Tempering chocolate sous vide

The story of chocolate and the science of tempering

Photo by Charisse Kenion on Unsplash

While humans have been drinking chocolate for millennia, they only worked out how to turn it into a block fairly recently (not much more than 100 years). The process involves multiple steps, starting with a cacao tree.

Step 1: Harvesting

The fruit of the cacao tree (genus Theobroma, translating from the Greek as ‘Food of the Gods’) contains bitter, astringent but otherwise tasteless seeds surrounded by a sweet pulp in a pod that grows straight out of the tree trunk. The tree itself is native to equatorial Central and South America, however post-Columbian events saw it spread to West Africa and Indonesia (via. Portuguese Brazil to their West African and Asian outposts) and these countries are now the world’s largest producers.

The seeds are mostly fat and protein (80%), while the remaining solids are cells containing bitter defensive chemicals. Originally, the pulp was prized as a sweetener or fermented into a alcoholic drink. This practice goes back to ancient Maya and Aztec civilisations. It seems likely that with time it was noticed that seeds left in the fermenting pulp changed their character and the first step to chocolate was taken (in those same civilisations).

Step 2: Fermenting

The cacao pods are broken open and the seeds and pulp piled in the sun. At first, naturally-occuring yeasts begin fermenting the pulp’s sugar into alcohol. Ultimately, the yeasts consume most of the available oxygen in the thick pulp and anaerobic fermentation takes over, mediated by lactic-acid bacteria (similar to dairy fermentation — e.g. yoghurt). The pulp is left undisturbed but then oxygen is re-introduced by turning the pile and the final stage of fermentation is completed aerobically by bacteria that produce acetic acid (as in the production of vinegar).

Throughout this process, the seeds themselves do not ferment. But, once the acidic stages are reached, the acidic pulp begins to break down the seeds through tiny perforations and causes their chemical compounds to react and produce more interesting flavours. The damage also activates enzymes in the seeds that break down proteins and carbohydrates into their flavourful building bocks (amino acids and sugars). Finally, the perforations allow flavours from the fermenting pulp to be absorbed by the seeds.

The last step is to clean the seeds from their pulp and dry them for preservation. The seeds are now ‘beans’ and can be exported to manufacturers in various countries for further processing.

Step 3: Roasting

The beans are an improvement over the raw seeds, but roasting the beans takes this further. The roasting temperatures (which are usually around 130C) are sufficient to initiate flavour-producing Maillard reactions. These reactions occur between the amino acids and sugars released by the seed enzymes during the fermentation phase. The Maillard reaction is also what gives seared meats, bread and coffee their flavours.

Step 4: Grinding

The roasted beans are broken open and their contents (nibs) are selected out to be ground. The nibs consist of fats (saturated) and solids (protein, fibre, carbohydrate). Fat content is about 50%. The nibs are ground to produce a fatty flavourful paste, known as cocoa liquor (rather confusingly, the term cocoa refers to processed cacao seeds).

The grinding of the cocoa liquor continues until the solid particles are not gritty in the mouth. The liquor can then be diluted with water or milk to make a drink. This drinking chocolate goes back thousands of years to ancient C. and S. Americal civilisations where it was highly-prized. However, block chocolate had to wait for mechanisation to be practical and is a much more recent invention. There are a few more steps to go.

Step 5: Pressing and conching

Warm cocoa liquor is pressed to separate out the fat (cocoa butter). The remaining solids are powdered to produce cocoa powder. The butter is reserved to make chocolate. Cocoa powder is often ‘dutched’ (the process was invented by a Dutchman). Natural cocoa powder is acidic and dutching adds potassium carbonate that raises the pH to make the powder neutral or slightly alkaline. This creates a milder flavour by modifying the chemical nature of the powder. It also means that natural and dutched cocoa powder are not interchangeable in recipes in which pH matters.

For chocolate, cocoa liquor is put through a device that rubs and scrapes the liquor in the presence of mild heat in a machine known as a conch (invented by Rudolphe Lindt in 1878, it had a shell-like shape). Other flavours (sugar, vanilla, powdered milk etc) are added now. Even with modern technology, this phase can be left to run for up to 3 days for high-quality chocolate. The purpose is to coat the solid particles evenly in cocoa butter and to evaporate flavour molecules, most of which turn out to be undesirable ones (as many as 80% are lost in this process). New desirable flavours are created by heating and time. It seems the gods are on our side.

The reserved cocoa butter is added during conching, mainly to adjust for the added sugar and any other solids, so keeping everything suspended in the butter. An emulsifier (soy lecithin) is frequently added for the same purpose.

Step 6: Tempering

All that remains now is to pour the chocolate into a mold and cool it. But this stage is not straighforward. The chocolate has to be tempered to produce the snap and bite that we expect from quality blocks.

Cocoa butter is composed of saturated fatty acids. Saturate fatty acids are mostly straight and can pack together closely. The fatty acids in cocoa butter are particularly regular and they pack together closely — cocoa butter is solid at room temperature.

