THE PROCESS IN MOTION

What goes wrong, and in which direction

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

▶ Play the process01Too long, with air02Too long, without air03Drying too slow04The worst is not the strongest

Too long, with air

A fermentation does not degrade progressively, it tips over, in a direction that says what has taken over. Too long with air, the bacteria that make vinegar take hold: acetic acid, sought in low doses as a ripe fruit note, becomes disqualifying. The cup turns sharp, then sour and hollow.

Read the stages without animation

1. Too long, with airA fermentation does not degrade progressively, it tips over, in a direction that says what has taken over. Too long with air, the bacteria that make vinegar take hold: acetic acid, sought in low doses as a ripe fruit note, becomes disqualifying. The cup turns sharp, then sour and hollow.
2. Too long, without airWithout air, other pathways open: butyric acid smells of rancid butter, propionic acid of cheese and sweat. They come from bacteria that live only in the total absence of oxygen, the Clostridia and the Propionibacteria. Their perception threshold is very low: a tiny proportion condemns a lot.
3. Drying too slowAfterward, drying that is too slow lets molds settle in: a taste of mold, earth, medicine, and a real health risk. Ochratoxin A is produced when water activity stays too high for too long: it is drying, not fermentation, that protects.
4. The worst is not the strongestA clearly vinegary lot is detected at the first cup and rejected; a slightly drifted lot passes the purchase tasting, ages badly and shows itself three months later. A foul-smelling bean, called a stinker, an isolated and severely defective bean, has often stayed at the bottom of a poorly emptied tank. Immature cherries, with a different sugar and a different pH, pull the whole lot toward astringency.

To remember: A fermentation does not degrade little by little: it tips over, and the direction says what has taken over.

Read the full article and its sources ↓

A fermentation does not degrade progressively, it tips over. And it tips over in a recognizable direction, depending on what has taken over.

• Too long, with airThe bacteria that make vinegar take hold. The cup turns sharp, then plainly sour and hollow.
• Too long, without airOther pathways open, producing smells of cheese, rancid butter and onion. This is the defect buyers call “fermented,” and it is the one that condemns a lot.
• Drying too slow afterwardMolds settle in. A taste of mold, earth, medicine, and a real health risk.
• Immature cherries in the lotThey do not have the same sugar or the same pH. They ferment differently and pull the whole lot toward astringency.

One thing to remember: the most serious defect is not the strongest. A clearly vinegary lot is detected at the first cup and rejected. A slightly drifted lot passes the purchase tasting, ages badly, and shows itself three months later.

The molecules of the defect, and the thresholds at which they are sensed

Fermentation defects carry trade names that cover identified chemical families:

• Acetic acidVinegar. Produced by the acetics from ethanol, therefore only in the presence of oxygen. This is a case where a compound is sought at low doses, as a ripe fruit note, and disqualifying beyond.
• Butyric acidRancid butter, vomit. Its perception threshold is very low, which explains why a tiny proportion is enough to condemn a lot.
• Propionic acidCheese, sweat. Same mechanism, same consequence.
• Phenolic compoundsMedicine, bandage. Associated with specific contaminations and with drying that is too slow.
• “Stinker”An isolated bean that has become severely defective, often a bean left at the bottom of a poorly emptied tank from a previous lot. A single one is enough to mark a cup, which makes it a problem of tank hygiene before it is a problem of process.

The health risk, for its part, is not played out in the tank but in drying: ochratoxin A is produced by molds that settle in when water activity stays too high for too long. It is drying, not fermentation, that protects.

Plate VIII, section on defects; plate IX, section “Drying, a floor and a ceiling”

The five pathways the plate does not name

The section describes the defects by their smells, cheese, rancid butter, onion, and by their immediate cause, too long and without air. The folded block names the molecules. What is still missing is the name of the organisms and of the pathways they take, and that is precisely what turns a list of symptoms into an explanation. Hackler and Särchen describe five fermentations beyond the three that the plate knows, and two of them alone account for the most serious defects of the trade.

One word is indispensable first: acetyl-coenzyme A. It is the crossroads of cellular metabolism, the molecule toward which the breakdown of sugars, of fats and of amino acids converge, and from which most synthesis pathways start again. Without this name, one cannot explain why a tank switches from rancid butter to heavy alcohol.

The butyric pathway, and the switch that follows it

The pyruvate from glycolysis is converted into acetyl-coenzyme A, then into butyric acid by specialized enzymes. The operation takes place only under strict anaerobiosis, that is, where oxygen is absent and would be toxic to the organism carrying it out. The bacteria responsible are the Clostridia, and their mechanism is heterofermentative: they do not yield only butyrate, they simultaneously produce acetic acid. That names the organism behind the rancid butter note, and explains why a tank that drifts that way also smells of vinegar without any acetic acid bacterium being involved.

