THE PROCESS IN MOTION

Who really detaches the sweet flesh?

Start the scene or choose a stage. The animation simplifies the mechanism; what follows gives the details and the available references.

▶ Play the process01The fruit starts02The yeasts finish03The acid helps04Removing a player to judge it

The fruit starts

The answer found everywhere holds that microbes cut up the pectins, the molecules that hold mucilage together as a jelly. A 2023 study ran seven treatments over thirty-six hours, removing now the bacteria, now the yeasts: in the first twelve hours, it is the enzymes of the fruit itself that work, to the point that the mucilage partly breaks down without any microorganism.

Read the stages without animation

1. The fruit startsThe answer found everywhere holds that microbes cut up the pectins, the molecules that hold mucilage together as a jelly. A 2023 study ran seven treatments over thirty-six hours, removing now the bacteria, now the yeasts: in the first twelve hours, it is the enzymes of the fruit itself that work, to the point that the mucilage partly breaks down without any microorganism.
2. The yeasts finishThen the yeasts finish: degradation is complete only in their presence. Hanseniaspora uvarum and Pichia kudriavzevii show the strongest activities. The bacteria play no decisive role.
3. The acid helpsIn 2002, Avallone and his co-authors found a weak bacterial activity; Leuconostoc mesenteroides produced no enzyme able to cut pectin, and adding strains that produce one sped nothing up. Their conclusion: the mucilage gets dissolved by the acid the microbes produce, more than it gets cut up.
4. Removing a player to judge itA 2023 modality held at pH 7, neutral, from start to finish puts this explanation in difficulty without eliminating it. The yeasts were removed twice, by two teams: tasteless coffee in 2020, incomplete mucilage in 2023. An independent replication is missing, across several origins.

To remember: The fruit starts, the yeasts finish, the acid helps: the sentence read everywhere credits the work to those who do the least of it.

Read the full article and its sources ↓

Here is the question everyone answers, and nobody has settled.

The answer read everywhere is simple: microorganisms make enzymes that cut up the pectins, and the mucilage comes loose. It is plausible, and it is probably wrong, or at least very incomplete.

One team tested the idea in 2002. It measured the cutting activity of the bacteria present: weak. It then deliberately added microorganisms known for this activity. Result: the degradation was no faster, and the cup did not change. Their conclusion is that the mucilage is not cut up, it is dissolved by the acid the microbes produce.

Another team took up the question in 2023, with a finer experiment: seven treatments in parallel for thirty-six hours, removing now the bacteria, now the yeasts, now both, and in one case holding the bath at neutral pH to prevent any acidification. Their answer is different and more precise. In the first twelve hours, the fruit's own enzymes do the work, to the point that the mucilage partly breaks down even without any microorganism. Then the yeasts finish. The bacteria play no decisive role.

In other words: the fruit starts, the yeasts finish, the acid helps, and the sentence read everywhere credits the work to those who do the least of it. It is a good example of what an explanation repeated a thousand times without being measured is worth.

The two protocols, and why they perhaps do not contradict each other

Avallone and co-authors, 2002. Isolation of bacteria from real fermentations, measurement of their pectinolytic activity, then inoculation of pectolytic strains. Two results: the measured activity is weak, and Leuconostoc mesenteroides proved unable to produce pectinolytic enzymes during the fermentation of an arabica. Inoculation allowed microbiological control of the process, without speeding up degradation or changing sensory quality. The authors' conclusion: degradation comes from acidification, organic acids solubilizing the pectic substances.

Study published in Yeast, 2023, volume 40, pages 425 to 436. Pulped coffee, seven treatments over thirty-six hours, with selective suppression of endogenous bacteria and yeasts, addition of pectinases or lactic acid, and one modality held at pH 7 from start to finish to neutralize the acidification hypothesis. Results: significant endogenous pectinolytic activity in the first twelve hours, to the point of partly damaging the beans in the absence of microorganisms; complete degradation only in the presence of yeasts; critical role of bacteria not found; Hanseniaspora uvarum and Pichia kudriavzevii showing the strongest pectinolytic activities.

The two perhaps do not stand in direct opposition. Avallone was looking for bacterial enzymes and found none, which the 2023 study confirms. His conclusion on acidification remains an explanation by default, which the pH 7 modality of 2023 puts in difficulty without eliminating it. What is missing to settle it is an independent replication of the neutral pH modality, across several origins and several varieties.

Avallone, Guyot, Brillouet, Olguin, Guiraud, Involvement of pectolytic micro-organisms in coffee fermentation, International Journal of Food Science and Technology, 37 (2002); Yeasts are essential for mucilage degradation of coffee beans during wet fermentation, Yeast, 40 (2023), pp. 425-436

One and the same work supports the two theses that the plate sets against each other

The section presents the enzymatic thesis as the one read everywhere and probably false. A work published in 2025 by a scientific publisher supports it without the slightest reservation, and that is a reason to re-examine it rather than file it away.

