Conching is a grinding problem and a ventilation problem at the same time

A finished chocolate is a powder held in fat

There is almost no water in a bar of chocolate. What there is instead is a mass of solid particles — cocoa solids, sugar crystals, and in milk chocolate a quantity of dried milk — suspended in cocoa butter, which is the only continuous phase in the whole thing. Everything that can be said about texture therefore comes down to two questions: how big those particles are, and how well the fat has coated them.

The tongue is a reasonably good instrument here and its limit is well established. Particles below roughly twenty micrometres aren’t perceived individually and the mixture reads as smooth; above that they are felt as grit, and the sensation gets rapidly worse as the size climbs. Refining a chocolate means taking the whole particle-size distribution under that threshold, and conching is the stage where the last of that work is done and the particles are finally wetted with fat.

The mechanical half of the job

Refiners break the sugar and cocoa solids down, but a freshly refined mass is a dry, crumbly powder because the newly created particle surfaces haven’t yet been covered by cocoa butter. Conching applies hours of shear to that powder, rounding the sharpest edges, coating each particle, and turning a dusty solid into something that flows. The transformation is dramatic to watch and it happens without adding anything at all in the first phase.

Fat and lecithin are usually added late, once most of the coating has been achieved, and the reason is efficient rather than mysterious. Shear works better on a stiff mass than on a fluid one, so adding the fat early would make the machine less effective at the very job it is there to do. That sequencing is one of the small pieces of process knowledge that separates a well-made chocolate from a merely adequate one.

The chemical half is about what leaves

A cocoa mass fresh from roasting carries a considerable load of volatile compounds, including acetic acid from the fermentation stage and other short-chain acids that read as sour, harsh or vinegary. Conching runs warm and open, with an enormous surface constantly being turned over, and a great deal of that volatile fraction simply evaporates. The measurable acidity of the mass falls over the course of it.

Residual moisture leaves at the same time, which matters more than it sounds, because water in a chocolate mass makes it thick and unmanageable by encouraging sugar crystals to stick to one another. Some further Maillard-type reaction also proceeds at conching temperatures, generating new aroma compounds while the raw ones are being driven off. So the flavour at the end isn’t the flavour at the start with the rough edges removed; it has been altered as well as stripped.

Time is a real variable and not simply a marketing one

Conching times vary enormously across the industry, from a few hours to several days, and longer is not automatically better. A very long conche produces a mellow, rounded chocolate at the cost of the bright fruit and acid notes that a fine-flavour bean was selected for in the first place. A short one keeps that character and risks leaving genuine harshness in.

Temperature is the other lever, and it interacts with time. A warm conche drives volatiles off faster and pushes further reaction, so a few hours hot can resemble a great many hours cool in some respects and differ sharply in others. Milk chocolate is generally conched cooler than dark because milk solids scorch, which is one of several reasons the two are not made on the same schedule even in the same factory.

Which of those is preferable depends entirely on what the bean is and what the maker is aiming at, and there is no consensus. Many small makers deliberately conche briefly because the origin character is the product; large-scale manufacture of a consistent, mild, familiar chocolate has different priorities and conches accordingly. Both positions are defensible and neither is the correct one.

What this explains about eating

A chocolate that feels waxy or heavy in the mouth usually has a viscosity problem, which is a coating and moisture issue rather than a fat-quantity issue, and it is why adding more cocoa butter to a badly conched chocolate improves it less than you would expect. A chocolate that feels sandy has a particle-size problem that no amount of tempering will hide.

It also explains why chocolate made at home from cocoa nibs in a domestic grinder is almost always disappointing. Reaching twenty micrometres takes many hours of continuous grinding under real shear, and the ventilation that removes the acids is a separate requirement again. The result of an afternoon of effort is usually gritty and sour, which is a fair description of what chocolate was before the machinery existed.