, but both are "a fermentation base containing sugar and microbes".

3. Looking inside mossto, in numbers (the research)

(Research block: from here on, published data.)

Properly made mossto holds the sugar of ripe cherries and the microbes that grew on it. What happens as fermentation proceeds can be followed in numbers. In an experiment measuring mucilage fermentation, while Brix (sugar) fell from 15.8 to 8.45, acidity nearly doubled from 20.02 to 42.69 mg/g and pH fell from 4.47 to 4.05 (Plaza-Dorado et al., Fermentation 2024). As sugar goes down, organic acids go up. Taste it and you get the sweetness of cherry, an alcoholic note like sake lees, and a prickle of fermentation on the tongue. Sugar, acid and alcohol are all present at once. So mossto can be read, by pH, Brix and its alcoholic character, for "what state it is in now".

4. Using mossto as a fermentation starter

Wine ferments its must into wine, but in coffee, mossto can be used as a starter. It is rich in the microbes and enzymes that get fermentation going. Research is beginning to back up this idea. In an experiment that cultured lactic acid bacteria from pulp that would have been discarded and yeasts from washing wastewater, each at high concentration, and used the mixture as a starter, the coffee earned high aroma scores without any commercial starter (Plaza-Dorado et al., Sustainability 2025). Low cost, and less waste.

Passing the starter on is called backslopping. In research on fermented foods, repeatedly carrying over the previous fermentation liquid took lactic acid bacteria that began at under 1% to dominance after 17 cycles (Fermentation 2024, 10:663). The more you repeat, the more the microbes you want are selected. There are three advantages: fermentation starts faster and more steadily; quality is more consistent; and there is no need to buy new cultures, which keeps costs down.

5. Using it safely

Mossto is not a cure-all. Mossto that has slipped out of control becomes a breeding ground for unwanted microbes and rot. So before using it we check its state by pH, Brix and smell. Good fermentation and rot are a hair apart. If you carry it over as a starter, measure it every time and use only what falls within the target range. Clean containers and low temperatures keep out contaminants and mould. Checking in numbers is the condition for using mossto safely.

6. What would be thrown away becomes a resource

Mossto, pulp and washing water are, in principle, the waste of processing. Circulate them as starters and you can stabilise fermentation without buying an external starter, and reduce waste and environmental load. It can be done without large equipment, which is the strength of a small farm. We consider this cycle as important as reproducibility of taste.

7. In summary

Mossto is an important by-product of coffee cherry fermentation with a large effect on the flavour of the bean. It forms differently from wine must and is different in nature, but its influence is just as great. Managed properly and used as a starter, it lets us make high-quality coffee reproducibly. We use mossto because it is full of the possibilities of fermentation.

Frequently asked questions

Is mossto an additive?

No. Mossto is the juice produced by the coffee cherry's own fermentation, not an external flavouring or foreign substance. We use it as a starter, carrying the microbes grown in the previous fermentation into the next.

Does using mossto give the same taste every time?

It gets closer. Research shows that repeatedly carrying over a starter lets the lactic acid bacteria you want become dominant, so fermentation proceeds faster and more steadily. But checking its state every time (pH, Brix, smell) is a precondition.

Is it the same as wine must?

The name has the same origin, but they are different things. Wine must is pressed grape juice, fermented itself into wine. Coffee mossto is a by-product of fermentation, used as a starter to help the next fermentation.

For the whole mechanism of fermentation see The four stages of fermentation. For methods that use mossto, see washed and anaerobic fermentation.

References

• Plaza-Dorado et al. (2025). Sustainability 17(5):1781. 10.3390/su17051781
• Plaza-Dorado et al. (2024). Fermentation 10(10):516. 10.3390/fermentation10100516
• Isolation and Identification of Novel Non-Dairy Starter Culture Candidates from Plant Matrix Using Backslopping Propagation (2024). Fermentation 10(12):663. 10.3390/fermentation10120663
• De Bruyn F. et al. (2017). Applied and Environmental Microbiology 83(1). 10.1128/AEM.02398-16