But, it is how the cocoa butter solidifies that matters. In fact it can soldifiy in at least five different configurations (usually referred to as Type I, II, III, IV and V). It is Type V we want — this has the most regular structure and tight packing and gives chocolate its pleasant snap and glossy patina.

Again, the gods are on our side — all of these Types have different melting temperatures and Type V has the highest melting point. Thus Type V can be selected by tempering.

Types I-IV melt in the range 15–28C, whereas Type V melts between 32–34C. That gives a 4C window (28–32C) in which Type V can be solid, while all the other Types are melted. If the chocolate is held in this range for a while to let some Type V develop, and then cooled slowly, the solidifying fats will preferentially attach themselves to the Type V form which will grow and coalesce.

The process is like seeding in crystalisation, and indeed the Types are often referred to as crystals by this analogy. Some previosly tempered chocolate can be ground and added to the mix to further promote seeding. The mixture is slowly stirred to encourage the crystals to link up in an orderly way.

In practice, the holding-temperature range is quite narrow and a bit tricky to maintain in a pot on the stove. That’s where sous vide precision comes in (at last).

Sous vide tempering

The no-mess method is to vacuum-seal chocolate in a plastic bag (e.g. foodsaver). Set the water bath to 45C and completely melt the chocolate. Replace some water in the bath with cold water and bring the temperature down to 28C (or leave the water and drop in some ice cubes — it takes a surprising amount though). Leave the chocolate at this temperature for a few minutes to allow Type V seed crystals to form, then reset the temperature to 32C and massage the bag every minute or so as the temperature rises to 32C. The chocolate is now tempered but still molten. If cooled (not too qickly) it should take on the Type V form throughout as it solidifies. The tempered chocolate can be held in the 32C bath until it is needed, and it will remain tempered for many hours.

Why can’t you just melt chocolate at 32C to begin with? It takes forever to melt (hours) if it melts at all. Chocolate has a kind of ’memory’ (hysteresis) that means its behaviour depends on the process of getting there.

Most modern chocolate is already tempered. So, you can keep back some of the chocolate before the intial melt, grate it and add it back in as a seed after the 28C step. This means opening the bag and resealing it and so makes the process a little more messy. A ziplock bag would be the bag of choice. Ensure the bag is completely dry before opening it, as even a trace of water in the chocolate will interfere with tempering.

A modified approach is to float a thin stainless-steel bowl in the water bath and temper the chocolate in that, going through the same water temperature changes. I haven’t tried this method. Details here.

Chocolate varieties

Dark chocolate is essentially the raw material — cocoa solids embedded in Type V crystallised cocoa butter with probably some vanilla, sugar and lecithin.

Milk chocolate is the same, with added milk powder to mellow the flavour. It was created by a Swiss chocolatier soon after his compatriot, Henri Nestlé, had produced the first powdered milk.

White chocolate uses no cocoa solids. It is just cocoa butter, milk powder and flavourings. Which probably explains why it tastes like soap.

Cheap chocolate substitutes other fats for some of the cocoa butter. As a result, it doesn’t have the same physical properties and can be soft and cloying. Some can include hydrogenated vegetable oils which may contain trans-fats.

Chocolate and health

Chocolate’s bad reputation masks a greater complexity.

Cocoa powder contains many biologically active compounds and is known to confer cardiovascular benefits. It is thought that this is mainly due to its polyphenols.

However, chocolate contains more than cocoa solids, and cocoa butter is a saturated fat. Even so, there are complications. The main fatty acid is stearic acid (also a significant component of bacon fat). On ingestion, a portion of stearic acid is converted into oleic acid by the body. Oleic acid is unsaturated and the main fatty acid in olive oil. Plus, stearic acid is unusual for a saturated fatty acid, it does not raise LDL (cholesterol) and in high doses can decrease it.

Sugar is not a natural component of chocolate — it is an added ingredient, so sugar levels can be selected to some degree by the consumer (within a manufacturer-defined range).

Chocolate has a fairly high Caloric value — around 5 Cal. per gram. By comparison, dairy butter has about 7 Cal. per gram and pure fat (such as olive oil) has about 9 Cals. So, any adverse health effects of chocolate might just be down to weight-gain from excess eating, which can be easy enough to do.

Eating chocolate can lead to a desire for eating more chocolate. There is much popular discussion about the addicitve properties of some cocoa compounds (as well as their psychotropic properties). None of this has stood up to scientific investigation (the concentration of these compounds is just far too low). We know this partly because if chocolate is swallowed by habitual chocolate eaters in capsule form, there is no compulsion to continue eating it. The more convincing explanation is not chocolate’s chemical makeup but rather its physical makeup. Type V cocoa butter crystals are fully melted at 34C, which is conveniently just under the temperature in our mouth (which in turn is just under body temperature). Hence, a quick ‘snap’ on biting is followed by a luxuriant melting creating an experience we want to repeat. Chocolate is known to release dopamine in the brain, and the resulting sense of pleasure is probably what drives a compulsion in some. Chocolate also has a cooling effect because it absorbs heat from the mouth as it changes from the solid to the liquid states, adding another dimension to the experience.

“Food of the Gods”, indeed.