What follows is the most useful point of the chapter. As butyric acid accumulates, the medium acidifies, and this acidity eventually no longer suits the bacteria that produced it. They then switch to a second pathway, called acetone-butanol, in which butyric acid is reduced to butanol, with production of acetone and ethanol. The consequence for the plate is a reformulation: the drift of a long, closed tank is not a continuous degradation, it is a chain of switches each triggered by the product of the previous one. A lot does not sink, it changes regime, and it changes because it succeeded in its previous regime.

The propionic pathway

It is carried by the Propionibacteria, the same bacteria that make the holes and the taste of certain pressed cheeses. They do not use the ordinary respiratory cycle but a detour called the dicarboxylic pathway, or Wood-Werkman pathway, after the two researchers who described it, and this detour works only under strict anaerobiosis. That ties the second molecule on the list of defects to its organism, the one that gives the note of cheese and sweat.

The formic pathway

It is carried by the enterobacteria, that is, by the very flora that section 03 describes as that of the start. Under anaerobiosis, an enzyme called pyruvate formate-lyase cleaves pyruvate into formic acid and acetyl-coenzyme A, which allows these bacteria to keep producing energy when oxygen disappears. The pathway at the same time yields ethanol, 2,3-butanediol, acetic acid and lactic acid. We now know what the enterobacteria make during the first hours, while waiting to be supplanted by the acidification they themselves helped trigger.

The citric pathway, and acidity as a weapon

It is carried by Aspergillus niger, a mold, under aerobiosis, from sucrose, and it requires an initial pH below two. This requirement is interesting for a reason that goes beyond coffee: a medium that acidic guarantees a low probability of contamination by other organisms. Acidification is not only a waste of metabolism, it is also a weapon of selection, and that sheds light on why the fall in pH of a tank is as much an ecological event as a chemical one.

The alkaline pathway, or what does not happen

This one starts not from sugars but from proteins, whose breakdown into amino acids releases ammonia and makes the pH rise to eight or more, instead of falling. It is carried by Bacillus, under aerobiosis, and the authors give it a subordinate role in coffee. This negative mention has value, because it says what does not happen: it avoids the belief that every fermentation acidifies, and it explains why pH remains a usable indicator for stopping, which it would cease to be if this pathway weighed more.

Hackler and Särchen, chapter 3: butyric pathway and acetone-butanol switch p. 30, propionic pathway p. 31, formic pathway p. 29, citric pathway p. 31, alkaline pathway p. 32. Glossary, pp. 112 to 118, for acetyl-coenzyme A and the Wood-Werkman pathway.

A defect is not a molecule, it is a threshold crossed

The section says that a fermentation does not degrade progressively, it tips over. That is right for the microbial population. It is less so for the molecules, and the distinction deserves to be made, because it changes how a tank is run.

Hackler and Särchen write that if the fermentation stops too early, it can lead to an incomplete metabolism, and that if it lasts too long, the microorganisms have more than enough time to produce undesirable by-products in sufficient quantities, which affects sensory quality. The phrase to remember is that one: in sufficient quantities. It says implicitly that these molecules are not absent before and present after.

The butyric acid of a lot condemned at the seventy-second hour was already there at the sixth; it was not perceptible. What tips is not the chemistry, it is the crossing of a perception threshold.

This reading explains something the list of molecules leaves unexplained: why butyric acid condemns a lot at tiny proportions while acetic acid is sought at low doses. Both are present all the time. Their perception thresholds differ by several orders of magnitude, and this gap alone makes one a ripe fruit note and the other a defect. Running a fermentation is therefore not preventing molecules from appearing, it is keeping them under their respective thresholds, which is a very different goal and a much more attainable one.

A lead, which Kornman reports without a source, extends this reasoning to a commercial consequence. Scientists are said to have proposed defining the ferment defect chemically rather than sensorially, by a detection threshold on certain ethyl esters, that is, molecules born of the meeting of an alcohol and an acid, responsible for the bad tastes in question. The same page adds that recent studies would suggest that the ferment defect is a microbial attack on the bean itself, and not the simple fermentation of the fruit surrounding it.

No reference accompanies these two claims, and they therefore cannot be carried as established facts. They are nonetheless worth writing down as leads, for a reason of use: a defect defined chemically, with a threshold and a method, becomes arbitrable between a buyer and a producer, whereas a defect defined sensorially never is. All the conflict in the trade over the word “fermented” comes from there.

What is missing to exploit the lead is perfectly identifiable: the name of the esters selected, the value of the thresholds, the matrix on which one measures, green bean or roasted coffee, the analytical method, and the work of correlating these thresholds with the verdicts of a panel. None of this is out of reach for a laboratory equipped with gas chromatography. In the meantime, the plate can say that the chemical definition of the defect is an open project, and name what prevents it from succeeding.

Hackler and Särchen, chapter 3, p. 23. Kornman, p. 12.