Hackler and Särchen write that mucilage consists mainly of pectin, that pectinases can degrade it, that this degradation is essential for the cherry to become a bean, and that without pectinases this would not be possible. This is the strongest formulation of the thesis, written in the indicative and without alternative. One must at once look at what it rests on: a review of the commercial applications of pectinases, which is not specifically about coffee.

In other words, the argument is that these enzymes know how to do this work in general, which nobody disputes, and not that they do it in a coffee tank, which is the question.

The most instructive part comes next. The same authors, six pages earlier, write that yeasts speed up the degradation of mucilage by producing enzymes, and that Saccharomyces cerevisiae, Wickerhamomyces anomalus and Pichia kudriavzevii keep a high pectinase activity even when acidity rises. This precision goes exactly in the direction of the trial published in Yeast in 2023, which the section mobilizes against the enzymatic thesis. The work thus holds simultaneously the two positions that the plate sets against each other, without seeming to notice that they conflict.

It is possible that this cohabitation is less incoherent than it looks, and untangling it is more useful than pointing it out. Three distinct questions are confused here under a single formula.

• Do pectinases work?Nobody doubts it, and the 2023 trial measures them. On this point, the work, Avallone and the 2023 trial agree.
• Whose are they?This is the only real disagreement. Avallone was looking for bacterial enzymes and found none; the 2023 trial attributes the first twelve hours to the fruit's enzymes and the rest to yeasts; Hackler and Särchen, when they are specific, name yeasts. The thesis truly contradicted is therefore the bacterial thesis, which is precisely the one the trade repeats.
• How much does the enzymatic share weigh against acidification?This is the question that none of the three sources closes. The modality held at neutral pH in 2023 puts the acid explanation in difficulty without eliminating it.

What would settle the matter is a measurement nobody has: the pectinolytic activity of a single tank, followed over time and attributed to its origin, fruit, yeasts or bacteria, for example by separating the fractions and comparing with controls in which each source is neutralized in turn. The 2023 trial comes close by subtraction, but it does not quantify the shares. As long as this breakdown is missing, the right sentence is not that the enzymatic thesis is false: it is that it is true of the wrong family of organisms, and that we do not yet know what it weighs.

Hackler and Särchen, chapter 4, p. 50 for the enzymatic thesis, and p. 44 for the pectinase activity of yeasts. Avallone and co-authors, International Journal of Food Science and Technology, 37, 2002. Yeast, 40, 2023, pp. 425 to 436.

Removing the yeasts to see what they do, twice and by two teams

The 2023 trial that the section mobilizes is not isolated. An experiment of the same type, and of the same logic, preceded it by three years, and it changes the weight of the argument.

Elhalis, Cox and Zhao deliberately removed the yeasts during the processing of a washed coffee, and the coffee obtained was, in the phrase taken up by Hackler and Särchen, tasteless. The result was published in 2020 in the International Journal of Food Microbiology, that is, in a peer-reviewed journal of a field demanding about microbiological protocols.

This type of experiment deserves to be explained for its own sake, because it is rare in this field and is worth more than the others. Nearly all the literature on coffee fermentation proceeds by addition: a strain is inoculated, compared to a spontaneous lot, and the conclusion is that the strain does something. An addition shows at best that an organism can act.

A subtraction shows something else, and something stronger: that without it, the thing does not happen. This is the difference between sufficient and necessary, and it is the only way to establish that an actor is indispensable rather than present.

The site can therefore write, without stretching, that the selective removal of yeasts was carried out at least twice, by two independent teams, three years apart, with different protocols, and that both times the coffee obtained proved defective: tasteless for Elhalis and his co-authors, with a degradation of the mucilage that remained incomplete in the 2023 trial. It is, in this field, one of the few results to have been reproduced.

Two reservations must travel with it, and they are practical in nature. The first is that removing yeasts requires an antifungal, that is, a product that is itself a treatment: without a control in which the antifungal is applied to a modality that must not lose its yeasts, one cannot exclude that the effect comes in part from the product and not from the absence of the organisms.

The second is that “tasteless” is a sensory verdict, and that neither the number of tasters, nor blinding, nor the scale used is given in the secondhand account available here. The article will have to be read before this phrase is carried as a measurement.

What would complete the case is simple to state: a third replication, on another origin, with the antifungal control described above and a blind panel. Nothing in these two trials requires rare equipment.

Hackler and Särchen, chapter 4, p. 43, after Elhalis, Cox and Zhao, International Journal of Food Microbiology, 2020. Comparable trial: Yeast, 40, 2023, pp. 425 to 436.