Temper Temper

To make chocolate look good, with a high gloss, and have a good snap it must be tempered. Tempering chocolate ensures that all the fat in the chocolate sets in the correct crystalline form.

To understand tempering we must first understand the structure of fat. The vast majority of the fats in chocolate are triglycerides. A triglyceride is a class of fats made up of glycerol linked to three fatty acids. Furthermore, three main fatty acids account for about 95% of the fat present. These three fatty acids are oleic acid (35%), stearic acid (34%), and palmitic acid (26%). Stearic and palmitic acid are saturated fatty acids, meaning all of the carbon atoms are saturated with hydrogen, while oleic acid is a monounsaturated fatty acid, there is one double bond which could be hydrogenated.
Fatty acids are called fatty acids because they have a long aliphatic carbon chain with an acidic carboxylic acid group at one end.

This diagram shows one of the ways these fatty acids are attached to the glycerol. Shown here is what is known as a SOP triglyceride because it has stearic acid attached to the first glycerol carbon, oleic acid in the middle, and palmitic acid in the third position. If we were to switch the oleic and stearic acids it would be a very different molecule.

As I said earlier stearic and palmitic acids are saturated while pleic acid is unsaturated. So our triglyceride is one of a subset of triglycerides found in cocoa butter known as SOS triglycerides. This set of triglycerides has oleic acid sandwiched in between two saturated triglycerides. SOS triglycerides make up 80% of the fat in cocoa butter. Because the fats in cocoa butter are primarily SOS triglycerides they melt over a short temperature range (room temp to body temp).

Cocoa butter is polymorphic which means that it can crystallize in multiple arrangements. Carbon is also polymorphic, it can be a diamond, graphite, or a bucky ball. While carbon has three polymorphs cocoa butter has six. These six forms were named by two different groups in the same year (1966). The chocolate industry uses Roman numerals I-VI as described by Willie and Lutton, while the oils and fats people prefer using Greek letters with superscripts and subscripts as set forth by Larsson (γ, α, β'₂, β'₁, β₂, β₁).

Form I (γ) melts from 61° to 67° F

Form II (α) melts from 70° to 72° F

Form III (β'₂) melts from 78° to 80° F

Form IV (β'₁) melts from 81° to 84° F

Form V (β₂)melts from 93° to 95° F

Form VI (β₁) melts above 97° F

Only one of these six polymorphs (form V) forms the shiny, firm chocolate that gives the nice snap when broken that we like. If we want to enrobe a truffle or a strawberry with chocolate and we want it to snap when we bite into it we need the chocolate to be in form V. Luckily chocolate comes from the factory in form V. If we can keep it there we will have lovely chocolates. Therefore our goal when we melt the chocolate is to never heat it above 91°-92° F.

Most chocolate melts between 89° -91° F even though all of the form V won't melt until you heat it above 95° F. Between 91° -95° F the chocolate still contains some form V microcrystals of cocoa butter. If you let the chocolate harden around these seed microcrystals it will all form form V chocolate.

If however you completely melt all of the precious form V crystals you need to start over by completely melting all of the chocolate. Be careful not to burn or separate the chocolate by heating it too high.

Next you should cool the chocolate rapidly to about 80° F and always use constant stirring. Cooling this low does allow some of the form III and IV crystals to form, but it gets a good start on the crystallization of the highly prized form V crystals. Now gently warm the chocolate to 86° F. This start the will start to melt the form III and IV so it should be held at this temperature for a few minutes, then warmed up to 91° to 92° F . This will finish melting all of the form III and IV crystals that were formed while cooling.

You can now test the chocolate to see if it is in temper by spreading a smear out on a piece of waxed paper, if it dries shiny and hard within 5 minutes your chocolate is tempered. Now that you have tempered chocolate you should eat it and enjoy. Just be careful about bloom (we'll get to that next time).

Seven Basic SI Units

There are seven basic SI units: the meter, second, ampere, kelvin, mole, candela, and kilogram. Of those seven only the kilogram is still defined by a man made object with no referent in nature. This means that in a vault outside of Paris there is a platinum and iridium cylinder (seen below nested in three glass jars) that cannot gain or lose mass. If you could get past security and cut off a small chunk it wouldn't have lost any mass at all. That's because it defines the kg. All masses are referenced to it. In 1889, 40 copies of this cylinder were made for national standards institutions around the world. These standards have all changed mass in different ways over the years; mass gain through dust accumulation or mass loss through cleaning. And although they can compare the copies with the original with an accuracy of better than 10^-9 kg it is still not good enough because thy don't know how the standard has changed. This therefore has led to groups trying to define the kilogram with a universally verifiable standard.

Two major approaches are being followed: atom counting and electrical approaches. The atom counting approach is easy enough to understand-count every atom in a sample then multiply by the mass of the atom and then you have the mass of the entire sample. Unfortunately we can't do this yet. What we can do is grow very pure crystals. The leader in the atom counting field is the Avogadro Project. They are seeking to make a single crystal silicon sphere, using laser interferometry to measure the diameter and then because they know the crystal structure of silicon they can calculate the number of atoms in that sphere.

Why use silicon? Because technology has been developed to purify silicon to a higher purity than anything we can make. Silicon used in the semiconductor industry is >99.99999% pure. It is made through a multi-step process first with zone refining and then a single crystal boule is grown from pure molten silicon. The isotopic make-up is then measured with a mass spectrometer and average atomic mass is then calculated. This single cryastal is then cut and polished into a perfect sphere. Alright it isn't perfect but it is nigh-perfect. To quote Wikipedia:

On the Ø 93.6 mm sphere, an out-of-roundness of 35 nm (undulations of ±17.5 nm) is a fractional roundness (∆r/r) = 3.7 × 10^–7. Scaled to the size of Earth, this is equivalent to a maximum deviation from sea level of only 2.4 meters. The roundness of that ACPO sphere is exceeded only by two of the four fused-quartz gyroscope rotors flown on Gravity Probe B, which were manufactured in the late 1990s and given their final figure at the W.W. Hansen Experimental Physics Lab at Stanford University.

The problems that they are facing are things like too much variation in the isotopes of the silicon and the thickness of the silicon oxide layer that grows on the sphere.

The second proposed method for redefining the kilogram is called the electronic kilogram. The idea is to design an incredibly sensitive balance that matches the weight of an object (the force exerted on an object by gravity) to an electromagnetic force produced by a coil of current carrying wire in a strong magnetic field. Then you could define the kg as 'the mass that can be suspended by the electromagnetic force generated when a specific amount of current flows.' So far the electronic kilogram has produced better results than the atom counting method, but it still can't provide an accuracy of a millionth of one percent every time.

Watt-Balance

1. Robinson, Andrew. The Story of Measurement. London: Thames & Hudson, 2007.