A coffee cherry is a pit surrounded by sugar. The pit is the two beans. Around it is a layer of sticky, very sweet flesh, the mucilage, then the skin. As soon as the cherry is picked, yeasts and bacteria begin to eat this sugar. That is all that “fermenting” means.

They do not eat it for nothing. By consuming the sugar, they give off other things: alcohol, acids, carbon dioxide, and dozens of aromatic molecules. It is this output that interests the producer, because part of it passes into the bean and ends up in the cup.

And why let it happen, rather than washing the sugar off with a jet of water right away? For three reasons, which must be kept separate because they are constantly confused. The first is mechanical: mucilage is sticky, and fermentation makes it let go. The second is about taste: the molecules produced change the flavor. The third is practical: a bean freed of its flesh dries much faster, and therefore molds much less.

Fermentation, respiration, maceration: three words used for one another

Strictly speaking, a fermentation is a metabolism that regenerates its electron carriers without an external acceptor: the pyruvate coming from glycolysis serves as its own acceptor, and yields, depending on the pathway, ethanol, lactate, acetate or butyrate. Respiration, for its part, uses an external acceptor, most often oxygen. The two coexist in a coffee tank, and it is precisely the passage from one to the other that structures the process.

Three other mechanisms take place at the same time and are not fermentations, which explains a good share of the misunderstandings:

• Enzymatic autolysisThe fruit contains its own enzymes, active after picking. They break down its walls without any microbe being involved.
• MacerationThe mere passage of soluble compounds from the medium into the bean, by diffusion. It is a transport, not a metabolism.
• Chemical acidificationThe acids produced solubilize the pectins by breaking down the network, independently of any enzyme.

A good share of the disagreements in the literature comes from authors measuring one and concluding about the other.

Wilbaux, Le café, sa préparation (1956); Coste, Le caféier (1968), ch. on preparation; professional training document

Two against twenty-seven, or why oxygen commands everything

The previous section contrasts fermentation, which gets by without an external electron acceptor, and respiration, which uses one. This is a fair distinction, but it does not say what each one yields to the cell that practices it. Yet it is this balance, and it alone, that explains why a microorganism changes pathway as soon as air becomes scarce, why an open tank heats up, and why a closed tank consumes more sugar for a comparable result. Hackler and Särchen set it out in two pages, and it comes down to two numbers.

Glycolysis, this chain of reactions common to nearly everything alive, cuts a six-carbon sugar into two molecules of pyruvate, which carry three each. The operation yields two molecules of adenosine triphosphate. This molecule is the cell's energy currency: everything a microorganism makes, transports or repairs is paid for in adenosine triphosphate, and the unit is the same in yeast, in bacteria and in a human muscle. Two units, then, for one sugar cut up, and this is the guaranteed share, the one collected whatever the conditions.

What happens next to the pyruvate depends on one thing only, the presence of oxygen. If there is some, the two pyruvates enter the tricarboxylic acid cycle, the oxidation loop that textbooks also call the Krebs cycle, then the electrons it releases go down a chain of carriers to the oxygen, which collects them. This descent yields twenty-five additional molecules. If there is no oxygen, the cell has no one left to hand its electrons to: it then gets rid of its pyruvate in order to recycle its own carriers, reducing it to ethanol and carbon dioxide, or to lactate, or to butyrate depending on the available pathway, and it gets nothing from the operation beyond the two units from the start.

Two against twenty-seven. The ratio exceeds thirteen. It must be added at once that these twenty-seven are a theoretical ceiling, that real yields are lower and vary from one species to another, but the order of magnitude does not move: respiring is more than an order of magnitude more profitable than fermenting.

This one datum explains two claims that the plate makes elsewhere without giving them a cause. The first is thermal. A cell that draws thirteen times more energy from a sugar also releases much more of it as heat, because no biological conversion is perfect and the lost share grows with the total converted. This is exactly what the sheet on micro-oxygenation measures: up to thirty-seven degrees in an open tank, against an excess of only zero point five to three point seven degrees above ambient in a closed tank. The difference is not one of insulation, it is one of metabolism.

The second claim is counterintuitive, and that is what makes it interesting. Knopp, Bytof and Selmar found more residual glucose and fructose in beans processed under aerobic conditions, in other words where oxygen was not lacking. One would expect the opposite, since oxygen activates everything. The explanation is purely arithmetic: under aerobiosis, a cell draws thirteen times more energy from each sugar molecule, so it consumes thirteen times less to do the same work.

Aerobiosis does not speed up the disappearance of sugar, it economizes it. This is also, in passing, the mechanical explanation for the fuller body of honey coffees, which Hackler and Särchen explicitly link to the fact that the coffee dries in open air and not under water: less sugar is broken down there, and more reducing sugars remain in the bean.

The consequence must finally be drawn for reading the claims. When a producer lengthens a closed-tank fermentation, he does not merely extend a duration, he keeps his population in the least profitable regime there is. The microorganisms compensate with volume: to live, they must burn far more raw material. This is the deep reason why a long closed tank exhausts the sugar and then switches to something else, the acids already produced first, and why the margin between the remarkable and the defective narrows so quickly there.

Hackler and Särchen, chapter 3, pp. 25 and 26 for the balance; chapter 3, p. 23 for residual sugars, after Knopp, Bytof and Selmar, 2006; chapter 6, p. 93 for the honeys. Tank temperatures: sheet 16, Cenicafé trial.